BOOKS 
BY 
PERCY G. STILES 
Human Physiology 
i2mo of 425 pages, illustrated. 
Cloth, $2.25 net. Third Edition. 
Nutritional Physiology 
i2mo of 306 pages, illustrated. 
Fifth Edition. 
The Nervous System and Its 
Conservation 
i2ino of 240 pages, illustrated. 
Cloth, $2.25 net. Third Edition. NUTRITIONAL 
PHYSIOLOGY 
BY 
PERCY GOLDTHWAIT STILES 
1i > 
ASSISTANT PROFESSOR OF PHYSIOLOGY IN HARVARD UNIVERSITY 
FIFTH EDITION 
PHILADELPHIA AND LONDON 
W. B. SAUNDERS COMPANY 
1924 Copyright, 1912, by W. B. Saunders Company. Reprinted September, 1914, 
and June, 1915. Revised, reprinted, and recopyrighted October, 1915. 
Revised, reprinted, and recopyrighted January, 1918. Re- 
printed April, 1920. Revised, reprinted, and recopy- 
righted January, 1922. Revised, reprinted, and 
recopyrighted October, 1924 
Copyright, 1924, by W. B. Saunders Company 
MADE IN U. S. A. 
PRESS OF 
W. B. SAUNDERS COMPANY 
PHILADELPHIA TO 
GSrabam Uugfc 
WHOSE DELICATE KINDNESS NO LESS THAN HIS 
SCHOLARLY POWER MADE MEMORABLE THE 
YEAR OF OUR ASSOCIATION PREFACE TO THE FIFTH EDITION 
The last three years have been productive beyond 
the average in contributions to our knowledge of nutri- 
tion. An earnest attempt has been made to give place 
to the more significant data in the chapters which fol- 
low. The discovery of insulin is probably the outstanding 
achievement which we are able to recount. So far as the 
work of revision consists in the addition to the text of 
new and concrete statements of fact the task is an easy 
one. What is much more difficult is to express the gradual 
shifting of emphasis from one matter to another. At 
one time psychologic factors in nutrition were especially 
stressed, later auto-intoxication was the dominant theme, 
just now it appears to be the vitamins. Indications of 
past enthusiasms can scarcely be removed from a book 
long in print by any means short of complete rewriting. 
If they are still traceable this may not be altogether un- 
desirable; each has had some justification and may re- 
ceive renewed attention at a future time. 
P. G. S. 
Boston, Mass., October, 1924. 
3 PREFACE 
The making of this book has been a study in elimination. 
It is, therefore, necessary at the outset to indicate the 
limits of its scope. It is intended that it shall be used 
with other books. Supplementary reading upon general 
biology, human anatomy, food chemistry, and dietetics 
is greatly to be desired. In the field of physiology itself 
many fascinating topics are entirely ignored and others 
treated in bare outline, with the purpose of subordinating 
all else to the subject of nutrition. Chemical formula? 
have been excluded from the text and used but sparingly 
in the notes. 
A certain preliminary knowledge of elementary science 
is assumed. The key-word of the following discussion is 
" energy." The success of the reader in gaining clear con- 
ceptions of what is presented will depend upon his famil- 
iarity with the meaning of that term. It is essential that 
he shall understand that energy is latent or potential in 
those chemical compounds which are susceptible of oxida- 
tion. He must have learned to recognize the possibility 
of its unending transformation. The more readily he 
thinks in terms of molecules, the more profitably he can 
read these chapters. 
Miss Ruth Bryant, Instructor in Biology in Simmons 
College, has borne a part in the work, which is to be de- 
scribed as collaboration rather than assistance. 
P. G. S. 
Boston, Mass. 
5 TABLE OF CONTENTS 
CHAPTER I page 
Introduction 11 
CHAPTER II 
The Energy Relations of Plants and Animals 20 
CHAPTER HI* 
The Nature and the Means of Digestion 28 
CHAPTER IV 
The Work of Muscles and Glands 36 
CHAPTER V 
Reflex Action 46 
CHAPTER VI 
The Alimentary Canal 54 
CHAPTER VII 
The Mouth-Swallowing, Salivary Digestion 63 
CHAPTER VIII 
The Movements of the Stomach 70 
CHAPTER IX 
Gastric Secretion and Digestion 80 
CHAPTER X 
The Small Intestine: Its Movements, Secretions, and 
Digestive Processes 90 
7 8 
TABLE OF CONTENTS 
CHAPTER XI PAGE 
The Large Intestine 101 
CHAPTER XII 
The Blood 109 
CHAPTER XIII 
The Circulation 120 
CHAPTER XIV 
The Absorption of Food-stuffs 132 
CHAPTER XV 
The Metabolism of Fats and Carbohydrates 139 
CHAPTER XVI 
Nitrogenous Metabolism 154 
CHAPTER XVII 
The Removal of the End-products of Metabolism 173 
CHAPTER XVIII 
The Estimation of Metabolism 183 
CHAPTER XIX 
The Energy of the Metabolism 192 
CHAPTER XX 
The Factors Which Modify Metabolism 203 
CHAPTER XXI 
The Maintenance of the Body Temperature 214 
CHAPTER XXII 
The Hygiene of Nutrition <. 222 TABLE OF CONTENTS 
9 
CHAPTER XXIII 
PAGE 
The Hygiene of Nutrition (Continued) 239 
Water; Meat; Sugar. 
CHAPTER XXIV 
Food Poisoning 252 
CHAPTER XXV 
Alcohol 260 
CHAPTER XXVI 
Internal Secretion 273 
CHAPTER XXVII 
The Nervous System 281 
CHAPTER XXVIII 
The Nervous System-Its Higher Work 291 
INDEX 299 NUTRITIONAL PHYSIOLOGY 
CHAPTER I 
INTRODUCTION 
The Modern Emphasis in Biology.-Living things are 
transformers of matter and energy. When we say trans- 
formers rather than generators we indicate the modern 
as contrasted with the old-time view. When we say that 
physiology is the physics and chemistry of living matter 
we suggest the same significant tendency to bring living 
and lifeless matter into direct comparison and to recog- 
nize the same laws as operating in both. The teaching 
that the same laws do hold sway in the living and the non- 
living is covered by the term ° mechanism " ; the earlier 
view that living things are not fairly to be compared with 
lifeless, and are to some extent exempt from physical 
principles and limitations, is expressed by the word 
" vitalism." We have every reason to believe that the 
principle of the conservation of energy holds as rigidly 
for the plant or the animal as for the clock or the loco- 
motive. This is perhaps the most important generaliza- 
tion of nineteenth century physiology. 
But while scientific workers are now seeking to analyze 
the reactions of organisms in accordance with the data 
furnished by chemists and physicists whose work has 
been with materials not living, it is probable that the diffi- 
culties of their problems are better appreciated than was 
the case a few years ago. Living matter is found to be 
more complex in structure and more varied in response 
11 12 
NUTRITIONAL PHYSIOLOGY 
than had been supposed. Physiologists are bound to be 
modest in their claims for progress. They are ignorant 
of many factors at work in even the simplest forms of 
plant and animal life. And the mystery of consciousness 
with its relation to nervous systems seems ever to defy 
approach. 
Free-living Cells.-About eighty years ago, at a time 
when investigators were profiting by important im- 
provements in microscopes, it was found that the larger 
Fig. 1.-Four types of free-living animal cells: A is the ameba, 
distinguished for its changeable form; B, the euglena, shows the 
peculiar feature known as a flagellum, a writhing filament, which 
is its means of locomotion; C is the paramoecium, or " slipper ani- 
malcule," which has a ciliated surface; D is the interesting form 
known as the stentor. 
plants and animals are made up of structural units as- 
sembled in vast numbers. These units are generally 
much too small to be seen without magnification. They 
are called cells, a term which is not especially appropriate, 
but not likely to be abandoned. Many microscopic 
forms, such as the swarming Infusoria of pools and ditches, 
are cells leading an independent existence. It will be 
helpful to consider what are the characteristic activities 
of such cells. They are for the most part equally charac- 
teristic of the higher forms. INTRODUCTION 
13 
The free-living animal cell takes something from its en- 
vironment and returns something to it. It takes into it- 
self a variety of organic substances together with small 
quantities of mineral salts. These constitute its food. 
It receives also a supply of oxygen. This is not ordinarily 
reckoned as a food and for a good reason. The term food 
is best restricted to material which can serve constructive 
purposes or at least be stored in the cell. The function 
of oxygen is not to promote constructive processes, but to 
release energy, a process of decomposition in which the 
stores of the cell are sacrificed. It is hard for the elemen- 
tary student to realize that a destructive change can be 
other than disastrous. Yet he has only to reflect that 
money becomes useful in its expenditure. It is much the 
same with organic reserves. The process in which oxygen 
reacts with substances within the cell, giving rise to 
simple oxidized products in place of complex material 
rich in potential energy, is called respiration. (The word 
is, indeed, frequently used as a synonym for breathing, 
but we shall use it in its chemical sense.) Respiration 
is often compared with combustion, and while the two 
are not identical in all their stages, the fact remains that 
the initial and the final conditions are essentially the same 
for both. The release of energy is generally just as great 
in the physiologic change as in the actual burning of like 
quantities of the cell constituents. 
The free-living animal cell is thus an accumulator of 
fuel and a furnace in which it is burnt. But this is a very 
imperfect comparison, for it has in addition the property 
of self-repair, and under favorable circumstances capacity 
for growth and reproduction. Cells multiply by cleaving 
into two similar parts, and the tendency to do this after 
a certain increase in size usually limits very definitely the 
dimensions to which a single cell may attain. When 
growth is taking place it is evident that not all the food 
is; serving as-fuel; a certain portion is becoming incor- 
porated with the more permanent-substance of the cell 
and is sb changed as to become entirely typical protoplasm. 
The process through which food becomes an integral part 14 
NUTRITIONAL PHYSIOLOGY 
of the cell is called assimilation. The word emphasizes 
through its root-meaning the attainment of likeness to 
the material of the cell and indirectly implies that the food 
was originally foreign in its nature. We use the expression 
in much the same sense when we speak of the assimilation 
of immigrant peoples. The word nutrition is used in 
about the same way as assimilation, the only distinction 
being that we speak of the nutrition of cells (or cell- 
aggregates), while we speak of the assimilation of food, 
the former term referring to the structure nourished and 
the latter to the supplies worked over for the purpose. 
The word digestion is best restricted to the preliminary 
stages of the assimilation process; its application will be 
defined later. 
Respiration has been said to be a process in course of 
which compounds are decomposed that their potential 
energy may be made available. The greater part of the 
released energy appears as heat. A smaller part mani- 
fests itself in movements through which resistances are 
overcome. The facts in regard to the production of 
energy are naturally better known for the larger animals 
than for the free-living cells, but " the whole is equal to 
the sum of its parts." The energy from respiration may 
in exceptional cases become kinetic, in the form of light 
or electric discharge (firefly, electric eel). The energy 
which shows itself in movement is of particular interest 
to us. Motion, when exhibited by animal cells, is almost 
always the expression of contraction, the word being used 
in a physiologic sense. So used, contraction does not mean 
diminution of volume, but does mean diminution of sur- 
face and active shortening in one or more dimensions. 
Although an increase in other dimensions attends such 
changes of form, we do not talk of the " expansion " of 
cells. It is the contraction which is the positive and 
forcible element in the movement. When this is said we 
intentionally leave out of account some types of movement 
occurring commonly in the plant world, in course of which 
the cells actually change their volume through gain or loss 
of water. Among free-living cells the type of move- INTRODUCTION 
15 
ment may be "ameboid," that is, a flowing of the cell 
contents to conform to an ever-changing outline. Con- 
tractile power may be limited in other cases to parts of 
cells, as in those forms which swim by the lashing of 
slender projections known as flagella, or by the waving 
of an animated nap or pile upon their surfaces, the cilia, 
of which more will be said. In all cases of energetic 
movement we feel justified in assuming that the source 
of the power is in destructive chemical reactions, and 
that a draft is being made upon the fuel stores of the 
cells. 
The Association of Cells.-When many cells are massed, 
as in the body of a worm, the situation of the single unit 
differs significantly from that of the cell leading an inde- 
pendent existence. First of all, its environment is made 
for it to a great extent by other cells. A very small 
minority are in direct contact with the outside world; 
the great majority are submerged among their fellows. 
The typical cell is, therefore, shut in from food supplies 
of the casual sort on which the free-living cell depends. 
It is remote from the oxygen of the surrounding air or 
water. A cell so situated would perish were it not for one 
of the most striking features of the larger organisms, a 
moving liquid medium, which bathes the cells and acts 
as a common carrier. This fluid supplies food and oxygen 
and removes wastes. 
The cells composing the body of any animal are of a 
common descent, but they have taken on widely different 
characters and have become adapted to particular func- 
tions. The cell which is in itself a complete living thing 
must perform all the essential activities for itself-the 
preparation of crude food, locomotion, etc. In the 
multicellular animal the individual cells have come to be 
far more restricted in their powers. Many have become 
passive structures serving only for mechanical support or 
surface protection. Such cells may or may not be living. 
Others, while clearly alive, have ceased to perform cer- 
tain functions. With limited exceptions movement is 
exhibited only by those systematically arranged cells 16 
NUTRITIONAL PHYSIOLOGY 
which form the contractile tissues. Almost all the cells 
require their food to be in solution and of a few standard 
forms. In other words, the primitive capacity to digest 
and assimilate every kind of nutriment has been lost, but 
by a wonderful co-operative activity the internal medium 
has been made a depot of those particular foods which can 
still be utilized. 
Fig. 2.-Drawings like the above are almost always made from 
tissues which have been prepared and colored by special means to 
make clear minute features: a Represents an ovum or egg-cell, the 
typical cell may be assumed to tend toward this spheric form; b is a 
cell from a compact tissue, to show how mutual pressure produces 
a faceted or polyhedral form; c is a contractile element such as 
occurs in the walls of the alimentary canal, it illustrates an elon- 
gated cell; d is an epithelial or lining cell of the order found on the 
inner surface of blood-vessels; this is an example of extreme flatten- 
ing-; e, from the nervous system, exhibits the possibility of a branch- 
ing development. 
As an animal grows larger its directly exposed surface 
becomes smaller in proportion to its weight. The trans- 
fers which must take place between the organism and the 
external world require ample surfaces, and they are secured 
by infoldings of the body wall at different places. The 
lining of the alimentary tract is an example of such an 
infolding and provides a large area for absorption. Among 
the higher forms the lining of the lungs constitutes a vastly INTRODUCTION 
17 
extended surface for gaseous exchange. The glands are 
organs in which are concealed great surfaces through 
which products of cell activity find an outlet. 
The specialization which groups the cells of the animal 
body in a number of classes each with its definite work to 
do also entails the dependence of each class upon the 
others. If we compare the life of a savage with that of a 
civilized man we shall find an analogy not too far fetched 
to be helpful. We have seen that the free-living cell is 
self-sufficient, and, indeed, its chances of survival are bet- 
ter when there are but few of its own kind in the neighbor- 
hood. Such organisms are competitive rather than co- 
operative. Somewhat in the same way the solitary savage 
may be capable of self-maintenance, having the skill to 
find and prepare his food and after a fashion to shelter 
and clothe himself. The civilized man is accustomed to 
look to many other men-and women-to supply his needs. 
Yet if the man, like the cell, loses something of ruggedness 
and resourcefulness through becoming a member of a com- 
plex society, he evidently gains time and opportunity to 
concentrate his efforts upon a special pursuit. It is very 
much the same with the cell. Our analogy fails, as such 
devices are prone to do, when we consider how the civil- 
ized man may become a hermit or a Robinson Crusoe, 
whereas no single cell detached from one of the higher 
forms can exist by itself for any length of time. 
Co-ordination.-We have emphasized above the services 
rendered to the organism by its internal medium. The 
composition of the circulating fluid is influenced by the 
ever-varying activities of all the organs and tissues. 
Accordingly, a contribution made to this medium by 
any group of cells may conceivably modify the conduct 
of any other groun. We shall meet with numerous 
instances of such influence exerted through the chemical 
products of one organ upon another or upon the system as 
a whole. When we speak of an animal as an individual 
we imply that the parts of the body constantly interact. 
It is this interaction which makes it appropriate to regard 18 
NUTRITIONAL PHYSIOLOGY 
the body with all its complexity as still forming a unity 
rather than a colony. The term co-ordination is employed 
to express this purposeful working together of all parts 
for the advantage of the whole. The means of co-ordina- 
tion may be chemical as just now suggested, but a more 
conspicuous agency in the highly developed types is that 
of the nervous system. While we must postpone until 
a later time any detailed description, we ought to indicate 
at this point the essential contrast between the two modes 
of co-ordination. One part of the body may affect an- 
other through the actual despatch of material to it. 
When the influence is through the nervous system instead 
of through the circulation there is no transfer of material. 
The nerves were once supposed to convey a fluid of strange 
properties, but the fact is established that they transmit a 
form of energy and not of matter. (The temptation to 
think of the nerves as conductors of electric currents and 
to compare the nervous mechanism with a telephone 
system is strong. Guardedly used, the comparison is 
valuable, but it is a symbolic rather than a literal repre- 
sentation of the facts. " Nerve impulses," so called, are 
not electric currents in the ordinary sense.) 
In a great nation the prosperity of any section must de- 
pend to a large degree upon the commercial exchanges 
taking place between that section and others. Its fac- 
tories may depend upon the mines of another province 
for coal and upon still another for raw material. Much in 
the same way a single organ of the human body is de- 
pendent upon others. A muscle, for instance, profits by 
the prepared foods or fuels forwarded to it from the di- 
gestive tract and by the oxygen borne to it from the lungs. 
The blood in this case is serving the purpose which is ef- 
fected by trains and steamers in the case of the nation. 
Nor do we find lacking in our illustration an analogy for 
the nervous communication between parts of the living 
body. The type of such intercourse is furnished by the 
telegraphic messages which pass incessantly from place 
to place. News, in itself immaterial, may affect the course INTRODUCTION 
19 
of local events just as surely and much more quickly than 
can the material exports of another region. Reactions 
produced through the nervous system are correspondingly 
sharp and prompt in developing. 
Blood and Lymph.-In the bodies of the higher animals 
the internal medium may be described as existing in two 
forms. In direct contact with the majority of the cells 
there is in scanty amount a fluid, the lymph. From this 
they draw their needful supplies of oxygen and food; 
into it they discharge their waste. The limited resources 
Fig. 3.-B is a blood-vessel of the smallest size-a capillary- 
with walls of flattened cells like that in Fig. 2, d. The blood flow- 
ing within is removed from direct contact with the cells (C, C), but 
dissolved substances may pass from one to the other through the 
capillary wall and the medium of the lymph (L). 
of the lymph at a given point would be quickly exhausted 
were it not that the blood is passing close by in vessels 
whose delicate walls permit the passage of material in 
both directions. The blood is in rapid movement and 
it is constantly renewing the oxygen of the lymph with 
fresh supplies just brought from the lungs. It is at the 
same time receiving from the lymph the accumulated 
waste. The lymph itself is not absolutely stagnant; at a 
given place there is a certain formation of fresh portions, 
while that which was previously present is displaced 
along channels known as lymphatics. CHAPTER II 
THE ENERGY RELATIONS OF PLANTS AND 
ANIMALS 
A chemical reaction can usually be assigned to one of 
two classes. Either it is exothermic, that is, attended by 
the evolution of heat, or it is endothermic, in which case 
heat must be supplied to cause its occurrence. When 
heat is evolved the products of the reaction are generally 
simpler and more stable than the original material. The 
most important reactions of this class are the oxidations. 
Heat, or other forms of energy applied from without, may 
effect the synthesis of complex substances rich in fuel 
value from initial material comparatively destitute of 
potential energy. 
Broadly speaking, the constructive chemical processes 
in nature are the work of the higher plants. Animals, as 
well as those forms of plant life which lack pigment, carry 
on for the most part reactions of a destructive character. 
This makes evident the dependence of all other forms of 
living matter upon the pigmented plants. It is through 
the agency of light-waves, a form of kinetic energy, that 
the synthetic reactions resulting in the formation of starch 
and other energetic compounds are made possible. When 
light is excluded from the green plant it has no advantage 
over the animal, but pursues a similar course of decomposi- 
tion. In fact, there is always going on in the plant, 
even when its constructive activity is most marked, an 
undercurrent of an opposite trend. Early writers com- 
monly exaggerated the supposed contrast between the 
chemical conduct of plants and that of animals. They 
were disposed to deny that an animal could execute any 
synthesis whatever. It is true that animal cells make no 
useful application of the energy showered upon them by 
20 ENERGY RELATIONS OF PLANTS AND ANIMALS 
21 
the sun's rays. Nevertheless they do carry on synthetic 
reactions, although to a limited extent. Since much energy 
is released within such cells by the prevailing oxidative 
changes it is not difficult to see that some portion of it may 
be applied to promote endothermic reactions. When a 
hydraulic ram supplies with water a house considerably 
above the level of the stream which operates the device, 
we understand that the result is made possible because a 
great deal of water falls that a little may rise. The prin- 
ciple of the conservation of energy is not violated here, 
nor is it when animal ceils erect from a portion of their 
food molecular structures more complex and energetic 
than anything in their current, supply. The formation of 
fat from sugar is a case in point. Weight for weight, the 
fat is more highly endowed with potential energy than is 
the sugar, but we must take into account the fact that the 
quantity of sugar entering into this common transforma- 
tion is much greater than the quantity of fat which can 
be produced. The energy in the product is more concen- 
trated, but not absolutely larger in amount than it was in 
the sugar. 
There are many species of green plants which are uni- 
cellular, just as there are many single-celled animal forms. 
It is suggestive to consider the reciprocal relations of one 
such plant and a solitary animal cell living beside it. A 
constant and pressing need of the animal is oxygen. Now 
oxygen is freely produced by plants when they avail 
themselves of the energy of light to carry on constructive 
processes. To this extent, then, the proximity of the 
plant to the animal is advantageous to the latter. This 
ceases to be true when light is succeeded by darkness. 
The animal meanwhile is giving off oxidized products, of 
which carbon dioxid is the most abundant. This, to- 
gether with water, is the very material out of which the 
plant can build its stores of starch and sugar. The out- 
put of the animal includes also various compounds of 
nitrogen. These, as well as the carbon dioxid, may be of 
service to the plant, although to be strictly accurate it must 
be added that they require some modification, usually 22 
NUTRITIONAL PHYSIOLOGY 
effected in nature through the action of bacteria. In 
view of these exchanges one is tempted to the hasty con- 
clusion that a single-celled green plant and a single- 
celled animal form a balanced system capable of con- 
tinued maintenance-in short, a microcosm. There is, 
however, a fatal obstacle to the continuance of the part- 
nership-the animal's need of organic food can only be 
satisfied by the sacrifice of the plant. To have a truly 
balanced system of an enduring character we must assume 
a multiplication of cells descended from the original uni- 
cellular plant, providing a surplus of vegetable tissue for 
the animal's consumption. This situation is sometimes 
realized in an aquarium. 
The give and take which has been illustrated for single 
cells is proceeding on a vast scale everywhere.1 It is 
hard to realize that the great harvests which support 
the races of mankind were formed for the most part from a 
gas present only very sparingly in the atmosphere, from 
water, and from the mineral salts of the soil. "The oak 
is but a foliated atmospheric crystal deposited from the 
aerial ocean that holds the future vegetable world in solu- 
tion" (Holmes). While we rely partly upon animal food 
(meat, milk, and eggs), this does not alter the fact of our 
absolute dependence upon the green plants, which in turn 
owe their growth to the translated energy of the sun. It 
is amusing to note the apparent travesty upon our human 
life which can be read into the contrasted conduct of green 
plants and of animals. The plants are conserving, while 
the animals are spendthrift. Plants create and distribute 
wealth. They seem like the thrifty and industrious mem- 
bers of society upon whose charity others less competent 
depend. One is reminded irresistibly of the parent at 
home and the son at college. If the father does not liter- 
ally subsist upon a diet of carbon dioxid and water that 
the son may have protein and alcohol, the approach to a 
parallel is too close for complacent attention. 
The Body and the Diet.-Turning now to the definite 
1 For a most interesting discussion of this matter see Arrhenius, 
"Worlds in the Making," Harpers, New York, 1908. ENERGY RELATIONS OF PLANTS AND ANIMALS 
23 
problems of human nutrition, let us consider the respective 
make-up of the body and the diet. There must evidently 
be a degree of similarity between them, inasmuch as the 
one has been built from the other. Similar compounds 
are met with in both, but, as we shall find, in quite unlike 
proportions. The body is mainly water. Water makes 
up about two-thirds of the total weight and forms even a 
larger percentage of the most active tissues. No material 
reduction of its quantity can be tolerated. Even after 
death from thirst the amount is surprisingly little dimin- 
ished. In the diet also water occupies the first place. It 
is likely to constitute fully five-sixths of the daily income. 
Its most obvious services are in connection with the 
absorption of food in solution and the removal of dissolved 
wastes. By its evaporation from the skin and the breath- 
ing passages it helps to keep the body temperature from 
rising above its normal level. 
Second to water among the substances which compose 
the body we find the group of bewilderingly complex 
compounds known as the proteins. A protein always 
yields the five elements-carbon, oxygen, nitrogen, 
hydrogen, and sulphur1-when subjected to analysis. 
Some members of the group contain phosphorus also. 
Merely to mention these constituent elements is to give 
no proper conception of the intricate manner in which 
they must evidently be combined. To appreciate this 
we need to consider the very long list of cleavage products, 
in themselves rather complex, which can be obtained by 
the decomposition of protein from a single source. The 
physiologic chemist is somewhat in the position of a person 
1 The elements occur in proteins in about the following percentages: 
C 53, O 22, N 16, H 7, S 1 per cent. Phosphorus when present 
amounts to 1 per cent, more or less. It is quite impossible to con- 
vey an adequate impression of the complex fashion in which the five 
or six elements are combined. Many years ago the following formula 
was suggested for hemoglobin, the red protein of the blood, which is 
exceptional in containing iron: 
C-srHjjo:; O228N195FeS3. 
It is not seriously maintained that these large numbers are precisely 
correct, but the order of their magnitude is probably typical. 24 
NUTRITIONAL PHYSIOLOGY 
who sees the various parts of a watch-the wheels, springs, 
jewels, etc.-lying loose upon the table of the watch- 
maker. He may gain a fair notion of the intricacy of the 
watch, though he may be very far from knowing how the 
parts were related in the time-piece. We do well to use 
the plural number in speaking of the proteins, for all recent 
work tends to emphasize the distinctive molecular pat- 
tern which characterizes each form and makes it differ 
definitely from every other. "There is one flesh of men, 
another flesh of beasts, another of fishes, and another of 
birds." This is excellent and altogether modern chemical 
biology. The presence of the element nitrogen in the 
proteins distinguishes them sharply from the other promi- 
nent compounds in both the body and its daily income. 
Nitrogen makes up about 16 per cent, of protein, and the 
value of this figure in calculations will be apparent later. 
It has been said that the proteins are second only to water 
in their abundance in the body. This is not true of the 
diet unless we have to do with a carnivorous animal. 
Among the herbivora, and almost always among men, the 
second place in the list of supplies is occupied by the 
carbohydrates. 
Third in quantity among the constituents of the body 
we find in most individuals the mineral compounds. 
These would not make so large a proportion if it were 
not for the skeleton. Bone is a tissue in which salts of 
lime are abundantly present. But in all the other tissues 
and in the fluids too we find a variety of salts, and it is 
well established that their presence is not accidental, but 
a matter of moment. Sodium, potassium, calcium, and 
magnesium at least, perhaps other bases also, must be 
kept in certain balanced relations if the life processes are 
to go on. The acids represented are chiefly hydro- 
chloric, phosphoric, and carbonic. Sodium chlorid, 
the one salt which we take pains to add to our food, 
is the one most abundant in blood and lymph. Potas- 
sium rather than sodium compounds predominate in the 
cells. ENERGY RELATIONS OF PLANTS AND ANIMALS 
25 
Next in amount to the salts in the body of average build, 
and not uncommonly exceeding them, are the fats.1 The 
word " fat " is used sometimes in a chemical and some- 
times in an anatomic sense. In the first case it denotes a 
compound of carbon, hydrogen, and oxygen, having a 
formula of a certain type. In the other usage the mean- 
ing is " adipose tissue," a form of connective tissue rich in 
such compounds. Fats have familiar physical characters. 
They are not soluble in water or to any great extent in the 
fluids of the body. They pass from solid to liquid form 
at moderate temperatures; the fats of the human body 
are regarde 1 as in a fluid state when under the influence of 
its warmth. Fats which are familiarly observed as liquids 
we commonly speak of as oils. No other common physio- 
logic compounds have so much latent energy awaiting 
release by oxidation. Fats are more plentiful in appar- 
ently lean individuals than might be judged. A con- 
siderable store of adipose tissue is to be found in any 
condition short of imminent starvation. 
It has been said above that carbohydrates usually have 
the leading place among the solid matters of the diet. 
This is owing to the large proportion of vegetable foods 
generally consumed. In the animal body the occurrence 
of carbohydrate is rather scant, and it is one of the chief 
problems of the physiologist to account for the daily dis- 
appearance of a great quantity of these compounds in the 
economy of the organism. The reader may already foresee 
what we shall later explain in detail, that this disappear- 
ance of carbohydrate is due in part to the fact that it is 
the fuel most constantly called upon to evolve energy, and 
in part to the ease with which the tissues transform to fat 
a surplus of these substances. Under the head of carbo- 
hydrates we distinguish the starches and the sugars. 
1 Fats are compounds which can be resolved into glycerin and 
organic acids. Those of chief interest in nutrition are the glyceride 
of palmitic, stearic, and oleic acids. The first mentioned has a com- 
position indicated by the formula C3H5(OOCH31C]5)3. The others 
are nearly related. The three common fats differ in their melting- 
points and in other respects. They are mingled in definite propor- 
tions to form the body fat of each animal species. 26 
NUTRITIONAL PHYSIOLOGY 
Starches1 are of high molecular complexity, imperfectly 
soluble, and tasteless. Sugars are cleavage products of 
starches or, if they occur apart from previously existing 
starches, closely resemble such cleavage products. They 
are of definite and moderate molecular weight, they are 
soluble, crystallizable, and sweet. They contain the same 
elements which are found in fats-carbon, hydrogen, and 
oxygen-but the percentage composition is wholly differ- 
ent and the structure of the molecule also. While the 
carbohydrates are energetic, their fuel value is less than 
half that of fats. 
We have now named this list of substances as uniting 
to form the body-water, proteins, mineral matter, fats, 
and carbohydrates-the order suggesting their relative 
abundance. We have said that in the diet water also 
takes the first place, but that carbohydrates ordinarily 
take the second. Either proteins or fats may stand third 
among the constituents of the diet. Often the amounts 
of the two are found to be about equal. A possible diet 
comprises 100 grams of protein, 100 grams of fat, and 250 
grams of carbohydrate, illustrating this equality. The 
mineral matter in the ration is not likely to exceed 20 
grams a day. Both the body and its income, of course, 
contain in small amounts substances which do not fall 
into any of the classes named. Such miscellaneous 
organic compounds are conveniently grouped as extrac- 
tives. Many of them are nitrogenous and represent dis- 
integration products of proteins. Reference to their 
significance will be made from time to time. 
At the very outset the double service of food to the 
1 The three elements in starch are present in proportions repre- 
sented by the formula C6H]0O5. But the number of atoms in the 
molecule is not correctly indicated by this formula; it requires to 
be multiplied throughout by an unknown, but considerable number. 
Sugars are of two common classes: the disaccharids, with the 
formula Cl2H22On, and the monosaccharids or hexoses, with the 
formula C6H12O6. Cane-sugar, malt-sugar, and milk-sugar are 
disaccharids. The hexoses of direct interest in nutrition are glucose 
(also called dextrose), fructose (or levulose), and galactose (a deriva- 
tive of milk-sugar). ENERGY RELATIONS OF PLANTS AND ANIMALS 
27 
organism was indicated. It represents both building mate- 
rial and fuel. If we liken the living body to a power-house, 
we see clearly how both kinds of supplies are required. 
Coal is the most bulky supply of the power-plant and 
the one on which its operation immediately depends. 
But there must also be new parts to replace those which 
wear out. The up-keep of the building calls for new 
wood-work, for paint, etc. A certain difficulty is encoun- 
tered in the attempt to show parallel conditions in the 
case of the body because the separation of the two func- 
tions is here much less sharp. Protein food, on the whole, 
has a peculiar title to be regarded as building material, 
but it is also an entirely available fuel. It is as if planks 
and beams designed primarily to repair the structure of the 
power-house were fed into its furnaces. The suggested 
comparison must not be pressed too far, for it conveys 
an impression of wanton destructiveness which we cannot 
assume to be just in the case of the organism. There are 
some minor supplies brought into the power-house which 
are not fuels nor precisely materials for repair. The oil 
is an example. Some of the extractives of the diet occupy 
an analogous position, being neither sources of energy nor 
of construction, but nevertheless favorably affecting the 
course of events. This comes near to our conception of a 
drug in relation to the processes of life. CHAPTER III 
THE NATURE AND THE MEANS OF DIGESTION 
It has been said that one of the results of the specializa- 
tion of cells is the loss of the primitive power to receive 
and utilize all kinds of food. The blood brings to the tis- 
sues of the body food of certain standard forms, and it is 
only these which can be used. The attempt to add various 
soluble foods to the blood by direct injection into the 
circulation has shown that in most cases such foods are 
offered in vain to the living cells. Milk introduced in 
this way is not a practical means of nutrition. Cane-sugar 
added in measured amounts to the blood is excreted 
promptly and in almost undiminished quantity by the 
kidneys. Thus it becomes clear that foods introduced 
directly into the blood are frequently treated like waste 
products, while the same foods after transformation in the 
alimentary canal are entirely acceptable to the body cells. 
The function of the alimentary canal is to work over the 
many foreign forms of nutriment into a few forms of the 
native type. From day to day the diet may be of quite 
variable character, but its variations hardly show them- 
selves in the composition of the blood. 
The term digestion is usually applied to those changes in 
the food-stuffs which precede absorption. To cover sub- 
sequent changes we use the word metabolism. 
One of the more evident characteristics of digestion is 
that it is a refining process. It effects a separation of the 
useful and the useless portions of the ration. This is 
a very conspicuous fact with the herbivora, whose food 
contains much woody material from which the available 
nutriment must be laboriously extracted. With man- 
kind, especially under modern conditions, a great part of 
28 29 
THE NATURE AND THE MEANS OF DIGESTION 
this work of separation is accomplished in the preparation 
of food, both industrial and domestic. In this way the 
task of the digestive organs is lightened, and, as we are 
often told, overeating is made easy. Some foods are 
quite devoid of residues when successfully digested and 
absorbed. 
The early writers, having little knowledge of chemistry, 
were naturally led to make much of the mechanical re- 
duction of food in the alimentary tract. Mastication sub- 
divides the food, and it was held that the later opera- 
tions, especially those of the stomach, were essentially 
further grindings of a similar sort. Such mechanical proc- 
esses as do occur continue to be of interest, but they are 
now seen to be preliminary to actual digestion. More- 
over, we shall see that it is easy to assume that they are 
of a more positive nature than is really the case. The 
human stomach is not a mill, though the gizzard of a 
bird may be fairly described by that word. 
In the eighteenth century the emphasis passed from 
mechanical factors to the process of dissolving the food. 
Solution is plainly one of the features of digestion, but it 
is a somewhat superficial one. Of course, it is natural to 
believe that solid food must become liquid before it can 
penetrate the intestinal wall, but mere solubility, as we 
have already seen, is not a guarantee of fitness for the 
use of the cells. Freely soluble foods like cane-sugar and 
milk-sugar require to undergo digestive changes just as 
definite as those carried out in the case of fats or coagulated 
proteins. It is often stated that the object of digestion is 
to produce diffusible substances. This statement is in- 
adequate, for diffusibility like solubility does not in itself 
determine the utility of a food. The sugars mentioned 
above are sufficiently diffusible, and the changes which 
they undergo before absorption serve a more fundamental 
purpose than the mere hastening of their passage through 
the lining of the intestine. . - 
In the light of modern chemical knowledge we can be 
somewhat specific in regard to the molecular aspects of the 30 
NUTRITIONAL PHYSIOLOGY 
digestive processes. They are probably always cleavages, 
large molecules giving rise to smaller. When the original 
molecule is of extraordinary size, as with proteins and 
starches, these cleavages have a serial character and a 
number of intermediate products must accordingly be 
formed. That is to say, the earlier products are in turn 
subjected to digestion. Such cleavages are generally, if 
not always, hydrolytic, that is, water enters into the 
reaction and its elements are found combined in the 
products. For the simpler instances of digestion, as in 
the case of fats and of the disaccharids,1 we can write 
precise chemical equations. We cannot do this with the 
same accuracy for the starches, and we are still farther 
from being able to express the exact manner in which the 
protein molecule undergoes hydrolysis. Yet we have 
sufficient evidence that the digestion is generally of a uni- 
form type. 
Some constituents of the diet need no digestion. This 
is the case with the mineral salts, so far as they are ab- 
sorbed, with the simple sugars (monosaccharids), and with 
alcohol. It is hardly necessary to say that water is also 
ready for reception into the body fluids. The numerous 
extractives are for the most part absorbed in the form in 
which they are eaten. A diet entirely predigested seems 
not to be practicable. If one were prepared it would have 
to contain advanced decomposition products of the pro- 
teins, which are bitter to the taste, and an amount of sugar 
which would be cloying and subject to fermentation. 
Digestion is anticipated to some extent by changes in 
our food which precede its actual arrival in the canal. 
The ripening of fruits and vegetables, as well as the corre- 
sponding processes in meat, illustrate this point. The 
influence of cooking is not so constantly of a sort to 
initiate digestion, yet in many instances it is so. For 
1 The following equation illustrates the hydrolysis of a disaccharid: 
C12H22O1J 4* H?O - 2CbHj2O8. 
This means that one molecule of malt-sugar, reacting with one mole- 
cule of water, gives rise to two molecules of glucose. 31 
THE NATURE AND THE MEANS OF DIGESTION 
example, when meat is boiled the common variety of con- 
nective tissue in it is converted to gelatin. This change 
is a typical hydrolysis, and if it were not executed in ad- 
vance it would be an early task of the gastric juice. When 
cooking is attended by considerable drying of the food it is 
less likely to count definitely toward digestion. Most 
proteins are coagulated by heat, and this change to solid 
form seems opposed to the general course of events in the 
alimentary system. The action of microscopic organisms 
upon food substances is in line more or less with normal 
digestion; the maturing of cheese is an example. But 
bacterial action may depart so far from the normal de- 
composition as to generate products of strongly poisonous 
properties, the so-called ptomains being among them. 
The Means of Digestion.-Hydrolytic cleavages closely 
resembling those of animal digestion may be caused to 
occur in various ways. Boiling food-stuffs with acids ac- 
complishes this. So does treatment with alkalis. Similar 
results follow the application of superheated steam. But 
the striking fact is that such changes as are brought about 
in the laboratory by violent reagents, high temperatures, 
or both in conjunction, are caused to take place in the 
stomach and the intestine by bland juices acting at the 
mild temperature of the body. The changes effected by 
these juices are often modified by the simultaneous 
activity of bacteria, but the presence of the latter, at least 
in man, is to be regarded as accidental and non-essential. 
The power to digest foods has been known for a long 
time to reside in the secretions which enter the alimentary 
tract. It was at first necessarily estimated simply by 
observing the progressive solution of solid food. The 
intimate nature of the process has become appreciated 
more recently. Comparison of the juices from different 
sources shows that they are individual and specific to the 
extent that each one, as a rule, acts upon certain classes 
of food and not on all. There is sufficient evidence for 
the belief that when a juice digests two or more classes 
of food-stuffs it contains separate and distinct reagents 32 
NUTRITIONAL PHYSIOLOGY 
for the performance of each line of work. We do not know 
the precise chemical nature of the active bodies, " diges- 
tive principles " as they were formerly called, but we know 
a great deal about the conditions of their working. 
When a digestive secretion has a single well-marked 
effect upon one sort of food material we say that it contains 
an enzyme. We are thus naming a body which we know 
chiefly by its power to promote a certain chemical reaction. 
It is not many years since an able writer protested against 
this confident reference to a substance where it is a property 
rather than a compound which we are observing. It will 
be admitted that it is doubtful whether anyone has ever 
seen that which is an enzyme and nothing else. What 
we see and handle are solutions possessing characteristic 
powers or dry preparations capable of furnishing such 
solutions. But it has seemed altogether reasonable to 
connect the property with a substance and we shall con- 
tinue to do so. Acting on this basis, we say of a juice 
which hydrolyzes starch that it contains a diastatic enzyme, 
and of one that acts in a parallel fashion on proteins that 
it contains a proteolytic enzyme. When the action is 
upon fats we call the enzyme lipolytic, or, using the sub- 
stantive, we call it a lipase. It is unfortunate that there 
is much confusion at present in the use of such terms; 
there are a great many more in use than there need be. 
The simplest plan is, perhaps, to fall back upon our Saxon 
and speak of enzymes as starch-splitting, protein-splitting, 
sugar-splitting, and fat-splitting. We shall take pains, 
however, in our detailed discussion to introduce various 
equivalent terms. We shall find especially that enzymes 
are often named with reference to their sources as well as 
to their powers. 
Enzymes are similar in many respects to the catalyzers 
of inorganic chemistry. Their presence accelerates reac- 
tions which in their absence might not be appreciable. 
We do not think of them as contributing either material 
or energy to the process. They suggest the oil in a machine 
which lessens the resistance of its parts to the driving force. THE NATURE AND THE MEANS OF DIGESTION 
33 
The enzyme in a digesting mixture is not forcibly compel- 
ling the molecules to disintegrate, but it is removing some 
hindrance to their spontaneous rearrangement. It is not 
definitely used up in this service. Accordingly, it follows 
that a limited quantity of a digestive juice, of course 
containing a still more limited quantity of enzyme, may 
be responsible for an amount of digestion practically 
unlimited. Unlimited time would be demanded for such 
a demonstration. (This form of statement should be 
qualified. Enzymes are somewhat unstable and liable 
to deteriorate.) 
When a process of hydrolysis takes place under the in- 
fluence of an enzyme and in a glass vessel, there must be a 
rising percentage of the products and a declining per- 
centage of the initial substance as the reaction goes on. 
The velocity of the transformation is found to diminish 
and at last it seems entirely arrested. A mixture now ex- 
ists which contains the first and the last members of the 
chemical system in proportions which have become con- 
stant. It is an instance of chemical equilibrium. The 
halting of the reaction does not mean that the enzyme is 
exhausted. If any means can be devised by which the 
accumulated end-products can be removed the hydrolysis 
will be continued. It was specified above that the trial 
should be made in a glass vessel. The reader will quickly 
recognize the important difference between such a con- 
tainer, from which nothing can escape, and the alimentary 
canal, from which active absorption processes withdraw 
the products of digestion. A clear field is thus provided 
for the continuance to substantial completion of the reac- 
tions which the enzymes are promoting. The contrast 
between laboratory conditions and those which prevail 
in the body did not escape the acute mind of Spallanzani, 
who was a pioneer among students of these matters. As 
early as 1777 he recorded that the solution of meat by 
gastric juice could be greatly facilitated by letting the 
digestive fluid fall drop by drop upon the food and to 
trickle away, bearing the dissolved products. 34 
NUTRITIONAL PHYSIOLOGY 
Enzymes are exceedingly sensitive to varying degrees of 
acidity and alkalinity in the medium. Most of them do 
not keep their efficacy if the solution is far from the 
neutral point. But they are somewhat individual in this 
as in other properties, the acid which is highly favorable 
for gastric digestion, for example, being quite prohibitive 
of salivary action. They are all destroyed when in solu- 
tion by temperatures somewhat short of boiling. Cold 
suspends their activity, but does not prevent its return 
upon warming. They are most effective at a temperature 
not far from that of the blood, though in general a few 
degrees higher. These relations between the enzymes 
and temperature are much like those established in the 
case of the simpler living forms. Having this in mind, 
one easily adopts the common practice of speaking of the 
killing of enzymes by heat. It must not be forgotten that 
this is a figurative expression. We are not justified in 
thinking of enzymes as living. Living organisms when 
they grow and multiply in a nutrient medium may de- 
compose it much as suitably assorted enzymes would do, 
and, in fact, the organisms in question are probably pro- 
ducing their own enzymes for the purpose. Formerly 
such living things as the yeasts and the bacteria were de- 
scribed as " organized ferments," and the detached en- 
zymes, incapable of self-multiplication, were called 
" unorganized ferments." These terms are not much used 
at the present time. Enzymes are assumed to be products 
of living cells and may be very characteristic fragments of 
the cell's fabric, but they are not independently living. 
The digestive changes to which we pay most attention 
are those which occur in the cavity of the alimentary 
canal, and which can be observed to take place also when 
the same mixtures are placed in the flasks and test-tubes 
of the laboratory. But we must not overlook the prob- 
able fact that similar changes are constantly occurring 
within the boundaries of every active cell. Intracellular 
digestion, presumably made possible by intracellular en- 
zymes, obviously takes place when a protozoan cell engulfs THE NATURE AND THE MEANS OF DIGESTION 
35 
a solid food particle, and has been thought to be as definite 
a process when a muscle-fiber of the human body nour- 
ishes itself at the expense of the surrounding lymph. 
While the great majority of enzymes are hydrolytic and 
favor reactions of the class which we have been discussing, 
it must be added that there are other enzymes associated 
with other orders of chemical change. Enzymes which 
promote oxidations are believed to play a most important 
part in the activities of the tissues. When reactions are 
hydrolytic they proceed with but little evolution or 
absorption of heat. When they are oxidative the release 
of energy is a most characteristic attendant condition. CHAPTER IV 
THE WORK OF MUSCLES AND GLANDS 
We cannot enter upon a description of the alimentary 
canal and its activities until we have devoted some space 
to the physiology of contraction and secretion. Movement 
is the most familiar manifestation of animal life. When 
visible to the naked eye it is the expression of the shorten- 
ing of elongated units-cells or fibers-associated to form 
contractile tissues. In the human body there are three 
principal kinds of these tissues. The obvious external 
movements of the limbs and the features, the act of 
breathing, etc., are produced by what we call the skeletal 
muscles. Contractile tissue of another order forms the 
walls of the heart and furnishes the power for its beating. 
A third kind occurs in the walls of the alimentary tract, in 
the blood-vessels, and elsewhere. 
The term skeletal applied to a type of contractile tissue 
implies relationship to the bones. It is easy to see that 
external movements are made effective through the connec- 
tion of the muscles which produce them with bones acting 
as levers. In some instances the term is a misnomer, for 
there are some small muscles histologically like the rest 
which do not act upon bones. The large and conspicuous 
muscles are attached, usually at both ends, to the bones. 
We can generally observe that one end is more freely mov- 
able than the other. The comparatively fixed end is called 
the origin of the muscle, the end more subject to move- 
ment is its insertion. 
What is called a skeletal muscle is a bundle in which 
we can distinguish an active and a passive part. There are 
36 37 
THE WORK OF MUSCLES AND GLANDS 
the true contractile elements and there is the connective 
tissue. The inactive substance forms a sheath enclosing 
the rest and also partitions which subdivide the interior. 
The arrangement is familiar in the cross-section of a piece 
of meat. The subdivision which is apparent to the un- 
aided eye is repeated on a microscopic scale until the 
finest meshes of the connective tissue enclose the hair-like 
individual fibers of the muscle. Each of these slender 
fibers is a miniature muscle in principle. The function 
of the connective tissue is often overlooked. While 
this part of the muscle is entirely passive in character, 
and scarcely to be considered alive excepting for a certain 
power of renewal after injury, it is quite necessary to the 
act of contraction. It may fairly be said to constitute a 
harness through which all the numberless, minute contrac- 
tile elements are enabled to unite their efforts. As the 
end of a muscle is approached the connective tissue in- 
creases in quantity at the expense of the typical contractile 
material. In most cases there is an extension of the 
muscle consisting of connective tissue only, and in a dense 
form, which attaches the whole to the bone. This is the 
tendon. It may be a long tough cord or it may form a 
wide thin sheet. A muscle deprived of its connective tis- 
sue would be simply a mass of unattached living fibers 
which might slip about among themselves, but which could 
not apply their combined tension to accomplish any ex- 
ternal effect. 
The fiber of skeletal muscle is a modified cell. It is 
exceptionally elongated; its length may be a thousand 
times its width. When it shortens it conforms to the 
general principle laid down in Chapter I, that is, it does 
not diminish in volume, but only in surface and, therefore, 
in length. How the chemical process which underlies 
the forcible shortening is made to contribute energy 
to carry it out has proved one of the most difficult prob- 
lems of physiology. It cannot be dealt with here. But 
the fact is to be emphasized that we are in the presence of 
a mechanism somewhat like the steam-engine, inasmuch 38 
NUTRITIONAL PHYSIOLOGY 
as it produces motion and does physical work at the cost of 
fuel destroyed. The resemblance goes farther than this, 
for both with the engine and with the muscle the ac- 
complishment of a measured amount of work is attended 
by seemingly wasteful evolution of heat. An engine is 
considered economical if it turns 15 per cent, of the energy 
resident in its fuel into horsepower. Muscles sometimes 
do better than this, but much of the time they are even less 
efficient. It is fair to point out that the heat set free as 
an accompaniment of muscle contraction is often of value 
to the animal. In our own case the temperature of the 
body must be kept above that of its usual surroundings. 
By far the largest part of the heat devoted to this main- 
tenance of a relatively high body temperature is produced 
in connection with muscular activity. Muscles are 
thus seen to be organs of heat production as well as 
organs to carry out movements. When the external 
temperature is high or the degree of muscular contrac- 
tion is greatly above the average, the heat evolved 
does become distinctly an embarrassment to the organ- 
ism. 
The source of the energy displayed in muscular activ- 
ity is chiefly the disruption of carbohydrate molecules. 
Sugar appears to be the preferred fuel of the muscular 
machine. While other foods are also available it may turn 
out that they have to be transformed to sugar before 
their energy can be utilized. Whether this is necessarily 
the case is now (1924) a matter of lively debate. When 
sugar is completely oxidized the only end-products are 
carbon dioxid and water, the same substances which 
would be formed by the literal burning of the sugar with 
an adequate supply of oxygen. These waste-products are 
very readily removed from the muscle, when its situation 
is normal, by the circulating blood. The carbon dioxid 
will almost immediately escape from the blood when it 
passes through the lungs. The water becomes part of 
the large total volume which is always passing into and out 
of the body, and may leave by all the main channels of 
excretion-the respiratory passages, the kidneys, and the THE WORK OF MUSCLES AND GLANDS 
39 
skin. It will be noted that the quantity of water leaving 
the body is constantly in excess of the income. Ordinar- 
ily the body excretes all the water which it receives plus 
the water which arises within it by oxidation. 
A distinction must be borne in mind between the com- 
pounds which for the most part make up the muscle and 
those substances which it is generally found to use as 
sources of energy. The solid part of the muscle is mainly 
composed of proteins. But, as just stated, it is most apt 
to destroy carbohydrates when at work. One is reminded 
of the fact that a steam-engine is composed chiefly of 
steel, but burns coal as its fuel. The comparison is some- 
what faulty, however, for it suggests a more radical differ- 
ence between structure and fuel than we can safely infer 
for the muscle. Under some conditions muscular work 
may involve some destruction of protein material. 
All that has been said of contraction up to this time 
applies equally to all three classes of muscle. Neverthe- 
less each type is adapted to its particular work by peculiar 
properties. Skeletal muscle is capable of quick shortening 
and prompt relaxation. A contraction may occur and 
the return to an extended condition be accomplished in 
one-tenth of a second. The trained finger of a pianist may 
strike a key ten times in a second. Such movements are 
in strong contrast with those executed by the form of 
muscle found in the viscera. The contractions of the 
stomach develop very slowly, are maintained for some 
time, and are correspondingly slow in fading out. Of 
course, it is true that skeletal muscles may also make 
prolonged contractions, as in keeping the body erect, carry- 
ing a suit-case, and in countless other instances. Experi- 
mental study has shown that such contractions as these are 
really compounded of successive brief twitches occurring 
too rapidly to permit relaxation. In view of this the 
possibility of having prolonged contractions in skeletal 
muscle does not invalidate the statement that it is essen- 
tially a quick-acting tissue. 
Muscle and Nerve.-The conception that muscular 
activity is due to the nervous system is probably suffi- 40 
NUTRITIONAL PHYSIOLOGY 
ciently familiar. Every skeletal muscle has its own strand 
of nerve-fibers placing it in connection with the brain or 
the spinal cord and under their control. If its connection 
is severed it becomes paralyzed and remains inactive, 
unless special local means, like electricity, are employed 
to excite it. Ordinarily we are justified in saying that 
skeletal muscle is not automatic, meaning that every move- 
ment which it makes is an indication of a previous act, or, 
as we say, a discharge on the part of the nervous system. 
Fig. 4.-The above represents somewhat diagrammatically a 
very small fraction of the length of a fiber of skeletal muscle. To 
include the entire element with the length proportional to the width 
we should have to extend this drawing to a length of several yards. 
The fiber is cylindric and enclosed by a more definite membrane than 
is usual with animal cells. The cross-marking is not a feature of this 
membrane, but stands for a peculiar organization of the protoplasm 
inside. Nuclei are seen here and there near the surface. The seg- 
ment shown is supposed to be the particular one about in the middle 
of the fiber within which falls the connection with the nervous sys- 
tem. A nerve-fiber (n./.) is seen making a junction with the muscle- 
fiber (M) through the so-called end-plate (e.p.). 
In somewhat sharp contrast is the behavior of the 
muscle composing the heart and of the form which is 
found in the viscera. These two kinds of contractile 
tissue are described as automatic, in the sense that they 
show a tendency to rhythmic contraction and relaxation 
even when deprived of their nervous connections. The 
automatic property of the heart is the cause of its beating. THE WORK OF MUSCLES AND GLANDS 
41 
In varying degrees the different portions of the alimentary 
canal exhibit the same power, not ceasing to shorten and 
to extend when observed entirely outside the body of the 
animal. While we emphasize this remarkable tendency 
to rhythmic activity, we must hasten to add that tissues 
showing such capacities are nevertheless subject to some 
nervous control. Thus the heart beats primarily because 
of the peculiar nature of its own substance, but varia- 
tions of rate and strength are constantly occurring as a 
result of the influence of the nervous system. In this 
connection it must be pointed out that such influence is not 
necessarily so applied as to excite increased activity, but 
may be inhibitory, that is, reducing the rate and force of 
the spontaneous contractions. A large place is now given 
to the inhibitory functions of the nervous system, and we 
shall meet with other examples of the restraint which it 
imposes upon various organs. A little reflection makes us 
realize that much of the highest work of the brain must be 
in the line of inhibition. A man is distinguished by the 
acts from which he refrains quite as much as by those 
which he performs. 
Muscular Tone.-It will be well before we go farther to 
make clear what is meant by tone (tonus, tonicity) in 
connection with the behavior of contractile tissues. 
Muscle is said to exhibit tone when it is not completely 
relaxed. Tone is thus a mild, sustained contraction. It 
seems rarely to be absent altogether, but may vary much 
in degree. Tone in the skeletal muscles gives them a cer- 
tain firmness and maintains a slight, steady pull upon their 
tendons. This is not likely to result in actual movement, 
because these muscles usually fall into antagonized 
groups, one of which opposes another. A heightened tone 
in the muscles of the arm may not change its position, 
since the force tending to bend it may be offset by an 
equal tension adapted to straighten it. Changes of tone 
in the walls of the hollow viscera, as the stomach, have a 
much more evident effect, since they alter the size of the 
cavity. One must discriminate carefully between stretch- 42 
NUTRITIONAL PHYSIOLOGY 
ing and tone change in such a case. A non-living, elastic 
sac may be distended by increasing its contents, but will 
react with a pressure proportional to the distention. A 
living organ which adapts itself to increased contents by a 
diminution of tone may exert no more pressure when full 
than when nearly empty. This principle is well illustrated 
by the urinary bladder. At one time this organ may have 
a capacity of a pint, and again its cavity may be nearly 
obliterated, but there is no strict correspondence between 
its size and the internal pressure. Indeed, a strong degree 
of tone and a high pressure may exist when the bladder 
is quite small. 
Glands and Secretion.-We have taken time and space 
to deal with the elements of muscular activity, and we 
must also give a place to another type of tissue and to its 
work. Some appreciation of the physiology of glands is as 
much a prerequisite of the study of the alimentary process 
as is a knowledge of the mechanism of contraction. Every- 
one understands that the nervous system throws the skele- 
tal muscles into their orderly activity, but the fact that 
the secretions of the body are often produced under 
nervous influences is not so familiar. Yet we do not have 
to look far for suggestive examples. The flow of tears 
as an accompaniment of an emotional experience is clear 
evidence that the small organs above the eyeballs which 
elaborate the tears are in connection with the brain and 
responsive to its changing conditions. A like relationship 
can be demonstrated for the glands that produce saliva 
and for those which secrete sweat. Secretion and con- 
traction are two manifestations of metabolism which are 
alike regulated by the nervous system. In fact, if we set 
aside for the present the phenomena of consciousness, it is 
doubtful whether we have any expression of the working 
of the nerve-centers beside these two. 
What, then, is a gland? The word is used sometimes 
to designate a large organ like the liver, the pancreas, or a 
kidney. Sometimes it is used with reference to a micro- 
scopic affair like an individual sweat-gland or one of the 43 
THE WORK OF MUSCLES AND GLANDS 
minute pits in the inner surface of the stomach or the in- 
testine. The fundamental structure is the same in both 
classes. The microscopic gland is a depression of a cel- 
lular surface-a pocket, one might say-out of which 
when it is active the secretion wells. The cells which 
Fig. 5.-The principle of glandular structure. In the upper 
figure a simple microscopic gland is supposed to be laid open by a 
section along its vertical axis. The cells are seen to surround a recess 
into which they discharge their secretion. Below, the same struc- 
ture is shown in its entirety, and in addition the encircling blood- 
vessels which contribute to make good the losses suffered by the 
secreting cells. 
bound its cavity are the producers of the secretion, and 
are in turn dependent for renewal upon the lymph which 
underlies them and the blood which is flowing close by. 
A superficial view would suggest that such a gland is a 
filtering device adapted for straining off certain portions 
of the blood. This is, however, an entirely inadequate con- 44 
NUTRITIONAL PHYSIOLOGY 
ception. Most secretions contain substances which were 
not present in the blood at all, and which must, therefore, 
have been elaborated by the cells of the gland. When we 
remember that the same blood flows through all the glands 
we cannot fail to be impressed by the variety of the 
products which are made from the same raw material- 
products as unlike as milk and bile, urine and saliva. 
A compound gland, like the pancreas, is an aggregate 
of numberless units, which are individually like the simple 
microscopic glands. Within the meshes of an abundant 
supporting tissue which is shot through with blood- 
vessels are these small pockets walled around with the 
characteristic cells of the gland. These ultimate recesses 
are called alveoli or acini. Each has a way open through 
which its liquid product may move toward an outlet. 
Usually there is a single main duct formed by the union 
of all the fine passages from the alveoli and bearing their 
combined contributions. A compound gland may have 
more than one duct. Glandular secretions may be 
discharged directly upon the surface of the skin, as in the 
case of the sweat, or they may enter cavities, as happens 
with the gastric juice, the pancreatic juice, and the secre- 
tion of the intestinal lining. The bile and the urine are 
two secretions which accumulate temporarily in special 
containers, the gall-bladder and the urinary bladder 
respectively, before they reach their final destination. 
Internal Secretions.-It may not be premature to add 
at this point that any organ may yield some peculiar prod- 
uct of its own life process to the lymph or to the blood as 
well as to the cavities of the hollow viscera and to the 
exterior of the body. A product of this kind which 
merges with the circulating medium instead of appearing 
distinct and separate from it is called an internal secretion.. 
One may maintain that every organ has such a secretion, 
for inasmuch as each has its unique chemical composition 
and its distinctive metabolism, it must give to the blood 
compounds which no other organ duplicates. As stated 
before, the actual make-up of the blood is the resultant 45 
THE WORK OF MUSCLES AND GLANDS 
of the action of all the tissues upon it. But we shall find 
that internal secretion is a function much more clearly 
attributable to certain organs than to others and most 
evident in connection with a few small structures like the 
thyroid and the adrenal, to which later reference must 
be made. To have an internal secretion an organ need 
not be a typical gland. No duct will be required to carry 
such materials as its cells turn over to the blood-stream. 
In some cases organs believed to work along these lines are 
spoken of as ductless glands. A word recently introduced 
as an equivalent of the term 11 internal secretion " should 
be given. It is the word " hormone," meaning a chemical 
messenger, a very convenient and suggestive expression. 
Absorption and Secretion.-Gland-cells have been said 
to draw upon the blood or the lymph for their raw mate- 
rial and to manufacture their secretions therefrom. In 
this process something enters the deeper boundary of the 
cell layer and in a more or less transformed state it is 
later discharged from the exposed surface. It is helpful 
to compare this operation with what takes place in the 
intestine when the products of the digestive cleavage are 
being removed to the circulation. When absorption is 
going on it is the exposed ends of the cells which receive 
dissolved substances and their deeper borders which are 
discharging to the fluids that underlie them. Such a 
process has been well called " reversed secretion," and 
there is the same possibility of an extensive making over 
of the transferred material in this case as in the other. 
In other words, the digestive products which are last 
detected in the intestine are not necessarily those which 
will be dealt out to the blood by the cells of the absorbing 
membrane. Both secretion and absorption are phenom- 
ena which can be completely carried out only by living cells. 
Each is probably promoted by a definite application of 
energy on the part of the cells concerned. In either case 
it is possible that there may be some transfer of material 
through the clefts between the cells as well as through 
the cell bodies. CHAPTER V 
REFLEX ACTION 
In the previous chapter it was pointed out that all the 
work done by the skeletal muscles is in response to the 
discharges of the central nervous system. For the other 
types of muscle-the cardiac and the visceral-it was 
shown that there is an inherent tendency to rhythmic 
activity, but that over these tissues also the nervous sys- 
tem exercises a regulation. Finally, it was stated that the 
glands likewise are subject to central government, al- 
though not to the same degree in all cases. We must 
now proceed to consider how the nerve-centers are them- 
selves prompted to throw muscles and glands into action. 
As we observe the body at work we cannot fail to be 
impressed with the timeliness of its adjustments. It is 
constantly meeting with emergencies and adapting itself 
to new conditions. If we are inclined to attribute all 
these quick adaptations to intelligent choice of courses 
to be pursued we shall find that we cannot long defend 
such an explanation of the facts as they occur. We can- 
not pretend that we think of each inequality of the 
pavement as we cross the street, or of each individual in 
the crowd through which wTe make our way. The balan- 
cing of our bodies, standing or walking, is not a matter 
about which we are given to deliberating. These things 
seem to take care of themselves. It is such adjustments 
which " seem to take care of themselves " that are called 
reflex actions. A reflex is an adaptive change, brought 
about through the agency of the central nervous system, 
to meet some new external condition. We may or may 
not notice the occurrence of a reflex. If consciousness is 
at all involved, it is incidental and not causal. Often 
46 REFLEX ACTION 
47 
the conscious effort is rather to prevent the reflex from 
taking place, as is apt to be true when we sneeze. Of 
some reflexes we are quite unlikely to be aware; this is the 
case with the narrowing of the pupil in response to in- 
crease of light. 
Let us now go into some detail and analyze carefully 
the reflex process. We have seen that the primary cause 
is an external change of some sort, the word " external " 
meaning outside the central nervous system and not 
necessarily outside the body. .The change which is at the 
root of the reflex is usually referred to as an external 
stimulus. It would be easy to give a long list of exam- 
ples. A foreign particle comes in contact with the larynx; 
its contact is the stimulus which develops the coughing 
reflex. Slight drying of the exposed surface of the eyeball 
is a common cause of the winking reflex. Irritation of the 
lining of the stomach is the most frequent of the many 
possible stimuli through which vomiting can be excited. 
External stimuli would fail of any extended effect if it 
were not for the nervous connections of the parts affected. 
In the last chapter the nervous system was spoken of as 
sending its impulses out to muscles and glands. But 
its work is twofold. It not only acts, but it is acted upon. 
Its fibers fall into two classes, those which are concerned 
with transmission of effects outward from the brain and the 
spinal cord, and those having the opposite function, the 
carrying inward of impulses started by external causes. 
The first class of conductors are usually called motor; the 
second, sensory. Both terms are open to objection, as a 
little consideration will show. The effects which the ner- 
vous system produces in the tissues of the body are not 
solely movements. The word " motor," then, is not in- 
clusive enough. It is better to substitute the word 
efferent,1 which means simply centrifugal, and which im- 
plies nothing whatever about the nature of the responses 
evoked. Efferent fibers may be motor, that is, exciting 
contraction, but they may also inhibit contraction, and 
1 Efferre, to bear away. 48 
NUTRITIONAL PHYSIOLOGY 
when they end in connection with the cells of glands they 
may be secretory. Probably also they may inhibit secre- 
tion. 
Just as we have found the word "motor" inadequate 
and have agreed to replace it by "efferent," so the word 
"sensory" does not properly indicate the whole service of 
the fibers which bear impulses toward the brain and cord. 
Sensory implies "productive of sensation," and we cannot 
assign such a property to all the millions of fibers which 
assail the centers with their communications. In the 
great majority of cases we do not feel any consequences 
of their activity. The term afferent is free from this 
objection and is the logical complement of efferent. If 
one hastens to ask what is the significance of afferent 
fibers which do not arouse sensation, the answer is simple 
and definite: They produce reflexes. 
If the first element in the reflex process is the applica- 
tion of an external stimulus, it is now clear that the next 
element is the afferent transmission of the impulses. 
What these impulses are cannot be discussed. It should 
be recalled that they are not fluid pulses nor electric cur- 
rents in the usual sense of the expression. They repre- 
sent energy of some kind in rapid, but not immeasurably 
rapid, motion. They pass along the nerves at rates in 
excess of 300 feet in a second, so that the longest paths in 
the human body are traversed almost instantaneously. 
When the afferent impulses reach the central nervous 
system the third event in the development of the reflex 
act occurs. This is localized in the brain or the spinal 
cord, and we may speak of it as "a central process " 
without committing ourselves as to its exact character. 
What we actually observe is that the arrival of the afferent 
impulses is followed by the appearance of efferent ones. 
It is not necessary to decide whether these efferent im- 
pulses are the same currents which just entered the in- 
tricate fabric of the central organ and which have found a 
path open through its mazes which has led them out 
again. That is one way of picturing the phenomenon REFLEX ACTION 
49 
and it appears to be gaining in favor. According to the 
older and more familiar view the impulses which come 
out are not those which went in, but a new set generated 
by an energetic metabolic process, a discharge on the 
part of cells in the brain or the cord. If this is the true 
conception the afferent impulses serve to "touch off" 
irritable nervous elements, much as these elements in their 
turn may touch off muscle-fibers or gland-cells. 
Fig. 6.-The principle of reflex action. The subject touches a hot 
object (7/). Afferent nerve-impulses traverse the route marked bv 
dots and dashes to the spinal cord (<S). Efferent impulses return 
promptly along the route marked by little crosses to the muscle (iff), 
which co-operates with others not shown to withdraw the finger 
from the stimulating surface. The situation of the co-ordinating 
center is left undetermined, whether in the brain or the cord. 
The fourth step in the evolution of the reflex is the 
efferent transmission. This may be said always to be more 
voluminous than the afferent flow which went before. 
Impulses go out by many channels, where but few were 
engaged in bringing them in. A great characteristic of 
the "central process" is the spreading of the initial stimu- 
lation, so that there seems to be no proportion between the 50 
NUTRITIONAL PHYSIOLOGY 
cause and the response. The number of nerve-fibers 
which can be excited by the slender proboscis of a mosquito 
as it pierces the skin of a sleeper must be very small. The 
reflex movement which results may involve a very large 
share of his skeletal muscles. 
The fifth and final occurrence completing the reflex is 
the reaction on the part of the muscles or the gland-tissue 
in which the efferent fibers end. As already indicated, this 
may be a movement, an outpouring of secretion, or it may 
have a negative character, the suppression of movements 
that would naturally have occurred, or possibly the with- 
holding of some secretion which would otherwise have been 
discharged. The illustrations of reflex action most often 
chosen are those in which an immediate, even abrupt, 
response is seen. Yet it is quite easy to find examples 
of gradual adjustment to the new external condition. 
Changes of color, the outward sign of changes in the 
blood-supply of the skin, when they occur on account of 
warming or cooling of the surface, are reflexes of this 
prolonged and gentle order. 
If there is any doubt as to whether a certain action is to 
be classed as a reflex, it may be tested according to the 
foregoing analysis. There must be an assignable stimulus, 
external at least as regards the central nervous system, 
there must be an afferent flow of the impulses resulting 
from the stimulation, a process within the bounds of the 
central axis, a return flow of impulses in multiplied volume, 
and the action itself. The more one thinks of the common 
course of events, the larger the number of actions which 
he finds he can place in this class. It becomes appropriate 
to ask what kinds of bodily activity are outside this depart- 
ment. To this question it can be replied that automatic 
actions, such as the beating of the heart, are to be dis- 
tinguished from reflexes. The nervous system is not re- 
quired to maintain the heart-beat. There are cases also 
in which the chemical composition of the blood reaching 
the centers modifies their behavior and causes them to send 
out certain impulses. Such cases do not fit our descrip- 51 
REFLEX ACTION 
tion of the reflex, since in them the stimulation is applied 
centrally and no afferent nervous mechanism is needed. 
Our breathing movements are determined to a great extent 
by such chemical conditions, but it is a fact that reflex 
disturbances of the breathing are so prevalent that it is 
often difficult to give just recognition to the two factors. 
We are accustomed to contrast sharply actions which 
are reflex with those which we regard as strictly volun- 
tary or deliberate. The distinction is a convenient one 
and not generally productive of confusion, but sometimes 
it becomes quite difficult to draw the line. It may be 
urged that all our conscious, intentional acts are performed 
in answer to external conditions which have risen to make 
an occasion for such new adjustments. So it might be 
argued that the writing of a word from a copy should be 
considered a reflex in which the retinal image of the copy 
furnished the external stimulus. Such images printed 
upon the retina of the trained pianist by the notes that are 
before him cause his fingers to drop upon the corresponding 
keys of the instrument. It may be claimed that this is a 
reflex action. Without denying the force of such reasoning, 
we shall do well to restrict the term to the class of responses 
for which we are quite sure that attention is unnecessary, 
and usually to those for which we have an inborn or at 
least a very early developed capacity. When we ask 
ourselves whether any act is really other than the result 
of external circumstances affecting an organism with its 
own past history registered in its structure, we find, al- 
most with a shock, that we are face to face with philo- 
sophic and ethical problems, responsibility and free will. 
Most of us like to believe that a place is to be reserved for 
a type of action, even though it may be rare and slight, 
which is not externally caused. 
The great difficulty encountered by the beginner in 
physiology lies in the attempt to realize the inevitable 
character of reflexes and their structural basis. He finds 
it hard not to read conscious purpose into acts which so 
constantly prove advantageous to the individual. When a 52 
NUTRITIONAL PHYSIOLOGY 
frog adroitly catches a fly it is natural to assume a desire 
and a design on the part of the frog. Scientific analysis 
nevertheless makes it appear far more probable that the 
fly is entrapped because the frog is a mechanical device 
adapted to do this thing over and over again. The eye 
receives the flitting shadow of the insect, the stimulus 
excites the brain, and the well-directed fling of the tongue 
follows. The reflex is not done away with when the part 
of the brain most likely to stand in relation to consciousness 
has been destroyed. We have to remember that much of 
the service of the eye is subconscious, as when it makes us 
turn aside from obstacles in our path. It is in this way 
that the eyes of the somnambulist assist in guiding his 
movements. Conscious attention is no more essential to 
such a use of the eyes in the waking than in the abnormal 
sleeping state. In fact, close attention to the balancing 
of the body is quite as likely to derange as to promote the 
reflex adjustment. 
Central Resistance.-Reflexes are not obtainable with 
the same ease at all times. We express this fact by saying 
that there are variations of resistance in the central ner- 
vous system. If reflexes are hard to bring about, we say 
that the resistance is high; if they occur with unusual free- 
dom and seem disproportionate to the exciting stimuli, we 
say that the resistance is low. Narcotics and anesthetics 
are said to raise the resistance, and their effect can be 
gaged by observing the degree of difficulty with which 
certain reflexes can be produced, or whether, indeed, they 
can be produced at all. Drugs of an opposite order, the 
true stimulants, make it easy to call out most reflexes. 
When one is distinctly under the influence of coffee, a noise 
may cause one to start, with a sharp contraction of many 
muscles. The auditory stimulus has an undue effect, and 
it is natural to assume that the conditions in the brain and 
cord are uncommonly favorable to the penetration and to 
the multiplication of nervous impulses. In poisoning by 
strychnin such an extension of conduction may exist that 
some trifling cause may precipitate a terrific and exhaust- REFLEX ACTION 
53 
ing convulsion. Clearly, then, a certain degree of central 
resistance is the most favorable condition for the activities 
of life. Any increase will tend to prevent needed adapta- 
tions to external changes, and any great decrease will make 
the reflex responses exaggerated, disorderly, and ill suited 
to their object. There is reason to suppose that the more 
frequently occurring reflexes become progressively easier 
to induce through a lowering of resistance in their particu- 
lar pathways. This brings us close to the subject of habit 
formation. 
Our emphasis has been constantly upon the advantage 
derived by the animal (or by man) from the possession of 
reflex capacities. When the environment is the accus- 
tomed one and the changes taking place are such as the 
species has often experienced, we find that almost every 
reflex is obviously beneficial. The reactions are such as 
maintain bodily equilibrium, secure nutriment, evade or 
defeat enemies, resist changes of temperature, all making 
for self-preservation. But it must be noted that an unin- 
telligent mechanism will act amiss in any environment 
which is sufficiently unlike the accustomed one. It will 
hardly be claimed that the reflexes exhibited by the novice 
on first going to sea help him in the struggle for existence. 
A number of reflex effects can be thought of which can 
scarcely be of value. Sneezing when going out into bright 
sunlight is one of these. Hiccups following immoderate 
laughter do not seem to be of any service, nor does laughter 
itself when induced by tickling. These instances, which 
on the whole have little importance, are mentioned simply 
to enforce the contention that the reflex mechanism, how- 
ever refined, is not directed in its routine performances 
by intelligence. Its structure determines its conduct. The 
finger laid upon hot iron is twitched away before the situa- 
tion is reasoned out, in fact, before pain is felt. Central 
connections exist which make the movement sure to occur. 
If we could rearrange those central connections we might 
conceive of a luckless subject who would not remove his 
finger from the stove, but would stand violently coughing 
while the injury proceeded. CHAPTER VI 
THE ALIMENTARY CANAL 
The single-celled animal digests its food within its own 
protoplasm, sometimes holding it for a while in a temporary 
cavity filled with fluid, the so-called food vacuole. In 
such intracellular cavities true digestive secretions contain- 
ing enzymes are doubtless at work. It is probable that 
single-celled forms may also secrete enzymes to the exte- 
rior and so modify food material which is near-by, but not 
yet enclosed. This appears to be the case with bacte- 
ria when they dissolve the solid gelatin in which they are 
growing. 
Among many-celled animals digestion of this second 
type, that is, external to the cells, becomes more con- 
spicuous. Their bodies are so formed as to contain spaces 
in which food may undergo digestion and from which the 
hydrolyzed products may be absorbed. In the sea- 
anemone a round opening or mouth leads to a cavity which 
is very large in proportion to the size of the animal. 
This primitive alimentary tract has no other opening. 
In the earthworm, a somewhat more highly developed 
form, a straight canal in the axis of the body leads from 
a mouth near the anterior end to an anus at the posterior. 
However much the alimentary systems of the higher ani- 
mals may be elaborated, each still represents a more or less 
winding passage between a mouth through which food is 
received and a vent or anus for the discharge of residues and 
excretions. The canal may be greatly lengthened through 
coiling. Some sections may be widened and others nar- 
rowed; the walls in some places may be thick and elsewhere 
thin. Local differentiation of this kind causes us to dis- 
tinguish in the human subject the familiar divisions of the 
54 THE ALIMENTARY CANAL 
55 
tract, as the esophagus, the stomach, the small and the 
large intestines. The lengthening of the system, it should 
be noted, does not merely increase its capacity, but multi- 
plies the surface available for the processes of absorption. 
A few anatomic expressions may well be defined at this 
time. Anterior, as we shall use the word, means toward 
Fig. 7.-I represents a protozoan cell-an ameba-which has 
enclosed a particle available for food (F). The particle occupies 
the center of a clear space or vacuole (V). Undoubtedly it is sur- 
rounded by a fluid having digestive powers. II is a diagrammatic 
section through the familiar sea-anemone. There is a relatively 
huge digestive cavity (<S) with a single opening to the exterior (fl/). 
Ill suggests the type of alimentary system found in the earthworm 
and in higher animals. Two openings exist, the mouth (Jf), de- 
finitely devoted to the reception of food, and the anus (A), used 
exclusively for the discharge of wastes. 
the head; posterior, away from the head. Dorsal means 
toward the back; ventral, toward the front. Right and 
left have their ordinary use. (In most figures, the subject 
being viewed from in front, right and left are reversed.) 
Reference must often be made to the body cavities. These 56 
NUTRITIONAL PHYSIOLOGY 
are the thoracic cavity above the diaphragm and within 
the cage of the ribs, the abdominal cavity below the dia- 
phragm, and the much smaller pelvic cavity bounded by 
the bones of the hip girdle. When we speak of these as 
cavities we do not mean that they contain any air-filled 
space. They are completely filled by the organs which 
they enclose plus a small quantity of fluid. Hence they are 
only potential cavities in life, becoming actual when their 
contents are removed in course of dissection. The thoracic 
cavity contains the lungs, nearly surrounding the heart, 
and is traversed by the esophagus. The abdominal cavity 
is filled almost entirely by the organs of digestion-the 
stomach, the small and large intestines, the liver, and the 
pancreas. The spleen at the left of the stomach is less 
certainly connected in its functions with the alimentary 
system. The kidneys lie in the dorsal body wall rather 
than in the abdomen. In the small pelvic cavity are the 
urinary bladder, the terminal part of the large intestine, 
and the reproductive organs. 
The mouth, the first division of the alimentary canal, 
scarcely calls for detailed description. Above, a bony par- 
tition separates it from the intricate spaces of the nasal 
passages. At the back this "roof" is prolonged as a 
mobile, muscular curtain-the soft palate. Below the 
edge of the soft palate a region is reached which is common 
to the alimentary and respiratory systems. This segment 
of the canal is known as the pharynx, though the term is 
extended also to the space behind the soft palate, which is 
above the normal course of food. The teeth and the 
tongue with its wmnderful muscular development are suffi- 
ciently obvious. Ducts from the salivary glands open 
into the mouth. We are rarely conscious of the situation 
of these openings, though in the dentist's chair we may 
notice the rapid flow of saliva from one which is opposite 
the upper molars. This is the place of entrance of the 
secretion of the parotid gland, situated before and below the 
ear, the gland usually affected in mumps. Under the 
tongue and within the sweep of the lower jaw-bone there Fig. 8.-The human alimentary canal shown diagrammatically: 
O is the esophagus; *■> is the stomach; S.I. suggests the small in- 
testine; C is the colon (see Fig. 14); R is the rectum. The connection 
between the stomach and the small intestine occurs behind the 
transverse colon, which also hides the pancreas. Fig. 9.-Relations of the mouth and nose. This is a vertical sec- 
tion through one nostril, and therefore slightly away from the 
midplane of the head. The convoluted character of the lateral 
wall of the nasal cavity is suggested. The connection between 
the nose and the throat will be seen behind the soft palate (P); 
L is placed in the larynx, above which is shown the spur of the epi- 
glottis; 0 indicates the course of the esophagus. It will be noted 
that the course taken by the food crosses the route of the breathing 
in the pharynx. THE ALIMENTARY CANAL 
57 
are, on either side, two other glands, the submaxillary and 
the sublingual, with ducts opening in the floor of the 
mouth. 
Below the root of the tongue there is a leaf-like pro- 
jection, the epiglottis, which juts backward as though to 
guard the entrance to the larynx. Through the larynx 
a way is open to the trachea and the lungs. At this point, 
therefore, the courses taken by the food and by the 
breath part company. From here the esophagus extends 
through the neck and the thorax, lying at first behind 
the trachea, and lower down passing back of the heart. 
Perforating the diaphragm slightly to the left of the mid- 
line it opens into the stomach. 
The stomach is the most expanded part of the aliment- 
ary canal. Its position is higher up than is generally as- 
sumed, so that it is well within the embrace of the lower 
ribs on the left side. It has a capacity varying greatly 
with the degree of its distention and with its variations 
of tone. After a full meal it may contain more than a 
quart. The form of the stomach also changes consider- 
ably from time to time, but we distinguish a large, rounded 
portion toward the left and a more conical region tapering 
off toward the right and joining the small intestine. The 
opening from the esophagus into the stomach is called the 
cardia, and that from the stomach to the small intestine is 
the pylorus. The pylorus is a trifle to the right of the mid- 
line. The upper border from the cardia to the pylorus is 
the "lesser curvature" of the stomach; a line drawn from 
the cardia around the convex left-hand side and thence 
along the lower margin to the pylorus is said to follow the 
"greater curvature." 
Leading away from the pylorus the small intestine de- 
scribes a short turn, within which is the pancreas. This 
first curve is called the duodenum. The remainder of the 
small intestine is a slender tube about 20 feet in length, 
coiled upon itself in a confusing manner. Two divisions 
are recognized, ths jejunum, continuous with the duodenum 
and the ileum, extending onward to join the large in- 58 
NUTRITIONAL PHYSIOLOGY 
testine. No sharp line of demarcation exists between 
these sections, but the latter is regarded as constituting 
somewhat more than half the whole. The ileum finally 
arrives at a point not far from the crest of the right hip- 
bone and there enters the large intestine. 
The large intestine is so called from its diameter, which 
is two or three times that of the small. It is quite as often 
Fig. 10.-The stomach with the pancreas and duodenum: C is 
placed at the cardiac opening of the stomach, while P is at the 
pylorus. A dotted line is used to complete the form of the pancreas, 
which discharges to the intestine near W. The shape of the stomach 
is of one moderately filled; with further distention the lower border 
of the organ would sag and the pylorus would cease to be the lowest 
point. B is the common bile-duct, which reaches the intestine be- 
hind at the same point at which the pancreas delivers its secretion. 
called the colon. Beginning with a small rounded pouch, 
the cecum, from which hangs the infamous appendix vermi- 
formis, it may be followed upward on the right side of 
the body to the level of the lower ribs. This part is the 
ascending colon. From here it bends sharply to the left 
and crosses the full width of the abdominal cavity. This 
horizontal segment is known as the transverse colon and 
lies close to the ventral body wall. Thus it passes in front 
of the duodenum and is in practical contact with the THE ALIMENTARY CANAL 
59 
stomach. At the left side of the body and near the spleen 
the descending colon begins. Its course is downward 
and backward, so that it passes behind the coils of the 
small intestine. Following the dorsal boundary of the 
cavity around to the middle line, the colon forms the short, 
curved region called the sigmoid flexure. The remaining 
section is the rectum, situated directly in front of the lower 
extremity of the spinal column within the pelvis and ter- 
minating at the anus. 
Almost everywhere the lining of the alimentary canal is 
pitted with microscopic glands. Those in the stomach 
furnish the gastric juice; those in the intestine, the intesti- 
nal juice. Besides these small glands and the salivary 
glands already mentioned, there are the pancreas and the 
liver, contributing secretions to the cavity of the digestive 
tract. The pancreas has been said to lie in the turn of the 
duodenum. It is thus under the pyloric portion of the 
stomach and behind the transverse colon. Its main duct 
discharges into the small intestine about 3 inches below the 
pylorus. A second, but very small, duct may open close 
by. The liver, which is the largest gland in the body, is 
fitted to the concave under surface of the diaphragm and is 
mainly to the right of the midplane. It is cleft into several 
lobes, from which ducts converge and unite as they ap- 
proach the duodenum. A single duct is finally formed and 
it enters the intestine at the same point as the chief pan- 
creatic duct. The arrangement serves to blend the two 
secretions, and is somewhat suggestive of the devices used 
with bath-tubs for mingling hot and cold water. 
The liver produces bile, and its channel of discharge to 
the duodenum is accordingly known as the bile-duct. 
This duct has a side branch which leads to a contractile 
sac embedded in the under surface of the liver, this reser- 
voir being the gall-bladder. Bile, as it flows down from 
the liver, may either find its way directly to the intestine 
or it may turn aside into the gall-bladder. The course 
taken will depend on the contraction and relaxation of the 
muscular walls of the ducts. Active contraction of the 60 
NUTRITIONAL PHYSIOLOGY 
gall-bladder when it is full may send a considerable amount 
of bile at one time into the intestine. The relation of the 
gall-bladder to the liver is like that of the urinary bladder 
to the kidneys, at least to the extent that its existence 
makes possible a continuous production of the secretion 
with an intermittent emptying. It has been shown, how- 
ever, that the bile is concentrated and otherwise modified 
Fig. 11.-This is an entirely schematic section across the human 
body in the mid-abdominal region: S indicates the spine; K, the 
kidneys; P is the peritoneum, the lining of the abdominal wall. 
It is prolonged from the back to form the mesentery (Af), which 
extends to and around the loop of intestine (7). The large unoc- 
cupied space shown does not really exist, for successive portions of 
the alimentary canal together with other organs completely fill the 
cavity. 
during its stay in the gall-bladder. The urine does not 
change its character distinctly while it is in storage. 
When the abdominal wall of an animal is cut through 
and laid back from the organs within, one's first impression 
is that the viscera are lying unattached in the cavity. 
They are, in fact, not adherent to the ventral or lateral 
portions of the wall. But if we take a loop of the small 
intestine at random and attempt to lift it from its resting- 
place, we find it attached to the middle of the back by a 
tough, transparent membrane, the mesentery. In the THE ALIMENTARY CANAL 
61 
mesentery can be seen blood-vessels, lymphatics, and 
nerves. This suspending sheet thus serves not merely for 
mechanical support, but also establishes connection be- 
tween the intestine and the circulatory and nervous sys- 
tems. The student is apt to find it hard to visualize the 
mesentery in its actual form; he is to imagine a membrane 
which at one edge extends to the entire length of the small 
intestine and to much of the large, while its other edge is 
condensed to be inserted into the space of a few inches 
before the spinal column. What results from these condi- 
tions has been described as "a ruffle or flounce." Al- 
though the mesentery is thin it is really a doubled sheet 
enveloping the intestine. This will be made clear by the 
diagram, which also shows how the mesentery is continu- 
ous with the exquisitely smooth, lustrous lining of the ab- 
dominal cavity, to which is given the name of peritoneum. 
Dissection of a small animal will give a comprehension of 
these anatomic facts which can scarcely be gained by 
reading. 
The stomach has a supporting membrane attached to it 
along its lesser curvature and uniting it to the liver, which 
is, in turn, anchored to the dorsal body wall. This mem- 
brane is, in effect, a mesentery for the stomach, but is called 
the lesser omentum. An extension of similar tissue hangs 
from the greater curvature like an apron over the intestinal 
coils and is called the great omentum. It may become 
a ponderous appendage from the fat which it sometimes 
accumulates. The continuation of the mesentery over 
the external surface of the intestine and the identical 
covering of the stomach form for these organs what is 
spoken of as their serous coat. 
The Finer Structure of the Alimentary Organs.-We 
have said that the internal surface of the stomach and of 
both intestines is provided with glands. The inner layer 
of the wall of the canal in which these glands occur is 
called the mucous coat or mucous membrane. This is in 
reference to the fact that its exposed cells produce the slimy 
substance, mucus, more familiarly associated with nasal 62 
NUTRITIONAL PHYSIOLOGY 
discharges. It probably acts as a lubricant in the digestive 
tract and also protects the lining cells from harsh contacts, 
both physical and chemical. The mucous membrane is 
depressed to form the recesses of the glands, and in the 
small intestine is raised into the microscopic prominences 
referred to as viZZi. Underlying it blood-vessels and nerve- 
fibers run thickly. 
Between the mucous layer and the serous coat on the 
outside of the intestine there is a development of muscle 
of the order described in a previous chapter as characteristic 
of most internal organs, that is to say, slow acting, more or 
less automatic, and much given to tone changes. In the 
small intestine there are two distinct muscular coats: the 
inner and thicker has its fibers at right angles to the axis 
of the canal, while the outer has them set parallel to this 
axis. The inner coat is hence spoken of as circular and the 
outer as longitudinal. There is no doubt of the' superior 
prominence of the circular coat in the production of intes- 
tinal movements. In the stomach the muscular organiza- 
tion is less simple and there are oblique elements in addi- 
tion to those which can be classed as circular and longi- 
tudinal. The colon has the circular coat, but instead of a 
complete covering of longitudinal muscle it has three 
bands of contractile tissue extending along its wall. 
At any point along the course of the intestine temporary 
closure may be effected by the contraction of the circular 
muscle. But there are certain places where such closure 
is far more frequent or, indeed, the usual condition. 
Where the esophagus joins the stomach an irritable band, 
the cardiac sphincter, is much of the time firmly contracted. 
There is, similarly, a pyloric sphincter guarding the open- 
ing between the stomach and the duodenum. Where 
the ileum enters the cecum a valve exists which is adapted 
to prevent the reflux of material from the colon to the small 
intestine. This, the ileocecal valve, is probably reinforced 
in its mechanical action by muscular support. Finally, 
the short anal canal is closed by an inner sphincter which 
is essentially a thickening of its own wall, and an external 
one composed of skeletal muscle. CHAPTER VII 
THE MOUTH-SWALLOWING; SALIVARY 
DIGESTION 
Mastication.-The hygienic importance of thorough 
mastication is undoubted, but there is little occasion for 
any extended analysis of a process so obvious. It is to be 
observed that the lower jaw does not have merely an up- 
and-down movement, but that it glides backward and 
forward and has some lateral play at the same time. The 
teeth, therefore, do not simply chop the food, but rub and 
grind it. The direct pressure which sound teeth can 
apply is incredible. It may amount to 270 pounds.1 
Thus the morsel on which this force is brought to bear is 
dealt with more harshly than if a barrel of flour were set 
upon it! In the work of mechanical reduction a larger 
part is borne by the tongue than is commonly recognized. 
The little member seems to be everywhere at once, thrust- 
ing food between the teeth, withdrawing it again, bruising 
and rasping it against the roof of the mouth. While this 
action is going on an intimate mixture with the saliva 
is accomplished. We must now proceed to a discussion 
of this the first of the digestive secretions. 
Mention has been made of the three pairs of glands which 
supply the saliva. Their united product is estimated to 
reach an amount of about 3 pints a day, equalling the vol- 
ume of the urine. If one finds it hard to credit such a 
statement, attention may be called to the copious character 
of the flow which is noted when one is interrupted at the 
moment of taking food. There is little secretion apart 
1 Cannon, "The Mechanical Factors in Digestion," Longmans. 
New York, 1911. 
63 64 
NUTRITIONAL PHYSIOLOGY 
from eating unless it is excited by chewing sundry things. 
At mealtime a large part of what is swallowed is saliva, 
and the proportion must be greatly raised by the practice 
of prolonged mastication, so-called Fletcherism. The 
formation of saliva is to be regarded as a reflex in which 
the primary stimulation is furnished by food in the mouth 
acting upon the endings of nerves excitable by its chemical 
ingredients and by its temperature more than by its mere 
contact. We have to do with something more than 
the typical reflex, however, because it is a familiar fact 
that the appeal to consciousness has much influence upon 
the flow of saliva. The "watering of the mouth" at the 
approach of acceptable food is a hard phenomenon to 
classify. It is a reflex, but it is one which would not 
occur in an unconscious subject. For such actions the 
term psychoreflex is often used. 
The saliva is a bland fluid which one would hardly sup- 
pose to be endowed with active powers of digestion. In 
some animals it does not have any apparent chemical ac- 
tion. Still, it has valuable properties which we shall do 
well to recognize. Whether it is a digestive juice or not, 
its physical effect is useful in mastication, since it softens 
the food, makes it cohere into the pellets which are pre- 
pared for swallowing, and later lubricates their transit to 
the stomach. Moreover, it has a defensive use, protecting 
the mouth from injury when food or fluid is taken too hot 
or when some corrosive liquid calls for dilution. As it issues 
fresh from the glands it is slightly alkaline in reaction. 
If it stagnates for a long time in the by-places of the mouth, 
as happens during sleep, and if it contains at the same time 
traces of carbohydrate food in solution, bacterial fermenta- 
tion may make it acid and the effect upon the teeth may 
be injurious. The value of an alkaline mouth-wash, like 
milk of magnesia, used at bedtime is evident. 
Human saliva contains various salts. Attention need 
be called only to its lime compounds, which are always de- 
posited more or less upon the back surfaces of the teeth, 
a process which reminds one of the formation of stalactites THE MOUTH-SWALLOWING; SALIVARY DIGESTION 
65 
and stalagmites in caverns. The hard crust that results 
is the tartar. It is not very unlike the original substance 
of the teeth in its chemical composition, and its occurrence 
might seem to indicate a mode of making good the wearing 
away of the teeth. Unfortunately we cannot regard it in 
this favorable light, for the lime salts are always contami- 
nated with food particles and bacteria. The deposit 
should be removed by the dentist at regular intervals. 
The three glands furnish slightly different varieties of 
saliva. Mucin, the essential compound in mucus, is pres- 
ent in the secretion of the two lower glands and not in 
that of the parotid. It gives to the saliva from the sub- 
maxillary and sublingual glands a ropy, mucilaginous 
character, which, of course, becomes more apparent when 
evaporation has concentrated the solution. This is illus- 
trated when the mouth is dried by rapid breathing during 
exercise and becomes furred with the residue of salivary 
mucin. This constituent of the saliva probably makes it 
superior to water as an agent for molding the food into 
pellets. The most interesting property of the secretion, 
its power to hydrolyze starch, may be discussed to more 
advantage after we have followed the food to the stomach. 
Clearly, there is not time enough for much digestive change 
in the mouth of a person of average habits. 
Swallowing.-The transfer to the stomach is a more 
complex matter than is likely to be realized. It involves 
an interruption of breathing and the protection of the nasal 
passages and the larynx against the intrusion of food. The 
first purpose is effected by the retraction of the soft palate 
against the back of the pharynx. The second is accom- 
plished by the drawing forward of the larynx toward the 
chin, a movement which can be plainly felt. By it the 
larynx is tucked under the root of the tongue. The epi- 
glottis was long supposed to bend down upon the larynx 
as a shield to ward off the food from this sensitive organ. 
Observation with the x-ray is said to show that it actually 
descends after the material has passed rather than in 
advance of it. The same motion which pulls forward 66 
NUTRITIONAL PHYSIOLOGY 
the larynx serves to widen the upper part of the esophagus, 
which is not at other times appreciably open. With the 
parts in this position the bolus of food is crowded back 
from its original seat upon the tongue and urged through 
the pharynx by the successive contraction of the bands 
of muscle which surround it. As soon as it is fairly within 
the esophagus the soft palate is lowered, the larynx is 
allowed to emerge from its covert, and the breathing can 
be resumed. Such quick and well-ordered adjustments 
give evidence of co-ordinated reflex action, the contact of 
inn 
Fig. 12.-An exaggerated representation of peristalsis. I and II 
are successive views of the same portion of the alimentary tube: 
P is the zone of contraction shifting downward and always pre- 
ceded by the zone of unusual relaxation (A). Ill is an imaginary 
section through II, showing the food bolus (h) slipping along in 
advance of the contracting region, its advance being facilitated 
by the relaxation below. 
the food morsel with one spot after another furnishing 
the requisite stimuli. We cannot go through the series 
of movements unless there is at least a little saliva to be 
swallowed, and we cannot arrest the march of events when 
it is once begun. 
The contractile tissue in the upper part of the esophagus 
is of the skeletal variety. Lower down this gives place to 
typical visceral muscle. Hence it is not strange to find that 
the advance of the bolus becomes progressively slower as 
it descends. The movement which is here taking place THE MOUTH-SWALLOWING; SALIVARY DIGESTION 
67 
is what is known as a peristalsis, and it is highly important 
that its principle should be understood. The most ob- 
vious feature is a ring of contraction setting in above the 
enclosed pellet, causing it to slide onward, and following 
it down by involving in succession each level of the tube. 
The mechanical application can be simply illustrated by 
propelling a glass bead through a soft-rubber tube by 
pinching repeatedly behind it with thumb and finger. A 
strict analysis of what occurs in the esophagus obliges us to 
recognize that the process is not so simple as it first appears. 
There seem to be two phases in what is called the peristal- 
tic wave. The eye detects chiefly the traveling contrac- 
tion, but this is apparently preceded by a zone of unusual 
relaxation, a region of inhibition. 
The peristaltic wave which is necessary for the propul- 
sion of solid food does not seem to be required to send 
liquid to the stomach. A swallow of water is shot swiftly 
from the mouth to the cardiac sphincter and arrives there 
distinctly in advance of the plodding peristalsis. When 
one drinks a glass of water, the swallows following in rapid 
succession, a single peristaltic wave ends the series. We 
shall find that the small intestine exhibits movements 
which are approximately the same in principle as those of 
the esophagus, but far slower and usually less energetic. 
Sensibility of the Esophagus.-It may be laid down as 
a general principle that the sensory equipment of the ali- 
mentary tract is most complete for the mouth and less and 
less developed as the course of the canal is followed. The 
lining of the mouth is manifestly sensitive to contact, heat, 
and cold, besides possessing the particular sense organs for 
taste. When any point in the mouth is stimulated, we can 
localize it with great accuracy. Interesting observations, 
which have been made upon the esophagus, show that its 
mucous membrane is quite responsive to heat and cold, but 
indifferent to pressure unless it is of such a degree as to 
stretch the underlying muscular coat.1 
1 Carlson and Braafladt, Amer. Jour. Phys., 1915, xxxvi, 153; 
Boring, Amer. Jour. Psy., 1915, xxvi, 1. 68 
NUTRITIONAL PHYSIOLOGY 
Salivary Digestion.-Within the stomach the accumu- 
lated food with a large admixture of saliva lies for some 
time with little motion. Here then salivary digestion must 
take place. The statement has been made that in some 
animals the saliva has only mechanical and protective 
functions. More frequently, however, it has the power to 
hydrolyze starch, forming malt-sugar as the chief end- 
product. This seems to justify the assumption that an 
enzyme is present, and it is variously named ptyalin, sali- 
vary amylase, or salivary diastase. Such an enzyme prob- 
ably plays an important part in the digestive processes of 
ruminants, animals which chew the cud. Human saliva 
acts upon starch with surprising energy. A simple demon- 
stration of the fact may be had by holding a bread-crumb 
in the mouth longer than is habitual, when it will grad- 
ually develop a mildly sweet taste. 
The prevailing opinion in regard to the amount of diges- 
tion accomplished by the saliva in man has undergone a 
change during the last few years. It is allowed a larger 
place than was formerly granted to it. The enzyme is 
extremely sensitive to acid. Inasmuch as the gastric juice 
is decidedly acid, it used to be claimed that salivary diges- 
tion could not proceed in the stomach. But it has come 
to be recognized that when a large mass of food is intro- 
duced into the stomach within a short time the gastric juice 
penetrates it rather slowly. A few minutes after the com- 
pletion of a meal we may picture the stomach-contents as 
being acidified near the surface, the acid slowly making its 
way inward, but having a neutral or even alkaline central 
portion. Salivary digestion will be continued in the stead- 
ily diminishing region not yet reached by the acid, and will 
cease only when the gastric secretion from one wall of the 
stomach meets that from the other. Any rotation of the 
contents would probably bring about an earlier distribution 
of the acid and arrest of starch digestion. No such rota- 
tion seems normally to occur. A factor which operates to 
postpone the destruction of ptyalin is the power of the 
proteins of the diet to engage hydrochloric acid in com- THE MOUTH-SWALLOWING; SALIVARY DIGESTION 
69 
bination. Since proteins are almost always present, the 
gastric glands must secrete acid enough to satisfy their 
capacity before there can be the excess of strictly free acid 
which will put an end to salivary digestion. 
If the mixed food is quite acid at the outset, it is hard to 
see how there can be any hydrolysis of starch brought 
about by the saliva. Yet we constantly eat acid fruits 
before our breakfast cereal and notice no ill effects. 
Starch which escapes digestion at this stage is destined to 
be acted upon by the pancreatic juice, and the final result 
may be entirely satisfactory. Still it is reasonable to as- 
sume that the greater the work done by the saliva, the 
lighter will be the task remaining for the other secretions 
and the greater the probability of its complete accomplish- 
ment. The power of saliva to convert raw starch to sugar 
is almost incomparably smaller than its capacity to digest 
starch which has been cooked. Raw starch exists in very 
dense grains which have to be dissolved from the surface 
inward. Cooking, especially boiling, utterly destroys these 
grains, and permits a reaction between the enzyme and 
the separated molecules of the carbohydrate. 
The change from starch to sugar seems not to be effected 
by a single reaction, but by stages. Physiologic chemists 
have studied extensively the numerous intermediate bodies 
which have a fugitive existence in the process. Most of 
these are covered by the term dextrins. It is sufficient for 
our present purpose to regard them as carbohydrates, 
simpler in their molecular structure than the original starch, 
but complex as compared with the familiar sugars. We 
have said that the chief product of salivary hydrolysis is 
malt-sugar or maltose. This is one of several sugars 
classed as disaccharids. By various means it can be hydro- 
lyzed further to form dextrose, a sugar of the simplest 
type, and one which is ready to be absorbed and to minister 
to the living tissues. Some dextrose is said to be formed 
in prolonged salivary digestion, but the cleavage lags when 
the maltose stage has been reached. CHAPTER VIII 
THE MOVEMENTS OF THE STOMACH 
It will be recalled (Chapter VI) that the stomach consists 
of a main rounded portion from which a much smaller 
conical segment extends to the right to join the duodenum 
at the pylorus. The larger part is called the fundus, the 
tapering region is the antrum. The muscular coats of the 
antrum are somewhat thicker than those of the fundus and 
show an especially conspicuous development of the circu- 
lar elements. There is no such contrast between the two 
parts of the human stomach as between the thin-walled 
crop and massive gizzard of the bird, but there is a faint 
suggestion of an analogous difference. The fundus is, 
indeed, primarily a place for the storage of food; the an- 
trum, while not a crushing mechanism, has distinctly 
greater motor properties. 
The antrum is considered to be set off from the fun- 
dus by the so-called transverse band. This is an irritable 
ring of the circular muscle which is often contracted enough 
to indent the outline of the stomach at this point, and which 
may occasionally create a temporary division of the gas- 
tric cavity into two parts. It has been called the sphinc- 
ter of the antrum, but it cannot fairly be compared with 
the cardiac and the pyloric sphincters, since these are 
habitually closed, while closure at the transverse band is 
rare. 
Regulation of the Cardiac Sphincter.-Food and drink 
entering the stomach pass the cardiac sphincter. The 
guardian muscle is usually more or less contracted. It 
relaxes upon the arrival of the peristaltic wave in the 
esophagus. If it is recalled that the peristalsis consists of 
a wave of inhibition running before a contraction, it is 
70 Fig. 13.-To suggest the probable appearance of the distended 
and active human stomach. Three marked waves of contraction 
are seen in the antral region. These are to be conceived of as pass- 
ing onward toward the pylorus. THE MOVEMENTS OF THE STOMACH 
71 
easy to see how the cardia may be opened at the moment 
when its muscular walls fall under the influence of the 
phase of relaxation. Closure will follow immediately, as 
the second or positive part of the peristalsis involves the 
sphincter. Attention has been called to the fact that liq- 
uids may outrun a pursuing peristalsis and arrive several 
seconds in advance of it at the cardiac opening. Under 
such circumstances it is said that the fluid remains at 
the bottom of the esophagus until overtaken by the wave, 
when the relaxation occurs which permits it to pass into 
the stomach. A person drinking with ill-advised haste 
may have the disagreeable experience of filling the esoph- 
agus enough to produce a painful distention. Relief 
comes abruptly when the peristalsis has made its way to 
the cardiac sphincter and secured an entrance for the 
liquid. 
While this has been the generally accepted description 
of the facts, it has been shown recently that the cardia is 
not always so firmly contracted. Observations by x-ray 
methods, to be described presently, have shown that for 
some time after a meal there may be a reflux from the 
stomach of a cat into the esophagus. Each escape of food 
evokes a local peristaltic wave which returns it to the 
stomach. Such an incident does not entail any movement 
of the throat muscles and is probably subconscious. As 
the period of digestion continues the sphincter becomes 
more tightly set and no longer allows any such return of 
stomach-contents. The increased tension has a simple 
explanation, which, like many another point about the 
stomach, we owe to W. B. Cannon. He has shown that 
the tension is developed in response to the rise of acidity 
in the liquid just within the cardia. Since the acid appears 
normally after each filling of the stomach, we have here 
an automatic provision for the establishment of the requi- 
site guard over this opening. The influence of the ner- 
vous system seems capable of nullifying the local effect of 
acid, since the sphincter may be relaxed to permit vomit- 
ing at times when the gastric contents are excessively acid. 72 
NUTRITIONAL PHYSIOLOGY 
The Fundus.-An important service of the stomach is 
to store food in relatively large quantities at mealtimes 
and to deliver it gradually to the intestine. A person who 
has been deprived of the stomach, or of most of it, by sur- 
gery is made aware of this when he finds it impossible to 
eat "a square meal," and is compelled to take small por- 
tions of food at short intervals. He is then serving his 
intestines somewhat as they are normally treated by the 
stomach. Storage is not the sole function of the stomach, 
but we do well to emphasize it. The fundus accommodates 
itself to its contents by tone changes, relaxing when food 
is being swallowed and afterward exerting a steady, mod- 
erate pressure which insures the filling of the antrum after 
every discharge at the pylorus. A lack of this tonic re- 
action may be a cause of serious disorders. 
The earlier writers often claimed that they could observe 
a definite and regular overturning of the contents of the 
fundus. In the light of more recent observations this does 
not seem to be usual. A German investigator fed to a rat 
three courses of food of contrasted colors. The animal was 
then killed and frozen. A section made through the hard- 
ened mass within the stomach showed distinct stratifica- 
tion. The food first taken was in the antrum and the 
lower part of the fundus, the second instalment was above 
the first, and the third was just under the cardia. It 
seems hardly probable that entirely liquid food could re- 
main thus stratified when one considers the extent to which 
the stomach is subjected to the influence of bodily move- 
ments. 
Hunger.-Very recently it has been established that 
there is a connection between certain contractions of the 
stomach-particularly of the fundus-and the sensation 
of hunger. The experimenters who have demonstrated 
this fact point out that we must distinguish between 
hunger and appetite in all discussions of this kind. Hunger, 
as they intend to have the word understood, is a recurring 
pang readily referred to the gastric region. It is experi- 
enced alike by adults and infants, by human beings and THE MOVEMENTS OF THE STOMACH 
73 
animals. Appetite, on the other hand, depends on psycho- 
logic factors (recollections of favorite foods, etc.). 
It will be worth while to recount the ingenious experi- 
ment by which it was made clear that the intermittent 
hunger sensation corresponds with a motor process among 
the contractile elements of the stomach. Washburn,1 
working under the direction of Cannon, devised a way to 
record the contractions of his own stomach. For this 
purpose a stomach-tube, bearing a collapsed balloon at the 
lower end, was swallowed by the self-sacrificing investi- 
gator. The balloon could then be inflated, and the tube 
brought into connection with a gauge consisting of a water 
column carrying a float and writing-point. Thus, if the 
stomach squeezed the balloon the tracing showed an ele- 
vation; as the stomach relaxed the curve declined. 
Washburn fasted until he could rely upon the occurrence 
of hunger pangs in spite of the discomfort to which he was 
subjected. The revolving drum which received the record 
was out of his sight. By pressing an electric key he could 
cause a mark to be made upon it at any time. He pressed 
this key whenever he experienced the sensation of hunger. 
The tracing showed that strong periodic contractions of 
the stomach coincided with the recorded gnawings. This 
result has been confirmed by Carlson,2 who has made sim- 
ilar experiments upon a man with an opening through the 
body wall to the interior of the stomach. 
It was observed in Washburn's experiments and in those 
of Carlson that the hunger contractions ceased when food 
was tasted and chewed. This indicates a reflex inhibition 
of the gastric musculature. It had already been noticed 
by Cannon that the tone of the fundus is temporarily 
lowered whenever food is swallowed. This constitutes 
what is called the "receptive relaxation," and it has an 
evident value since it facilitates the transfer of material 
from the esophagus to the stomach. 
We are told that when a long fast is undertaken the 
1 American Journal of Physiology, 1912, xxx, 309. 
"The Control of Hunger in Health and Disease," University of 
Chicago Press, Chicago, 1916. 74 
NUTRITIONAL PHYSIOLOGY 
characteristic hunger pangs are not experienced after two 
or three days, ft is probably true that insufficient feeding 
is more distressing than downright fasting. Such suffer- 
ings as those endured by the Greeley party at Cape Sabine 
afford a confirmation of this opinion. 1 
Thirst.-When the body loses water at a rate which 
threatens to reduce the normal percentage in the system 
thirst is experienced and steadily gains in urgency. It 
may become one of the most grievous distresses known to 
man. The central fact in this condition seems to be the 
dryness of the mouth and throat. This is to be referred 
to a suspension of salivary secretion. The organism, de- 
prived of its normal supply of water, is conserving its store 
so far as possible.1 
Certain statements have previously been made concern- 
ing the sensibility of the esophagus to common forms of 
stimuli. Carlson and others have reported corresponding 
facts with reference to the mucosa of the stomach. They 
find that it is relatively difficult to evoke sensations from 
this surface. Pressure, to be effective, must be of such in- 
tensity that its transmission to deeper tissues has to be 
assumed. Heat and cold are productive of recognizable 
sensory impressions, but it has long been an unsettled 
question whether the mucous membrane itself is respon- 
sible for the effect or whether this depends on temperature 
changes in the .skin. Carlson's subject gives such a 
prompt reaction when a small piece of ice is pressed against 
the lining of his stomach that the investigator believes 
there must be a true local sensitiveness to this kind of 
stimulation.2 
x-Ray Studies of the Stomach.-While much can be 
learned of the behavior of the stomach through experi- 
ments involving its exposure by surgical procedures, the 
ideal method is clearly one which leaves the animal in its 
normal condition. Such a method became available when 
1 Cannon, Proceedings of the Royal Society, B, xc, 1918, 283. 
2 See also Hertz, "The Sensibility of the Alimentary Canal," 
Oxford University Press, 1911. 75 
THE MOVEMENTS OF THE STOMACH 
the x-ray was first turned to account to observe visceral 
movements. The image of any part of the body projected 
by means of the x-ray shows the bones in clear contrast 
with the softer parts, but scarcely outlines the organs. If, 
however, any harmless substance opaque to the x-ray is 
introduced into the contents of the alimentary canal, it 
becomes possible to recognize the situation of this sub- 
stance so long as it remains sufficiently concentrated. 
More than this, if the cavity is well filled its outline is, of 
course, identical with that of the included material. The 
x-ray picture will then show the changing contour of the 
organ in silhouette. The compounds most used to secure 
opacity to the x-ray are the salts of bismuth, generally 
the subnitrate or the subcarbonate. 
The most numerous experiments of this sort are those of 
Cannon, and the cat has been the favorite subject. When 
the animal has had a full meal of bread-crumbs and milk 
with the bismuth salt evenly mixed in the mass the x-ray 
shows the entire form of the stomach. The fundus has an 
even outline and preserves it unchanged from hour to 
hour, except that a very gradual contraction takes place. 
The antrum is traversed by deep but slow-moving peris- 
taltic waves, which originate near the transverse band and 
pass to the pylorus. The tendency of such waves must be 
to force successive portions of the food into the intestine, 
but in the great majority of cases the waves bear down 
upon a tightly closed pyloric sphincter. The only possible 
result is then an eddying movement, the contents advanc- 
ing only to rebound from what is, for the time, a blind 
pouch. This favors the reduction of the larger morsels and 
helps to secure at length the formation of the smooth, 
creamy "chyme." But it is probable that most people 
have an exaggerated notion of the mechanical powers of 
the stomach. 
The waves which pass over the antrum arise in the cat 
with strange regularity at intervals of ten seconds. Each 
wave takes about half a minute to make its way to the 
pylorus, so there are commonly three creases to be seen, 76 
NUTRITIONAL PHYSIOLOGY 
all shifting with a motion of clock-like slowness toward the 
outlet. During the prolonged period required for the 
emptying of the stomach of the cat-eight hours or more- 
it is evident that the total number of the waves may be 
over two thousand. When the pyloric sphincter momen- 
tarily relaxes, under influences to be discussed presently, 
the peristalsis of the antrum naturally drives more or less 
of its contents into the duodenum. 
Nervous Control of the Gastric Movements.-Muscular 
elements of the order found in the stomach have been 
said to have an automatic property. We have insisted, 
however, that this fact does not exclude the influence of the 
central nervous system. There is abundant evidence so 
far as the stomach is concerned that the musculature of 
the organ is played upon by efferent impulses. If it is 
separated from the central nervous system many of its 
reactions take place in a nearly normal manner, but we 
cannot assume that its adjustments are as well timed and 
decisive as they were before. Laboratory trials show that 
the impulses which are sent to the stomach may either 
accentuate or abate its spontaneous movements. In 
other words, they may either excite or inhibit the contrac- 
tile elements. 
Of the two types of nervous control, the inhibitory 
seems to be of particular significance. Complete arrest of 
the peristalsis of the antrum may be brought about. This 
happens in the cat when the animal is enraged or terrified, 
and, indeed, when it seems merely to be restless. Cannon 
has again and again seen the peristaltic notches fading 
away from the z-ray profile of the antrum when the animal 
has wTearied of being kept under restraint, and is manifest- 
ing its feeling by switching its tail and struggling. He has 
seen the regular activity resumed when the cat has been 
pacified. Similar facts have been demonstrated for the 
rabbit. Since we generally believe that the higher the 
grade of an animal's development, the more extensive the 
command of the nervous system over its organs, we have 
every reason to think that unpleasant emotions may be THE MOVEMENTS OF THE STOMACH 
77 
accompanied in man also by inhibition of the gastric 
movements. We shall have occasion to enlarge upon this 
matter in connection with the Hygiene of Nutrition. 
The Pyloric Sphincter.-The regulation of the escape 
of the stomach-contents to the intestine has long been a 
subject of interest. It was in 1833 that William Beaumont 
published his observations upon the stomach of Alexis 
St. Martin, a young Canadian trapper, who had suffered 
a gunshot wound in the left side, in consequence of which 
he had a permanent gastric fistula.1 The impression 
which went abroad from this celebrated case has seemed 
to convey to the less scientific writers the idea that 
the pylorus has a power almost akin to intelligent inspec- 
tion whereby it permits the passage of certain portions of 
the chyme and refuses egress to other portions. This 
notion recalls amusingly the teaching of Van Helmont, 
in the seventeenth century, that the soul of man resides in 
the pylorus. It cannot be said that all the conditions 
affecting the discharge from the stomach are entirely clear, 
but much progress has been made in this direction. 
The sphincter is influenced both by the physical consist- 
ency and by the chemical reaction of the gastric contents. 
The contact of coarse, angular particles with the adjacent 
mucous membrane seems to reinforce its contraction, so 
that such material tends to be kept longer in the stomach. 
A more important factor with this sphincter, as with the 
cardiac, is the acidity of the chyme. It was stated above 
that when the stomach-contents becomes distinctly acid- 
ified, the tone of the cardiac sphincter is increased. The 
acid in this case is acting upon the lining below the irritable 
ring. Comparison of the two sphincters shows it to be a 
principle applicable to both-that acid acting immediately 
above favors their relaxation, while acid below causes them 
to tighten. If this is the main factor in regulating the 
pylorus, the first opening will occur when the acidity in the 
antrum has reached a certain point. The next peristaltic 
1 See Osler, "An Alabama Student and Other Essays," Oxford 
University Press, American Branch, New York, 1909, 159-188. 78 
NUTRITIONAL PHYSIOLOGY 
wave will transfer a little chyme to the duodenum. At this 
instant there will be acid material both above and below 
the sphincter. The action from below appears to predom- 
inate, so that closure will be established and maintained 
until the acid in the duodenum is either neutralized or re- 
moved somewhat from the pylorus. When the stimula- 
tion from below is no longer effective the acid above will 
cause a second gaping of the sphincter, followed as before 
by prompt and decisive contraction. A more efficient 
mechanism to insure gradual delivery to the intestine 
without distending it locally can scarcely be conceived. 
When the latest portions of a meal are leaving the stomach, 
the first which went out may have reached the colon, and 
intermediate fractions may be undergoing digestion in 
numerous loops of the intervening small intestine. 
Our meals are usually of a mixed character, including 
proteins, fats, and carbohydrates. For purposes of experi- 
ment single food-stuffs may be fed to an animal and the 
rate of departure from the stomach noted for each. The 
x-ray has been employed for this purpose. Carbohydrates 
have a striking tendency to escape rapidly to the intestine. 
The discharge of proteins and of fats is relatively much de- 
layed. The difference observed may be referred to the 
following facts. Proteins combine with acid and so delay 
the establishment of effective acidity at the pylorus. Car- 
bohydrates do not have this power. Fats do not unite 
with the gastric acid, but they lower the rate of its pro- 
duction and hence retard the rate of discharge to the 
intestine. There is an increasing recognition of the pos- 
sibility that after the stomach has been emptied some 
reflux into it from the intestine may occur. 
Vomiting.-The occasional expulsion of the stomach- 
contents through the cardia and esophagus is accomplished 
as the result of a reflex movement in which the chief mus- 
cles involved are not the coats of the stomach, but the 
contractile tissue of the diaphragm and abdominal wall.. 
When these contract simultaneously a high pressure is 
thrown upon the stomach. Such a pressure may accom- 79 
THE MOVEMENTS OF THE STOMACH 
pany the act of straining or bracing the body for lifting, 
but does not ordinarily result in vomiting, it would appear, 
because of the resistance offered by the cardiac sphincter. 
In the crisis of nausea, however, convulsive movements 
of this kind take place with inhibition of the sphincter. 
With the passage open, each application of intense pressure 
to the stomach may drive a portion of its contents to the 
exterior. The palate meanwhile has assumed the same 
position as for swallowing; the larynx is drawn forward and 
is shielded by the root of the tongue, which is depressed and 
grooved. At every descent of the diaphragm the capacity 
of the thorax is increased, and as no air is permitted to 
enter the lungs the esophagus is dilated. 
During the act of vomiting the fundus is said to contract 
steadily upon its diminishing contents. This is not a 
movement powerful enough to secure the emptying of the 
stomach, but adapts it to be gripped effectively by the 
muscles of the body wall. The transverse band is at the 
same time strongly contracted and the antrum has a very 
small volume. The pyloric sphincter is said to be closed, 
but it is a familiar fact that bile from the duodenum may 
be pressed backward into the stomach under the stress of 
violent vomiting. Profuse salivation precedes and accom- 
panies the act. Such a reflex has a manifest value when 
it serves to remove from the stomach material which might 
prove poisonous. It occurs, however, under many circum- 
stances when it seems not to have any advantageous re- 
sult. Its apparent uselessness in seasickness has already 
been alluded to. It appears equally illogical when, in 
pregnancy, it is excited by irritation of the pelvic nerves. CHAPTER IX 
GASTRIC SECRETION AND DIGESTION 
It was in the eighteenth century that the chemical 
factors in digestion were first clearly separated from the 
mechanical. The accounts which have been preserved 
of the experiments of Reaumur (1752) and of Spallanzani 
(1777) are of extraordinary interest. An entertaining 
summary is to be found in Foster's "Lectures on the His- 
tory of Physiology," Chapter VIII. These ingenious 
investigators were the first to show that digestive changes 
may be caused to take place outside the body and in the 
absence of any mechanical process whatever. They 
obtained small quantities of gastric juice from various 
animals, mixed it with food in flasks and test-tubes, and 
watched for signs of alteration. Spallanzani, in particular, 
succeeded in bringing about considerable solution of his 
samples. 
From that time to the present studies of digestion have 
continued. Where the pioneers were forced to be content 
with observing the dissolving of solid food, their successors 
are drawing inferences regarding the transformations of 
unseen molecules. A deeper insight into the meanings 
of digestion became possible as the new science of organic 
chemistry was swiftly advanced by the researches of 
Wohler, Berzelius, and Liebig. As was pointed out in 
Chapter III, it is the molecular change which is significant 
and not the physical. Experiments in which material is 
introduced into the alimentary canals of animals and later 
withdrawn for analysis are constantly compared with 
trials in which the digestion takes place throughout in the 
thermostats of the laboratory. 
The stomach is popularly supposed to have a very large 
share in the total work of digestion. It cannot, however, 
be claimed that it is indispensable to man. It forms, as 
80 81 
GASTRIC SECRETION AND DIGESTION 
already indicated, a convenient place of deposit for food 
and a gradual feeder of the small intestine, but it is mean- 
while the seat of preliminary digestive changes which 
greatly facilitate the further advance of the process. 
Attention has been called to the fact that salivary diges- 
tion is continued for a time in the almost stationary con- 
tents of the fundus. When this is stopped by the penetra- 
tion of the acid gastric juice it is superseded by a new type 
of digestion in which the proteins are the food-stuffs acted 
upon. The gastric hydrolysis of proteins is generally re- 
ferred to as peptic digestion. Before we discuss it in 
detail we must consider the nature and the circumstances 
of formation of the gastric juice. 
This secretion is the product of the numerous, relatively 
simple glands with which the mucous coat of the stomach is 
provided. Beaumont has vividly described the appearance 
presented by the lining of St. Martin's stomach directly 
after a meal. The surface, usually of a pale gray, flushed 
deeply, and the gastric juice welled up in glistening beads 
from the invisible mouths of the glands. Its manner of 
breaking out resembled the rising of perspiration from 
the pores of the skin. The empty1 stomach may have 
a well-marked film of mucus upon its walls, but its active 
secretion is limpid and free flowing. The volume which 
the human stomach produces in twenty-four hours is 
apparently large; Carlson estimates it at 1500 c.c. (3 
pints) in the case of his subject with a gastric fistula. 
The Acid of the Gastric Juice.-Repeated reference has 
been made to the acidity of the stomach-contents. The 
early investigators were surprised and puzzled when they 
were forced to recognize that the acid in question is largely 
free hydrochloric. When we consider that this acid can- 
not be made industrially except by decomposing a chlorid 
with the still stronger and more corrosive sulphuric acid 
and in an earthen container, we can appreciate their 
1 The "empty" stomach commonly contains air, saliva, an appre- 
ciable quantity of gastric juice, and frequently bile. The liquid 
contents may amount to one or two ounces. 82 
NUTRITIONAL PHYSIOLOGY 
feeling. For here is what we call a strong mineral acid 
proceeding from delicate living cells and from fluids of 
neutral reaction. The formation of hydrochloric acid by 
the cells of the gastric glands has become much more 
intelligible in the light of modern chemical teachings. 
The current theories cannot be presented here. It is 
evident that when the elements of an acid are withdrawn 
from a neutral fluid it must, theoretically at least, be 
rendered alkaline. If we may accept a claim which has 
lately been denied we have a capital illustration of the 
refinement of the mechanism by which the body preserves 
uniform internal conditions in the fact that when gastric 
juice is being secreted, the urine, usually acid to common 
indicators, becomes alkaline. Thus the normal chemical 
equilibrium of the blood is maintained sacred from disturb- 
ance. The juice secreted into the antrum is said not to be 
acid. 
Mention has been made of some ways in which the acid 
of the stomach modifies local conditions. We have seen 
that it gradually checks salivary digestion. It is the most 
important controlling factor for the two sphincters. 
Other features of its action must now be presented. 
Among these is its distinct antiseptic infleunce. Spallan- 
zani noticed that pieces of meat undergoing digestion in 
gastric juice passed into solution without putrefying. 
Similar pieces kept for an equal time in water had radically 
spoiled. This was a significant observation at the time, 
because digestion and putrefaction had been regarded by 
many as identical. We know now that putrefactive de- 
composition of protein is due to the influence of swarming 
micro-organisms, and that hydrochloric acid in the con- 
centration usual in the gastric juice restrains the develop- 
ment of such forms. 
The average strength of the acid in man is given as 
0.2 to 0.3 per cent., rising sometimes to 0.5. Such a concen- 
tration by no means suffices to sterilize the stomach-con- 
tents, but in all probability it destroys many kinds of bac- 
teria, including some which might become the cause of dis- 83 
GASTRIC SECRETION AND DIGESTION 
ease. Others it undoubtedly weakens, so that they multi- 
ply less rapidly when the chyme passes on to the small 
intestine, where the conditions for bacterial growth are 
more favorable. It is not surprising to find that the species 
of organisms which thrive most in the stomach are those 
which themselves produce acid. The "sour stomach" com- 
monly referred to is a stomach in which the generation of 
lactic acid from sugars is actively taking place. This is a 
process very like the familiar souring of milk. Indeed, 
milk is one source of such acid fermentation in the stomach. 
Excessive acidity, whether due to the native juice or to the 
activity of bacteria, may be a cause of discomfort and a 
hindrance to digestion. Cannon has lately shown that 
acidity above a certain degree delays the departure of food 
from the stomach, and this is easily comprehensible when it 
is recalled that after each brief relaxation the pylorus re- 
mains closed until the acid which has just passed has been 
neutralized or dispersed. Acidity within limits is a neces- 
sary condition of gastric digestion, and this will be dis- 
cussed later. 
The Secretion of the Gastric Juice.-The glands of the 
empty stomach seem to be quite inactive. The natural 
supposition that they begin to secrete when food comes in 
contact with the mucous membrane is not borne out by the 
results of experiments. It is most important to note that 
the juice may start somewhat in advance of the arrival of 
food. The stomach as well as the mouth may be said to 
water at the contemplation of a meal. Abundant evi- 
dence of this fact has been furnished by the extraordinary 
experiments of the Russian physiologist Pawlow upon 
dogs. When a permanent opening has been made to the 
interior of a dog's stomach a little gastric juice may issue 
when the dog is merely shown food which he likes. 
That it is unnecessary to have actual contact with the 
stomach wall is still better shown in the case now to be 
described. A dog having a gastric fistula is subjected to a 
second operation, by which the esophagus is severed and 
the portion connected with the pharynx made to open 84 
NUTRITIONAL PHYSIOLOGY 
through the skin of the neck. Whatever is swallowed by 
the dog is now returned to the exterior. The pleasure of 
eating is not impaired. To maintain nutrition suitable 
food may be introduced directly into the stomach. When 
the dog chews and swallows a meal he is quaintly said to 
have a "Scheinfutterung"-a fictitious feeding. This 
proceeding is accompanied by a steady flow of the secre- 
tion from the gastric fistula. 
The secretion obtained under circumstances like the 
above is called the psychic secretion. This term serves to 
emphasize the fact that a mental element is a necessary 
incident of the reaction. The conditions governing gastric 
secretion seem to be quite parallel with those which regu- 
late the movements of the stomach, and their importance 
in hygiene is equally evident. There are a number of 
individuals who have in various ways lost the power to 
swallow food, commonly because of the closure of the 
esophagus. Their lives are preserved by feeding through 
gastric fistulas established by surgery. These unfortu- 
nates find it to their advantage to attend to the idea of 
eating, and to taste and chew portions of their food at the 
time when it is being introduced into their stomachs. 
After a brief period of fictitious feeding the flow of 
gastric juice into the stomach of a dog may continue for 
two or three hours. In view of this we must conclude 
that, however important a favorable mental state may be 
for the initiation of the process, it need not be its accom- 
paniment throughout. We cannot pretend that a meal 
receives our constant attention during any such interval. 
Nevertheless we are hardly likely to overestimate the 
necessity of securing a normal start. If the initial cir- 
cumstances are not favorable the secretion may be long 
delayed or even lacking. 
Now that we have reserved a due place for the psychic 
element, we must pass on to consider the other factors 
which modify the activity of the glands of the stomach. 
Much has been learned from the surprising operations of 
Pawlow and others, who have succeeded in dividing the GASTRIC SECRETION AND DIGESTION 
85 
stomach of the dog into two parts without robbing either 
of its connection with the circulatory and nervous systems. 
When the dog has recovered from the immediate effects 
it may be said to have two stomachs. Either or both may 
communicate with the exterior by a fistula, while one still 
retains its normal relations with the esophagus and small 
intestine. This arrangement makes it possible to place 
food in one stomach and to obtain and measure the un- 
mixed juice secreted into the other. The evidence goes 
to show that when the glands in the lining of one sac are 
active there is corresponding activity on the part of those 
in the other. 
A most significant fact is at once noted when such a dog 
is under observation. There are kinds of food, perfectly 
appropriate for the animal's nutrition, which may lie in 
the stomach without exciting any flow of the juice. This 
is said to be true of bread, white of egg, starch, and some 
sugars. The same articles of diet would be met by an 
abundant gastric secretion if they had been eaten with 
enjoyment by the hungry dog. It makes a radical differ- 
ence, then, whether these materials enter the stomach 
through the mouth and attract the favorable notice of 
the animal, or whether they are slipped through a fistula, 
a proceeding which would probably not be recognized by 
him as a mode of feeding. 
On the other hand, there are some things which do cause 
an outbreak of gastric juice by their mere presence in the 
stomach and in the absence of any psychic factor. To a 
somewhat limited extent this is the case with water, 
though only, it appears, when there is enough of it to dis- 
tend the stomach slightly. The best-known excitant of 
the secretion is meat, and the property is said to belong to 
the extractives or minor substances in this food, and not to 
the proteins of which it is chiefly composed. Meat causes 
a considerable flow of the juice, but there is a much longer 
initial delay than when the psychic element has its normal 
place. In fact, the total quantity produced when meat is 
introduced into a dog's stomach without attracting his 86 
NUTRITIONAL PHYSIOLOGY 
attention is decidedly less than when it is eaten in the 
natural way, and when the psychic and chemical agencies 
are combined. 
It is unfortunate that our knowledge of this matter has 
been drawn so largely from a carnivorous animal. Meat 
might be expected to stimulate the stomach of the dog 
more surely than other foods. How far the secretion may 
be elicited by placing other compounds in the stomach is 
imperfectly known. Milk is credited with some power to 
call it forth, but its superiority to water seems doubtful. 
Alcohol is said to have a positive action of the same kind. 
It is claimed that the dextrins, the intermediate bodies 
produced in salivary digestion of starch, have the property 
of starting the gastric flow. If this is true it is interesting 
as establishing a connecting link between the two successive 
processes, and making it apparent how one may tend to 
insure the setting in of the other in due time. A link of 
this sort exists between gastric digestion and pancreatic 
secretion, as we shall have occasion to point out. The 
condiments, such as pepper and spices, have a reputation 
for stimulating the discharge of gastric juice, and un- 
doubtedly do so when they favorably affect the flavor of 
the food. They are known to increase the blood flow in the 
lining of the stomach, which would perhaps help to con- 
tinue the secretion process when once under way, but 
whether they can actually initiate it apart from their 
psychic effect remains uncertain. 
When gastric secretion is well started there is provision 
for its maintenance as long as the stomach contains food. 
It appears that some of the early products of digestion 
act after the manner of the extractive substances of meat 
and excite the glands to continued activity. The acid 
itself has been found to be absorbed by the cells lining the 
antrum and to set in motion a train of events leading to 
the same result. This form of stimulation will evidently 
cease only with the departure of the last portions of the 
acid chyme. The flow of gastric juice is retarded by 
fats. GASTRIC SECRETION AND DIGESTION 
87 
Digestion in the Stomach.-The gastric juice is usually 
said to contain two enzymes. Recent work indicates the 
presence of a third. The two familiar ones are pepsin and 
rennin. The third is the gastric lipase. Certain writers 
have questioned whether we ought to speak of pepsin and 
rennin as distinct individuals, suggesting rather that there 
is in the secretion a single body having two sets of proper- 
ties. We need not enter into such a discussion; we shall 
for the present continue the convenient usage of speaking 
of pepsin and rennin as two substances. 
Rennin.-The fact that extracts of the stomach wall 
cause the curdling or coagulation of milk has been known 
from very early times. Such extracts, usually derived 
from the stomach of the calf, have long been in use in the 
manufacture of cheese. Rennet is the industrial term for 
an extract with this property; rennin is the scientific term 
for the supposed enzyme contained in it. Cheese curd 
consists of the bulk of the protein of milk which has 
undergone an obscure chemical change and has passed into 
an insoluble form. From a physical standpoint this is an 
anomaly among the digestive processes. We look to see 
solids becoming liquids, while in this curious instance a 
liquid becomes a solid. It may be suggested that this pre- 
vents an unduly rapid passage of milk to the small in- 
testine. The curd when formed has to undergo solution 
like any other solid. 
The action of rennin becomes the more enigmatic when 
it is noted that it is found in the stomachs of animals 
which do not have milk in their normal diets. Milk is 
curdled by extracts of various organs other than the digest- 
ive glands and by some vegetable juices. In the human 
stomach a very firm curd may be formed when a large 
quantity of cows' milk is taken at one time. The dense 
mass may be slow to digest. Human milk is said never to 
set into such a tenacious coagulum, and this is natural, 
since, regarded as a solution of proteins, it is much more 
dilute than the milk of the cow. 
Peptic Digestion.-The chief enzyme of the gastric juice 88 
NUTRITIONAL PHYSIOLOGY 
is the one commonly called pepsin. Its relations with the 
acid of the stomach are so close that many writers urge 
that we should speak rather of "pepsin-hydrochloric acid," 
the term suggesting the existence of a compound of the 
two which is responsible for the action on the food. The 
power to digest proteins is manifested only with an acid 
reaction, and is permanently lost when the mixture is made 
distinctly alkaline. The conditions which permit peptic 
digestion to take place are, therefore, precisely those which 
exclude the action of the saliva. 
When protein in solid form, such as boiled white of 
egg, is subjected to the influence of gastric juice the pieces 
swell and become softened. Later they are dissolved. 
When the trial is made with protein which is originally in 
solution, such as unboiled white of egg, there is no visible 
evidence of change. But there are physical and chemical 
tests which can be employed to show that digestion is as 
definite a change in this case as in the other. An early 
manifestation of this fact is the loss of the property of 
coagulation on heating. Later there are indications that 
the molecules are undergoing cleavage. At each successive 
stage there is a gain in the power of diffusion, a reduction 
of viscosity, and a diminution in the number of precipitants 
which can be employed to throw the protein out of solution. 
Physiologic chemists have studied minutely the charac- 
teristics of the hydrolytic products during the advance of 
peptic digestion. They have attempted to identify nu- 
merous compounds, each of which has a transient existence 
and is then itself hydrolyzed. For our present purposes it 
would be unprofitable to dwell upon such questions of detail. 
Certain of the earlier cleavage products are included under 
the general name of proteoses or albumoses; others, arising 
later and of a simpler character, are called peptones. 
Roughly speaking, there is a parallel between salivary and 
peptic digestion. In either case, molecules of great size- 
of starch and protein respectively-are subdivided pro- 
gressively, first with the formation of somewhat complex 
bodies-dextrins and proteoses-later to form maltose in GASTRIC SECRETION AND DIGESTION 
89 
the first instance and peptones in the second. The corre- 
spondence is imperfect in several ways; for example, mal- 
tose is a single substance, while there appear to be a number 
of peptones. It will be remembered that maltose itself 
is slowly transformed by long-continued action of saliva. 
Quite similarly, the gastric juice will in time effect a fur- 
ther digestion of the peptones, but this advanced diges- 
tion seems normally to be postponed until the intestine is 
reached. 
Gastric Lipase.-Down to a recent time it was held that 
fat underwent no true digestion in the stomach. It was 
recognized that some forms of fat, butter, for example, 
must be melted, and that the fat in the adipose tissue of 
meat might be released from the enclosing cells when their 
protein portion was dissolved. These, however, are mere 
physical changes. Attentive study has brought out the 
fact that there is, after all, some hydrolysis of fat in the 
stomach, though it is probably slight. An enzyme with 
this action is accordingly assumed and is spoken of as 
gastric lipase. It is only the most finely divided (emulsi- 
fied) fats which seem to be appreciably affected. The 
products of this decomposition as well as of the later fat 
digestion in the intestine are glycerin and free fatty acids. 
Summary.-The material passing the pylorus is com- 
paratively dilute and normally free from coarse particles. 
It is acid in reaction, both the native hydrochloric acid and 
the acids formed by fermentation contributing. Much 
of the food is as yet practically undigested. On the other 
hand, some progress has been made in the transformation 
of cooked starch into sugar. The proteins are partially 
peptonized. If milk has formed a part of the diet, it will 
have been curdled and redissolved. Fats may have been 
liquefied and scattered, but are not likely to have been 
extensively hydrolyzed. On the whole, gastric digestion 
may fairly be described as preliminary in character. CHAPTER X 
THE SMALL INTESTINE: ITS MOVEMENTS, 
SECRETIONS, AND DIGESTIVE PROCESSES 
The intestinal content is propelled on its winding way 
by peristaltic movements. These are similar in their 
mechanical principle to the waves of muscular contraction 
passing down the esophagus in the act of swallowing. 
They are, however, much slower and gentler in character. 
As a rule they do not run an uninterrupted course from the 
pylorus to the ileocecal valve, though such a phenomenon 
is an occasional possibility. More commonly a wave will 
travel a limited distance and then fade out, leaving the 
material which it was moving to rest for a time in some 
depending loop. If this is true, the progress of the food is 
intermittent; x-ray observations upon human subjects 
have led to the estimate that it takes four or five hours for 
the passage through the whole length of the small intestine. 
This time may be assumed to vary widely with the indi- 
vidual and the diet. Reduced to an average rate the data 
quoted above give us about one inch per minute. 
Intestinal peristalsis, like the movements of the stom- 
ach, is governed mainly by local mechanisms. Yet in this 
case as in the other the central nervous system may exert 
an influence tending to accelerate or to suppress the ac- 
tivity of the muscular coats. The second or inhibitory 
action is the more marked. A question much discussed 
is whether the direction of peristalsis in the small intestine 
is at all subject to reversal. The sum of the evidence at 
present supports the view that such a reversal (antiperis- 
talsis) is possible, but only under conditions which are 
clearly abnormal. The circumstances tending to bring it 
about have been fully discussed by Alvarez.1 Ordinarily 
1 "The Mechanics of the Digestive Tract," Hoeber, New York, 
1922. 
90 THE SMALL INTESTINE 
91 
it is plain that the intestine is distinctly specialized to 
act in one way rather than the other. 
Not all the muscular contractions exhibited by the small 
intestine have a progressive character. Frequently a loop 
which contains food will become creased at short intervals 
by rings of constriction which do not shift their position, 
but remain stationary for a little. The internal effect is to 
create a series of small pouches holding separate portions of 
the chyme. After this condition has persisted for a mo- 
ment the regions originally contracted become relaxed, and 
new contractions set in at points midway between them. 
Under the influence of such movements the food is con- 
stantly shifted about and subdivided, but it is not driven 
steadily in one direction. This "marking time" on the 
part of the small intestine is referred to as rhythmic seg- 
mentation. Inasmuch as it serves to alter the contact 
relations between the intestinal contents and the lining, 
it probably favors absorption. Some writers have made 
much of the effect which these contractions may be as- 
sumed to have upon the flow of blood and lymph in the 
walls of the canal. When pressure is applied at brief 
intervals to tissue containing these fluids the result may be 
described as a massaging action, hastening the circulation 
and crowding out some of the lymph. This again must 
tend to promote the absorption process. 
A longitudinal movement of individual loops is often 
described. Two neighboring turns of the intestine may be 
seen to glide the one upon the other, coming to rest after 
slipping a short distance, and presently reversing the direc- 
tion of their travel. This form of activity cannot defi- 
nitely further the progress of the contents, but when it 
affects segments enclosing liquid material we may suppose 
that the food and juices are tilted about and brought into 
relation with the largest possible area of absorbing surface. 
The upper part of the small intestine appears to be more 
active than the lower. Consequently it tends to free 
itself rather promptly of contained food. The name jeju- 92 
NUTRITIONAL PHYSIOLOGY 
num, meaning empty, reminds us that this behavior must 
have been noted centuries ago. 
The Secretions Entering the Small Intestine.-In 
Chapter VI it was stated that this part of the canal re- 
ceives the contributions of the liver and the pancreas, as 
well as of the microscopic glands in its own mucous mem- 
brane. The bile and the pancreatic juice, it will be re- 
membered, enter just below the pylorus. The intestinal 
juice is produced by all parts of the extensive lining, but 
more abundantly in the upper than in the lower segments. 
The three secretions have some characters in common. 
They are all alkaline in reaction, owing to the presence in 
them of sodium carbonate. This confers upon them in a 
considerable degree the power to neutralize acids. As 
the acid chyme from the stomach meets the alkaline secre- 
tions in the duodenum there must be more or less carbonic 
acid gas evolved. This may be helpful to the digestive 
process, since the tendency will be to lighten the texture 
of the food particles, much as dough is lightened by the 
agency of yeast. 
It is not merely the acid from the stomach which may be 
combated by the alkali of the juices below; there are two 
other sources of acid to be taken into account. One of 
these is found in the bacterial fermentation, chiefly of 
sugars, which goes on in the intestine. The second is the 
entirely normal formation of free acids occurring in the 
course of fat digestion. So far as the first class of acids are 
neutralized the products are mainly lactates and buty- 
rates; the fatty acids may be converted into soaps. There 
is no guarantee of an exact proportionality between the 
acids and the alkali, and it is impossible to say which will 
be in excess in a particular part of the canal. Generally, 
however, the resulting reaction of the mixture is not far 
from the neutral point. 
The united volume of the three secretions is held to be 
very large, but any estimate tends to mislead, since 
throughout the length of the intestine we have the with- 
drawal of water keeping pace approximately with its in- THE SMALL INTESTINE 
93 
flow. In a certain section selected for observation the 
bulk of the contents may show little change during a long 
period, and yet there may have been profuse secretion 
entirely disguised by counterbalancing absorption. In 
this consideration we see indicated an important service 
shared by all the digestive secretions, that of supplying 
liberal quantities of water to act as the solvent and carrier 
of food-stuffs destined for absorption. If absorption were 
to continue without compensatory secretion, a final stage 
might be reached in which the cavity of the intestine would 
be drained of all fluid, while the walls would be crusted 
with a dry residue suggestive of boiler scale. 
The Pancreatic Juice.-In considering the causes of 
gastric secretion the importance of the central nervous 
system called for emphasis. This is not true to the same 
extent of the government of the pancreas, though here 
also it is held that the nervous system plays a part. A 
chemical means of control is better known. As has been 
hinted before, there is an intimate relation between the 
gastric activity and the later awakening of the pancreas 
from its state of repose. The arrival of acid from the 
stomach in the duodenum causes a timely outflow of pan- 
creatic juice. 
This might be supposed to be an instance of reflex action, 
like the production of saliva when acid is taken into the 
mouth. It has been shown, however, that it has another 
explanation. The acid, striking into the lining membrane 
of the duodenum, initiates a series of reactions which have 
been studied in detail, and which lead at last to the for- 
mation of a substance of quite definite chemical properties 
called secretin. This finds its way into the circulation, 
and, like a drug, is swept far and wide. It stimulates the 
pancreas to produce its secretion, augments the formation 
of bile by the liver, and probably excites the glands in the 
wall of the intestine to greater activity. In the light of 
these facts it becomes clear that a vigorous gastric digestion 
with the strongly acid chyme which results goes far to 
insure a normal intestinal process. 94 
NUTRITIONAL PHYSIOLOGY 
The pancreatic juice in man is an abundant secretion- 
a pint or more daily-and, in marked contrast with the 
saliva and the gastric juice, it has relations with all three 
principal classes of food-stuffs. It contains an enzyme 
which some have thought to be identical with the ptyalin 
of the saliva, but generally called amylopsin or pancreatic 
amylase. Thus the progress of starch digestion, inter- 
rupted for a time in the stomach, is now renewed. The 
slight acidity which may exist in the intestine is not likely to 
prevent this type of digestion from going on to completion. 
Beside acting on starches, the pancreatic juice continues 
and greatly accelerates the hydrolysis of fats which has 
been barely begun in the stomach. The enzyme concerned 
is called steapsin in the older books, but by more recent 
writers lipase. The immediate products are glycerin and 
fatty acids. A secondary formation of soaps is a possibil- 
ity already indicated. When an oil undergoes digestion 
it breaks up at an early stage into microscopic drops and is 
said to be emulsified. This subdivision clearly multiplies 
the surface of contact between the food and the digestive 
juice, and has an effect corresponding to that of mastication 
upon solids. It is, therefore, most helpful to digestion, 
but it is not to be confused with digestion itself. 
The statement is commonly made that the pancreatic 
juice continues the work of pepsin upon proteins by virtue 
of an enzyme called trypsin. While this is approximately 
true, it calls for a certain qualification. If the juice is 
carefully collected as it comes from the main duct of the 
pancreas without being allowed to mingle with other se- 
cretions or even to touch the lining of the intestine, it is 
reported that it is usually incapable of hydrolyzing pro- 
teins. This power it gains in a striking degree when it has 
been mixed with ever so little of the intestinal juice, the 
succus entericus, as it is sometimes called. The interaction 
of the two secretions is described as resulting in the 
"activating" of the pancreatic juice. The natural infer- 
ence is that the inactive fluid contains in solution a body, 
not yet deserving the name of enzyme, but ready to be- THE SMALL INTESTINE 
95 
come one by a quick transformation. An inactive ante- 
cedent body of this kind is termed a zymogen; in this specific 
instance, trypsinogen. Acting on the assumption that 
there is a definite substance in the intestinal juice capable 
of changing trypsinogen to trypsin, physiologists have 
given the name of enterokinase to the agent concerned. 
Bile, also, is capable of activating pancreatic juice. 
Tryptic digestion differs in characteristic ways from the 
peptic process which has been described. Acid which is 
essential to digestion in the stomach is antagonistic to the 
pancreatic type. Trypsin does its work best in a nearly 
neutral mixture. In the normal course of events it acts 
upon material already partly hydrolyzed, but it has the 
power to carry through all its stages the digestion of native 
protein. If the food is a solid, mere inspection reveals a 
difference between peptic and tryptic solution. In the 
former case there is marked swelling, as previously stated; 
in the latter there is progressive corrosion, a shredding or 
honeycombing of the specimen. 
When chemical methods are employed in the study of 
tryptic digestion, it is found to run a course roughly parallel 
with that of the gastric digestion of proteins. Correspond- 
ing intermediate bodies-proteoses-are described in both 
instances, though it is said that some stages in the tryptic 
process are passed so rapidly as to seem almost to be omit- 
ted. What distinguishes the pancreatic proteolysis most 
radically from the peptic is the facility with which the 
peptones are broken down into still simpler bodies. We 
have made the statement that these late cleavages take 
place only very slowly under the influence of gastric juice. 
So it becomes clear that trypsin is distinctly adapted to fol- 
low after pepsin in the accomplishment of protein digestion. 
Peptones are compounds which are simple by comparison 
with standard proteins, but which are still too complex 
to be given precise chemical formulas. When they are 
hydrolyzed, most of the products come within the knowl- 
edge of the organic chemist so definitely that their molecu- 
lar structure can be confidently expressed. To the student 96 
NUTRITIONAL PHYSIOLOGY 
it must be admitted that such formulas do not appear 
simple, but if he is disposed to resent the use of the word 
he is to reflect that these molecules stand in some such rela- 
tion to the original protein complex as the bits of the mosaic 
bear to the whole design. It is with some such an idea 
that the Germans have called them Bausteine, that is, 
the building-stones, from which a new architecture can be 
constructed. The simplest products of the tryptic process 
are conveniently called amino-acids. 
The Intestinal Juice.-This copious secretion was for- 
merly regarded as having little to do with digestion. The 
present disposition is to credit it with a very considerable 
share. When the pancreatic juice is prevented from enter- 
ing the intestine it remains possible to keep up the nutri- 
tion of the animal, and one must conclude that the intes- 
tinal juice is successfully preparing more than one kind 
of food for absorption. Samples of the secretion have often 
been obtained from loops of the intestine disconnected from 
the remainder of the canal. Different workers give vary- 
ing accounts of its properties. 
The feature of its digestive action concerning which there 
is the most general agreement is the hydrolysis of the more 
complex sugars, the disaccharids. Of these sugars, three 
are commonly present, and there appear to be three en- 
zymes adapted to act upon them. Maltose arises princi- 
pally from the salivary and pancreatic digestion of starch. 
It is hydrolyzed to dextrose by an enzyme, which is best 
called maltase. Lactose, or milk-sugar, is similarly con- 
verted into equal parts of dextrose, and the less familiar 
sugar galactose by the enzyme lactase. Saccharose, or 
cane-sugar, gives rise to dextrose, and a sugar of different 
properties, levulose (fructose), under the influence of the 
enzyme, invertase. 
When an extract is prepared from the thoroughly minced 
lining of the small intestine it can be shown to have the 
power to cause proteoses and peptones to undergo hydro- 
lysis, though it is said not to act upon the original unmodi- 
fied protein. This is equivalent to saying that such an THE SMALL INTESTINE 
97 
extract can parallel the later work of trypsin, though lack- 
ing its power to initiate digestion. The enzyme implied 
has been named erepsin. It is regarded as an open ques- 
tion whether this enzyme normally enters the cavity of the 
intestine or does its work within the confines of the cells 
from which it can be extracted. We may conceive that 
when an animal is deprived of its pancreatic secretion it is 
still able to digest proteins, pepsin beginning the digestion 
and carrying it to a stage at which the products are suscep- 
tible to the action of erepsin. The presence of enteroki- 
nase in the intestinal juice has just been noted. 
The Bile.-The secretion of the liver cannot be regarded 
in the same light as the digestive juices mentioned hereto- 
fore. It is not secreted merely after meals, but is always 
flowing through the ducts which converge from the several 
lobes of the liver. It is not necessarily entering the in- 
testine at all times, since the gall-bladder provides a place 
for its temporary storage, as described in Chapter VI. 
While the production of bile never ceases, it does show an 
acceleration during the digestive periods, and this is be- 
lieved to be in response to the stimulating effect of secretin. 
Bile attracted the attention of physicians in very early 
times, its conspicuous color and intensely bitter taste giving 
it a certain distinction. It entered largely into ancient 
theories of disease and of medicine. We have traces of 
these facts in the root-meaning of such words as bilious, 
choleric, and melancholy. In popular estimation bile is 
a poison arising now' and then in the system and causing 
digestive disturbances. Patient study has shown that the 
bile is a complex mixture, and that it numbers among its 
constituents some which are waste-products and others 
which have a favorable effect upon the progress of diges- 
tion and absorption. It stands, therefore, in a position 
intermediate between that of the gastric juice, which is 
formed solely to advance digestion, and that of the urine, 
which is composed of material useless to the body. 
The pigments of the bile are counted as waste substances. 
A red one predominates in carnivorous animals. The bile 98 
NUTRITIONAL PHYSIOLOGY 
of the herbivora is green; human bile may be green, yellow, 
or orange. These pigments show in their chemical nature 
a close relationship with the red coloring-matter of the 
blood, the important compound hemoglobin. All the evi- 
dence goes to show that the bile-pigments are modified 
fractions of the great hemoglobin molecule, and that their 
abundance is an indication of the amount of destruction 
suffered by the red corpuscles of the blood. They do not 
contain the iron previously present in the hemoglobin; this 
element seems to be conserved. These pigments are rela- 
tively insoluble and are not always successfully carried to 
the intestine by the bile. When they deposit in solid form 
in the gall-bladder they contribute to the formation of 
"gall-stones," aggregations in which another compound, 
the waxy cholesterin, may be included. The pigments are 
usually more or less altered by bacterial action in the 
course of their journey through the canal, and eventually 
become the chief coloring-matter of the feces. 
The bitter taste of bile is due to two organic salts of 
high molecular weight. These seem to have a totally dif- 
ferent significance from that of the pigments. They 
are not lost to the body, but are absorbed from the lower 
part of the small intestine and are presumably secreted 
again and again. This phenomenon has been spoken of 
as the "circulation of the bile-salts." The withholding 
of these bodies from the alimentary tract tends to derange 
digestion and, in particular, to diminish the absorption of 
fats. When bile is tested by itself it shows only the 
feeblest digestive powers, yet pancreatic digestion is 
greatly promoted by its presence, and it may be likewise 
an ally of the intestinal juice. 
Light is thrown on the properties of the bile by observing 
the condition of jaundice. This disorder is commonly 
caused by the more or less general plugging of the bile- 
ducts with mucus. The secretion cannot make its escape 
from the liver in the normal way and some of it enters the 
circulation. Bile-pigments make their appearance in the 
white of the eye and in the skin. The urinary pigment, THE SMALL INTESTINE 
99 
which is always closely related to the pigments of the bile, 
is much increased. The ill feeling which usually attends 
the condition may be due in part to the mildly poisonous 
effect of the abnormally retained bile constituents. It is 
likely to be aggravated by indigestion. The bile-salts are 
lacking and the capacity to digest the food and promptly 
absorb the end-products is greatly reduced. Bacterial 
action in the intestine may become pronounced. This 
last fact has suggested that the bile may be an antiseptic. 
It cannot be shown to have anything like a universal action 
of this kind, but it is very probable that it has a selective 
one, favoring one type of organism and restraining another. 
Even though it had no such influence, the intestinal 
bacteria might be expected to multiply in its absence, for 
the simple fact of delayed absorption would suffice to bring 
this about. Our best defense against excessive fermenta- 
tion and putrefaction is in the early and complete removal 
of the food from the sphere of action of micro-organisms. 
Summary.-The secretions flowing into the small intes- 
tine supply enzymes in sufficient number and variety to 
accomplish the digestion of all common foods. The trans- 
formation of starch to maltose begun in the stomach is 
completed by the pancreatic amylase. The resolution of 
proteins into the simple structural units from which their 
molecules are built is carried out under the influence of 
trypsin and perhaps of erepsin. The last-named enzyme 
is supposed by many to work upon the tryptic products as 
they pass through the lining cells on their way to the blood. 
Fats are hydrolyzed by the pancreatic lipase, with the 
formation of glycerin and fatty acids, the latter being 
in some measure converted to soaps. The disaccharids are 
changed to monosaccharids, a work attributed to the in- 
testinal juice. Fermentation caused by bacteria has been 
taking place along with the strictly normal processes. 
The most evident products are organic acids, which may or 
may not be fully neutralized by the alkaline secretions. 
Protection Against Self-digestion.-There has been 
much discussion of the fact that the proteolytic enzymes 100 
NUTRITIONAL PHYSIOLOGY 
in the digestive tract do not ordinarily attack its mucous 
membrane. They may do so after death and when the 
tissues are in an abnormal condition, as in case of gastric 
ulcer, the juices may strongly antagonize the healing pro- 
cess. It is often asserted that there is a definite chemical 
difference between the proteins of living and of lifeless 
matter. A recent explanation of the resistance which 
living cells offer to digestion is based on the apparent fact 
that such cells form bodies which have the capacity to neu- 
tralize enzymes in fixed proportions. The name of anti- 
enzymes has been applied to such protective substances.. 
The ability of the tissues to withstand digestive agents 
is thus made closely comparable with immunity to the 
toxins of diseases. CHAPTER XI 
THE LARGE INTESTINE 
The material passing into the colon is dilute and much 
reduced in volume as compared with the chyme passing out 
of the stomach. In the lower part of the small intestine 
the absorption of water more than counterbalances the 
secretion, hence the shrinkage of the contents. But 
since the end-products of digestion are being absorbed also, 
there is no tendency toward extreme concentration. In 
the large intestine there is but little secretion, and the con- 
tinuation of the absorption of water reduces the contents 
at last to a nearly solid consistency. 
The colon appears to be of very unequal value to animals 
of different classes. In the carnivora the work of diges- 
tion and absorption is so nearly finished by the small in- 
testine that very little remains to be done. The small 
quantity of matter having a potential food value may be 
accompanied into the large intestine by enzymes, which 
may there carry further their digestive action. Such food, 
however, is likely to be a negligible amount, and the digest- 
ive powers of the mixture at this point are unreliable. 
In the herbivora, the food being bulky and refractory, 
a considerable portion may arrive undigested in the colon. 
These animals have very capacious ceca, in which great 
masses of contents seem to be held for long intervals. 
The digestion occurring there may be partly effected by 
the native secretions, but it is believed to be largely the 
work of bacteria. 
The average human being resembles the carnivorous 
type rather than the other. Numerous cases have been 
observed in which no use was made of the colon, and it was 
never difficult to maintain nutrition. When the large in- 
testine is no longer traversed the discharges are watery and 
rather voluminous, but they contain only small percentages 
101 102 
NUTRITIONAL PHYSIOLOGY 
of valuable nutriment. The fluid escaping from the end of 
the small intestine or the beginning of the large is described 
as entirely inoffensive, which indicates that putrefactive 
Fig. 14.-The colon with special reference to its movements: 
T is placed near the seat of frequent sustained or tonic contraction. 
From here backward to the cecum (C) antiperistalsis is of common 
occurrence, but this segment is swept also by occasional waves in 
the opposite direction; V indicates the position of the ileocecal valve, 
which prevents reflux to the small intestine under the influence of 
antiperistalsis; beyond T the infrequent movements which take place 
have always a progressive character; 5 and S, are regions often 
found to contain stationary contents; K is the kink referred to in 
the text; R is the rectum, and Sph is the double sphincter at the 
anus. 
decomposition of protein is usually confined to the colon. 
Protein putrefaction has sometimes been held accountable 
for many ailments, and from this view has arisen the com- 
mon teaching that man would be better off without a THE LARGE INTESTINE 
103 
large intestine. Comment upon this opinion may well be 
reserved for the chapter on the Hygiene of Nutrition. 
The Movements of the Colon.--The average rate of 
progress in the large intestine is low. A factor tending to 
make it so has recently been brought to notice through the 
studies of Cannon and others. The ascending colon has a 
property not duplicated elsewhere in the canal, that of 
habitual antiperistalsis. As repeated instalments pass the 
ileocecal valve the cecum is filled and the accumulating 
contents reach higher and higher levels, but instead of 
thrusting onward this gathering mass, the circular mus- 
cular coat most of the time urges it backward. The result 
is said to be much like what is seen in the antrum of the 
stomach, although here the direction of the movement is 
reversed, that is, the waves of contraction press the mix- 
ture downward into a pouch, from which the only escape 
is by an eddying reflux through the crawling ring. It is 
assumed that the ileocecal valve prevents any return to the 
small intestine. A deliberate hindrance to progress at this 
point may be of service so far as it secures a more perfect 
absorption of digestive products and the retrieving of sur- 
plus water. The antiperistaltic waves are described as 
starting from a zone in the transverse colon near the mid- 
line of the body. 
Another type of movement often witnessed is exhibited 
by the little pouches or sacculations which are character- 
istic features of the large intestine. These undergo dis- 
tention as the fecal matter is pressed into them and from 
time to time they react and empty themselves by con- 
tracting. This is called "haustral churning"; it occurs in 
such a localized but irregular fashion that it has been 
likened to the dropping of piano keys under invisible 
fingers. 
An interesting description of the behavior of the human 
colon has been furnished by Hertz. He has made use of 
food impregnated with bismuth salts, and has employed 
the x-ray to follow its progress. Earlier accounts left 
somewhat in doubt the means of transferring the contents 104 
NUTRITIONAL PHYSIOLOGY 
of the ascending colon to the transverse. Tt now appears 
that this is effected not so much by a peristalsis as by the 
vigorous contraction of the entire ascending colon, with 
the result that its cavity is almost abolished and the 
enclosed material decisively driven toward the spleen. 
Such marked contractions occur only at long intervals 
and, as Hertz believes, most commonly after a meal. 
At such a time the distal as well as the proximal portions 
of the colon are especially subject to agitation. This 
seems to indicate that the outpouring of nerve impulses 
upon one part of the tract is likely to be extended to other 
segments.1 
Beyond the midpoint referred to above those move- 
ments which occur are invariably progressive, but are 
separated by long periods of repose. If we adopt a recent 
estimate we can make the following general statement: 
The foremost portion of a meal, that which was first to 
pass the pylorus and the ileocecal valve, may be expected 
to be near the spleen at the end of the ninth hour. When 
defecation occurs all the colon beyond this point may be 
freed of its contents. If this happens once in twenty-four 
hours, the age of the food residues discharged will evidently 
vary from thirty-three hours, for the matter longest 
retained, down to nine hours, in the case of that which 
barely came within the section evacuated. There must be 
extremely wide departures from these figures in individual 
subjects. 
The descending colon is reported to be found empty, as a 
rule, when observed with the x-ray. This is taken to mean 
that it is an irritable segment which is stimulated to ad- 
vance all that enters it to the sigmoid flexure without de- 
lay. The sigmoid flexure, on the contrary, permits a rela- 
tively large accumulation before it is excited to contract. 
This region of the colon is interesting from a biologic stand- 
point, because it seems to be an adaptation to the erect posi- 
tion of the body. Quadrupeds hold the large intestine, 
roughly speaking, in a horizontal plane, and with them 
1 Hertz and Newton, Journal of Physiology, 1913, xlvii, 57. THE LARGE INTESTINE 
105 
the effect of gravity on the contents is immaterial. These 
animals have no sigmoid flexure, the descending colon in- 
clining toward the midline, and joining the rectum without 
the S-shaped connection. When the erect position is as- 
sumed there will naturally be a tendency on the part of the 
fecal material to settle toward the anus. The develop- 
ment of the sigmoid in the apes and in man provides a 
place of lodgment for the burden and spares the rectum 
from a constant distention. 
The sharp bend between the sigmoid and the rectum, 
amounting almost to a kink, such as one may see in a rub- 
ber tube that has been doubled, is not easily passed by the 
feces. It is only when the quantity has become consider- 
able that a vigorous peristalsis overcomes the resistance 
and fills the rectum. Here for the first time the pressure 
arouses distinct sensations. If defecation is postponed the 
tone of the rectum may be lowered and these sensations 
cease to be felt. If the occasion favors, the anal sphincters 
are inhibited and the rectum is emptied by its own peris- 
talsis, reinforced by external pressure developed through 
contractions of the abdominal muscles and the diaphragm. 
The action of the skeletal muscles at this time resembles 
that in vomiting, but is, of course, much more largely 
voluntary and more sustained in character. When defeca- 
tion takes place peristalsis may not be limited to the 
rectum. Material is commonly brought down from the 
region of the splenic flexure and forms the latest part of 
the evacuation. 
The Feces.-Two classes of material may be mingled in 
the contents of the colon: the residues of the diet and the 
excretions of the alimentary tract including its glands. 
The proportion existing between these two is a variable 
one. Physiologists have lately come to regard the food 
residues in the feces, under average conditions, as forming 
a less prominent element than was formerly supposed. 
A correspondingly increased importance is assumed by the 
excretions. The feces passed during long fasting are evi- 
dently made up exclusively of bodies of the second class. 106 
NUTRITIONAL PHYSIOLOGY 
Such feces, while small in amount, may have practically 
the same composition as those formed upon a moderate 
and digestible diet. This suggests that the increased 
quantity which results from eating does not imply a greater 
residue so much as a greater production of secretions along 
the active canal. A mass very much like normal feces 
may gather in an isolated loop of the intestine to which no 
food is admitted. Among the substances originating from 
the tract rather than from the food are the modified bile- 
pigments, cholesterin or its derivatives, mucus, and de- 
tached cells. 
Bacteria, living and dead, intimately mixed with the 
numerous products of their own life activity, are promi- 
nent in the bowel discharges. These organisms can in one 
sense be said to have their origin in the diet, since that was 
the source of the primary seeding or infection. From 
another point of view they are largely developed at the 
expense of the body itself, for they have multiplied in the 
intestine and may have lived in part upon its secretions as 
well as upon the food. The gases of the colon are due to 
their activities. 
True food residues include the indigestible matter of the 
diet and some undigested food-normally but little of the 
latter. Absorption is strikingly efficient with most sub- 
jects and reasonable diets. Not more than 10 per cent, of 
most foods goes to waste. (The solids of an average daily 
ration amount to about 500 grams. The dry material in 
the day's feces is not usually more than 40 grams. Even 
if it were all derived from the diet this would represent 
a waste of only 8 per cent.) The principal indigestible com- 
ponent is the cellulose of vegetable tissues. When it is 
abundant, as in a diet containing coarse, woody matter, 
such as is eaten by the rabbit, the feces are given an im- 
mense bulk. The same result is secured in a measure 
when man adopts a ration in which fruits and vegetables 
enter freely. Much cellulose is an impediment to com- 
plete digestion and absorption of the proteins and carbo- 
hydrates which it envelops, but the foods in which it THE LARGE INTESTINE 
107 
occurs are cheap and the waste involved does not open 
an economic question of moment. Some bacterial decom- 
position of cellulose is said to take place in the colon and 
may be of slight advantage to us, not that we profit by the 
products of such fermentation, but that a freer exposure 
of proteins and starch may be insured. 
Most writers have held that a moderate amount of in- 
digestible material is a desirable feature of the diet. This 
teaching was emphasized by the celebrated Sylvester 
Graham early n the last century. Lowell has referred to 
him as an "apostle of bran." Later authorities have not 
recommended such wholesale loading of the tract with 
husks and fibers, but an admixture of such elements has 
been generally advocated. The favorable effects include 
the well-recognized stimulation of peristalsis and probably 
a better distribution of food along the canal. The indi- 
gestible fraction of the food has been spoken of as ballast 
and also by the expressive name of "roughage." 
Under abnormal circumstances the loss of valuable food 
through the stools may become extensive. This will be 
the case when absorption is interfered with either by an 
acceleration of peristalsis or in other ways. Different 
cathartics bring this about in a varying manner, some by 
hastening the rate of propulsion, and others, especially the 
salines, giving the intestinal contents a concentration and 
a chemical character which forbids absorption. A mild 
disturbance of this nature is more apt to result in a waste 
of fats and fatty acids than of other forms of food. 
Rectal Feeding.-While the large intestine of man is not 
called upon in normal conditions to absorb much nutri- 
ment, it has some reserve pow'er to do so. This is often 
of a certain value in sickness, when it may be possible 
to tide over a critical time and to keep up a measure of 
strength by introducing suitable foods into the colon. 
Fluid mixtures used in this way must be of low osmotic 
pressure, a fact which, unfortunately, rules out the sugars 
excepting in small amounts. Milk and eggs are much em- 
ployed. Experience has shown that the body makes a 108 
NUTRITIONAL PHYSIOLOGY 
partial use of the proteins offered, even though they have 
not undergone digestion. The results are better when arti- 
ficial digestion has been brought about in advance. So far 
as we know there is no flow of efficient digestive secretions 
in response to the introduction of food through the rectum. 
It is believed that nutritive enemata may in part at least 
enter the region of prevailing antiperistalsis, and that this 
must favor their long retention and better utilization. 
It is fair to say that less reliance is placed upon rectal 
feeding than was the case a few years ago. Stimulants 
may be given through the large intestine and water to 
allay thirst. CHAPTER XII 
THE BLOOD 
The intestinal lining is a barrier between the mixture 
of food and secretions within the canal and the blood 
which flows through the neighboring vessels. The main 
problem to be dealt with in treating of absorption is the 
transfer of portions of the intestinal contents to the blood. 
We shall find that a certain fraction of the incoming nutri- 
ment is carried for a time in the lymph, but this is destined 
before long to blend with the main current of the circula- 
tion. It will be for our advantage to become somewhat 
familiar with the make-up and service of the blood before 
we discuss the entrance into it of the products of digestion. 
Blood is a carrier. Regarding it as such, we can con- 
veniently subdivide its functions according to the classes 
of material which it conveys. Most people will think of it 
as, first of all, the bearer of food to the tissues. This is, 
indeed, a matter of prime importance. The food, during 
periods of digestion, is added to the blood mainly as it 
flows through the capillaries of the wall of the alimentary 
canal, and taken from it by the various parts of the 
living body in proportions corresponding with the degree 
of the local activity. The largest tax is that levied by the 
skeletal muscles. During fasting food for current needs 
is added to the blood from various reserve stores. An 
unfailing supply of oxygen is a need even more urgent 
from moment to moment than the presentation of food. 
Oxygen is added to the blood in the lungs, and is with- 
drawn from it as it makes its round through the body, 
the quantity consumed in different organs being an even 
better measure of their individual metabolism than is 
their appropriation of food. Where oxygen is taken from 
109 110 
NUTRITIONAL PHYSIOLOGY 
the blood, carbon dioxid is returned to it. This indicates 
that the blood is a bearer of wastes. The excess of carbon 
dioxid makes its escape during the next transit of the 
blood through the lungs. Other waste substances are 
gathered by the blood as it flows near the active cells. 
The major part of these is destined for excretion by the 
kidneys, and these organs receive so large a share of blood 
that the entire volume must come under their influence 
within a short space of time. A minor fraction of the 
waste finds an outlet in the bile, the sweat, and through 
the intestinal wall. 
It has already been pointed out that every organ has a 
chemical constitution and a metabolism peculiar to itself. 
Therefore each organ must make certain demands upon the 
blood not exactly duplicated elsewhere. What is more 
important, each organ gives rise to products unlike those 
formed by any other part. In a number of cases these 
products can be shown to have far-reaching effects. Their 
existence has been mentioned in Chapter IV. Recognizing 
the large part which they play in the economy of the organ- 
ism, we may state at this point that an essential service 
of the blood is the transportation of internal secretions. 
Another function of the blood, and one often overlooked, 
is that of equalizing the temperature of different regions. 
One is reminded of the arrangement of a heating system in 
a house where a fire burns in a furnace and a circulation 
of air, hot water, or steam disperses the heat through the 
rooms above. Interruption of this circulation will allow 
the upper stories of the house to cool off, while the base- 
ment is overheated. In the living body heat production 
is a function of all active tissues. The ancients believed 
that it was the particular duty of the heart. This organ is, 
in fact, distinguished for its rapid evolution of energy, but 
it must be remembered that it is of small bulk. A much 
larger share of the aggregate heat production is borne by 
the skeletal muscles. These are supplemented to some 
extent by the liver and the other great glands of the body. 
Blood passing through a tissue which is undergoing lively THE BLOOD 
111 
metabolism will have a higher temperature as it leaves than 
it had when it entered. As it flows on it will communicate 
some of this surplus heat to resting structures in which 
little or no heat production is going on. This is the case 
with the connective tissues and with the skin. 
At the surface the blood loses heat, at least under all 
conditions which can be called normal. The cooler blood 
then returns to the large vessels, merges with the heated 
blood from the muscles and glands, and brings the 
temperature of the mixture to an average which seldom 
varies materially. The means by which this standard tem- 
perature is maintained will be discussed at length in a 
later chapter. It will be well to point out even at this 
time that the skin is subject to considerable changes of 
temperature, and that our sense of being warm or cold 
depends entirely on the condition prevailing at the sur- 
face. If we turn again to our illustration of a house heated 
by a "circulation" of hot air or water, we shall recognize that 
here, as in the living body, there is a constant escape of heat 
to the environment. The temperature of a pane of glass 
in a window of the house will be influenced both by the 
internal and the external state of affairs. The glass may be 
warmed as hot air is wafted against it from within or cooled 
by a gust from without. So the skin may be warmed by a 
waxing of the blood-current close beneath it or chilled by a 
passing draft. 
Blood may be described as a red fluid, but inspection 
with the microscope shows at a glance that its redness is 
not due to a coloring-matter in solution and uniformly dis- 
tributed, but to the presence of minute solid bodies in sus- 
pension. These are the red corpuscles. The liquid in 
which they are swept about is the plasma. The corpuscles 
make up something less than one-half the total volume. 
In view of this, it is surprising that the blood can have such 
a free-flowing character and find its way through the cap- 
illaries so readily. One would anticipate that such a 
mingling of solid particles with fluid would result in the 
formation of a highly viscid mass. That the actual condi- 112 
NUTRITIONAL PHYSIOLOGY 
tion is so different must be due to the absolute smoothness 
and the great pliability of the corpuscles. 
The Red Corpuscles.-The individual corpuscle is usu- 
ally seen in the form of a disk slightly hollowed on its sur- 
faces. It is about five times as wide as it is thick. A very 
slight unbalanced pressure acting from one side will con- 
vert it into a saucer- or even a cup-shaped form. It is re- 
markably elastic, and springs back into its original shape 
as soon as there is no longer a force acting to distort it. 
Most cells of the body are variable in outline and in size, 
but one red corpuscle is as much like another as though all 
had been made in the same mold. The diameter does not 
vary perceptibly from inch. This is the figure for 
human blood; there is a standard size of corpuscle for each 
animal species. As the corpuscles are very minute so 
they are almost inconceivably numerous. The total 
number in the blood of a man must be reckoned in trillions. 
The red corpuscles are often spoken of as cells, but their 
claim to rank as such is questionable. Regarding them 
Fig. 15.-Red blood-corpuscles. Several are shown in different 
positions. The hollow centers are evident. In one case two cor- 
puscles are overlapped to show that they are transparent. They 
tend to run into piles, as shown at the right. A saucer-shaped form 
is represented. 
from an anatomic standpoint, they lack a feature which is 
reckoned an essential of the cell; namely, the nucleus. 
On the physiologic side there is little reason for suppos- 
ing them to be alive. The fairest view, probably, is to 
look upon each red corpuscle as a modified and, in a sense, 
degenerate cell. A single substance has come to constitute 
a very large percentage of its make-up. This is the 
peculiar, iron-containing protein, hemoglobin. We shall 
not be far wrong if we consider the corpuscle to be a packet 
of hemoglobin moving passively at the mercy of the cur- THE BLOOD 
113 
rent. Its function is clear and definite. Hemoglobin has 
the property of combining with oxygen whenever the gas 
is freely present in the surrounding medium. It has also 
the property of releasing this oxygen when it comes into a 
situation where there is a relative scarcity of the element. 
The hemoglobin of the blood, as can be seen from what 
has been said, is ever passing from one state to another, as 
it alternately adds oxygen to itself and parts with it. 
Evidently it can be described as existing in two varieties: 
a form fully charged with detachable oxygen (oxyhemoglo- 
bin), and a form from which this loosely engaged oxygen 
has been removed (reduced hemoglobin). The first is the 
prevailing variety in arterial blood fresh from the lungs, 
and it gives to such blood a bright scarlet color. The 
venous blood, returning from the tissues, still contains a 
goodly proportion of oxyhemoglobin, but mingled with it 
there is now a variable amount of the reduced compound. 
Reduced hemoglobin is of a dark color, and venous blood, 
in consequence, inclines toward a purple. The sharply 
contrasting red and blue so commonly used in diagrams to 
distinguish arteries from veins greatly exaggerate the 
actual difference. 
The red corpuscle, it will now be clear, is a bearer of 
oxygen. A service like this must be dependent on the 
extent of surface exposed for the absorption and the dis- 
charge of the gas. The disk is obviously more efficient 
than a sphere would be, for it has more surface in pro- 
portion to its mass. Small corpuscles likewise must be 
quicker to load and to unload than large ones, and this 
helps us to understand why the largest corpuscles in nature 
are not found in large animals, but in those like the frog 
and the fish, which do not have a very intense metabolism. 
There is an additional reason why small corpuscles are 
better fitted to meet the demand for a great oxygen supply: 
the capillaries must be of a size to admit the corpuscles, 
and an animal with large corpuscles cannot have the 
closely woven capillary net which becomes a possibility 
when the diameter of the corpuscles is reduced and which 114 
NUTRITIONAL PHYSIOLOGY 
brings the oxygen into closer relations with the cells which 
require it. 
The red corpuscles originate, generally speaking, by the 
transformation of cells in an unexpected locality. This is 
the so-called "red marrow" which fills the small spaces that 
abound in the enlarged extremities of the long bones. 
Here there is always going on a progressive change in the 
composition of the cells, in course of which hemoglobin 
replaces most of their original substance. The cell nuclei 
are eventually lost and the newly formed corpuscles de- 
tach themselves and drift away in the blood-stream. Their 
hollow centers strongly suggest the deficiency due to the 
loss of the nuclei. It is an interesting fact that when re- 
covery is taking place after hemorrhage, red corpuscles 
containing nuclei or presenting other signs of immaturity 
are often to be found in samples of the blood. This makes 
it seem as though in meeting the emergency corpuscles 
in an incomplete stage of their development had been 
pressed into service. 
The first glance at a specimen of blood under the micro- 
scope leaves the impression that the corpuscles are all of one 
unvarying type. Closer observation shows here and there 
among the host of colored elements bodies of another or- 
der, the white or colorless corpuscles. There is but one of 
these to 500 or 1000 red. The white corpuscles are not 
flattened, but more nearly spheric. They are, however, 
of no fixed form, and many of them have the property of 
ameboid movement. This is good evidence that they are 
to be considered living, and it can be shown further that 
they have nuclei and conform to our conception of com- 
plete cells. Much that is of interest is known of them, and 
the relation which they bear to the checking of bacterial 
infection is of the utmost importance. We shall not enter 
upon a discussion of this fascinating subject. 
The Plasma.-Aside from the transportation of oxygen, 
all the chief functions of the blood could apparently be 
fulfilled by the plasma. The standard foods of the tissues 
are here. So also in much smaller amounts are the non- 
gaseous wastes. Carbon dioxid, the most abundant of all THE BLOOD 
115 
waste-products, is carried jointly by the plasma and the 
corpuscles. It is evident that we must expect a solution 
meeting such manifold requirements to be of a highly 
complex nature. This is eminently the case. 
More than nine-tenths of the plasma is water. The 
proportion is little increased by drinking and little reduced 
by thirst. Its constancy is secured by kidney activity 
coupled with rapid exchanges of fluid which take place be- 
tween the blood and the lymph and tissues. When a great 
draft is made upon the water of the blood, as is the case in 
profuse sweating, the volume is restored by taking in water 
from outside the vessels, and this may mean a marked 
loss of body weight. The blood itself shares less than 
might be anticipated in such a loss.1 Its total volume 
in the adult of average build has been variously estimated 
at from 3 to 5 quarts. The earlier calculations led to 
the higher figures; 4 quarts (8 to 9 pounds or yg- of the 
weight) is a reasonable standard for us to adopt. 
Among the substances in the plasma which can be 
classed under the general head of foods, proteins take 
the first place. They form about four-fifths of the total 
solids. This high proportion does not correspond at all 
with the make-up of an average diet, in which, as we 
have seen, proteins take the second or even the third 
place. It is usual to distinguish three varieties of protein 
in the plasma, serum-albumin, serum-globulin, and fibrin- 
ogen, but indications are multiplying which support the 
belief that the actual number of proteins with clearly 
individual characters is much greater. Our knowledge 
of these bodies, their mode of origin, place of formation, 
and the particular value of each one in the system, is in a 
highly unsatisfactory state. The impression prevails that 
the proteins of the plasma constitute a relatively stable 
and permanent mass not subject to depletion and, accord- 
ingly, not actively produced under ordinary conditions. 
1 Haldane and Priestley found that the drinking of 5 liters of 
water in two hours and fifteen minutes did not produce any distinct 
dilution of the blood as judged by the percentage of hemoglobin. 
The urinary output reached more than 5 liters in eight hours. 
Journal of Physiology, 1, 1916, 296. 116 
NUTRITIONAL PHYSIOLOGY 
Carbohydrates, which occupy the leading position in the 
rations ordinarily chosen by civilized man, are very scantily 
represented in the plasma. The principal one present is 
the monosaccharid, dextrose, known also as glucose, or 
grape-sugar. The percentage of dextrose figured for the 
whole blood is usually near 0.1, rarely so much as 0.2. 
This means from 1 to 2 grams of sugar in a liter of blood, 
and limits the total amount in circulation to 10 grams at 
most. Such a quantity seems insignificant when it is 
remembered that 100 grams of sugar may easily be formed 
in the digestion of a moderate meal. The sugar is not 
readily increased by free feeding of carbohydrates, nor 
does it noticeably diminish during long fasting. The ex- 
planation of this singular constancy will be given later. 
Fat, or its derivatives, is found in the plasma in a pro- 
portion of a similar order to that of sugar. It is much more 
subject to variation, rising notably after a meal in which 
there was much fat. Plasma obtained from a dog at such 
a time may exhibit the phenomenon of developing a true 
cream, the fat gathering at the surface. During starva- 
tion the blood is not necessarily poor in fat, since it is 
likely to be engaged in carrying this form of food from 
places of storage-the adipose tissue-to the muscles and 
elsewhere to be oxidized. 
Those compounds in the plasma which we can confi- 
dently designate as waste-products occur only in the small- 
est quantities. Carbon dioxid is, of course, an exception, 
being very abundant. Among the non-gaseous wastes 
destined to be dealt with by the kidneys, urea is the only 
one which is easily detected. This is the compound in 
which seven-eighths of the nitrogen is carried from the 
body. The fact that the waste substances are kept down 
to such a low level in the blood is evidence of the remark- 
able efficiency of the kidneys and the supplementary or- 
gans of excretion. It is also a reminder of how rapid and 
copious is the circulation. No portion of the blood can 
long escape passage through the glands which have this 
striking power to hold it to a standard composition. THE BLOOD 
117 
If the mixed solids from a sample of plasma are inciner- 
ated, we have left a small mass of ash or mineral matter 
amounting to about 1 per cent, of the whole blood. By 
far the largest component is sodium chlorid, the chief salt 
of the diet, and the only one which we deliberately add to 
our food. Other bases represented are potassium, calcium, 
and magnesium. Besides chlorids, we find carbonates and 
phosphates, the former having greatly involved relations 
with the carbon dioxid of the blood.1 To the list of 
components native to the blood might be added many 
minor constituents, some of which are judged to be 
present because of certain properties displayed by the 
blood rather than because they can be chemically identi- 
fied. Illustrations of such are afforded by the internal 
secretions, enzymes, and the immune bodies. Individual 
peculiarities of metabolism, susceptibility, and resistance 
must depend on substances of this obscure class. 
Coagulation.-Blood when shed shows the familiar 
property of clotting. This is a most valuable quality, 
since it provides for the automatic checking of ordinary 
bleeding and also forms a protective shield, the scab, be- 
neath which the healing process may go on. The im- 
portance of coagulability is emphasized by the rather fre- 
quent observation of cases in which it is lacking, and in 
which serious or even fatal hemorrhages follow trifling 
injuries. The essence of the process is a chemical change 
affecting one of the minor proteins of the plasma in such a 
way that it passes into a solid form and cements together 
the red corpuscles. The original protein is the one already 
named as fibrinogen. The modified form after coagulation 
is called fibrin. The actual amount of fibrin is extremely 
small (perhaps 2 to 4 parts in 1000 of blood), but it is not 
difficult, when we consider its gummy character, to under- 
stand how it can convert a liquid medium into a stiff jelly 
by knitting together the suspended corpuscles. 
1 Sulphates will also be found in such a sample of ash. They are 
scarcely to be discovered in normal blood, but arise from the burn- 
ing of proteins. 118 
NUTRITIONAL PHYSIOLOGY 
The entirely natural impression that coagulation is the 
result of exposure to the air is erroneous. The matter has 
been the subject of the most painstaking studies, of which 
we can give only the briefest summary. The formation of 
fibrin is an instance of enzyme action, and forcibly recalls 
the curdling of milk in the stomach under the influence of 
rennin. The resemblance is not complete in every re- 
spect, but is still suggestive. If we are to assume that an 
enzyme exists in the blood at the time of clotting and not 
before, we must establish its origin. Reducing the facts 
to the. barest essentials, we may make the following state- 
ment: normal blood contains in great numbers minute and 
extremely perishable bodies known as the blood-plates. 
These are much smaller than the red corpuscles. When 
Fig. 16.-The beginnings of a lymphatic. Blood capillaries are 
suggested (B-B). Associated with them is the supplementary chan- 
nel (L) through which surplus fluid may pass away. To complete 
the picture these vessels must be conceived to be embedded in a 
mass of cells. 
the surroundings are normal, as is presumably the case 
within the vessels, they keep their integrity, or at least do 
not disintegrate en masse. When they are brought in con- 
tact with foreign surfaces they do undergo prompt decom- 
position and, of course, their constituent material is dis- 
solved in the plasma. Something derived from the blood- 
plates sets in motion the series of chemical reactions which 
leads at last to the perfecting of an enzyme, thrombin, 
capable of transforming the soluble fibrinogen into the 
insoluble fibrin. 
Lymph.-This term is usually made to stand for the 
fluid outside the blood-vessels. Used in this way, it THE BLOOD 
119 
includes the liquid filling the microscopic intervals between 
neighboring cells, the larger spaces which often occur in 
the loosely woven connective tissues, and also the contents 
of an inconspicuous system of tubular channels known as 
lymphatics. A recent writer, Starling, has suggested 
that a distinction should be made between the fluid in the 
lymphatics and that which is not enclosed in vessels of any 
sort. He would restrict the term lymph to the first appli- 
cation, and would speak of the other as tissue-fluid. This 
usage appears highly desirable, but is not as yet widely 
current. 
Lymph, in the sense of tissue-fluid, cannot be collected 
for analysis. Considering the delicacy of the capillary 
wall and the probable freedom with which exchanges take 
place through it, there is reason to believe that this fluid 
must closely resemble the blood-plasma. When a tissue 
is the seat of an active metabolism the income and outgo 
of its cells must tend to alter the composition of the adja- 
cent lymph and to make it less like the plasma. The 
general effect will be to reduce the oxygen to a low level, to 
raise the carbon dioxid content correspondingly, to con- 
sume 9 fraction of the organic food, and to'add miscella- 
neous v aste-products. The lymph which can be obtained 
by cutting one of the larger lymphatics of the body has 
these characters. This lymph may not be precisely like 
the mixture which exists in close contact with the living 
cells, but it is probable that the differences are not great. 
According to a view formerly universal lymph can work 
its way from any interstice of an organ into the branches 
of the lymphatics, and so to larger vessels of the same 
class. Most authorities now hold that there is a definite 
separation between the unwalled spaces of the tissues 
and the interior of the true lymphatic system. If this 
is the correct conception, we have good reason to dif- 
ferentiate lymph from tissue-fluid. We may adopt either 
view provisionally without being seriously misled. Lymph 
contains white corpuscles and blood-plates. Since it has 
in it some fibrinogen it may coagulate, but owing to the 
absence of red corpuscles the clot is frail and tremulous. CHAPTER XIII 
THE CIRCULATION 
We must postpone still further our account of the ab- 
sorption of the products of digestion until we shall have 
made clear the general course followed by the circulating 
blood. We have quoted the estimate that the body con- 
tains 8 or 10 pounds of blood. At a given moment about 
one-fourth of this may be assumed to be in the thorax 
(the heart, the lungs, and the great blood-vessels), a 
fourth in the skeletal muscles, a fourth in the liver, and the 
remaining fourth elsewhere. The ceaseless movement of 
this large volume of liquid is maintained by the beating of 
the heart. 
This organ consists of two halves, right and left, com- 
pletely separated, so far as their cavities are concerned, by 
a middle partition. Regarded as a mass of muscle, the 
heart is single; considered as a pump, it is double. Each 
half is, in a literal sense, a force-pump. Each side shows 
us two communicating chambers, an auricle above and a 
ventricle below. The auricles receive the blood, which the 
corresponding ventricles will shortly discharge. The 
vessels leading to the auricles are called veins; those which 
convey the blood from the ventricles are called arteries. 
The auricles have thin walls, while the ventricles are 
fitted for their task by heavy muscular development. 
The left ventricle has much more power than its fellow, 
and the necessity for this will soon be evident. 
A single great artery, the aorta, springs from the left 
ventricle. Its branches reach all parts of the body. Sub- 
dividing repeatedly, they introduce the blood at last into 
the capillaries, the innumerable vessels of the smallest 
order through whose exquisitely thin walls take place the 
120 THE CIRCULATION 
121 
exchanges with the lymph already described. The word 
"capillary" means hair-like, but the description falls 
far short of indicating the actual slenderness of these mi- 
croscopic tubes. They are so narrow that the corpuscles 
pass through them practically in single file. The capillar- 
Fig. 17.-In this diagram, as is usual in such cases, right and left 
are reversed. This is as though the observer were looking at an- 
other subject. The short pulmonary path is to be traced from the 
right ventricle to the left auricle (P. C.). Alternative routes are 
suggested for the passage of the blood through the greater circula- 
tion from the left ventricle to the right auricle. The blood which 
traverses the digestive tract (Z>) passes through a second set of 
capillaries in the liver (L) before it can return to the heart. Note 
that the liver has in addition a separate arterial supply of blood. 
ies are short and soon unite to form the smallest veins. 
These lead the blood back toward the heart, joining as 
they go to form larger and less numerous channels, until at 
the last there are but two great veins. These enter the 
right auricle, one from above and one from below. The 
sweep of the blood from the left ventricle through the body 122 
NUTRITIONAL PHYSIOLOGY 
at large and back to the right auricle is called the systemic 
circulation. 
From the right auricle the blood descends to the ven- 
tricle of the same side, and is sent forth again, this time 
through the vessel known as the pulmonary artery. 
This immediately forks into branches which plunge into 
the two lungs. The smaller pulmonary arteries lead to 
rich capillary networks which are wrapped around the 
numberless air-sacs. Here the corpuscles are recharged 
with oxygen and the carbon dioxid of the blood is reduced 
to a standard amount. Four pulmonary veins return the 
blood to the left auricle. The relatively short journey 
of the blood from the right ventricle to the left auricle 
by way of the lungs is called the pulmonary or lesser cir- 
culation. 
It is necessary to call attention to the fact that the ad- 
jectives "arterial" and "venous" are not used in a sense 
which exactly corresponds with the meanings of the nouns 
"artery" and "vein." The adjectives have a chemical 
significance; the nouns, am anatomic one. Arterial blood 
is blood fully oxygenated; venous blood is blood more or 
less deficient in oxygen. But an artery, as has been said, 
is merely a vessel carrying blood away from the heart. 
The blood within will be arterial if we are observing the 
systemic circulation, and venous if it is in the pulmonary. 
So the systemic veins are filled with venous blood, while the 
pulmonary veins carry that which has just been brought 
up to the arterial standard through coming into relation 
with the air in the lungs. 
From what was stated above with respect to the distri- 
bution of the blood, it is evident that less than one-fourth 
of the whole volume is in the pulmonary circulation at one 
time, but the student must be cautioned against the con- 
clusion that the pulmonary circuit is traversed by less 
blood than passes through the systemic pathways. The 
quantities passing along the two routes are inevitably 
equal, for it is, after all, the same blood which runs through 
each in turn. If a slack chain is traveling over two wheels, THE CIRCULATION 
123 
as shown in the diagram (Fig. 18), the links pass the two 
in equal numbers, and yet there are constantly more links 
on their way from L to R than from R to L. 
When we speak of a heart-beat we mean a co-ordinated 
contraction of the heart's peculiar muscle. The phe- 
nomenon includes two phases: a brief and not forcible 
contraction of the auricles followed by a longer and much 
more powerful closing in of the ventricles upon their cavi- 
ties. The ventricular contraction is the essential factor in 
propelling the blood. Valves between the auricles and the 
ventricles permit the latter to fill during their period of 
B L 
Fig. 18.-For explanation of figure see text. 
relaxation, and forbid the backward flow from the ventricles 
when they are emptying themselves. Other valves at the 
commencement of the aorta and the pulmonary artery 
allow blood to pass out during each contraction of the 
ventricles, but not to return from either artery into the 
heart when the resting phase ensues. The action of the 
two sets of valves is precisely on the principle of the two 
which make a syringe-bulb an effective force-pump. The 
right and left halves of the heart act practically at the 
same time. 
Either ventricle when full may contain 5 or 6 fluidounces 
of blood. As it contracts it reduces its capacity and dis- 124 
NUTRITIONAL PHYSIOLOGY 
charges blood in like measure. At the conclusion of the 
active period it may have nearly obliterated its cavity, or 
the effort may fail while there is still considerable blood 
within. Estimating an average output from one ventricle 
to be about 4 ounces, or 100 c.c., it is apparent that forty 
beats will suffice to discharge from one side of the heart 
an amount of blood equivalent to the entire quantity of 
circulation (40 x 100 = 4000 c.c., or 4 liters). In other 
words, it will require less than one minute for all the blood 
to pass successively through both sides of the heart. When 
this statement is made it should be remembered that 
certain routes in the systemic circuit are many times longer 
than others (compare the path to and from the feet with 
that to and from the esophagus), and some corpuscles may 
revisit the heart two or three times while others are making 
one prolonged journey. 
It would be out of place to enter here upon a discussion 
of the value of the auricles. From a mechanical point of 
view they contribute but little of the energy required for 
driving the blood. They serve to accommodate the 
gathering volume of blood which the veins bring in during 
the rather prolonged contraction of the ventricles, and they 
secure a more efficient filling of the lower chambers of the 
heart. While their muscular development is slight, their 
automatic power is peculiarly marked, and it is a fair 
statement that the ventricle, under normal conditions, is 
stimulated to perform each beat by an influence radiating 
to it from the auricle. If there is an interruption of certain 
strands of tissue which normally unite the two, they cease 
to maintain the same rhythm, the auricle continuing at 
the accustomed rate, while the ventricle beats much less 
frequently. When the heart is dying the last signs of 
pulsation are in the auricles. 
The Portal Circulation.-In our general description of 
the course followed by the blood on its way through the 
body it was stated that when it has passed through a set 
of capillaries it is gathered up into veins to be returned 
to the right auricle. An important departure from this THE CIRCULATION 
125 
order is now to be indicated. The blood which has trav- 
ersed the small vessels of the digestive tract (including the 
stomach, both intestines, the pancreas, and the spleen) 
would be expected to make its way back to the heart 
through branches of the systemic veins. The actual ar- 
rangement is as follows: A large vessel receives all the 
blood from this region and carries it into the liver, where 
a second set of capillaries is entered. When these unite 
it is to form short veins discharging into the chief vein of the 
body, just below the diaphragm and practically within the 
boundaries of the liver. The channel leading from the 
organs of digestion to the liver occupies a unique position. 
Inasmuch as it is formed from a capillary system it seems 
to be a vein, but since it also supplies a capillary system 
it may be contended that it is an artery. Its structure 
favors the view that it is a vein and it is so called. The 
vessels which gather the blood from the digestive tract 
and distribute it inside the liver are said to form the portal 
system; the chief conductor described above is called the 
portal vein. An important consequence of this arrange- 
ment is that the absorbed products of digestion, so far as 
they are in the blood rather than the lymph, are brought 
under the influence of the liver before they go elsewhere. 
The question will naturally be raised whether the liver 
is supplied exclusively with venous blood. Provision is 
made for a supplementary supply through what is known 
as the hepatic artery, a vessel which is an offshoot of the 
general arterial system, and which, of course, introduces 
into the liver blood rich in oxygen. An analogous condi- 
tion may be noted in the lungs, where the main currents 
are the venous streams coming from the right side of the 
heart, but where there are also interwoven in the tissue 
much smaller vessels which take their rise in the arterial 
tree springing from the left ventricle. 
The problems of the circulation fall into two classes: 
There are those which are purely physical and which can 
be approximately reproduced and more or less successfully 
studied in lifeless models. There are also those which have 126 
NUTRITIONAL PHYSIOLOGY 
to do with the behavior of the organs concerned when they 
are viewed as living structures. Under the first class are 
included the facts of blood-pressure, velocity of flow, the 
resistance overcome, etc. The interpretation of such data 
is almost a science in itself and is known as hemodynamics. 
Among the questions of the second sort are the mysterious 
automaticity of the heart and the nervous government of 
the quantity and the distribution of blood flow. These 
difficult matters may well be left to be briefly dealt with 
toward the end of the book, when the general work of the 
central nervous system will be presented. We must, 
however, devote some space at this time to the fundamen- 
tals of hemodynamics. 
The Character of the Blood Flow in Vessels of Different 
Classes.-When an artery is cut the blood escapes in a 
forcible jet which may spring to a distance of several feet. 
The stream is not steady, but mounts and declines in a 
rhythm corresponding with the heart-beat. A good deal 
of pressure must be applied to restrain the bleeding. If a 
vein is severed the flow of blood is rapid and copious, but 
easily repressed. It is uniform so far as can be judged. 
These observations lead to the conclusion that the blood 
in the arteries is under a high average pressure, with large 
fluctuations from moment to moment. The pressure in 
the veins is evidently very low. 
The diminution of pressure between the arteries and the 
veins can be simply explained. When the blood is started 
on its course through the systemic circulation the high 
pressure which it exerts against the elastic wall may be 
regarded as a measure of the energy which the ventricle 
has impressed upon it. When it draws near the right side 
of the heart, the goal of its journey, its abated pressure is 
the sign that the initial energy has been spent. How has 
it been consumed? There is but one possible answer: It 
has been transformed into heat in overcoming the friction 
encountered in the vessels. Analogous conditions can be 
demonstrated for any tubular system through which liquid 
is driven. In the mains which carry a city water-supply THE CIRCULATION" 
127 
from a pumping-station to distant points there is a similar 
decline in pressure along each line as it is followed farther 
from the fountain head. (The assumption is made that 
we have to do with pipes which are on the same level 
throughout.) 
In any such system the cutting down of the pressure will 
be abrupt at any point on the route where there is an 
unusual impediment to the flow. This is the case in the 
body where the blood-steam is so extensively subdivided 
to enter the smallest vessels. Subdivision of channel 
means multiplied surface for friction. Thus there occurs 
a radical drop in pressure between the smallest arteries in 
which we can measure it and the veins of similar size. 
The highest pressure developed anywhere must be in the 
left ventricle when it is discharging the blood, for it is 
then to be regarded as the starting-point of the circulation. 
The lowest pressure which is ever registered is probably in 
the same ventricle when it is beginning to fill, for now it is 
the terminus of one of the two circuits. 
The swelling of the arteries which promptly follows each 
ventricular contraction is what we recognize as the pulse. 
It is the sign of a newly introduced portion of blood dis- 
tributing itself in the vessels. We have said that the veins 
show no such rhythmic enlargement. We have, therefore, 
to account for the conversion of the intermittent flow in the 
arteries into the constant flow in the veins. The principal 
factor concerned is the marked elasticity of the arterial 
trunks. It will be helpful to refer to the artificial devices 
which serve the same purpose in connection with force- 
pumps. A familiar one is the air-chamber. This is a 
large container communicating with the outflow-pipe. 
Whenever a stroke of the pump drives water along this 
pipe a certain share proceeds directly toward the outlet, 
while another fraction turns aside into the air-chamber. 
During the return stroke when no water is issuing from the 
pump barrel the air which a moment before underwent 
compression in the chamber tends to regain its original 
volume, and in so doing forces water through the lateral 128 
NUTRITIONAL PHYSIOLOGY 
branch and thence along the main pipe. The intermittent 
delivery through the valve is transformed more or less 
successfully into a continuous flow through the remote 
outlet. 
The neutralizing of intermittency in the blood system is 
referable to the same principle, but the elastic compensator 
is not to be found as a single localized feature; it is discov- 
ered in the universal capacity of the arteries to stretch and 
to regain their former size. At the beginning of the aorta 
or of the pulmonary artery we have complete intermittency, 
the blood alternately forging ahead and halting. But each 
portion of blood ejected from the heart finds room for 
itself partly by distending the arteries and not altogether 
by driving forward the blood which is before it. Hence, 
when the ventricles relax and the outpouring of blood 
ceases there is still an onward movement in the smaller and 
more distant arteries, because the larger ones, which were 
momentarily overdistended, are now contracting and send- 
ing along part of their accumulation. The farther we go 
from the heart the more largely the driving of the blood is 
to be attributed to the reaction of the stretched arterial 
walls and the more nearly uniform it becomes. This does 
not mean that the whole power keeping up the circulation 
is not to be sought in the heart-beat; it merely means that 
this energy may be stored temporarily by these elastic 
structures and rendered back again. 
The facts we have been treating may be expressed in 
another way. The arterial tree forms a reservoir of con- 
siderable capacity. Within it is an amount of blood so 
large that the single contribution of the ventricle makes 
a rather small addition to it. The escape of the blood 
through the terminal twigs cannot cease while there is 
so much stored under a high pressure in the aorta and its 
branches. The heart may omit or "drop" a beat without 
noticeably diminishing the flow through the capillaries. 
A standstill will be reached only when the arteries have 
attained a degree of contraction such that the internal 
pressure is no higher than that in the veins. The homely THE CIRCULATION 
129 
illustration (Fig. 19) which accompanies this may be help- 
ful. The pump delivers water intermittently to the leaky 
trough, keeping it filled to a level which is nearly constant, 
though fluctuating a little in the rhythm of the strokes. 
Meanwhile the escape of water through the cracks is 
all but uniform in its rate. One important difference be- 
tween the pump and trough, on the one hand, and the cir- 
culatory system, on the other, lies in the fact that in the 
Fig. 19.-At I the discharge is in gushes with pauses between- 
the type of the expulsion of blood from the heart At C the escape 
through the cracks is at a practically constant rate; this is true of the 
blood flow through the capillaries. 
first case the driving force is gravity; in the second, it is 
the reaction of the enclosing elastic walls. 
A set of facts which it is well to separate in one's thought 
as completely as possible from considerations of pressure 
is the body of data respecting the linear velocity of the 
blood. By this is meant the rate of advance of the aver- 
age corpuscle. In any vessel the stream runs more swiftly 
in the central axis and lags along the walls. The velocity 
in the arteries rises arid falls as does the pressure, but, on 130 
NUTRITIONAL PHYSIOLOGY 
the whole, is relatively high. The aorta is passed at a 
speed of at least a foot in a second. The veins also are 
traversed at a high velocity, though the figures are some- 
what lower than for the arteries. The movement in the 
capillaries is in sharp contrast to that in both arteries and 
veins, being exceedingly slow, perhaps inch in a second. 
It appears that a corpuscle may take as long to go through 
a capillary link which would scarcely span the breadth of a 
pin-head as to travel from the heart to the brain. 
When it is remembered that the actual service of the 
blood to the tissues is rendered in the capillaries (since all 
other vessels have walls too thick to permit free diffusion), 
the value of the slow passage is obvious. At the same time, 
one recognizes the desirability of the rapid transit to and 
from this department of the system. The heart itself and 
all the main vessels may be thought of as accessory to the 
capillaries. The explanation of the slowing of the stream 
in the small channels is entirely simple, yet often mis- 
apprehended. Whenever an artery forks to form two 
branches, these are individually of smaller cross-section 
than the parent stem, but their combined cross-section is 
greater. The result is the same that is seen when a river 
widens or deepens-the current slackens. If the river 
broadens into a lake the current may become impercep- 
tible, yet we know that the water is still setting forward 
toward the outlet. 
Are we then to believe that the capillary system is many 
times wider than the aorta or the great veins? There is no 
escape from this conclusion: their number is so vast that, 
despite their infinitesimal size as single conveyors of the 
blood, collectively they form the broadest division of the 
entire path. If they fail to suggest a lake, an analogy may 
be found in the tangled swamp in which a stream loses 
itself, breaking into many sluggish arms, from which at 
last the waters converge to resume a rapid course over a 
narrow bed. The acceleration noticed as the veins are fol- 
lowed toward the heart is merely a sign that as their num- 
ber grows less their combined cross-section also contracts. THE CIRCULATION 
131 
The Movement of the Lymph.-Just as we find veins 
returning from every part of the body, we can make out, 
though with much greater difficulty, small vessels bringing 
lymph in the same general direction, that is, toward the 
thorax. Like the veins, they unite as they draw near the 
heart, and the great majority eventually contribute to a 
trunk known as the thoracic duct. This springs from the 
union of many branches in the abdominal cavity, pierces 
the diaphragm, and can be traced upward in front of 
the spinal column until it empties into a great vein which 
is bringing the blood from the left shoulder toward the 
right auricle. A comparatively insignificant group of 
lymphatics centers at an outlet in the corresponding posi- 
tion on the right. 
The onward movement of the lymph in its channels is 
parallel with that of the venous blood, but is incomparably 
slower. It is sometimes almost entirely arrested. In the 
last chapter it was stated that we cannot confidently say 
whether the lymphatics drain all the microscopic spaces 
in the tissues or whether they rise in small definite enclos- 
ures. In either case they are bearing away a certain over- 
flow of fluid and may be regarded as supplementing the 
service of the veins. As to the cause of the halting move- 
ment of their contents, the simplest statement that can 
be made is somewhat as follows: The formation of new 
lymph in all the organs crowds away the lymph previously 
in the beginnings of the lymphatics, and this is the central 
fact to be considered. The energy required is derived 
partly from the heart, since liquid may be forced out of 
the capillaries by its transmitted pressure, and partly from 
other sources too obscure to be discussed. The lymph 
is generally referred to as a carrier of waste, but a partial 
exception must be made in favor of the lymph coming 
from the intestinal area during digestion. This lymph 
may contain absorbed food, principally fat. It was in the 
mesentery that lymphatics distended with milky liquid 
were first seen. They were called lacteals because of their 
appearance, and this term is still used in a local sense. CHAPTER XIV 
THE ABSORPTION OF THE FOOD-STUFFS 
If two unlike solutions are separated by a membrane, 
such as a sheet of parchment or some artificial substitute, 
they will usually tend to equalize both in composition and 
concentration. When, for example, potassium chlorid and 
sodium chlorid solutions are placed on opposite sides of 
such a partition, each salt proves its ability to pass through 
the barrier, and in the course of time there will be uniform 
mixtures in both compartments. This is said to show that 
the salts are diffusible and that the membrane is permeable. 
Different membranes are permeable in very different 
degrees, and the freedom with which various salts pass 
through a particular membrane is also far from constant. 
Some substances may appear freely soluble and may go 
readily through ordinary filters, but may hardly diffuse at 
all. This, as a rule, is the case with the proteins. If a 
mixture of unboiled white of egg and sodium chlorid is on 
one side of a membrane and the other side is washed with 
running water, nearly all the salt will escape, leaving the 
protein practically undiminished. The process by which 
diffusible salts are encouraged to separate themselves from 
substances which cannot accompany them through the 
membrane into the water beyond it is called dialysis. 
The products of digestion are, in general, much more 
diffusible than the food-stuffs from which they are derived. 
Starch, even when boiled for a long time, does not make 
its way through ordinary membranes; the sugars do so 
with relative ease. Fats are not even soluble in water; 
soaps and glycerin are diffusible compounds. Peptones 
arising from the hydrolysis of proteins have some power 
to penetrate membranes, and the simpler amino-acids pass 
132 133 
THE ABSORPTION OF THE FOOD-STUFFS 
still more freely. We might picture the situation in the 
intestine somewhat as follows: The blood flows steadily 
beneath a rather complex cellular wall, the other surface 
of which is bathed by the mixed products of digestion and 
the digestive secretions. Peptones and amino-acids, su- 
gars, soaps, and glycerin, being formed in relative abun- 
dance in the intestine, diffuse into the blood, which contains 
little of these bodies. If the blood were to stand still the 
small volume in direct relation with the absorbing surface 
would accumulate digestive products until it held them in 
the same concentration in which they exist in the canal. 
Then a state of equilibrium would be established and no 
further transfer to the blood would occur. 
Detailed observation shows that the facts of absorption 
cannot be expressed in this simple manner. It has already 
been hinted that large allowance must be made in such a 
case for the fact that the membrane under examination is 
alive. Its cells may discharge material at one surface 
quite unlike that which they receive at the other. They 
probably have a considerable metabolism. This means 
that energy is set free within their borders and a share of it 
may be applied to the moving of the absorbed food. We 
have previously called attention to the parallelism be- 
tween secretion and absorption. 
Before we can go further with this discussion something 
must be said concerning the place of absorption and the 
minute anatomy of the structures involved. There is a 
measure of absorption from the stomach. Until rather 
recently this organ was not credited with any marked 
powers of this kind, unless it were in the case of alcohol. 
This is a highly diffusible compound and its prompt 
entrance into the circulation is noteworthy. When one 
says of a glass of wine, <£This goes to my head," the state- 
ment is literally true. The alcohol strikes through the 
walls of the stomach at once and is borne to all parts of the 
body, including the brain. On the other hand, water is 
not at all freely absorbed when it is taken into an empty 
stomach. It is known to pass the pylorus in practically 134 
NUTRITIONAL PHYSIOLOGY 
unchanged volume. Some poisons produce no positive 
effects while in the stomach, but exert their action promptly 
when they pass to the small intestine. 
As regards the sugars and the products of peptic diges- 
tion, it is now believed that there is a considerable absorp- 
tion of these substances from the stomach. They disap- 
pear most rapidly when they are present in high concentra- 
tion, and the process is promoted by condiments which 
increase the blood flow under the gastric mucous mem- 
brane. When all reasonable allowance is made for the 
part played by the stomach in absorption, the fact remains 
undisputed that the major part of the work is done below 
the pylorus. Furthermore, since, as has been said, there 
is usually little valuable material left to be recovered by the 
colon, it becomes evident that the small intestine is of 
central importance. 
A striking peculiarity of the small intestine is the ex- 
tension of its internal surface. This is effected, first, by 
the numerous cross-folds which cut into its cavity, and, 
second, by the microscopic projections which stud its 
lining. These are the villi. An individual villus may be 
described as a minute finger-shaped process. It rises 
above the general surface in a contrast to the glands, which 
sink below; the villus is a peg, as the gland is a pit. Ob- 
viously the existence of the villi increases many times over 
the number of cells in contact with the intestinal contents. 
These cells are described as columnar; they are prisms 
standing side by side, with their larger surfaces in contact 
and their smaller ends presented to the interior of the in- 
testine and to the loose internal tissue of the villi. A 
certain share of absorption may take place through the 
crevices between the cells, but the main transfer of material 
seems to be through their own protoplasmic bodies. 
The interior of a villus is filled by a confusing assortment 
of cells. There is evidence that some of these are con- 
tractile. Microscopic examination of the living intestinal 
lining is reported to show villi alternately extending and 
retracting like marine organisms on the bottom of an THE ABSORPTION OF THE FOOD-STUFFS 
135 
aquarium. Between the internal cells there are probably 
intervals containing lymph, and near the central axis of the 
villus is a rather definite lymphatic channel. It is a small 
branch of the general lymphatic system and opens one 
Fig. 20.-This is a conventionalized drawing to show the essen- 
tials in the structure of a villus. The lining cells of the intestine are 
shown as in section. Within is seen a tangle of capillaries, and at the 
very core of the villus a lymphatic (L). The loose tissue, which in 
reality exists inside the villus, has been ignored for the sake of sim- 
plicity. 
way by which food can pass from the seat of absorption 
to the veins in the thorax, there to mingle with the blood. 
Between the exposed cells of the villus and the lymphatic 
at its core is interposed a net of capillaries carrying blood 
which has come from the neighboring aorta, and which 136 
NUTRITIONAL PHYSIOLOGY 
will flow through the liver before it returns to the heart. 
It may be said at once that, of the two possible routes for 
absorption, the portal system is the more important. 
Such food as enters the lymphatics will not have to run 
the gauntlet of the liver on its way to the general cir- 
culation . 
The statement has been made that the cells lining the 
intestine do not act in a way that can be imitated by life- 
less models. The fact is covered by the expression that 
they exercise selective powers. Too much might easily be 
inferred from this phrase; there is the same danger which 
was pointed out in connection with the pyloric sphincter, 
the inclination to credit the cells with something more like 
intelligence than it is right to assume for them. It is 
reasonable to believe that however complex and unex- 
pected may be the behavior of the cells concerned, a 
mechanistic explanation would be apparent if our knowl- 
edge were sufficiently full. What is meant by selective 
action can be readily illustrated. 
If a comparison is made in the laboratory to determine 
the relative rates at which dextrose and magnesium sul- 
phate make their way through an ordinary membrane, the 
sugar will lag behind the salt, although both pass with 
relative freedom. If a mixture of the two is introduced into 
the living intestine the impression is totally different. 
The sugar is absorbed, while the magnesium sulphate is 
kept back. We say that the mucous membrane is imper- 
meable to this salt, though we can hardly picture the 
peculiarity of structure which makes it so. A salt which 
is refused absorption by the intestinal wall will act as a 
laxative, for it will hold back from absorption a large quan- 
tity of water and this will be swept through by the peris- 
talsis. An investigator has called attention to the fact 
that all the common precipitants of calcium are denied 
passage into the blood, and may, therefore, be reckoned 
as cathartics. These include, beside the sulphates, the 
phosphates, the citrates, and the tartrates. 
Again and again we find as we pursue the subject that THE ABSORPTION OF THE FOOD-STUFFS 
137 
laboratory tests give us little indication of what may be 
expected of the intestine as an absorbing mechanism. 
Some substances usually held to be indiffusible pass into 
the circulation with comparative readiness. Even egg- 
albumin, a protein of enormous molecule, may enter the 
blood. This was recently proved by the observation that 
after eating a number of raw eggs the albumin may be 
found in the urine. It evidently reaches the capillaries to 
some extent before it can be hydrolyzed by the digestive 
enzymes, even at a time when these are presumably pres- 
ent and active. The application of mild poisons to the 
lining of the intestine causes it to behave much more like 
the typical indifferent membrane. Thus a weak solution 
of sodium fluorid (which does not visibly disorganize the 
cells) wipes out the selective properties which have been 
instanced. This makes it seem the more probable that 
the normal processes require the application of energy, and 
that accordingly they cannot continue after the death of 
the cells. 
A mosaic surface formed of cells, each one of which is a 
living body, is far from comparable with a homogeneous 
partition. Even if we leave out of account the crevices 
between the cells which may bear a part in absorption, as 
already noted, we must consider the individual cell to be 
an elaborate structure. Its surface layer is undoubtedly 
different in its chemical nature from its interior. There is 
good reason to believe that the exposed border differs 
entirely from the end which abuts on the connective tissue. 
A result of this complex organization is the existence of a 
property that might be called "polarity," that is, a capacity 
to act in one direction rather than the other. A somewhat 
ponderous expression for the same idea is found in the 
phrase "irreciprocal permeability." This means special- 
ization for absorption, and strongly suggests that if the 
cells could be reversed in their relation to the interior of the 
intestine they would begin to absorb from the lymph and 
secrete into the canal. Gland-cells may be said to have 
irreciprocal permeability in this reversed sense. 138 
NUTRITIONAL PHYSIOLOGY 
The departure of the intestinal lining from the behavior 
of a common membrane is still more obvious when we note 
that absorption may be accompanied by chemical trans- 
formation. The food-stuffs which leave the interior of the 
canal do not of necessity reappear in the circulation in the 
same form. Two possibilities exist: either the process of 
digestive cleavage may be continued during the passage 
through the wall, or a recombining of the products of diges- 
tion may be accomplished. The first action is thought to 
occur to some extent when the peptones formed in the 
cleavage of proteins are moving toward the blood. It will 
be recalled that the enzyme erepsin, having the power to 
decompose peptones still farther, is obtainable from 
these cells, and it is quite possible that its action is intra- 
cellular. 
The opposite property, that of synthesizing, is illustrated 
by the behavior of the products of fat digestion. We have 
seen that the pancreatic enzyme hydrolyzes fats, with the 
formation of fatty acids and glycerin as the first result of 
the cleavage, and that the fatty acid may be changed to 
soaps, though we do not know how extensively they under- 
go this second change. It follows that it is these bodies 
which disappear from the intestine, but they are not to be 
found at all freely in the contents of the lymphatics. 
In their place we have neutral fat evidently reconstructed 
during the transfer. Microscopic study of the cells 
through which the material has been passing shows that 
the border adjoining the cavity of the intestine is without 
fat droplets, but that these occur deeper down, increasing 
in size toward the other border. The appearance strongly 
indicates that some compounds other than neutral fats 
entered the cells and underwent a transformation while 
progressing through the protoplasm. CHAPTER XV 
THE METABOLISM OF FATS AND CARBOHY- 
DRATES 
It was pointed out early in our treatment of the subject 
that foods serve a twofold purpose: to some extent they 
are built into the body as relatively permanent parts of its 
structure, while in a much larger degree they are steadily 
oxidized, yielding their energy to maintain its activities. 
The proportion between the two divisions of the supply 
cannot be constant. Clearly, the fraction of the diet 
devoted to construction must be larger in childhood than 
in adult life. There may be other periods during which the 
constructive work is notably prominent, for example, re- 
covery from illness or from fasting, pregnancy, and perhaps 
athletic training. Apart from these times we must assume 
that the actual building of tissue is a very small item. In 
other words, the living matter of the body is comparatively 
stable and needs only slight though perfectly definite 
contributions to insure its up-keep from day to day. 
The food-stuffs entering the circulation may be destined 
for immediate destruction or for storage. Throughout 
long terms of our lives a fair balance is preserved between 
the income and the consumption of these compounds. 
If we receive in one day certain quantities of proteins, 
fats, and carbohydrates, and there is evidence of an exactly 
equal decomposition of the three classes of material, we 
cannot say with precision whether the oxidation affected 
the particular food eaten or corresponding matter stored 
previously, but in either case the condition of the system 
at the beginning and at the end of the twenty-four hours 
is the same. We must now proceed to discuss the possibil- 
ities of transformation and retention of the different food- 
stuffs, and we shall find that the story is most simple in the 
case of the fats. 
139 140 
NUTRITIONAL PHYSIOLOGY 
Fat Metabolism.-It has been customary to say that 
fat never becomes anything else until it is decomposed 
with release of energy. This statement may be too sweep- 
ing to cover all the conditions which arise in disease and 
has come to be questioned even for health. We have 
traced the fat from the walls of the intestine into the 
lymphatics. From the smaller branches it must find its 
way to the thoracic duct, and through this vessel it goes 
to merge with the general blood-stream. In the blood, the 
lymph, and the tissues at large fat is present in a small 
percentage. We must now give attention to the special 
provision made for the storage of fat in what is called 
adipose tissue. 
The word "fat" is used in two senses. In its strict 
chemical meaning it describes a certain type of compound, 
and it is this usage which we have thus far employed. 
But when we speak of the fat of meat we include something 
more. We mean a form of connective tissue in which the 
cells hold a large accumulation of fat in the chemical sense. 
Under the microscope this tissue is seen to be composed of 
a fibrous network holding within its meshes these distended 
cells. The fat which they contain is in drops of such a size 
that the protoplasmic portion of each cell seems a mere 
envelope, while the nucleus is crowded to one side. So it 
happens that while the fat is really an inclusion, it forms a 
very large percentage of the whole mass. When a piece 
of adipose tissue is subjected to the action of gastric juice 
the fibers and protein of the cells are rapidly digested, and 
the actual fat, being liberated, rises to the top and floats 
as a clear layer of oil. 
Of course, the amount of adipose tissue varies widely 
with the individual. Still it is more abundant in subjects 
of spare build than is usually supposed. The hollow shafts 
of the long bones, such as those of the arms and legs, con- 
tain what is called the white marrow. This is typical 
adipose tissue. A large deposit is to be found at the back 
of the abdominal cavity, where it closes round the upper 
parts of the kidneys. Flakes of it occur in the mesentery 
and on the surface of the heart. It is developed in the THE METABOLISM OF FATS AND CARBOHYDRATES 
141 
deep eye-sockets. In subjects better nourished there will 
be more or less of this tissue widely distributed over the 
body occupying a position between the skin and the 
underlying muscles-the so-called subcutaneous fat. 
This may be indefinitely increased in the obese. Another 
characteristic of obesity is the gathering of adipose tissue 
in the great omentum, the sheet of membrane hanging 
from the lower border of the stomach. When much fat 
is present in this situation the ventral wall of the body has 
a double burden, one layer of this reserve material out- 
side and a second within the abdominal muscles. 
We shall postpone to a later time any discussion of the 
factors which influence the accumulation of fat in the sys- 
tem. A point previously made is to be insisted upon, that 
the fat of the body is not derived solely, nor, indeed, chiefly, 
from the fat of the food. We shall presently consider to 
what extent it is formed from the other food-stuffs. 
Whether there is a great or only a moderate amount, it 
will serve to maintain the activities of the muscles during 
periods of insufficient feeding or absolute fasting. When 
an animal has died from starvation but little fat can be 
found in its tissues. The reduction of the fat presumably 
present at the beginning of inanition has been estimated to 
have reached 97 per cent, of the supply when death finally 
supervenes. The power to endure starvation is naturally 
greater for an animal or a man having a large initial store. 
For each species the body fat has a nearly constant 
character. There is a certain ratio maintained between 
the several fatty acids, and, as a result of this, a definite 
melting-point. There cannot be such a diversity here, as is 
the case with the proteins of different animals, but there 
is a similar appearance of individuality. Accordingly, 
when one animal preys upon another of a different species 
and is nourished at the expense of its victim, it does not 
store precisely the form of fat which it has eaten, but 
modifies it to conform to its own standard. The making 
over is a still more marked phenomenon when a vegetable 
oil is eaten and transformed into animal fat. 142 
NUTRITIONAL PHYSIOLOGY 
The question whether the body can under any cir- 
cumstances transform fat to sugar has been much dis- 
cussed. For a long time the weight of evidence seemed to 
be decidedly against such a change. Many physiologists 
are now convinced that the glycerin fraction of fat can be 
worked over to form dextrose. This represents only about 
one-ninth of the molecular mass of fat. Whether the 
fatty acids can possibly yield sugar is still a matter of 
controversy. Some attractive theories of muscle-con- 
traction seem to require a belief in such a transformation. 
(See p. 38.) 
The Metabolism of Carbohydrates.-The sugar of the 
blood is usually called dextrose or glucose.1 As a matter of 
fact, it cannot be strictly correct to speak of a single sugar 
in the plasma, for there are probably three at least. 
Glucose, however, is undoubtedly the principal one, and 
so far as we know the possible services to the body are prac- 
tically the same for all. Reference has already been made 
to the fact that the quantity of sugar in the blood is small, 
but singularly constant. It is now time to explain how this 
constancy is maintained. 
It has been stated that the body contains relatively 
little carbohydrate in spite of its large supply. When it 
is considered that the entire volume of blood contains less 
than 10 grams of sugar, though the amount absorbed after 
a single meal may be as much as 100 grams, it appears 
strange that there should be, as a rule, no significant in- 
crease in the percentage circulating during the period of 
digestion. The solution of this problem was achieved in 
great measure by the French physiologist Bernard, near 
the middle of the last century. Knowing that the incom- 
ing sugar passes to the liver, he anticipated that this organ 
might have the power to take the surplus from the passing 
stream and store it temporarily in some form. 
Glycogen.-Investigation showed that there could be 
obtained from the liver of a well-fed animal (rabbit) con- 
1 The word glucose has a commercial use which does not corre- 
spond closely with its strict, scientific meaning. (See page 247.) 143 
THE METABOLISM OF FATS AND CARBOHYDRATES 
siderable quantities of a carbohydrate resembling starch. 
This substance is called glycogen. Its presence within the 
cells of the liver can be demonstrated in microscopic prep- 
arations. Its molecule is of unknown size, and it is 
capable of undergoing digestion in the same manner as 
vegetable starch with the formation of sugar. The amount 
may be strikingly large, reaching, in the rabbit, one-fourth 
the total weight of the liver, deducting the weight of the 
blood usually contained in it. In the human liver it does 
not attain to such a high percentage, but may still equal 
something like 10 per cent, of the net weight of the organ. 
This means that a full-sized liver may hold 150 grams of 
glycogen. 
Bernard's interpretation of his discovery was somewhat 
as follows: The liver is the carbohydrate bank of the body. 
Like any bank, it is subject by turns to deposit and with- 
drawal. Its hoard is increasing when much sugar is ar- 
riving from the intestine, for it is then diverting the surplus 
from the blood of the portal circulation. The change by 
which sugar is made into glycogen is clearly just the re- 
verse of the digestive process, a dehydration and a con- 
densation to form larger molecules as contrasted with the 
familiar hydrolytic cleavage. The liver cells seem to be 
stimulated to make this change by the rise of the per- 
centage of sugar in the portal blood. When absorption 
ceases it may be assumed that the sugar of the blood in 
general sinks slightly in amount. This condition appears 
to cause a reversal of the prevailing reaction in the liver, 
the stored glycogen is gradually transformed to- sugar, 
and this passes out to renew the supply in the circulation. 
The approximate constancy of the sugar in the blood is thus 
accounted for in the main by the power which the liver 
possesses to remove or return it, according to the shifting 
conditions.1 
1 An animal deprived of the liver cannot be kept alive much 
beyond twenty-four hours. During the limited period of its sur- 
vival it has been shown that the sugar of the blood falls to an ex- 
ceedingly low level and that a transient improvement in general con- 
dition follows the injection of dextrose into the circulation. 144 
NUTRITIONAL PHYSIOLOGY 
The making of glycogen from sugars occurs only during 
life. The converse change from glycogen to sugar takes 
place freely after death, and is doubtless due to an enzyme. 
It might be anticipated that a comparatively short period 
of fasting would suffice to exhaust the glycogen of the liver. 
As a matter of fact, there is a large reduction in the first 
day, but the removal then proceeds slowly and is scarcely 
ever completed. The disappearance is greatly hastened by 
muscular activity, most effectually by the intense con- 
vulsions produced by strychnin-poisoning. For human 
subjects it has been shown that glycogen is consumed 
rapidly under the influence of iced baths. (How we are 
able to judge of the abundance or scarcity of glycogen in 
living men will be explained in another connection, p. 186.) 
For some time after Bernard first called attention 
to the "glycogenic function" of the liver, the fact that gly- 
cogen is deposited also in the skeletal muscles was over- 
looked. In these tissues it does not reach any such per- 
centage as may be found in the liver, but inasmuch as the 
muscles form nearly half the entire mass of the body, a 
small percentage means a large aggregate. Collectively, 
the muscles have commonly been estimated to hold an 
amount equal to that in the liver, and there is a growing 
impression that they contain even more. The total gly- 
cogen in the system may probably be as much as 400 
grams, or nearly a pound. The question which now calls 
for consideration concerns the importance of maintaining 
such a strict constancy in the sugar-content of the blood. 
Some light is thrown on this matter when we observe 
the result of artificial increase of sugar concentration. 
This may be brought about by injecting sugar solution 
into the blood-vessels. If this is done freely there is 
excessive lymph-formation and other evidence of deranged 
conditions in the circulatory system. A symptom to 
which especial attention must be called is the appearance 
of sugar in the urine under such circumstances. The 
kidneys are so organized that any distinct rise of sugar in 
the blood leads to the excretion of the excess. Thus the THE METABOLISM OF FATS AND CARBOHYDRATES 
145 
composition of the blood is restored to the standard, while 
potential food is lost to the tissues. Such a waste of sugar 
is less likely to follow abundant feeding of foods rich in it 
than to occur after the experimental procedure just de- 
scribed, but it may nevertheless result from the selection 
of peculiar diets. It is then called alimentary glycosuria 
and it is induced somewhat readily in some persons by 
eating sugar, but hardly ever in wholly normal subjects 
by eating starch. The differing reaction is presumably 
explained by the fact that sugar requires a brief digestive 
treatment and is then rapidly absorbed. Starch, on the 
other hand, has to pass through serial stages of digestion, 
and the absorption of the resulting sugar is extended over 
a longer period. In the first case we may suppose that the 
inrush of sugar overwhelms the liver, which is unable to 
arrest all of it. That which goes by raises the sugar con- 
tent of the blood above the level at which it begins to 
pass into the urine. 
If for any reason much of the glycogen in the liver or 
the muscles is quickly resolved into sugar the blood must 
be affected quite as though the added sugar had come from 
the intestine. Glycosuria will ensue. Certain changes in 
the circulation are known to cause such a flooding of the 
system with sugar and the appearance of a part of it in the 
urine. A most interesting instance of such glycosuria is 
that following an experience of strong emotion. It 
seems that one of the results of the disturbance in the 
central nervous system is the conversion of a large amount 
of glycogen into dextrose. With acute insight a physiolo- 
gist has pointed out that this release of sugar is a helpful 
reaction under the circumstances. The occasion of emo- 
tion is usually an occasion for strenuous action, perhaps for 
flight or for giving battle, and the muscles may be re- 
inforced by the increased supply of their preferred fuel 
brought to them.1 
The regulating action of the liver and the muscles upon 
1 Cannon, "The Bodily Effects of Pain, Hunger, Fear, and Rage," 
Appleton, New York, 1920. 146 
NUTRITIONAL PHYSIOLOGY 
the carbohydrate distribution may be paralleled, in part at 
least, by an analogy. Let us compare the active tissues to 
a mill turned by the waters of a stream. The water- 
supply to the mill is to be compared with the sugar-supply 
to the cells which derive their energy from it. A meal is to 
the body as a storm is to the mill-stream-it adds to the 
volume of the power-producing element. The dam by the 
mill is like the kidney in its relation to the accumulated 
store; if the water rises above the crest of the dam it flows 
over and passes on down the stream without having con- 
tributed its energy to the turning of the machinery; if 
the sugar rises above a certain level it begins to escape, 
with its potency for work lost to the organism. More- 
over, the capacity of the liver and the muscles to hold back 
carbohydrate suggests the function of a broad mill-pond. 
The larger the pond above the dam the more successfully 
the irregularities due to alternating rain and drought will 
be offset and the less likely will be a wasteful overflow when 
a storm follows a term of low water. The conversion of an 
intermittent supply into a constant one is the function of 
the mill-pond, and it is equally the service of the tissues 
holding glycogen. 
While a relatively sudden rise of sugar in the circulation 
may produce glycosuria, a slight chronic excess over the 
normal may have an entirely different effect. If the diet 
is supplying day by day a little more carbohydrate than 
the body is oxidizing, the surplus may be transformed to 
fat. The opinion that starchy and saccharin foods are 
fattening has scientific support as well as common observa- 
tion in its favor. Glycogen formation is limited and we 
may suppose that fat-building takes place when the gly- 
cogen reserve is at its maximum and still more sugar is 
offering. An important advance wTas made in physiologic 
knowledge when Liebig called attention to the fact that a 
cow's milk contains an amount of fat utterly out of pro- 
portion to the scanty supply furnished by the food of the 
animal. It had been believed that no such transformation 
could be accomplished by animal tissues, and that all fat THE METABOLISM OF FATS AND CARBOHYDRATES 
147 
founcl in the body or in the secretions must have been re- 
ceived as fat. Liebig fell into error when he stated that 
the milk-fat had been made solely from proteins and not at 
all from carbohydrates, but he had taken a notable step in 
recognizing the possibility of changing one food-stuff into 
another. Quantitative experiments soon showed that car- 
bohydrates must be given a very prominent place among 
fat-forming materials. 
The steps through which sugar is transformed into fat 
are little understood. A comparison of the composition 
of the two makes it evident that a great deal of oxygen has 
to be removed in the reactions. This element is never set 
free in animal metabolism; in the present instance it is 
separated in the form of carbon dioxid. This is not merely 
a theoretic consideration, but a condition which can be 
demonstrated by experiments upon an animal rapidly 
gaining fat. A wood-chuck, for example, when it is eating 
voraciously of starchy food and gaining steadily in fat, 
breathes out more carbon dioxid than can be accounted 
for by the oxygen consumed. The excess is a by-product 
of the process in which sugar with its high percentage of 
oxygen is made over into fat with a much lower per- 
centage.1 
Recourse may be had once more to analogy. Bernard 
compared the liver to a bank, but we may extend the com- 
parison to the whole body. The products of digestion are 
the daily deposits; the oxidations stand for the daily 
payments. The bank will have a convenient cash balance 
on hand from which to meet current demands. This is 
the function of glycogen. The.cash in the bank will 
be but a small fraction of its total resources, and its varia- 
tions from hour to hour will signify but little as regards the 
stability of the institution. So the glycogen of the body 
1 Without assuming that the process is fully understood, French 
authorities have suggested that the principles involved may be shown 
in the following equation: 
13(C6H12O6) = C55H104O6 + 23(CO2) + 26(H2O). 
(Shafer, " Text-book of Physiology," vol. i, p. 933.) 148 
NUTRITIONAL PHYSIOLOGY 
is a small reserve and may vary by 50 per cent, within 
twenty-four hours. The body-fat, like the securities held 
by the bank, is a large accumulation and less subject to 
change. If for some time the deposits are in excess of the 
withdrawals, the officials of the bank will, of course, make 
new investments. The parallel is clear: the body receiving 
more carbohydrate than it is expending will not allow it to 
go on increasing in the liver and muscles, but will begin to 
convert it to fat. Unhappily, the correspondence becomes 
uncertain at one point: a bank which is subject to a 
"run" may sell its bonds and other holdings that it may 
have cash to pay its depositors. The obvious suggestion 
is that the fat of the body will be reconverted to sugar 
when there are demands to be met and no incoming food. 
We have pointed out (p. 142) that this is not certainly 
known to occur; the oxidation of fat during starvation 
has been held to proceed without any such previous 
change; 
Using another metaphor, though still a financial one, it 
may be said that the glycogen is like a checking account 
which a man uses to pay his routine expenses, drawing 
upon it often and recruiting it at longer intervals. Such 
an account is sometimes nearly wiped out and then at one 
stroke largely increased. The fat of the body is like a 
savings bank deposit, gathered slowly, drawn upon only 
in emergencies, and, it may be added, gaining by com- 
pound interest in many cases. 
Two facts readily suggest themselves which may be 
used to explain the low limit of glycogen storage. For 
one thing, its physical properties would probably make a 
high percentage of it undesirable. In the second place, 
there is a distinct economy in substituting fat for glycogen 
because fat represents more energy in proportion to its 
mass. An individual who carries 20 pounds of fat in adi- 
pose tissue may wish to be rid of a part of it, but if he were 
compelled to bear a load of glycogen equivalent in energy 
his burden would amount to about 45 pounds. 
The Pancreas and Carbohydrate Metabolism.-e THE METABOLISM OF FATS AND CARBOHYDRATES 
149 
have repeatedly compared the carbohydrate of the body 
to money. Just as it is the eventual function of money to 
be spent, so it is the destiny of carbohydrate to be oxidized 
that its latent energy may be turned to account. This 
oxidation takes place chiefly in the muscles, to some extent, 
doubtless, in the glands, the gray matter of the nervous 
system, and the absorbing cells of the intestine. The in- 
tensity of the local process will in every case be proportional 
to the heat and other forms of energy developed. Consid- 
ering the wide distribution of this purposeful destruction 
of sugar it is surprising to find it all dependent upon an 
obscure action of the pancreas. 
This organ has been dwelt upon previously as a most im- 
portant contributor of digestive juice to the intestine. It 
would be anticipated that the removal of the pancreas 
from an animal would be followed by defects of digestion 
and assimilation. This is probably justified by the results 
of a trial, but the effect upon digestion is overshadowed by 
a consequence hardly to be foreseen. This is the almost 
complete loss on the animal's part of the power to oxidize 
dextrose. In other words, a process not occurring in the 
pancreas at all, but in the tissues at large, is arrested by the 
removal of this gland. What is the natural explanation of 
this condition? Evidently that something proceeds from 
the pancreas through the circulation to other parts of the 
body, without which the cells in general are incapable of 
decomposing the sugar. 
This imperfectly known product of the pancreas is an 
example of what has been referred to as a hormone. One 
is tempted to think of it as an enzyme, but it cannot be 
accurately described by this word. If it were a typical 
enzyme we might expect it to cause the destruction of 
sugar in solutions to which it has been added. No marked 
disappearance of sugar occurs when the experiment is 
made. In view of this it is wiser not to commit ourselves 
as to the precise character of the body in question. The 
fact has seemed to be that neither the pancreatic extract 
by itself nor an extract of muscle will particularly pro- 150 
NUTRITIONAL PHYSIOLOGY 
mote the oxidation of sugar, but that a combination of 
the two is necessary. 
The fact that some product of the pancreas is necessary 
to insure the oxidation of sugar has recently been confirmed 
by a remarkable observation.1 Experimenters were en- 
gaged in recording the symptoms exhibited by one animal 
after another following the removal of the pancreas; 
diabetes set in with uniformity until, quite unexpectedly, 
a negative case appeared. Under scrutiny it turned out 
that the animal which escaped diabetes was a pregnant 
female. It was then surmised that the retention of the 
power to utilize sugar was due to the presence of pancreatic 
tissue in the embryos. The inference proved to be correct 
for, directly after the birth of the young, the mother 
developed diabetes with full intensity. 
Diabetes.-If, owing to a lack of the pancreatic hormone, 
the ability to utilize sugar is lost, the continued absorption 
of carbohydrate will raise the percentage in the blood, with 
the result which always ensues under this condition. The 
kidneys will steadily excrete the surplus sugar. Unlike 
alimentary glycosuria, this overflow of sugar will not be 
limited to times of free feeding with carbohydrates, but 
will attend the ingestion of the most moderate amounts. 
(As a matter of fact, sugar will still be excreted when 
carbohydrates are excluded from the diet and in fasting. 
The explanation of this fact must be deferred to the next 
chapter.) The difficulty of keeping up nutrition when the 
cells can make no use of glucose will be evident in a measure 
even at this point, and we shall find additional reasons 
later. 
The complications which set in when a case of diabetes 
approaches a fatal issue are of a nature not yet indicated. 
It is not the simple difficulty resulting from the loss of the 
power to oxidize sugar which is encountered; toward the 
last the metabolism of fat pursues an abnormal course. 
The fact seems to be that as soon as a stage is reached when 
1 Carlson, Orr, and Jones, Journal of Biological Chemistry, xvii, 
1914, 19. THE METABOLISM OF FATS AND CARBOHYDRATES 
151 
dextrose can no longer be destroyed the clean and complete 
oxidation of fat likewise becomes impossible. The system 
begins to suffer from the accumulation of acid by-products 
originating from fat. An acidosis is said to exist. These 
are responsible for the coma and, to a great extent, for 
other symptoms which signify that the end is near. The 
curious dependence of proper fat metabolism upon the 
simultaneous destruction of sugar has been picturesquely 
described by saying that "fat has to be burned in the flame 
of carbohydrate." 
The attempt to exclude carbohydrates from the diet 
and to live chiefly on fats with a moderate protein allow- 
ance results in a serious acidosis.1 The subjects of such 
experiments become miserable in the extreme and soon 
loathe the carefully prepared dishes (bacon, eggs, creamed 
chicken, etc.) which are set before them. Analysis of the 
urine shows the presence of the toxic substances. 
We have seen that in fasting there is at first a free use 
of glycogen and then a substitution of fat for carbohydrate 
in the metabolism. When the replacement has become 
nearly complete there may be difficulty in oxidizing so 
large a quantity of fat and symptoms of acidosis may be- 
come more or less marked. This is largely an individual 
matter and accounts in part for the fact that some people 
find fasting highly disagreeable while others do not mind it. 
When fasting is enforced by an attack of indigestion the 
same metabolic derangement may be induced, prolonging 
the nausea and malaise. These are the familiar cases 
which terminate so happily as soon as food can be taken. 
It is difficult to draw a line between diabetic and non- 
diabetic human beings. We have at one extreme those 
whose power to store and utilize carbohydrate is never 
demonstrably overtaxed, at the other those who cannot 
oxidize any sugar. All gradations between are represented. 
In a doubtful intermediate class are those subjects who 
have glycosuria when they indulge at all freely in carbo- 
1 Higgins, Peabody, and Fitz, Journal of Medical Research, xxxiv, 
1916, 263. 152 
NUTRITIONAL PHYSIOLOGY 
hydrates. They are said to have a low tolerance for 
sugar. There is reason to think that they can neither ox- 
idize it rapidly nor turn it readily to glycogen. The two 
capabilities are strangely linked. 
The fact is now well established that persons with a low 
sugar tolerance invite the development of grave diabetes 
if they allow themselves diets productive of glycosuria. 
The pancreatic function seems to fail progressively under 
such treatment. Apparently this organ, unlike some 
others, cannot be strengthened by systematic demands 
upon it; if it is weak there is no advisable course but to 
spare it.1 
Insulin.-As soon as the responsibility for diabetes was 
definitely fixed upon the pancreas physicians began a 
series of attempts to treat the disease by the use of vari- 
ously prepared extracts of the organ. For many years 
their efforts were fruitless. We can now account for the 
repeated failures in a simple way. In ordinary prepara- 
tions of pancreatic material the enzyme trypsin is acti- 
vated, and it destroys at once the internal secretion which 
it has been desired to obtain. A special technic is requisite 
to avoid this untoward result. 
There are in the pancreas certain cell-groups which 
do not contribute to the digestive juice discharged through 
the ducts of the gland. These are known as the Islands 
of Langerhans, and it has long been assumed that it is 
their function to manufacture the hormone which regu- 
lates carbohydrate metabolism. They have frequently 
been found degenerated in postmortem studies made 
upon victims of diabetes. We now have proof that the 
view generally held is correct. A principle of the utmost 
value in the control of diabetes can be derived from these 
cells. Because of its source it is called Insulin or Iletin. 
The first successful quest of such an agent was achieved 
by Banting, Macleod, and Best of Toronto. It had been 
suggested that the product might be secured from a 
pancreas in which the part originating the juice had been 
1 Joslin, "The Treatment of Diabetes Mellitus," Lea & Febiger, 
Philadelphia, 1923. Also Williamson, The Practitioner, cv, 1920,233. THE METABOLISM OF FATS AND CARBOHYDRATES 
153 
caused to atrophy. If this condition could be brought 
about the interference from trypsin would be avoided. 
The desired degeneration was induced by tying the ducts 
of the pancreas and keeping the animal alive for some 
weeks. The islands remained normal and yielded an 
extract having remarkable properties. It has since been 
found possible to prepare Insulin from glands fresh from 
the slaughter-house. Alcoholic extracts are used. 
The internal secretion of the pancreas is ineffective 
when taken by mouth because it is decomposed in the 
digestive tract. When it is injected under the skin and 
thus introduced into the circulation it promptly lowers 
the concentration of the blood-sugar. If the dose is 
rightly measured this will prevent the excretion of sugar 
in the urine. More important than this is the fact that 
the tissues begin to utilize dextrose in the normal way. 
The liver regains the power to store glycogen. The 
tendency to acidosis is counteracted. The reduction of 
blood-sugar by insulin does not necessarily stop at the 
normal level; if it proceeds farther marked prostration 
results. In the course of Insulin treatment for diabetes 
such reactions are warded off by eating oranges or glucose 
when the sensation of weakness is experienced. 
The improvement of diabetic patients receiving Insulin 
has been extraordinary and it is clear that we have here 
one of the greatest of medical discoveries.1 At the same 
time it should be stated that the treatment does not cure 
diabetes; it is palliative in character. By its help life is 
made comfortable and efficient, but the dependence on 
the injections must probably continue. However, the 
alternative is semi-starvation, greatly restricted activity, 
and uncertainty as to the future. All clinicians emphasize 
the fact that diabetes attacks chiefly individuals who have 
allowed themselves to become overweight. In later life 
at least the spare subject is practically insured against it. 
Among the underfed populations of Central Europe dia- 
betes is one of the few diseases which has definitely 
decreased. 
1 Macleod, Physiological Reviews, iv, 1924, 21. CHAPTER XVI 
NITROGENOUS METABOLISM 
Our knowledge of the history of the products of protein 
digestion has been much extended in the past few years, but 
there are still many uncertain passages. Any account 
which can be given must be held subject to revision. 
Nevertheless the course of nitrogenous metabolism, in its 
broader aspects, is tolerably clear. The present interpre- 
tation is most easily appreciated when earlier conceptions 
have been briefly reviewed. 
Not long ago it was held that the earlier products of 
tryptic digestion were promptly absorbed, and that the 
formation of the late products, the amino-acids, was rather 
an accidental and, possibly, an unfortunate occurrence. 
It was believed that the simplest bodies could not serve all 
the purposes of nutrition. Such compounds were known 
to arise in the laboratory experiments, but their formation 
in the intestine under strictly normal conditions was ques- 
tioned. The opinion prevailed also that the peptones 
which disappeared from the canal were reconstructed at 
the time of absorption and represented thereafter by the 
protein of the blood. The old impression that but little 
in the line of synthesis could be expected of the animal 
tissues was distinctly influential. 
About 1901 it was shown that a dog can be nourished 
when the nitrogenous food which it receives is in the form 
of the most advanced products of tryptic hydrolysis. A 
quantity of lean meat had been digesting with pancreatic 
juice for a period of months. The resulting mixture 
contained only bodies of a simpler order than the ones 
on which nutrition had hitherto been supposed to depend. 
Food so prepared is not attractive, but it can maintain an 
154 NITROGENOUS METABOLISM 
155 
animal in fair condition. Physiologists accepted the 
evidence and granted that the proteins of the organism 
could be synthesized-sometimes at least-from amino- 
acids. Shortly after this change in our conceptions the 
discovery of erepsin was announced. It was recognized 
that the existence of this enzyme made it more probable 
that digestion should normally run its full course rather 
than be terminated at an early stage by the intervention 
of the absorbing cells. It became apparent that even 
though the material leaving the intestine might have the 
relatively complex character which we associate with the 
peptone stage of digestion, the products delivered to the 
interior of the villi by the cells might have undergone 
further cleavage. The ability of the animal to turn to 
account such simple bodies in synthesizing its own proteins 
became clear. 
In the chapter on Intestinal Digestion (Chapter X) the 
statement was made that the various amino-acids have 
been called the "building-stones" of metabolism. In the 
course of digestion they are separated, and after absorption 
or during the very act of absorption, they are to some 
extent assembled again. Many facts bearing upon this 
process have been brought to our attention by the recent 
studies of physiologic chemists concerning the constitu- 
tion of protein molecules. All that has been done of late 
in this direction has served to emphasize the variety of in- 
dividual structure comprehended under the term protein. 
When such compounds from various sources are subjected 
to decomposition, either by digestion or by other means, 
the assortment of amino-acids obtained differs with the 
particular protein under investigation. 
Some, which one is tempted to regard as perfect proteins, 
yield the full list of amino-acids as at present known. 
Others, seemingly defective when judged by this rather 
arbitrary standard, fail to yield certain members of the 
series. This variation has an important bearing on mat- 
ters of nutrition. It can no longer be maintained that all 
substances characterized as proteins are equivalent in 156 
nutritional physiology 
their power to minister to growth and repair. Among 
an increasing number of defective proteins now recognized 
gelatin is the best known. Its behavior in the system calls 
for exposition. 
Gelatin.-This familiar compound belongs to the ill- 
defined group often spoken of as the albuminoids. These 
may fairly be regarded as proteins which have undergone 
a more or less definite degeneration both in function and 
in chemical structure. They are found as intercellular 
substance in the connective tissues and also in the dead 
and dry surface layer of the skin. They make the chief 
substance of the hair and nails. Gelatin itself is derived 
by boiling certain varieties of connective tissue, including 
bone and tendon. It gives nearly average percentages of 
the five elements present in ordinary proteins. When this 
fact was discovered in the early days of organic chemistry 
it was urged that gelatin must have a value in nutrition 
quite equal to that of any nitrogenous food.1 
Trials were made on a large scale in the pauper institu- 
tions of France. It was found very definitely that the 
free use of gelatin led to indigestion and that it soon 
became repugnant to the subjects, however hungry they 
might be. These effects were later found to be correlated 
with inadequacy to maintain the weight and strength of 
animals. An animal cannot be said to be perfectly nour- 
ished if it is losing more of any element day by day than it 
is receiving. This is eminently true of the nitrogen in the 
income and the outgo. "Nitrogenous equilibrium," 
expressing the equality of the two, is a phrase we shall use 
freely. Now nitrogenous equilibrium cannot be estab- 
lished by feeding gelatin in place of all other protein, no 
matter how skilfully the experiment is conducted. 
For many years the insufficiency of gelatin to make good 
the losses from the tissues remained a mystery. It has 
1 The early history of gelatin is related entertainingly by Lewes, 
"Physiology of Common Life," Edinburgh, 1859, vol. i, 129. The 
whole work is of interest as setting forth tlie prevailing opinions of 
the period. NITROGENOUS METABOLISM 
157 
been amply explained by the findings of chemists in our 
own time. Gelatin yields most of the building-stones re- 
quired for the new construction, but it does not furnish 
them all. Therefore, it is impossible to make blood-pro- 
teins or the proteins of the living cells from its cleavage 
products. It is useless to increase the quantity of the 
amino-acids if the variety is not great enough to supply the 
details of the molecular pattern to be wrought. What is 
true of gelatin is true of a number of proteins from vege- 
table sources. They do not give all the groupings needed 
in the constitution of the more elaborate animal proto- 
plasm. 
In some cases a single protein may be adequate for nutri- 
tion, supplying a complete 'assortment of amino-acids. 
But it will be seen that successful nutrition is more cer- 
tainly to be secured by using proteins from various foods. 
This is our practice, save in the important ease of the milk- 
fed infant. Milk actually contains proteins of more than 
one order, so that the exclusive use of this food does not 
narrow the selection of building units so greatly as might 
be supposed. The chief protein of milk contains the ele- 
ment phosphorus and is perhaps of somewhat unusual 
complexity. 
A very important body of work bearing upon this 
matter has been reported by Mendel and Osborne of the 
Sheffield Scientific School. They have conducted feeding 
experiments of the most elaborate character, in course of 
which small animals have been supplied with single pro- 
teins. Non-nitrogenous and mineral constituents have 
been liberally furnished. By this procedure individual 
proteins from various sources have been placed on trial. 
The investigators have shown that the proteins employed 
may be placed in three classes: those which suffice for all 
purposes of nutrition, including growth, those which can 
sustain life but not growth; and those which are quite in- 
adequate for either purpose.1 
1 Lusk, "The Fundamental Basis of Nutrition," Yale University 
Press, 1914. This is a most valuable reference. See also Underhill, 
"The Physiology of the Amino-acids," Yale Press, 1915. 158 
NUTRITIONAL PHYSIOLOGY 
Interesting facts have appeared respecting the second 
class of proteins mentioned above. White rats limited 
to such forms of nitrogenous food may be kept alive for 
months during the period in which a normal animal makes 
a steady growth and yet remain stunted, their weight 
nearly constant throughout. When they are at length 
granted a more favorable diet growth may begin and con- 
tinue until a full stature is attained. Mendel has pointed 
out that this postponement of growth and its subsequent 
accomplishment may be comparable with an arrest of de- 
velopment in man which should continue up to the age of 
forty and then be followed by full and normal growth. This 
statement is based on the ordinary longevity of the rat. 
In an introductory chapter the protein molecule was 
likened to a watch with its many dissimilar parts associ- 
ated in the one possible way to secure a desired result. 
One or more of these parts might be missing without their 
absence being apparent to the untrained person as he 
looked into the works. He could nevertheless observe the 
fact that the watch would not go. This is quite parallel 
with our progress toward an understanding of the failure 
of gelatin and other proteins to serve all purposes in nutri- 
tion. Just as the watch-maker, with his special knowledge, 
easily detects what is wanting, so the physiologic chemist 
is now able to say with much accuracy what particular 
amino-acids are lacking in his feeding experiments. As 
the defective watch may be made serviceable by the addi- 
tion of certain bits of mechanism, so in a measure an in- 
sufficient diet may be made adequate when extra amino- 
acids are supplied. 
Mendel has made it clear that we cannot control the 
supply of amino-acids, in the case of an experiment, so 
precisely as might be supposed. This is because of the 
micro-organisms which flourish in the canal. These 
simple plants take to themselves a share of the food and 
build it into their own substance. As they constantly die 
and are subjected to digestion they constitute a source of 
proteins, and so finally of amino-acids, which may be quite 
different from those which it was designed to furnish. NITROGENOUS METABOLISM 
159 
It is necessary now to approach a subject of some 
difficulty. We must attempt to show why a given quan- 
tity of protein fed-say 100 grams-cannot contribute an 
equal quantity to the protein supply of the body. When 
protein of one kind undergoes complete hydrolysis and 
protein of a new kind is to be made from the resulting 
cleavage products, certain of the building-stones will be 
needlessly plentiful, while others will be relatively scarce. 
We have seen that if a single one of these structural units 
is not furnished, there is complete failure to synthesize, 
the new compound. Similarly, if the second body is to 
contain a large percentage of an amino-acid which is but 
, scantily represented in the first, the possible formation is 
definitely limited. The principle is easily illustrated. 
Suppose that in a club of 100 members there are 25 Demo- 
crats. It is desired to elect for purposes of debate the 
largest possible body, consisting of Democrats and Re- 
publicans in equal numbers. Evidently, this body will 
comprise 25 men of each party. There will be 50 men un- 
related to the new organization. We may change the 
comparison: A house is pulled down and another is to be 
erected from the timbers. If the second house is of an 
architecture entirely unlike that of the first, there will be 
many unavailable pieces to discard and the new building 
will be smaller than the old. It is not at all unlikely that 
the misfit fragments of building material will go into the 
cellar of the new house, later to be used as fuel. This is 
just what the body does with the misfit amino-acids. So 
far as they do not find place in the mosaic which is put 
together they serve as producers of energy. 
Again, a better analogy suggests itself: The structure of 
a molecule of food-protein, previously compared with that 
of a watch or a house, may be likened to the type set up to 
print a page. The letters, some of which occur frequently 
and some rarely, stand for the amino-acids. The type is 
allowed to fall apart, the symbol of digestion. It is to be 
set up again to print different matter. If the language and 
vocabulary are much as at first, it may be possible to com- 160 
NUTRITIONAL PHYSIOLOGY 
pose nearly a whole page before the lack of some letter 
brings the proceeding to a standstill. But some shrinkage 
will be inevitable and when the type-setting has to halt 
there will be some unused letters. The shrinkage will be 
much greater if the second page is to be printed in a lan- 
guage other than the original. Suppose, for example, the 
type used in English composition is next devoted to Ger- 
man. The resulting difficulty is readily foreseen-the 
letter z is uncommon in English, but frequent in German. 
Hardly a line can be perfectly set up before this letter will 
be vainly sought. Almost the whole collection of type will 
be useless for composing. This is analogous to the attempt 
to minister to animal growth with some isolated vegetable 
protein of exceptional constitution. Offering the cleavage 
products of gelatin to the cells is like giving the compositor 
incomplete fonts of type. He cannot set up connected 
matter if some of the letters are not to be found. 
It seems natural to assume that the closer the structural 
resemblance between the proteins digested and those to be 
synthesized, the more economically the transformation can 
be accomplished. In fact, we have definite evidence that 
the quantity of protein required to maintain a balance may 
be very much greater for one type than for another from a 
different source. The reference is to the experiments of 
Thomas. This investigator made a series of trials upon 
himself, making liberal use throughout of non-nitrogenous 
food and, combined with this, proteins of various kinds 
to ascertain the amount of each necessary to keep him in 
equilibrium. Some of his results are tabulated below:1 
Meat protein 30 grams. 
Milk protein 31 " 
Rice protein 34 " 
Potato protein 38 " 
Bean protein 54 " 
Bread protein 76 " 
Indian corn protein 102 " 
It is not strange that meat and milk should stand first 
when rated on a basis like this. Meat may be expected 
1 Lusk, loc. cit. NITROGENOUS METABOLISM 
161 
to have a constitution closely resembling that of the body 
which is to be nourished, while milk is nature's food for the 
growing animal. The good showing made by rice and 
potato is rather more surprising, and, when we consider 
what vast populations depend largely upon these staples, 
we must conclude that there could scarcely have been a 
better choice. 
Our knowledge of the place and the manner of protein 
sythesis is incomplete. The cells which line the intestine 
and receive the digestive products were formerly held 
to bear a large part in the work. The fact that these cells 
may be made to yield erepsin, an enzyme capable of break- 
ing down the more complex nitrogenous bodies, does not 
exclude the possibility that dehydrations and condensa- 
tions may still take place within them. The enzyme may 
be modified under some conditions so as to be inactive. 
It is even conceivable that it may facilitate the combining 
of the building-stones. Some enzymes, like other catalysts, 
may favor the progress of reactions either in one direction 
or the reverse, according to the proportions of the sub- 
stances present at the moment. There is an increasing 
body of evidence that the liver-cells play an important 
part in the manufacture and storage of proteins. 
Not long ago it was usual to make much of the blood 
proteins in outlining the probable course of nutrition. 
These are abundant constituents of the plasma and it was 
natural to conceive of them as being made in the intestinal 
wall from the products of protein digestion, offered to all 
the tissues, and locally digested to furnish the groupings 
needed for growth and repair. During fasting it was sup- 
posed that the cells imperatively requiring to be preserved 
still made a draft upon the proteins of the blood. At a 
time when these could not be produced by the intestine 
various reserve tissues were called upon to supply them. 
The present way of picturing the process is different and 
distinctly more simple. The amino-acids from the intes- 
tine are believed to remain uncombined in the blood and 
to be appropriated by each tissue according to its need. 162 
NUTRITIONAL PHYSIOLOGY 
When the intestine does not contribute the requisite 
material it is released to the blood by other organs in which 
reserve protein is being hydrolyzed. In this case as well 
as during feeding the amino-acids in transit are free rather 
than combined. The blood proteins accordingly cease to 
be emphasized as standard food-stuffs. It is no longer 
believed that they are continually consumed and re- 
placed. The presence of amino-acids in the blood can 
be demonstrated by delicate tests. The amount is small, 
but we have only to recall the disproportion between the 
quantity of sugar in circulation and its importance in 
metabolism to realize that we need not make much of 
this fact. Money is vastly important to our comfort, but 
the currency of the nation seems insignificant in com- 
parison with its total wealth. 
It is not easy to estimate the extent of protein synthesis 
which normally takes place. Of course, it is more promi- 
nent during growth than during adult life. The present 
impression is to the effect that only a very small fraction 
of the usual protein income is thus used. Most people 
can materially reduce the quantity of protein in the diet 
and still remain in nitrogenous equilibrium. When the 
lowest level at which this is possible has been attained it 
is still true, as we have just pointed out, that the amount 
of new protein constructed is but a fraction of that sup- 
plied. It is, therefore, certain that under average con- 
ditions by far the larger part of the nitrogenous food eaten 
never exists in the form of proteins after absorption. We 
must now consider the destiny and value of the uncombined 
building-stones. 
The surplus amino-acids, either free or in simple com- 
binations, are borne away from the intestine in the portal 
blood. Accordingly, these digestive products, like the 
sugars, are brought under the influence of the liver-cells. 
They undergo a transformation in this organ-and very 
probably elsewhere-which has important consequences. 
This is the process described as "deaminization." To 
deaminize an amino-acid is to remove from it the group NITROGENOUS METABOLISM 
163 
to which it owes its name, the radicle NH2. One of the 
products of this reaction will be non-nitrogenous, the other 
will contain nitrogen in a greatly increased percentage. 
We cannot claim to know all the steps which are gone 
through in this connection, and we shall not discuss those 
which are known. We shall emphasize simply the final 
results. 
The chief nitrogenous compound which issues from the 
series of reactions affecting surplus amino-acids is urea. 
This is apparently of no further use in the system. It is 
destined to circulate until it shall find its way into the 
urine. The efficiency of the kidneys is so remarkable and 
the whole blood volume is carried through them at such 
short intervals that the percentage of urea in normal blood 
is kept very low. Urea seems to be a most convenient 
form for nitrogen elimination; it is highly soluble and 
diffusible, inert, and, comparatively speaking, non- 
poisonous. If a man eats 100 grams of protein in twenty- 
four hours he will excrete some 30 grams of urea, an 
amount which represents about seven-eighths of the total 
nitrogen passing into and out of the body. This urea is 
not all made by the liver, but a large share of it proceeds 
from the deaminizing activities of this organ. Most of 
the sulphur from those amino-acids which contain the 
element is oxidized and leaves the body as sulphates. 
What is the principal non-nitrogenous compound 
produced from the amino-acids? There seems to be 
no doubt that it is dextrose. Evidence in support of 
this belief has been derived from the study of diabetes. 
Whether this condition is experimentally induced or devel- 
ops spontaneously, it is found that, if the case is one of full 
severity, sugar excretion goes on even when no carbohy- 
drate is fed, and, indeed, throughout long periods of fast- 
ing. This sugar might be assumed to have come from the 
fat of the body, but it can be more surely attributed to the 
protein which is being decomposed. The following con- 
sideration shows this: the nitrogen and the sugar in the 
urine of the fasting diabetic patient maintain, in many 164 
NUTRITIONAL PHYSIOLOGY 
cases, a singularly constant ratio; 1 gram of nitrogen is ac- 
companied by 3.6 grams of dextrose. They rise and fall to- 
gether. This seems to prove that the two must have a 
common source, which can only be protein. Again, the 
feeding of amino-acids to the diabetic increases his loss of 
sugar; the feeding of fat does not have this effect. A simple 
calculation shows that 100 grams of protein fed may give 
rise within the body to about 58 grams of sugar. The 
seriousness of diabetes will now be better appreciated than 
has been possible up to this time. The organism loses not 
merely the support normally secured from the chief carbo- 
hydrates of the diet, but, in great part, that ordinarily 
furnished by protein food. 
From all this it appears that much of the protein which 
we eat serves only to supply the tissues with carbohydrate. 
The impression is likely to be that this is a roundabout 
and not an economical way to provide sugar, which might 
have been taken as such at the outset. The facts may 
fairly be employed to support the modern teaching that 
excess of protein is to be avoided, but we have already 
shown the necessity for allowing more than is actually to 
be reconstructed after the digestive dismembering. Recog- 
nizing as inevitable the discarding of amino-acids, we can 
see the desirability of having them made to furnish a 
simple standard food like sugar, valuable for its store of 
energy. The possibility of glycogen formation from pro- 
tein naturally follows. The glycogen of carnivorous 
animals presumably has such an origin. The maintenance 
of the sugar of the blood during long fasting is also ascribed 
to the disintegrating protein of the tissues. Given dex- 
trose in such quantities, the production of fat from protein 
becomes at least theoretically possible. Broadly speaking, 
we may claim for protein that it can do all that any form 
of organic food can do for the system. Yet this does not 
impair the statement, equally to be recognized, that car- 
bohydrates and fats should form much the larger part of 
the income. 
After the constructive requirement has been met, all ad- NITROGENOUS METABOLISM 
165 
ditional protein seems to entail unprofitable labor on the 
part of the liver in deaminizing the cleavage products, the 
presence of various substances of a possibly detrimental 
nature in the circulation, and an activity on the part of the 
kidneys which may amount to an abuse of these important 
excretory organs. There is this general contrast between 
the behavior of proteins and non-proteins in the body: 
the former give rise to rather complex waste-products 
imposing a task upon the liver and kidneys; the latter 
are oxidized cleanly to carbon dioxid and water, two com- 
pounds which are eliminated with ease. A fuller discus- 
sion of these facts will be undertaken in the chapter on The 
Hygiene of Nutrition (Chapter XXII). 
Folin, of the Harvard Medical School, has classified the 
facts of protein metabolism in a particularly clear and 
helpful form. He distinguishes two lines of transforma- 
tion, the endogenous and the exogenous. Under the head 
of endogenous metabolism he traces the various steps in 
the history of those building-stones which are erected into 
the proteins of the blood and other tissues. The narrative 
is continued in the account of the rather gradual and steady 
crumbling which such tissue-proteins undergo. What is 
loosely called the wear and tear of the cells gives rise to 
definite end-products, one of which Folin holds to be of 
particular value in estimating the extent of such decompo- 
sition. This is the substance creatinin, which accompanies 
urea out of the body and which is far less subject to fluctua- 
tion. The urea excreted rises and falls with the protein 
ration, but the creatinin is not markedly responsive to 
dietetic variations. It is believed, therefore, that endog- 
enous metabolism, a necessary feature of animal life, is 
relatively independent of feeding, at least while nutrition 
is satisfactory. 
Exogenous metabolism is an expression to cover all the 
reactions affecting the uncombined amino-acids. Hence 
it includes the formation of urea, dextrose, and whatever 
substances may be formed from the nitrogenous cleavage 
products in the liver. It may be extended to take in the 166 
NUTRITIONAL PHYSIOLOGY 
secondary production of glycogen or of fat from surplus 
sugar originating in this way. In contrast with endogen- 
ous metabolism its amount varies widely. With a low 
protein diet the exogenous changes will be but a fraction 
of what they will become with abundant protein. One 
may be tempted to conclude that in fasting the metabolism 
will be wholly endogenous. The insight of a German writer 
has served to show us that this is not so. We have spoken 
at length of the assembling of amino-acids fresh from the 
intestine to form the standard proteins of the various 
tissues. Now there are differences of constitution be- 
tween the proteins of the various organs. When a par- 
ticular tissue, muscle, for example, is to be nourished at 
the expense of some other, a true local digestion is neces- 
sary, and once more there must be the selecting and 
rejecting of amino-acids. Those not available may be 
deaminized quite as if they had come from the seat of the 
original digestion. This type of activity whereby one kind 
of tissue is sacrificed to build up another is best illustrated 
in the case of the salmon before spawning. No food is 
taken; the muscles steadily waste and the roe increase at 
their expense. 
While all proteins are complex in their molecular struc- 
ture, there are some which are distinguished above the 
others by the intricacy of their organization. The presence 
of phosphorus in certain proteins suggests such an emi- 
nence. This is a characteristic of the proteins in cell 
nuclei, though phosphorus is found in other compounds 
also, for example, in the casein of milk. When such pro- 
teins decompose in the body phosphates are among the 
metabolic products. 
The same peculiar proteins of the nuclei which yield 
phosphates give rise to a number of closely related organic 
bodies known collectively as the purins. These undergo 
various transformations under the influence of enzymes 
and are very largely converted to a single end-product, 
uric acid. If we measure the output of uric acid we shall 
have an indication of the nuclear metabolism, provided 
that two conditions are fulfilled. These are, first, that NITROGENOUS METABOLISM 
167 
excretion is keeping pace with production, and, second, 
that no purins are furnished by the diet. When the food 
includes much nuclear material (meat especially) the uric 
acid which the body has to eliminate is the sum of two 
fractions, one endogenous, an inevitable product of the life- 
processes, the other exogenous, derived from the current 
ration. 
Vitamins.-We often need to remind ourselves that there 
is an important distinction between proteins and proto- 
plasm. Living matter may fairly be considered to consist 
very largely of proteins-with much water, of course- 
but other components are doubtless necessary to its exist- 
ence. That this is true of certain mineral compounds 
has become familiar. There is increasing evidence that 
various organic bodies other than those produced by the 
digestion of proteins are also essential. It has been pro- 
posed to call such compounds vitamins. An amin is a 
nitrogenous body of a particular type, and the prefix con- 
veys the idea that the one referred to is indispensable. 
The objection that the term is too narrow and specific 
seems to be well taken. We do not know that all such 
valuable bodies have the molecular structure of amins. 
To designate them as "accessory substances" would be 
better, but the other usage is rather firmly established. 
The belief in vitamins has proceeded from the study of 
certain diseases. One of these, the best suited to our dis- 
cussion, is beriberi. It is technically described as a mul- 
tiple neuritis, that is to say, a wide-spread degeneration 
of the nerves. The great majority of cases have occurred 
among populations which subsist largely upon rice. Here 
and there the disease, or one much like it, has been recog- 
nized where rice has not figured prominently in the ration, 
but the diet has always been monotonous. Various theo- 
ries have been advanced to account for the disorder, but the 
one most in favor is that it is a "deficiency disease," due 
to the lack of a particular nutritive substance. A com- 
pound, the want of which produces such disastrous effects, 
obviously deserves to be called a vitamin according to our 
previous description. 168 
NUTRITIONAL PHYSIOLOGY 
Some years ago it was shown that a decline with symp- 
toms resembling those of beriberi could be induced in 
pigeons by limiting them to a diet of "polished" rice, that 
is, rice without the husk or pericarp. The inclusion of the 
pericarp in a ration otherwise similar would prevent the 
derangement or remedy if it had not gone too far. Em- 
ploying our modern terminology, we shall be inclined to 
say that the pericarp in this case furnishes a vitamin absent 
from the enclosed kernel of the rice. A leading worker 
in this field believes that he has isolated the pure vi- 
tamin of beriberi and ascertained its chemical structure, 
which is not of high complexity. He identifies it with a 
body obtainable from meat and from yeast. It follows 
that either meat or yeast-and doubtless other materials- 
added to a diet of polished rice may be expected to render 
it adequate. 
It will be well briefly to outline the supposed course of 
events when the supply of vitamin is cut off. The com- 
pound is assumed to be most urgently needed in the white 
matter of the nervous system. For a time the demand can 
be met by levying a tax upon other tissues, such as the 
muscles, but the withdrawal of a necessary component 
will result in the complete breakdown of such cells. The 
consequence is a destruction of protoplasm, which appears 
to be utterly out of proportion to the minute quantity of 
the vitamin transferred to the nervous system. It is like 
the wrecking of complex machinery to secure a supply of 
bolts and rivets. While this stage continues the individual 
rapidly loses weight and strength, but does not manifest the 
acute nervous symptoms. These supervene when it is no 
longer possible to collect the vitamin even at such a ruinous 
cost. 
Another disorder which apparently should be regarded 
as a "deficiency disease" is scurvy. The chronicles of 
explorers, especially of Arctic voyagers, are filled with 
accounts of this distressing malady. Many readers will 
recall the painful situation on board Dr. Kane's ship, when 
the majority of the party lay helpless in their bunks and 169 
NITROGENOUS METABOLISM 
their commander set up a mirror to cast into their midst 
the first beam of the returning sun. Among the symptoms 
of scurvy are prostration, intense soreness of the mouth 
with spongy swelling of the gums and loosening of the 
teeth, hemorrhages under the skin, and remarkable fragil- 
ity of the bones. We readily recognize the signs of scurvy 
in Benvenuto Cellini's vivid narrative of his imprisonment 
in the Castle of St. Angelo. The disease, like beriberi, has 
occurred chiefly among those whose diet has been lack- 
ing in variety. Certain articles of food, such as fresh meat, 
potatoes, and lemons, have been reputed to ward off or cure 
scurvy, and have been described as "antiscorbutics." 
It is commonly supposed that the want of a definite 
vitamin is responsible for scurvy. Analogous facts prob- 
ably hold for other disorders. Pellagra may be one of these, 
though there is no clear evidence of a single curative sub- 
stance.1 Malnutrition in infancy may often be referred to 
such deficiencies. There is interesting evidence that the 
fat of milk is superior to most other fats as a promoter of 
growth. This probably means that it carries in minute 
amount an admixture of some essential accessory. The 
same is believed to be true of orange-juice, long known to 
be beneficial to babies. We are brought to a new apprecia- 
tion of the value of a varied diet. The more numerous the 
foods which we eat, the more certain we are to include all 
the miner compounds which we require. It is, further- 
more, a question whether some of these essential bodies 
may not be decomposed by cooking or by prolonged 
preservation of food. This consideration gives at least a 
measure of support to the teaching that we should make 
a considerable use of uncooked articles and be chary of 
canned goods. Nevertheless, many writers have grossly 
exaggerated whatever dangers exist in this direction.2 
In the opinion of many observers the vitamin question 
1U. S. Public Health Service, Hygienic Laboratory, Bulletin 
No. 120, 1920. 
2 Funk, Journal of State Medicine, xx, 1912, 341. Harrow, "Vita- 
mines, Essential Food Factors," E. P. Dutton & Co., New York, 
1921. 170 
NUTRITIONAL PHYSIOLOGY 
admits of a more specific enunciation. There are at least 
three types of essential accessories which may be called, 
in view of their properties, "fat-soluble A," "water- 
soluble B," and "water-soluble C." The first is a pro- 
moter of growth, the second, of the normality of the ner- 
vous system. Want of the first causes stunting, and, 
under some circumstances, an inflammation of the eyes. 
Lack of the water-soluble B leads to polyneuritis or beri- 
beri. Water-soluble C is the antiscorbutic vitamin, the 
agent preventing scurvy. It seems to be less stable than 
the others. The fat-soluble body is found not only in 
butter and glandular fats (such as cod-liver oil), but in 
green vegetables, particularly in growing tips and leaves. 
Milk is a reliable source of vitamins. Its inclusion goes 
far to insure the adequacy of any diet.1 Yet nutrition in 
infancy is promoted by giving orange-juice which is a 
reliable source of the accessory C. The fruit juice is 
particularly needed to supplement pasteurized milk. 
Rickets.-This is a serious and frequent disorder of 
nutrition. The prominent feature is a failure of the skele- 
ton to acquire the lime necessary to give it rigidity. The 
soft bones become bent under the weight placed upon 
them and deformities result. Analysis of the blood in 
such cases shows abnormal ratios between calcium and 
phosphoric acid. Foods containing vitamin A have gen- 
erally been found to correct rickets and there has been a 
disposition to credit this accessory with the beneficial 
property. But there is ground for claiming that a specific 
"antirachitic vitamin"-sometimes called D-is required 
for development.2 While rickets may be remedied by 
additions to the diet we must give due recognition to the 
fact that it can, apparently, be cured by sunlight. The 
double possibility has doubtless been productive of con- 
fusion in the search for causes and preventives. Thus a 
child who is much in the open may escape the disorder 
1 McCollum, "The Newer Knowledge of Nutrition," Macmillan, 
New York, 1922. 
2 Park, Physiological Reviews, iii, 1923, 106. 171 
NITROGENOUS METABOLISM 
while subsisting on a ration which would be inadequate 
for another less fortunate1/ situated. It seems well es- 
tablished that the beneficial rays are those in the ultra- 
violet region of the spectrum and it is most important to 
bear in mind that these cannot pass through window-glass. 
The brightest interior lacks something of the hygienic 
virtue of outdoors. It is not strange that rickets is for 
the most part a disease of the cold and dark months of 
the year. Cod-liver oil has long been relied upon as a 
remedy for rickets and it has been suggested that the power 
has been developed by the fish because it frequents depths 
beyond the effective reach of the sunlight. 
A recent review of these matters by Hess1 contains in- 
teresting suggestions. It is pointed out that the higher 
animals cannot synthesize their vitamins, but must obtain 
them from plants. Hence milk and flesh do not yield for 
human nutrition any accessories of the animal's own mak- 
ing, but only such as it was able to secure in its vegetable 
food-supply. It does not produce, but only concentrates 
these valuable materials. If such an animal has an inferior 
ration it may furnish less of the vitamins than it would 
under more favorable conditions. Food values may thus 
vary more than we have assumed. Another contention of 
Hess, and a reasonable one, is that for every case of well- 
developed deficiency disease there must be many incipient 
and hardly to be recognized-numerous types and degrees 
of malnutrition-all to be benefited by a more inclusive 
diet. 
A SUMMARY OF METABOLISM 
Fats are hydrolyzed in the alimentary canal to fatty 
acids and glycerin. To an uncertain extent there is soap 
formation. These products are largely combined to form 
fat during the passage through the lining cells of the 
intestine. Fat is stored chiefly in adipose tissue. Its 
eventual service is to be oxidized to carbon dioxid and 
water with release of its energy. 
1 Journal of the American Medical Association, Ixxvi, 1921, 693. 172 
NUTRITIONAL PHYSIOLOGY 
Carbohydrates enter the circulation in the form of simple 
sugars, mainly dextrose. There is little sugar in the blood 
at any one time. Much is dehydrated by the cells of the 
liver and muscles to make glycogen, subject to reconversion 
to sugar when required. A surplus may be converted to 
fat. The possibility of the converse change from fat to 
sugar is generally held to be unproved. The final value 
of carbohydrate is like that of fat; its energy is set free 
through the respiratory oxidation, and the end-products 
again are carbon dioxid and water. The internal secretion 
of the pancreas is necessary to bring about this destruction. 
Proteins are hydrolyzed to simple compounds (amino- 
acids or combinations of these), and these are used for the 
synthesis of the new proteins-of types peculiar to the 
species-which can-be utilized for growth or repair. A 
large surplus of uncombined amino-acids is usual; these are 
dealt with by the liver and other tissues, and the best- 
known resulting products are urea-destined to be excreted 
-and dextrose, for which all the possibilities exist that 
have been mentioned above. The amount of wasting suf- 
fered by nitrogenous tissues as a part of their life process 
is believed to be indicated by the extent to which the prod- 
uct creatinin is eliminated. All proteins yield sulphur 
compounds among their decomposition products. Some 
yield phosphorus compounds also, and among them are the 
proteins which give rise to much of the uric acid which re- 
quires to be removed. 
Reference: Mendel, "Nutrition: The Chemistry of Life," New 
Haven, Yale University Press, 1923. CHAPTER XVII 
THE REMOVAL OF THE END-PRODUCTS OF 
METABOLISM 
The statement has repeatedly been made in varying 
form that the bulk of the food is taken for the sake of its 
potential energy. Either at once or after storage it is 
oxidized, and the energy turned to account for tempera- 
ture maintenance and for the performance of muscular 
work. The main products are carbon dioxid and water. 
These are likewise the chief products formed when familiar 
fuels are burned outside the body. Wood, coal, and gas 
yield the two in great quantity and only small amounts of 
other compounds. Hence the primary problems of excre- 
tion concern the manner of elimination of carbon dioxid 
and water. 
The water leaving the body during twenty-four hours 
may be 2 or 3 kilograms. The carbon dioxid discharged 
in the same period is not commonly in excess of 1 kilogram. 
Nevertheless, we say that carbon dioxid is the leading 
waste-product of animal life. This is justified by the con- 
sideration that by far the larger part of the water which we 
measure is merely water previously received in the same 
state. To this large volume the tissues have added a mod- 
erate quantity of water-say 250 grams-which is a true 
metabolic product. This has been formed by the oxidation 
of compounds containing hydrogen. The water output 
of the body is inevitably greater in the long run than the 
water income. This fact may be disguised on single days 
by water retention. 
Carbon Dioxid Elimination.-Respiration has been 
defined as che process within the living cells in course of 
which complex organic molecules are decomposed and 
173 174 
NUT' PHONAL PHYSIOLOGY 
more or less completely oxidized. Carbon dioxid is the 
most conspicuous product. The respiratory exchanges 
occur in the different tissues in a measure corresponding 
with the extent to which they severally evolve energy. 
The skeletal muscles lead in amount of respiration (and 
of carbon dioxid set free), both because of their great mass 
and by reason of their activity. The glands, especially 
the liver and the kidneys, contribute largely to the total. 
I 
H 
Al 
-B 
m 
Cap 
Al 
Cap. 
Corp. 
Fig. 21.-I is intended to suggest the form of an air-sac of the 
lung overlaid with a network of capillaries belonging to the pul- 
monary system. II is an imaginary section through such an air- 
sac. B in both I and II is the minute bronchial tube through which 
the air is renewed. Ill is a bit of detail from II, still more en- 
larged, showing two capillaries (Cap.) conveying corpuscles (Corp.). 
The air is close by (Al), yet two partitions intervene, the capillary 
wall and the wall of the air-sac. 
So does the heart. Other tissues have a minor part in the 
general respiration. Some which are passive and stable, 
like cartilage, can have but little. 
The carbon dioxid formed by the cells is first transferred 
to the lymph. The concentration of the gas in the lymph 
leads to its passage into the blood. A gas will always pass 
from a higher to a lower concentration when the two solu- 
tions are placed in communication. The delicate capillary REMOVAL OF THE END-PRODUCTS OF METABOLISM 
175 
wall between the lymph and the passing blood offers prac- 
tically no impediment to the movement. It will be re- 
membered that the blood which enters upon the very short 
journey through the capillaries is arterial; that which 
enters the minute veins only a fraction of an inch away is 
reckoned venous. It has parted with a large share of its 
oxygen and has received carbon dioxid. It is swept on 
without significant change to the right side of the heart 
and thence to the lungs. The development of these organs 
is such as to multiply the surface of contact between the 
blood and the air within them. The capillaries of the 
pulmonary circulation are wrapped about innumerable 
elastic sacs, the walls of which are as thin as those of the 
capillaries themselves. There is, accordingly, a double 
partition between the blood and the air, but it is of a nature 
to permit free gaseous exchange. 
If the air in the sacs were not renewed it would accumu- 
late carbon dioxid in increasing amount, while its oxygen 
would progressively diminish. This tendency is normally 
counteracted through the effects of breathing. The lungs 
have no power to move of themselves; the changes which 
they undergo are due to the displacement and return of the 
thoracic walls. This is not the place to analyze the breath- 
ing movements. The muscles employed are of the skeletal 
order. Being so, they are not automatic, and it follows 
that every breath taken is the expression of a separate and 
distinct act on the part of the central nervous system. 
Each time the chest is made larger the air presses in along 
the breathing passages to fill the space created for it in 
the host of widened sacs. The return of the chest walls 
to their first position reduces the capacity of the sacs, and 
air is pressed out along the same channels by which it 
entered. The action is that of a pair of bellows not pro- 
vided with the usual inlet valve. 
It is not to be conceived that we empty and refill the 
air spaces of the lungs with each breath. We usually expel 
something like one-fifth or one-sixth of the air contained 
and replace that fraction with fresh air. When allowance 176 
NUTRITIONAL PHYSIOLOGY 
is made for the rather long and capacious passages between 
the air-sacs and the nostrils the impression that our breath- 
ing is rather ineffective becomes strengthened. To offset 
this idea we must remember that the movements occur 
fifteen or eighteen times a minute, providing thus for at 
least two fairly complete renewals of the whole volume of 
air within that time. Still it is a fact that when the breath- 
ing is rarely deepened beyond the constant habit, some 
portions of the lungs, notably their upper extremities, are 
but little subject to extension and contraction. By the 
deepest possible breathing we can increase the proportion 
of the air removed and replaced to perhaps three-fourths of 
the total at a single movement. A quiet breath, unmodi- 
fied by the influence of attention, muscular activity, or 
any other temporary condition, is said to amount to about 
500 c.c. (30 cubic inches or 1 pint). Reckoning sixteen 
breaths in a minute, this will mean 8 liters of air breathed 
in that interval, about 500 in an hour, or 12,000 in a day. 
Fresh air has but a small content (0.03 to 0.04 per cent.) of 
carbon dioxid. The air expired has 4 per cent., more or 
less; 4 per cent, of 12,000 liters is 480 liters, a fair average 
volume to represent the daily output of this gas. This 
quantity, chaxiged from volume to weight with correction 
for temperature, is about 800 grams. 
The air to which the blood is exposed in the lungs is at 
least as rich in carbon dioxid as that which we breathe out. 
Coming into relation with air of such a composition the 
blood by no means frees itself of its large carbon dioxid 
content. It carries on to the left side of the heart and so to 
the general arterial system some five-sixths of the carbon 
dioxid which it contained when it entered the lungs. The 
actual amount in venous blood is in the neighborhood of 
45 in 100 c.c. of blood. As much as 38 c.c. in 100 will 
usually remain in blood which is counted arterial and which 
carries a maximum of oxygen. Carbon dioxid does not 
seriously interfere with the capacity of the blood to carry 
oxygen, and the converse is equally true. It may be well to 
state that the color of blood varies with the extent to which REMOVAL OF THE END-PRODUCTS OF METABOLISM 
177 
the corpuscles are charged with oxygen and is independent 
of the carbon dioxid present. 
While there is no question of the propriety of calling 
carbon dioxid a waste-product, it does not follow that the 
system would be benefited by its complete removal. 
How far this is from being the case has been shown by the 
important experiments of Yandell Henderson. He has 
demonstrated that any considerable lowering of the carbon 
dioxid below the high standard noted above as character- 
istic of arterial blood results in marked prostration, often 
involving the suspension of breathing and perhaps result- 
ing fatally. The inference is that a certain concentration 
of carbon dioxid is a desirable source of stimulation to the 
nervous system and especially to the respiratory center. 
The intimate connection between this gas and breathing is 
manifest when its percentage in the blood is ever so little 
increased. A noteworthy deepening of the respiration 
promptly results. Since this is true it is not strange that 
a reduction of the carbon dioxid should cause inhibition of 
the breathing movements. 
Water Elimination.-Water leaves the body by all the 
possible excretory routes. Statements regarding the 
proportion taking this or that course can have but little 
value, so great is the variation under different circum- 
stances. If we exclude the effects of exercise and of un- 
usual temperatures we may expect to find somewhat more 
than half the whole amount to be removed by the kidneys. 
The daily volume of the urine is customarily set down at 
1200 to 1500 c.c. The remaining excretion of water will 
be almost wholly accounted for by the perspiration and by 
evaporation from the breathing passages. Of these two, 
the former is commonly more considerable. Some loss of 
water will occur in the feces, but normally this is not to be 
compared with the quantities discharged in the three ways 
just mentioned. 
When the external temperature is high the water passes 
in increased amounts through the skin and the perspira- 
tion may greatly exceed the urine. The urinary secretion 178 
NUTRITIONAL PHYSIOLOGY 
may shrink somewhat or may be kept near the standard as 
a result cf the water drinking which is stimulated. The 
water discharged through the skin during the hot weather 
and also during muscular activity does not serve primarily 
as a vehicle of waste, but rather as a means of ridding the 
body of heat. This matter will be discussed at length at a 
another time. The evaporation from the respiratory tract 
probably varies less widely than the perspiration. The 
body seems bound to saturate all the air that is breathed, 
so that this loss increases with the volume of breathing 
and is greater when the air is dry than when it is humid. 
The Kidneys and the Urine.-The chief significance of 
the kidneys is their function of excreting the distinctive 
products of protein metabolism. A secondary service is 
the disposal of inorganic salts. The two glands are placed 
to the right and left of the spinal column at the level of the 
lower ribs. Each kidney receives a large short artery from 
the aorta which passes between them. Each returns a 
large vein, not to the portal system, but to the chief venous 
trunk of the body. The kidneys, in consequence of this 
arrangement, constitute short cuts or "shunts" in the cir- 
culation, and are perfused by an exceptionally large quan- 
tity of blood. No portion of the blood can long escape 
their influence. The urine discharged by the many tubu- 
lar units of the kidneys is conveyed through the ureters, 
contractile vessels which lead to the bladder. This is a 
saccular organ capable of accommodating much urine when 
dilated, and of contracting again to nearly complete expul- 
sion of its contents. Its walls of muscle (the same type 
found in the alimentary canal) are obviously under ner- 
vous control and much subject to reflexes. 
Urine of average composition is a complex solution con- 
taining some 3 or 4 per cent, of dissolved solids. The 
leading substance is urea, the chief nitrogenous waste of 
the system, and the index, according to Folin, of the 
exogenous metabolism. Its origin has been discussed. 
Evidently the duty of the kidney is less to manufacture 
urea than to select and remove from the blood the urea 179 
REMOVAL OF THE END-PRODUCTS OF METABOLISM 
originating in the liver and elsewhere. Second in abun- 
dance among the urinary constituents we ordinarily find 
the mineral salts. The quantity of these depends in a 
Fig. 22.-The kidneys and the urinary bladder. The two kidneys 
are shown within an outline which suggests the body cavity. Their 
advantageous connections with the chief artery and vein of the sys- 
tem are indicated. Below is the bladder reached by the two ureters. 
These vessels enter the bladder low down and behind-not at the 
level where they disappear from the figure 
large measure upon the amount in the diet, and as sodium 
chlorid is the one taken most freely, so it will generally be 
the principal inorganic compound in the urine. The 180 
NUTRITIONAL PHYSIOLOGY 
chlorids of the mixture are accompanied by phosphates and 
sulphates. These are not to any extent salts which have 
been eaten, but, like the urea, they represent modified 
fragments of protein molecules. The phosphates come 
from a limited class of proteins, largely from those of the 
cell-nuclei; the sulphates arise from all proteins. 
The minor ingredients of the urine are very numerous. 
Those of most interest are the bodies which carry the ni- 
trogenous waste over and above that handled as urea. 
To one unfamiliar with organic chemistry a list of their 
names can have little meaning. The substance creatinin, 
already mentioned, is provisionally regarded as indicative 
of the rate of true endogenous metabolism, the inevitable 
gradual wasting of the nitrogenous tissues. Another com- 
pound which has attracted much attention on account of its 
apparent relation to several pathologic conditions is uric 
acid. A certain amount of this is produced during fasting 
and is not increased by the taking of many kinds of food. 
The addition to the diet of meats leads to a larger formation 
of uric acid. A maximum quantity is elaborated when 
glandular tissues, such as liver, kidneys, and sweetbreads, 
are eaten. These articles contain an exceptional propor- 
tion of nuclear material rich in the proteins just referred 
to as sources of phosphates. The same proteins are evi- 
dently uric-acid formers. The chief peculiarity of uric 
acid is its slight solubility, which renders its complete 
excretion difficult and uncertain. Retention of this crys- 
talline substance has been held accountable for the pain- 
ful symptoms of gout and of a good deal that goes by the 
inclusive name of rheumatism. 
We have by no means exhausted the list of normal urin- 
ary constituents, but the student must be referred to other 
sources for the details. The most commonly occurring 
compounds of an abnormal character are sugar and albu- 
min. The significance of the sugar should be clear in the 
light of what has been said. Its transient appearance as 
a result of free consumption does not indicate a diseased 
condition, but only dietetic indiscretion. The possibility REMOVAL OF THE END-PRODUCTS OF METABOLISM 
181 
of emotional glycosuria will also be recalled (page 145). 
Very frequent or continuous elimination shows that the 
body has not the usual power to oxidize its sugar. The 
kidneys are not generally at fault; the defect is in the 
metabolism of the pancreas or elsewhere. An abundant 
escape of albumin (presumably drawing upon the protein 
mass of the blood) is commonly due to a disordered condi- 
tion of the kidneys. It takes place, for example, in Bright's 
disease. 
Urine when freshly secreted is ordinarily acid to litmus. 
It may be alkaline when much vegetable food is eaten, and 
becomes so on standing in any case. The change is due 
to a bacterial fermentation whereby urea is transformed 
into ammonium carbonate. An ammoniacal odor develops 
in connection with this alteration and the liquid is likely 
to become turbid. The deposition of a sediment under 
such conditions is no cause for anxiety. It has often been 
represented by unscrupulous quacks to be a serious symp- 
tom. The urine of the herbivora, which is normally al- 
kaline, becomes acid when the animals are fasting, and it 
may be pointed out that they are then carnivorous-living 
upon their own flesh and fat. 
How much urine is secreted depends largely on the quan- 
tity of water taken, so far at least as this is in excess of the 
perspiration. Kidney activity is stimulated by almost 
any dissolved substance foreign to the standard composi- 
tion of the blood. The nitrates, for example, are absorbed 
rather freely from the intestine and afterward removed by 
the kidneys in a large volume of water. They, therefore, 
belong to the class of bodies known as diuretics. The 
active principle of coffee and tea has the same action. In 
securing their own elimination such compounds may 
promote the excretion of others. Diuretics bring about 
their effect partly through modifying the circulation, and 
partly, it is believed, through direct influence upon the 
kidney cells. As regards the first mode of working, it may 
be said that the kidney is readily responsive to increased 
blood-flow, especially if it is attended with high arterial 182 
NUTRITIONAL PHYSIOLOGY 
pressure. Difficulties with the heart, if they entail re- 
tarded circulation and lowered pressure, frequently lead to 
deficient output. 
Other Factors in Excretion.-The lungs and the kidneys 
perform so large a share in the disposal of metabolic waste 
as to leave relatively little of the work undone. The 
feces, however, include small quantities of miscellaneous 
excretions, and it is assumed that the precise part borne by 
the intestine and the liver (in the separation of bile) could 
not be taken by the kidneys. The modified bile-pigments 
of the feces illustrate this specific action. The share of the 
skin in the removal of waste is popularly overestimated. 
The belief that "the pores must be kept open" lest poisons 
gather in the system is so fruitful of wholesome practices 
that one is reluctant to question it. Candor requires, 
however, that the physiologist repudiate the moral in the 
story of the Italian boy, who died because the surface of 
his body had been sealed with gold paint for a few houts. 
If the case is authentic, he must have died because of the 
character of the application and not from toxic products 
of his own evolving. Volunteers have submitted to have 
the skin shellacked and have not suffered any other ill 
effects than sensitiveness to heat and cold. 
Perspiration is almost purely a mineral solution and the 
salts it carries could doubtless be cared for by the kidneys. 
When made profuse by severe exercise, it contains in small 
amounts some organic constituents like those of the 
urine, but its highest possible rating as a vehicle of nitrogen 
excretion is not impressive. The same may be said of the 
assistance rendered by the skin to the respiratory tract. 
Carbon dioxid passes from the skin in measurable quanti- 
ties when there is abundant perspiration, but the largest 
loss which can occur in this way seems insignificant when- 
compared with the discharge from the lungs. CHAPTER XVIII 
THE ESTIMATION OF METABOLISM 
It is more than fifty years since the first well-equipped 
laboratory for the quantitative study of human nutrition 
was opened in Munich. Before that time much had been 
accomplished in the analysis of foods, the measurement of 
rations, and the examination of urine, but no satisfactory 
knowledge of the general metabolism could be had until 
means should be devised to entrap and measure the gas- 
eous outgo of the body. This difficult task was accom- 
plished by the construction of the first respiration chamber, 
now one of several in various centers of scientific research. 
In the long run there must be a correspondence between 
the food and the metabolism, the income and the outgo, 
but on a single day there is no necessary agreement be- 
tween them. This is radically demonstrated on a day of 
fasting, when the income is nil and the outgo is consider- 
able. It is to the excreta that we attend, therefore, when 
we wish to judge to what extent various materials have 
been broken down in the body. Studies of the food may 
be valuable, but in our first discussion we shall limit our- 
selves to the simple case of the subject without income. 
A great deal can be learned about the metabolism by deter- 
mining two chemical elements-the carbon and the nitro- 
gen of the waste-products. Other facts can be ascertained 
when the quantity of oxygen consumed is noted. Water 
excretion is frequently measured also. 
Nitrogen Elimination.-The fact is already familiar 
that the nitrogen leaving the system is found almost 
wholly in the urine. An additional fraction is in the 
feces. Concerning this latter item it will be remembered 
that we cannot easily say how largely it is a residue signi- 
183 184 
NUTRITIONAL PHYSIOLOGY 
fying incomplete absorption and how far it is a true waste- 
product. If feces are discharged during long fasting the 
nitrogen contained must be the body's own contribution. 
On the whole, the fecal nitrogen is nowadays regarded as 
an excretion unless it is clearly excessive. The nitrogen 
of the perspiration can usually be ignored. 
When we assume all the nitrogen eliminated to have 
come from the decomposition of proteins, a certain'error 
always exists, but it is not great enough to be considered 
in the present elementary treatment of the subject. 
Nitrogen constitutes about 16 per cent, of protein. Ac- 
cordingly, the excretion of 16 grams is taken to stand for 
the destruction of 100 grams of protein. This is not far 
from an average amount when the diet is freely chosen. 
In the opinion of an increasing number of authorities it 
is higher than it should be for the best nutritional condi- 
tion. To arrive at an estimate of the protein metabolized 
we multiply the quantity of nitrogen in the outgo by 6.25 
(an operation which is equivalent to dividing by 16 to 
find 1 per cent, and multiplying by 100 to obtain the total). 
This was a familiar procedure before the erection of res- 
piration chambers made possible a complete survey of the 
metabolism. 
Carbon Elimination.-A subject excreting 16 grams of 
nitrogen may be expected to excrete something like 200 or 
250 grams of carbon in the same period. This will be in 
the respiratory carbon dioxid so largely as to make the 
urinary and fecal carbon appear insignificant. Figures 
from an actual experiment are: 
In the respiration 208 grams. 
In the urine 6 " 
In the feces 11 " 
Total 225 " 
This carbon may have been furnished by all three types of 
body substance-the proteins, fats, and carbohydrates- 
in numberless possible combinations. The amount ot 
protein decomposition has already been fixed at 100 grams. 
Such an amount of protein must have yielded in its falling THE ESTIMATION OF METABOLISM 
185 
apart a quantity of carbon represented by the percentage 
of that element in the compound. The actual percentage 
is about 53, so that in this instance 53 grams of the 225 
may be ascribed to protein as a source. The remaining 
carbon, 172 grams, must have been derived from non- 
nitrogenous material. How far it has come from fat and 
how far from carbohydrate we cannot exactly determine 
without additional data. 
It is of value to know all the circumstances of such a 
trial. If the twenty-four hours under consideration is the 
first fasting day and the diet of the day before has been the 
ordinary one, we may assume that the subject entered 
upon the experimental period with a fair stock of glycogen. 
This will be used rather freely at the outset, but more 
and more slowly as the hours pass. Carbohydrate, accord- 
ingly, contributes largely to the support of the organism 
during the first day of abstinence, and thereafter bears but 
a very small part. To say that the glycogen of the body 
is used up in a single day of fasting would not be correct; 
the fact is rather that the rate of consumption diminishes 
sharply. In proportion to this diminution in the use of 
carbohydrate the fat is called upon increasingly. For any 
day of hunger after the first it is substantially true that 
the individual is living on protein and fat.1 
Let us continue the discussion of our numerical illus- 
tration with the added statement that the day is the second 
rather than the first in a fast. The 172 grams of carbon 
from non-protein material may now be attributed to fat. 
The percentage of carbon in fat is about 77. A simple cal- 
culation (dividing by 77 and multiplying by 100, or, what 
is the same thing, multiplying our first quantity by 1.3) 
gives 223.6 grams of fat as the amount destroyed in the 
1 The following figures are from Publication 77 of the Carnegie 
Institution, 1907: Average metabolism of several subjects on the 
first and third days of fasting, 
I 
III 
Protein 
60s 
78g 
Fat 
135 
155 
Glycogen 
no 
21 186 
NUTRITIONAL PHYSIOLOGY 
body during twenty-four hours. The total metabolism 
is then 100 grams of protein and 223.6 grams of fat. It is 
not safe to conclude from this that the loss of weight 
will prove to be just equal to the sum of the two items. It 
may be found to be either more or less, the result depending 
chiefly on the relation of the income and outgo of water. 
Generally, during a fast there must be a large and con- 
tinuous loss of water from the body. The destruction of 
tissue liberates water in the proportion of nearly 400 grams 
for each 100 grams of protein disintegrated. This water 
tends to be eliminated. If it were not, the percentage of 
solids in the body in starvation would steadily decline. 
Could there be conditions under which all the non-pro- 
tein carbon could confidently be assigned to carbohydrate 
sources? Not in the fasting state nor commonly on a 
mixed diet. The case might be approximated by giving 
ample rations with minimal fat and maximal carbohydrate 
for days together. This is nearly equivalent to the nutri- 
tion of the herbivora. If our supposed human subject 
yielded the amounts of carbon and nitrogen already 
quoted while adequately fed upon protein and' carbo- 
hydrate, we should not be much in error in assuming 
that the carbon from non-protein had been evolved from 
starches and sugars metabolized. Carbon forms about 40 
per cent, of carbohydrate,1 and, if we reckon according to 
the same principle as before, we find that 172 grams of 
carbon could have come from 430 grams of carbohydrate, 
more or less. Under more ordinary conditions of feeding 
-and on the first day of a fast-both carbohydrate and 
fat would share with protein the sustaining of the body's 
activities. 
The Respiratory Quotient.-The modern respiration 
chamber is a small room with impervious walls and care- 
fully controlled ventilation. The carbon dioxid of the 
air drawn off is either determined directly or estimated 
from measured samples bearing a known relation to the 
1 Just 40 per cent, of dextrose, about 42 per cent, of cane-sugar, 
and about 44 per cent, of starch. THE ESTIMATION OF METABOLISM 
187 
total volume. In chambers of the best type the oxygen 
consumption is also ascertained. If we know both the 
carbon dioxid production and the oxygen absorption we 
can, of course, compute the ratio between the two. The 
value of this ratio, based on the volumes and not the 
weights of the two gases, is known as the respiratory quo- 
tient. It is figured by dividing the volume of carbon dioxid 
by the volume of oxygen. So determined it has most of the 
time the character of a proper fraction, or, rendered as a 
decimal, it is less than one. This is a way of saying that 
the carbon dioxid discharged is generally less than the 
oxygen which has disappeared in the exchange. 
Every molecule of carbon dioxid holds combined the 
equivalent of a molecule of oxygen. It follows that if all 
the oxygen were devoted to the formation of carbon dioxid 
the two volumes would be equal and the ratio between 
them would be unity. The failure of a part of the oxygen 
to reappear as carbon dioxid indicates that it has been 
combined in some other way. It has actually gone to form 
the second great respiratory product, the water of the 
metabolism. The interest which physiologists feel in the 
respiratory quotient springs from the fact that it varies 
with the prevailing employment of one kind of material 
or another in the general oxidation which is going on. 
The decimal value of the ratio is elevated in proportion 
to the prominence of carbohydrate in the process.1 It is 
lowest when fat is bearing a principal part. Since, at the 
beginning of a fast, carbohydrate is called upon to meet 
the requirement, while its place is taken by fat a few hours 
later, the respiratory quotient will show a decline which 
marks with exactness the shifting of the current. 
It would be beyond the scope of this discussion to show 
how the respiratory quotient can be made the basis of 
equations which determine how much fat and how much 
carbohydrate are broken down to give a certain output of 
carbon dioxid. Suffice it to say that the possibility exists, 
1 Lusk, "Elements of the Science of Nutrition," Third Edition, 
W. B. Saunders Co., Philadelphia, 1917, p. 61. 188 
NUTRITIONAL PHYSIOLOGY 
and is highly fruitful of results in the quantitative studies 
of nutrition laboratories. The meaning of the respiratory 
quotient is sometimes altered by temporary conditions. 
Perhaps the most interesting of these is the peculiar in- 
crease in carbon dioxid outgo exhibited by an animal which 
is rapidly fattening on a diet rich in carbohydrates. Such 
an animal may show for days together a respiratory quo- 
tient in excess of unity, that is, it produces carbon dioxid 
not accounted for by the observed oxygen intake. This 
extra carbon dioxid is explained satisfactorily as having 
come from the carbohydrate undergoing transformation to 
fat. (See also Chapter XV, page 147.) 
Equilibrium.-Our nearly uniform weight, maintained 
for periods of years, suggest that income and outgo are 
often nicely balanced. Complete equilibrium demands 
strict equality between the income and outgo of water, of 
mineral matter, of nitrogen, and of carbon. The realiza- 
tion of these conditions is not likely, though it is often 
closely approached. Partial equilibrium, that is, equality 
between intake and output for one class of compounds with 
inequality for another, is more common. The most fre- 
quent striking of a balance is between the nitrogen of the 
food and that of the excreta. Nitrogenous equilibrium is 
the rule rather than the exception. The tendency of the 
body to establish this correspondence, in spite of wide 
variations in the diet, was noted long ago. It was said 
that the organism refused to store protein when supplied 
with large amounts of this kind of food. This is true in the 
narrow sense that the body does not add freely to the adult 
measure of its living tissues when offered extra protein. 
Yet, as we have seen, the return of all the nitrogen fed 
does not of necessity mean that the body has retained no 
part of the protein supplied to it. The chief reason why 
there is such a marked disposition to make the output of 
nitrogen equal the income is found in the fact that all the 
amino-acids beyond the small quantity used for protein 
synthesis are deaminized. So long as this is the case, 
raising the nitrogen of the food must result merely in THE ESTIMATION OF METABOLISM 
189 
adding to the urea excreted. The non-nitrogenous resi- 
dues may find more or less permanent lodgment in the 
tissues in the form of glycogen or fat. 
Nitrogen retention is to be expected during growth when 
the protein syntheses are more extensive than the endogen- 
ous decomposition. The recovery from illness or from a fast 
is another instance when the body protein must definitely 
increase. This is really only a special case of growth. A 
liberal protein supply has been demonstrated to accelerate 
the healing of extensive wounds.1 Changes of climate or 
the pursuit of athletic training may encourage some 
degree of protein storage. And without seriously qualify- 
ing what has just been said it may be stated that abun- 
dant nitrogenous food may have the same effect, but the 
nitrogen retention secured by forced feeding is always 
limited to a very small fraction of the protein given. 
The roughly maintained equilibrium, which is, after all, 
a striking example of the adjustments of the organism, is 
to be traced to the singular reliability of the appetite. 
This is the agent which prompts so surely to the taking of 
extra food when one exchanges an inactive life for one of 
bodily activity. The most radical changes in the total 
metabolism are unlikely to lead to lasting variations in 
body weight beyond slight gains and losses, which, by the 
way, are often the reverse of what was anticipated. Ex- 
ercise, which is supported by large oxidation, may even 
result in some increase of weight, showing that the appe- 
tite has rather more than met the precise need of the body. 
The average recruit in a training camp gains weight in 
spite of unwonted and fatiguing exertions. 
Carbon Retention.-When a quantitative comparison 
is made between the compounds in the diet and those ex- 
creted it is not infrequently found that carbon is being 
stored, though the nitrogen of income and outgo may be 
balanced. What can be inferred as to the nature of the 
substance added to the tissues? Just as in the previous 
case where we desired to interpret the meaning of the 
1 Clark, Johns Hopkins Hospital Bulletin, xxx, 1919, No. 339. 190 
NUTRITIONAL PHYSIOLOGY 
carbon loss during fasting, we have to consider the re- 
spective share taken by carbohydrate and by fat. As be- 
fore, it is important to know the condition of the subject 
prior to the trial. If the day is the first of feeding after 
a fast there will be some recruiting of the gylcogen in the 
body, and a part of the carbon retention may be attributed 
to a gain of this material. Otherwise, when carbon is 
stored in the midst of a period of liberal feeding, the prob- 
ability is that fat rather than carbohydrate has been de- 
posited. 
Applying the same factors as in the earlier instance, we 
multiply the retained carbon by 1.3 if the circumstances 
point to its having been held as fat. An excess of 10 
grams of carbon in the income over the outgo would be 
assumed to indicate the addition of 13 grams of fat to the 
supply in the body. An amount of this magnitude would 
not show itself decisively in the weight, being easily dis- 
guised by the temporary gain or loss of water that might 
occur at the same time. 
Starvation.-The statement has been made that when 
food is withheld the system at first makes large use of 
glycogen to supply its needs, but presently must draw 
on its fat reserves. From the first it is at the same time 
breaking down protein. In a prolonged fast the fat of 
the body may be almost entirely used up. When this 
condition is reached an increased destruction of protein 
may set in, and it is a sign that the end of the siege 
is not far off. This is signalized by the "premortal rise" 
of nitrogen excretion. It has been noted in starving 
animals and in tragic fashion in the personal experience 
of Loewy, a German physiologist, reduced by the re- 
strictions of the war.1 * 
The losses suffered in starvation by the various tissues 
are extremely unequal. There is clear evidence that the 
most essential organs are protected from wasting by the 
consumption of those that can be better spared. In a cat 
deprived of food for thirteen days and losing about one- 
1 Lusk, Physiological Reviews, i, 1921, 523. THE ESTIMATION OF METABOLISM 
191 
third of its original weight the fat was reduced by 97 
per cent., the liver by 53, the muscles by 30, and even 
the bones by 14. Yet the heart and the central nervous 
system had scarcely diminished at all. (The losses were 
estimated by comparing the organs of the fasting animal 
with those of a normal cat.) 
Dogs survive starvation for incredible periods. Kuma- 
gawa kept one fasting until its death on the ninety-eighth 
day. Hawk compelled a dog to go without food for one 
hundred and seventeen days, nursed it back to vigorous 
health, and then conducted a second fast of one hundred 
and four days without killing the unfortunate animal. 
The loss of weight in these long periods was about 60 per 
cent. The Russian Awrowrow denied a dog water as well 
as food; death came on the sixtieth day when the body 
weight had been reduced to about two-fifths of the normal. 
Human subjects are said to have fasted for fifty days. 
Several times volunteers have been under close sur- 
veillance for periods of more than a month without food 
and valuable scientific data have been recorded. Per- 
haps the model study of this kind is the one made upon 
Levanzin at the Carnegie Nutrition Laboratory.1 The 
fast continued for thirty-one days. The loss of weight 
was nearly 22 per cent. (The change was from 133 pounds 
to 104.) The study of the excreta indicated that this loss 
must have been the sum of about 8 pounds of fat and 20 
pounds of average nitrogenous tissue which, it should be 
remembered, contains 75 per cent, or more of water. No 
doubt the glycogen present in the beginning was quite 
completely removed. Levanzin was in fair condition at 
the end of the period, though in an unhappy frame of mind. 
1 Benedict, Carnegie Institution of Washington, 1915, Publica- 
tion 203. 
General reference: Sherman, "Chemistry of Food and Nutrition," 
Macmillan, New York, 1923. Also Bayliss, "The Physiology of 
Food and Economy in Diet," Longmans, New York, 1917. CHAPTER XIX 
THE ENERGY OF THE METABOLISM 
The initial statement in this book-that living things 
are transformers of matter and energy-is a text to which 
we have closely adhered. In recent chapters the emphasis 
has been placed upon transformations of matter. We 
shall now pass on to speak of the energy evolved by animals 
and particularly by the human body. The fundamental 
facts are presumably clear. The energy of the income is 
potential in the complex molecules of the food. It is 
released in the oxidative decomposition processes of life 
and made kinetic. It appears chiefly-often solely- 
in the form of heat. Measurements of the heat production 
of living organisms are generally to be accepted as indic- 
ative of the total energy production. Certain exceptions 
to the rule will soon be noted. 
Fuel Values.-Since energy can be transmuted from one 
form to another it is possible to make the units which stand 
primarily for one kind do duty for all. It is our constant 
practice to use the units of heat to measure all the energy 
of metabolism. The unit which we shall employ is the 
large Calorie, approximately defined as the amount of 
energy required to raise the temperature of 1 kilogram of 
water one centigrade degree. The large Calorie is prefer- 
ably distinguished from the small by the capital C. The 
small calorie is of the large; no further reference to 
it will be made. When a combustible organic substance 
of a standard composition is completely oxidized a definite 
quantity of heat is evolved. The heat produced by 
oxidizing 1 gram of any compound is its fuel value. 
The highest fuel value recorded is that of hydrogen, 
about 34 Cal. This is the amount of heat produced when 
192 THE ENERGY OF THE METABOLISM 
193 
a gram of hydrogen gas (11 liters) is oxidized to water. 
One gram of carbon oxidized to carbon dioxid gives nearly 
8 Cal. These two illustrations do not bear directly on our 
physiologic inquiry, for the body does not use the free 
elements for oxidation, but their compounds. It is, there- 
fore, of more interest to turn to the fuel values of carbo- 
hydrates, fats, alcohol, and proteins, since these are the 
actual sources of heat and kindred energy. The calorific 
value of a compound is not precisely that of the carbon and 
the hydrogen contained in it and not yet bonded to oxygen, 
although some early work of a useful kind was based upon 
that assumption. It is to be noted that the oxygen in the 
physiologic compounds reduces their potency; the less they 
contain the more largely they will consist of elements sub- 
ject to oxidation. This is the main reason why fats have 
fuel values greatly in excess of those of carbohydrates. 
A gram of fat contains nearly twice as much carbon as a 
gram of sugar. It also contains much hydrogen with un- 
satisfied affinities for oxygen. 
The actual heat production observed when a gram of 
starch is burned is a trifle more than 4 Cal. Sugars, which 
are slightly richer in oxygen than starch is, have a little 
lower fuel value. The figure (4 Cal.) is fairly representa- 
tive of carbohydrates as a class. The oxidation of 1 
gram of fat liberates about 9.3 Cal., or 2| times as much as 
starch. A gram of grain alcohol fully oxidized gives an 
intermediate quantity, about 7 Cal. All these non-nitrog- 
enous compounds are made to yield the same simple prod- 
ucts, namely, carbon dioxid and water, whether they are 
destroyed by literal burning outside the body or by the 
metabolic processes. We shall see shortly that the energy 
which is found to be set free in their oxidation can be 
proved to be equal in the two cases. 
Protein stands somewhat apart in its behavior. A gram 
of dried protein burned in oxygen gives nearly 6 Cal. 
But some of the products generated in such a laboratory 
test are not those which the body forms from protein and 
excretes. Urea, for example, is not found after the actual 194 
NUTRITIONAL PHYSIOLOGY 
burning of protein. Urea itself has a certain capacity for 
oxidation and a low but distinct fuel value, something 
like 2.5 Cal. per gram. The residual fuel value in urea is a 
sign that some portion of the energy latent in protein is 
constantly lost to the animal economy. Bacteria may 
profit by it, but it seems not to be available for the higher 
living forms. There are certain products other than urea 
which remain after the decomposition of protein molecules 
and which likewise represent unused energy. Some of 
these minor products accompany the urea in the urine, 
while others are mingled with the feces. To make ac- 
curate allowance for the energy lost with these incompletely 
oxidized compounds is a difficult matter. The estimate 
arrived at credits to a gram of protein 4 Cal. or a little 
more. It is a coincidence with very convenient results 
that this is almost exactly the same as the figure for car- 
bohydrate. 
The Total Daily Metabolism.-The widest limits be- 
tween which the metabolism of an adult may vary may be 
set down as 1000 and 10,000 Cal. per day. The lowest 
level will be approached when there is complete rest and 
protection from cold during the twenty-four hours. The 
maximum will be reached, if ever, when a large, powerful 
man performs the heaviest muscular work while under ob- 
servation. The food taken on a single day influences 
the result less than would be anticipated. (As already 
pointed out, the appetite follows the metabolism rather 
than precedes it.) High protein feeding has an effect 
which will be discussed later. 
Of course, the average for an individual will generally 
fluctuate much less than is suggested above. It will 
rarely fall below 1500 or rise much above 3000 under the 
conditions of city life. It is probably reasonable to fix upon 
2400 as a mean. If for sixteen hours of waking we allow 
100 Cal. per hour,"and for eight hours of sleep 60 per hour, 
the estimate adds up to 2080. For the performance of 
any great amount of muscular work a good deal larger total THE ENERGY OF THE METABOLISM 
195 
must be assumed.1 Farmers in widely separated countries 
have been found to employ rations representing about 3500 
Cal. per day. Army rations usually furnish as much as 
4000. Maine lumbermen have reached twice this figure.2 
Active boys in a superior preparatory school (St. Paul's, 
Concord, N. H.) are served with meals that sum up to 
4350 Cal. daily and buy enough outside to bring the 
total to 5000.8 It will be suggestive if we consider how 
much of a single food-stuff would be required to furnish a 
total of the lower order. 
Two thousand Calories could be obtained by the oxida- 
tion of about 500 grams of starch or sugar. The meeting 
of the energy requirement is not the only qualification to 
be demanded of a day's dietary. A ration of pure sugar 
is obviously not to be recommended, though the consump- 
tion of candy to Such an extent may be entirely possible 
for some subjects. The quantity of fat needed to afford 
the same heat value will be in the vicinity of 216 grams. 
One can hardly conceive of eating clear fat to this amount. 
Making a similar calculation for alcohol, we find that a little 
less than 300 grams of this compound will theoretically 
meet the need. A palpable absurdity is apparent. While 
the attempt to use alcohol exclusively as a fuel for the 
body would evidently be disastrous, it is interesting to 
consider that a lamp burning 300 grams of absolute alcohol 
in a day would equal a human being as a source of energy. 
The flame would be a very small one and the compari- 
son gives one a feeling of dissatisfaction and disappoint- 
ment. 
Since protein is nearly equivalent to carbohydrate as a 
source of energy, the theoretic ration of protein wrould be 
the same as that of sugar, namely, 500 grams. The prac- 
1 Benedict has suggested the following standard: 
8 hours' sleep at 65 Cal. 520 Cal. 
12 sedentary hours at 100 Cal. 1200 " 
4 hours' light exercise at 170 Cal. 680 " 
2400 Cal. 
2 U. S. Department of Agriculture, Bulletin No. 149, 1904. 
3 Lusk, loc. cit., p. 559. 196 
NUTRITIONAL PHYSIOLOGY 
tical impossibility of eating so much protein is evident 
when it is remembered that protein is acceptable only when 
softened and expanded with liberal quantities of water. 
Lean meat, which is not strictly a straight protein food, 
but which approaches that composition, is three-fourths 
water. White of egg, in which the solid portion is almost 
all protein, is seven-eighths water. The attempt to dry 
such material and so to reduce it to the smallest bulk 
produces a mass resembling hardened glue or shellac. 
To subsist on the proteins of meat alone, the fat having 
been removed, one would be compelled to eat some five 
pounds daily. Travelers have described certain peoples as 
living almost wholly on meat or fish, but this does not mean 
a pure protein diet, the fat undoubtedly figuring largely. 
The Eskimos necessarily eat but little carbohydrate, for 
they can obtain no vegetable food of importance; it seems 
plain, however, that they have increased both fat and pro- 
tein consumption and not protein alone. 
The foregoing suggestions make clear the inconveniences 
and the unhygienic aspect of any attempt to live on a 
single type of food. It is a fact, furthermore, that one 
could not under any conditions continue indefinitely to eat 
only non-nitrogenous food. Protein metabolism never 
ceases and a certain nitrogenous income must be provided. 
We cannot, therefore, judge the fitness of a diet solely by 
its heat value. It must measure up to a reasonable stand- 
ard in this respect, but it must also include a suitable pro- 
portion of protein. Another criterion, not so commonly 
insisted upon, is that a sufficient quantity and variety of 
mineral compounds shall be supplied. Of course, these 
scientific characterizations of the diet are inadequate unless 
attention is paid also to attractiveness and digestibility. 
When a subject freely chooses his food, unprejudiced 
by chemical knowledge, he is apt to make use of all three 
classes of food-stuffs to a considerable extent. Carbo- 
hydrate will usually amount to more than half the total 
solids of the ration, while protein and fat show a curious THE ENERGY OF THE METABOLISM 
197 
tendency to be taken in nearly equal weights. An ex- 
ample of such a selection, perhaps the one most frequently 
quoted, is the following: 
Protein 100 grams 410 Calories. 
Fat' 100 " 930 " 
Carbohydrate 250 " 1025 " 
2365 " 
Of late years, observation having been extended to large 
numbers of people, it has become evident that Americans 
of student and professional classes rarely choose to eat as 
much as 100 grams of protein. 
Calorimetry.-We have been speaking of the amounts 
of heat set free in the oxidation of various food materials, 
and of the energy liberated by animals and men as an ac- 
companiment of their metabolism. The determination of 
such data requires the use of calorimeters. These are of 
various forms, but have the same underlying principle. In 
case a food sample is to be burned it is enclosed in a 
small chamber, preferably in an atmosphere of oxygen, and 
its combustion is initiated by means of an electric spark. 
The heat is imparted to a large mass of water surrounding 
the chamber and can then be estimated by the elevation of 
temperature observed in this water. This is an elementary 
and somewhat sweeping statement; numerous corrections 
would be necessary in practice. 
One of the earliest attempts to gage the heat production 
of an animal consisted in confining it within a chamber 
having double walls and ice between. The amount of ice 
melted could thus be made the basis for the estimate de- 
sired. The employment of a water calorimeter evidently 
does away with the unnatural chilling which must have 
been entailed in this primitive trial. The large and costly 
calorimeters in modern laboratories for the study of human 
nutrition are of the water type. No adequate idea of the 
difficulty involved in such work is likely to be grasped by 
reading this condensed presentation. Ventilation must be 
maintained and allowance made for the warming of the 
circulating air. The evaporation of water must be ac- 198 
NUTRITIONAL PHYSIOLOGY 
curately measured, for if this were ignored a great deal of 
heat produced in the metabolism would escape measure- 
ment. (In the evaporation of 1 kilogram of water heat to 
the amount of 536.5 Cal.1 will be made latent.) Changes 
in the body temperature cannot be overlooked. If, for 
example, a human subject equivalent in heat capacity to 
50 kilograms of water should experience a rise of tempera- 
ture amounting to 0.5° C. during an experiment, 25 Cal. 
would have been retained in his body. If, instead, there 
were a fall of 0.5 degrees there would have been a discharge 
of 25 Cal., and the observed heat output would have been 
greater than that due to the current metabolism. 
Now the question may be asked whether vze can be sure 
that the observed heat production (corrected for body 
temperature changes and supplemented by the heat loss 
through evaporation) is a just representation of the total 
energy set free. May there not be other forms of energy 
than heat? What are the facts concerning muscular 
work? We cannot say positively that no energy eludes 
the calorimeter, but the impression is constantly being 
confirmed that any such escape must be insignificant. As 
to the energy of movement it can be shown that under most 
conditions this will eventuate in heat. This is an import- 
ant matter and deserves to be carefully illustrated. 
Let us take for an example the case of the heart. This 
organ does a great deal of work in forcing the blood through 
the vessels. Does this work appear as heat so as to be 
arrested and recorded by the calorimeter? There is no 
doubt that it does. The conversion is effected as the re- 
sistance to the blood-flow is encountered and overcome. 
The heart produces some heat within itself, and some addi- 
tional heat due to its metabolism is made to appear in all 
parts of the body. Wherever the blood is driven there is 
friction, which is a means of transforming the energy of 
motion into heat. The same statement applies to the 
breathing movements. Work is done, in the physical sense 
1 This is the figure at the freezing-point. It increases slowly with 
rising temperature. THE ENERGY OF THE METABOLISM 
199 
of the term, each time the ribs are lifted, but with their 
return to the expiratory position this work is reconverted 
into heat. So, too, all ordinary forms of exercise may be 
shown to result in heat production. 
Nevertheless it is possible to devise conditions under 
which a part of the metabolic energy will not be given to 
the calorimeter. Suppose, for example, that the subject 
within the apparatus is employed in taking books from the 
floor and placing them upon shelves. As long as he pur- 
sues this form of activity a share of his evolved energy will 
become potential and be lost to direct observation. It 
may be said to exist as "energy of position" in the mass 
which he has elevated. In other words, it has been stored. 
If, after a day of this labor, he should occupy himself in 
removing the books from the shelves and laying them upon 
the floor, he would be giving to the calorimeter more heat 
than that actually produced as a result of his metabolism. 
When the books had all been returned to the original level 
the sum of the calories for the two periods would justly 
represent the energy production of his body. One could 
conceive of two calorimeters placed side by side, in one of 
which a man might turn a crank operating a shaft which 
should pass into the second chamber and there revolve a 
wheel against the resistance of a brake. Most of his energy 
would be registered by the first calorimeter, but a fair pro- 
portion of it, standing for most of his muscular work, would 
be apparent in the second. 
Direct and Indirect Calorimetry.-We have said else- 
where that the proof of the validity of the principle of the 
conservation of energy for living things was a great achieve- 
ment of the nineteenth century physiologists. The 
method of this proof may now be outlined. We have just 
seen that the total energy production of animals, including 
man, may be satisfactorily measured in calorimeters, 
provided that the amount of evaporation is known and 
any changes of body temperature considered. This pro- 
cedure is direct calorimetry. Now, of course, it is possible, 
during the same period to collect the excreta and to deter- 200 
NUTRITIONAL PHYSIOLOGY 
mine the character and the amount of the metabolism 
according to the principles explained in the last chapter. 
Knowing the metabolism-so many grams of protein, of 
fat, and of glycogen destroyed-we may credit to each of 
these its respective fuel value and calculate the number of 
calories which could theoretically result from just such 
oxidation. We may then compare the energy as deter- 
mined from the living organism and the energy which 
should have been liberated in the formation of the measured 
wastes. 
To make the matter plain, we may refer once more to the 
case on which the numerical estimation of metabolism 
was previously based. The subject was credited with a 
metabolism of 100 grams of protein and 223.6 grams of fat. 
(This, it may be recalled, is the usual assumption when 
the day is one of fasting and does not closely follow carbo- 
hydrate feeding.) The apparent heat value of the calcu- 
lated metabolism will be as follows: 
100 grams of protein at 4.1 calories = 410 Calories. 
223.6 grams of fat at 9.3 " = 2079 " 
Total 2489 " 
This bit of reckoning is an instance of indirect calorimetry. 
It is found in practice that direct and indirect calorimetry 
lead to results so nearly identical as to make it certain that 
the discrepancy between them is accidental. The two 
figures differ in modern work by less than 1 per cent. 
The correspondence between the observed and the cal- 
culated calories means this: that the animal displays no 
powers of movement or heat production which cannot be 
referred to the oxidation of organic compounds in its body, 
and so eventually to the stores of chemical energy in its food. 
It is an engine, a transformer, and not a creator of energy. 
A glimpse of this conception came within the vision of the 
brilliant Count Rumford more than one hundred years ago. 
He had become convinced that heat must be a form of 
energy rather than a substance.1 His conclusions had been 
1 Jordan, "Leading American Men of Science," Holt, New York, 
1910. THE ENERGY OF THE METABOLISM 
201 
drawn in part from the observation that there seemed to be 
no limit to the quantity of heat obtainable from small 
masses of metal in the process of drilling. With a flash 
of insight he looked at the horse which was walking in a 
circle to move the drill, and queried whether the heat in the 
iron turnings might not be a derivative of the muscular 
work of the animal, and whether the same energy had not 
in some sense pre-existed in the food given to it. The 
verification of his induction was long deferred, but has at 
length been made entirely conclusive. The credit for 
this demonstration is shared by many workers, but most 
conspicuously by Rubner, in Germany, and Atwater, in 
this country. 
Let us return once more to our quantitative illustration. 
If the day of the experiment had been one of ample feeding 
instead of fasting, and the diet had consisted of protein 
and carbohydrate, the metabolism might have been held 
to have involved these materials rather than protein and 
fat. The supposition has already been entertained and 
may now be made a basis for indirect calorimetry. From 
p. 188 we copy the amounts of protein and carbohydrate, 
multiplying by 4.1, the value common to both: 
100 grams protein 410 Calories. 
430 " carbohydrate 1763 " 
Total 2173 
Comparing this sum with the 2489 Cal. previously de- 
termined, we see that for a given production of carbon di- 
oxid the evolution of heat will be greatest when fat is the 
exclusive non-protein source, and least when the metabo- 
lism is as nearly as possible on a carbohydrate footing. 
The difference is some 15 per cent. 
If both carbohydrate and fat had participated in the 
decomposition underlying the observed excretion, the heat 
of the metabolism might have had any value intermediate 
between 2173 and 2489 Cal. The figure obtained by 
direct calorimetry might in such a case of mixed metabolism 
be used to determine the part borne by carbohydrate and 
fat respectively. But, as indicated before, the respiratory 202 
NUTRITIONAL PHYSIOLOGY 
quotient can be used to solve the problem. Direct calor- 
imetry is rarely resorted to in these days. 
There is a fair proportionality between the consumption 
of oxygen and the energy set free, whatever the assortment 
of the materials undergoing oxidation. While fat has a 
much higher fuel value than carbohydrate, it also requires 
much more oxygen for the catabolism of an equal quantity. 
Lavoisier's plan of measuring metabolism on the basis of 
oxygen absorbed did not entail any gross errors. The 
simpler forms of apparatus for determining metabolism 
depend on this principle. They are much used in hos- 
pitals.1 
1 Benedict, Boston Medical and Surgical Journal, clxxxviii, 1923, 
567. CHAPTER XX 
THE FACTORS WHICH MODIFY METABOLISM 
The circumstances which radically affect the quantity 
and kind of decomposition going on in the body must al- 
ready be evident. There can be no hesitation in placing 
first among these conditions muscular activity. Minimal 
metabolism attends the most nearly complete state of 
rest which can be secured. It is somewhat lower during 
sleep than during a like period of lying awake, doubtless 
because the waking subject cannot so successfully abolish 
muscular contraction. When one is sleeping metabolism 
obviously continues in certain of the breathing muscles 
and the beating heart. We cannot doubt that it proceeds 
also in the glands, the muscular coats of the alimentary 
canal, the nerve-centers, and many other localities. 
Basal Metabolism.-This expression has come to be a 
very familiar one in medical circles of late. What is 
meant is the rate of heat production in a state of rest and 
in what is called the "post-absorptive" condition, that 
is to say at a time when the digestion and absorption of 
the last meal can be assumed to be complete. This must 
be realized twelve to fifteen hours after eating. The 
subject must be relaxed, comfortably warm, but awake. 
The metabolism is determined by indirect calorimetry 
as explained in the preceding chapter or, quite satis- 
factorily, by a calculation based on the measured con- 
sumption of oxygen. 
The result of such trials is usually expressed in Calories 
per square meter of body surface per hour. The normal 
for an adult male is well established; it is close to 40. 
A variation of 10 per cent, upward or downward is not 
regarded as significant. The extent of the body surface 
203 204 
NUTRITIONAL PHYSIOLOGY 
has been estimated by ingenious and laborious procedures 
and formulae have been deduced which make it possible 
to obtain it approximately for any individual whose 
weight and height are known. An average value is 1.75 
square meters. Evidently the basal metabolism for a 
man of this stature must be 40 X 1.75 or 70 Calories per 
hour. 
Age and Basal Metabolism.-In the newborn the basal 
figure is below the adult quantity, but within a few months 
it rises to exceed it. At five years of age it is near 60, at 
twelve it is still as much as 50. These determinations have 
showed us that children have a high food requirement for 
bare maintenance over and above which we must provide 
liberally for their growth and great muscular activity. 
With advancing age the basal rate very gradually declines 
and in the octogenarian it may have fallen to 35. The 
standard of 40 Calories per square meter per hour, quoted 
as applying to the vigorous adult male, is not reached by 
the undernourished. Prolonged fasting has been known 
to bring it down to 28 or thereabouts. 
Sex.-The basal rate for a woman is about 7 per cent, 
below that for a man, about 37 against his 40. This 
probably does not mean that the protoplasm of the female 
body is less active than that of the male; it means rather 
that the amount of living tissue is less in the body of a 
woman than in that of a man having the same surface. 
If we compare average types it is plain that adipose tissue 
is more abundant in the female. The muscles are dis- 
guised by its presence so that they are inconspicuous even 
in very strong women. Adipose tissue has little metab- 
olism of its own: it follows that a subject having much 
of it is not so large in terms of active protoplasm as the 
lean individual of equal height and weight. 
The Ductless Glands and Basal Metabolism.-A large 
body of data secured in recent years has proved that the 
basal metabolism rises and falls with changes in the de- 
livery of the internal secretions by the tissues which 
prepare them. This is most firmly established in the case THE FACTORS WHICH MODIFY METABOLISM 
205 
of the thyroid, an organ of which more will be said in 
Chapter XXVI. Its activity is known to be under the 
influence of the nervous system. An excessive production 
of the thyroid hormone increases the rate of metabolism. 
In some instances of this kind the basal figure has been 
nearly doubled. Subjects who are oxidizing their bodily 
stores so rapidly can hardly be kept from progressive loss 
of weight. They have to eliminate so much heat that their 
skins are constantly warm and moist. They cannot toler- 
ate heavy clothing. Less commonly cases of the opposite 
type are seen in which the thyroid has a subnormal 
activity. The skin is cool and dry, there is inertia and a 
tendency to gain weight, while the basal metabolism may 
be 20 per cent, or more below the standard. The adrenal 
is another organ which affects the rate of decomposition. 
Feeding and Metabolism.-When food is being ab- 
sorbed the metabolism according to our definition is no 
longer basal. It is definitely elevated. But the effect of 
food is, in general, so much less than the effect of muscular 
activity as to be readily concealed by changes in the 
exercise taken. Thus, while it is broadly correct to say 
that a fasting man metabolizes less material than a man 
who has abundant food, the relation will be immediately 
reversed if the starving subject is compelled to work more 
actively than his fellow. An animal which is denied food 
is economical in its metabolism, but the economy is 
secured chiefly through its decided tendency to remain 
quiet. When the influence of muscular activity is ex- 
cluded so far as possible we discover some suggestive facts 
with reference to the different food-stuffs. 
Suppose that a man's metabolism is determined for 
two successive days, the first of fasting, the second of 
feeding, without increased exercise. Let us assume that 
the metabolism of the first day turns out to be 2000 Cal- 
ories. The food allowance for the second day may be 
designed to supply 2000 Calories. (This must be the net 
value of the ration, the amount actually assimilated.) 
It can be predicted that the metabolism of the second 206 
NUTRITIONAL PHYSIOLOGY 
day will not remain at 2000, but will rise somewhat in 
consequence of the taking of food. The increase will not 
be striking; the new total may be something like 2200 
Calories. A far greater increment would have resulted 
from the performance of moderate muscular work. The 
precise extent of the advance in metabolism following 
feeding will be conditioned by the make-up of the diet. 
If protein is a prominent feature the acceleration will be 
more evident than if the food furnished is mainly non- 
nitrogenous. 
The increase of metabolism above the fasting level when 
the mixed diet supplied has the value of the apparent 
fasting requirement-2000 Calories in the hypothetical 
case above-may be, as indicated, in the vicinity of 10 
per cent. If the food fed is practically all carbohydrate 
the ensuing rise of metabolism will probably be less than 
this, perhaps 6 per cent. A similar limited effect follows 
the consumption of fat. When an animal is given protein 
to cover its theoretical need the heat production for the 
next twenty-four hours may be augmented by 30 per 
cent, or more. The facts are expressed by saying that 
protein has a high specific dynamic effect compared with 
the non-nitrogenous foods. Protein is abundant in those 
articles of diet which have the name of being heating foods. 
The description is seen to be accurate. The eating of 
much meat in warm weather must add to the discomfort 
of the eater. The consequence is something like that of 
opening a furnace draft when the house is already too hot 
for the pleasure of its tenant. On the contrary, high pro- 
tein rations are appropriate for those who have to face 
cold weather. 
It is not clear just how the specific dynamic effect of 
feeding is brought about. The obvious suggestion is that 
it is an expression of the work required of the alimentary 
tract for the digestion of the ration. It might be supposed 
that the motor activities of the organs and the secretory 
processes taking place at the same time would call for 
increased oxygen supply and result in greater production 207 
THE FACTORS WHICH MODIFY METABOLISM 
of carbon dioxid. But this does not seem to be the case. 
The canal may be made to work upon innutritions material 
to the full extent usual with normal food; it can be stim- 
ulated by cathartics and yet no increase of metabolism 
is discovered. The whole mechanism operates with re- 
markable economy. It is not digestion itself, but the 
absorbed products which must be assumed to speed up 
the oxidations in the body. 
Metabolism and Work.-Very moderate activity suffices 
to double the basal rate of heat production. More vigor- 
ous exercise raises the figure in many cases to 200 or 300 
Cal. per hour, while we have records of as much as 600 
Cal. per hour. Waller has made many measurements of 
the metabolism of men variously employed. The calcu- 
lation was based upon the quantity of carbon dioxid 
exhaled through a mouth-piece into a "Douglas bag," a 
container expanding like a pair of bellows. Thus tested, a 
longshoreman doing time work produced 284 Cal. and on 
a piece work basis 430 Cal. per hour.1 The highest records 
have been made by a professional bicycle rider exerting 
himself to the utmost upon a stationary bicycle inside a 
calorimeter. The total for a day observed upon an athlete 
working to the limit of endurance may reach 9000 Cal. 
Changes so extreme as these could hardly fail to attract 
early attention. Lavoisier in the eighteenth century 
noticed that exercise led to increased consumption of 
oxygen. Everyday experience of the heating and the 
"winding" effect of activity would lead to the clear im- 
pression of heightened metabolism. 
When it had become plain that muscular contractions 
must involve increased destruction of material, the ques- 
tion arose as to the nature of the substance sacrificed. 
Chemical teaching in the middle of the nineteenth century 
was dominated by the influence of Liebig. It was his view 
that the performance of muscular work could be supported 
only by the consumption of protein. The impression 
was a natural one, since muscle is so largely composed of 
1 Journal of Physiology, lii, 1919, fix. 208 
NUTRITIONAL PHYSIOLOGY 
protein. He recognized that carbohydrates and fats were 
oxidized in the respiratory process, but held that heat 
alone and not movement resulted from their metabolism. 
The distinction which Liebig attempted to draw between 
the service of protein and non-protein material was des- 
tined to be wiped out in consequence of a certain memorable 
trial. The reference is to the so-called "Faulhorn experi- 
ment" of Fick and Wislicenus. 
By the year 1860 the conception of the convertibility 
of energy from one form to another had become familiar. 
A valuable datum, the mechanical equivalent of heat, had 
become available. (One Calorie is equal to the energy 
consumed in raising 426.5 kilograms 1 meter.) The fuel 
values of a number of substances were known. Two young 
scientists, Fick and Wislicenus, conceived a project for 
testing the prevailing belief in the unique service of pro- 
tein as the support of muscular activity. They knew that 
the nitrogen of the urine would afford the approximate 
measure of the protein undergoing destruction in a given 
interval. If a known amount of work were done it would 
be possible to find out whether the protein consumed in 
the same time would account for all of it. 
They ascended the Faulhorn, a mountain rising 1956 
meters above the lake at its foot. In reaching the top 
each experimenter must have done an amount of work 
represented by the product of his weight by the vertical 
height attained. (Much additional work must have been 
done also-by the heart, the breathing muscles, and in the 
execution of other movements-for which no credit is 
given in the calculation.) The figured work, therefore, 
was a minimum quantity. For Fick it was 129,096 kilo, 
grammeters; for Wislicenus, a heavier man, it was 148,656. 
The investigators collected their urine while on the path 
and for some hours after completing the ascent. They ate 
only non-nitrogenous food, that the excretion might not be 
increased merely as a result of diet. The nitrogen found 
in the urine indicated in each case the destruction of be- 
tween 30 and 40 grams of protein. The highest theoretic 209 
THE FACTORS WHICH MODIFY METABOLISM 
contribution which such a quantity of protein could 
have made to the work done would have been only half 
the known total, and, of course, a smaller part of the actual 
performance. The inference could not be escaped; some- 
thing other than protein had been used as a source of 
muscular energy. 
The figures obtained by Fick and Wislicenus have been 
revised and corrected by critics in view of later discoveries, 
but their chief significance has never been modified. At 
the time of this celebrated experiment the respiration ap- 
paratus at Munich had not been built. As soon as this 
plant was in operation under the direction of Voit and 
Pettenkofer it became possible to learn much more about 
the effects of exercise upon metabolism. It was soon es- 
tablished that instead of being the sole fuel employed to 
furnish muscular energy, protein is not even the principal 
one. The nitrogen excretion of a man on a mixed diet 
does not increase notably in a period of work as compared 
with what it is during rest. On the other hand, his out- 
put of carbon dioxid, like the calories, is a reliable index of 
the degree of his activity. The contracting muscles evi- 
dently employ non-nitrogenous material for oxidation when 
the usual conditions of supply are maintained. 
The question which naturally follows is as to whether 
carbohydrate or fat is the preferred fuel. This cannot be 
discussed at length, but it seems fair to say that both may 
be used to good purpose and with nearly equal economy. 
There is some ground for thinking, as already noted 
(p. 38), that fat must undergo transformation to sugar 
or a nearly related substance before it can perform this 
function. The herbivorous animals may be assumed to 
work most of the time at the expense of carbohydrate 
oxidized. The carnivora use much more fat, though it 
is to be borne in mind that the large quantities of protein 
which they consume are a source of sugar within the body. 
Gram for gram, it must be remembered, sugar and fat are 
not equivalent. One gram of fat has the energy of approxi- 
mately 2| grams of carbohydrate. Quantities of the two 210 
NUTRITIONAL PHYSIOLOGY 
food-stuffs which are in this proportion (and hence equal in 
fuel value) are said to be isodynamic. The principle may 
be illustrated as follows: 50 grams of fat is withdrawn from 
a day's ration and 113 grams of starch substituted. A 
simple computation will show that the heat value of the 
diet has neither been increased nor diminished. The 
calories lost through the removal of the fat are 465, while 
those introduced with the starch are practically the same. 
Of course, the possibility of such substitutions is much 
limited by considerations of palatability and individual 
variations of digestive power. 
The fact that carbohydrate and fat are freely used to 
yield energy for muscular movement does not exclude pro- 
tein from such service. A carnivorous animal can be kept 
for a long time strong and active upon a diet composed 
almost wholly of protein. Yet it remains possible that 
carbohydrate is the chosen fuel during feeding of this 
kind. When enough protein is given to secure nitrogen 
equilibrium and to furnish energy to the full extent needed 
for the work done, the uncombined amino-acids must give 
rise to abundant sugar, and it may well be that this is the 
chief factor in the muscular metabolism. An engine is 
made of iron and steel, but its regular fuel is coal; a muscle 
is made essentially of protein, but protein is not its usual 
fuel. Protein food must be needed in definite amounts for 
the original development of muscles and also for the in- 
crease in their substance under conditions of training. It 
is not clear that any considerable supply is needed for their 
best working when the desired level of development has 
been reached. This distinction between growth and 
operation has important bearings upon theories of hygiene. 
The Influence of Cold.-Exposure to cold greatly in- 
creases the non-nitrogenous metabolism of men and all 
warm-blooded animals. But when we observe their reac- 
tions closely we find that, for the most part, they constitute 
a special case of muscular activity. The organism is 
stimulated to reflex responses, among which we recognize 
shivering, general tenseness, and briskness of movement. THE FACTORS WHICH MODIFY METABOLISM 
211 
There is some evidence of a moderate rise of heat produc- 
tion not associated with muscle contraction in response 
to external cold. This matter will be explained more 
fully in Chapter XXI. 
Mental States and Metabolism.-After hearing of the 
striking correspondence between the degree of muscular 
work and the extent of metabolism, one is disposed to ask 
what are the facts regarding mental work. When it is 
said that mental states have no distinct influence, apart 
from that which is secondary to the changes in tissue 
activity which may attend them, a feeling akin to disap- 
pointment is often manifested. Yet a little reflection will 
convince one that positive effects are scarcely to be ex- 
pected. The central nervous system, important as it is, 
constitutes, after all, only 3 or 4 per cent, of the body. 
Most of its substance is stable and relatively passive in its 
nature. If its really active cells were to double their 
average metabolism the addition to the heat production 
and carbon dioxid elimination of the subject would not 
be significant in the totals observed. A slight change in 
the state of the muscles would suffice to offset and dis- 
guise it. 
An emotional experience is much more than a cerebral 
phenomenon. In times of excitement the skeletal muscles 
are played upon by the nervous system and metabolism 
in consequence may be largely increased. But this is not 
the direct result of the brain process; it is merely a special 
case of muscular activity. An emotion, pleasurable or the 
reverse, is a kind of exercise and often one of marked inten- 
sity. This is recognizable in the increased heart action, 
in the quickened breathing, and in the contractions which 
bring about characteristic attitudes and expressions. 
Mental application of a kind which can be more or less 
successfully separated from these muscular accompani- 
ments cannot be shown positively to affect the metabolism. 
A short time ago a trial was made at Middletown, Con- 
necticut, in which a group of students in Wesleyan Uni- 
versity took bona-fide examinations in a respiration cham- 212 
NUTRITIONAL PHYSIOLOGY 
ber which was at the same time a calorimeter. Each man 
took his turn, spending three hours over his paper and 
experiencing the usual anxiety and strain attendant on 
such a proceeding. On other days each subject spent a 
like period in the chamber engaged in copying printed 
matter. Thus it was possible to compare in about twenty 
cases the metabolism of a period of brain work with the 
metabolism during a time in which similar muscular move- 
ments were made, but in the absence of conscious effort. 
There was no distinct difference. 
More recently a similar result was obtained in the case 
of proof-readers.1 These people certainly concentrate to 
good purpose, and it will be agreed that their work is exact- 
ing. But it is pursued by the adept with a minimum of 
postural strain; in fact, it is consistent with very complete 
relaxation. Waller and de Decker found no change in the 
metabolism when their subjects entered upon or discon- 
tinued work. 
Brain cells undoubtedly have peculiar metabolic prod- 
ucts and make demands upon the blood for supplies of a 
somewhat different order from those required by any 
other tissues. But their distinguishing wastes can hardly 
be recognized when mingled with the outgo from so many 
other organs, and it is equally difficult to discover just 
what they appropriate for their nutrition. The notion 
that certain articles of diet are brain "foods" rests on very 
unsafe assumptions. The popular association of phospho- 
rus with the brain and its activity has no more justifica- 
tion than could be claimed for sulphur or any element 
present in the proteins. 
From what has been said it will be evident that the diet 
must be increased for the support of muscular work, but 
that no more food is needed for the student occupied with 
his books than for the same man at leisure. The chances 
are that his leisure days will be spent in less sedentery 
fashion than his days of application, and that his appetite 
will lead to a larger consumption in his so-called resting 
time. 
1 Journal of Physiology, liii, 1920, cv. THE FACTORS WHICH MODIFY METABOLISM 
213 
Maximum and Minimum Excretion of Carbon and 
Nitrogen.-By way of summarizing certain facts of im- 
portance the following statements may be made: 
The maximum excretion of carbon results from the 
performance of the heaviest muscular work. Carbon 
excretion is little affected by feeding. 
The minimum excretion of carbon occurs when rest is as 
complete as possible. Protection from cold favors a low 
carbon secretion chiefly because it permits muscular relax- 
ation. 
The maximum excretion of nitrogen results from high 
protein feeding. Nitrogen excretion is little influenced 
by muscular activity. 
The minimum excretion of nitrogen occurs not in fast- 
ing, as might be hastily concluded, but when non-nitrog- 
enous food is liberally supplied. CHAPTER XXI 
THE MAINTENANCE OF THE BODY 
TEMPERATURE 
All animals are producers of heat, and must, therefore, 
maintain temperatures above those of their surroundings 
unless the evaporation of water more than compensates 
for the heat of metabolism. We speak, however, of warm- 
blooded and cold-blooded animals as though some were 
far more liberal than others in their calorific output. 
This is actually the case. But a better distinction be- 
tween the two classes may be found in the fact that some 
animals allow themselves to be warmed and cooled readily, 
while others keep near to a constant standard of internal 
temperature. We call a frog a cold-blooded animal, but 
what we really mean is that it accepts for its own the tem- 
perature of its environment. On a very hot day the frog 
may be as warm as a man; in winter the frog is chilled 
through and through, while the tissues of a human being 
everywhere save at the surface are kept at the same tem- 
perature as in summer. 
Animals which have a fixed standard to which they ad- 
here under all ordinary conditions are spoken of as homo- 
thermous. This trait is shared by birds and mammals. 
Our interest, of course, centers in man's remarkable capac- 
ity to keep his deeper tissues at the same level in spite of 
radical external changes and equally sweeping changes in 
his own metabolism. The means by which this result is 
attained differ somewhat with different animals of the 
homothermous order. We shall confine our attention 
almost entirely to the human problem. 
The common clinical thermometer bears the mark 
"Normal" opposite the point on its scale corresponding to 
214 THE MAINTENANCE OF THE BODY TEMPERATURE 
215 
98.6° F. (In all the following discussion the familiar 
Fahrenheit system will be used.) The standard just 
quoted is for the mouth. The body temperature is often 
estimated by placing the thermometer elsewhere, as in the 
armpit or the rectum. The rectal temperature is the most 
reliable, and is usually higher by half a degree, or a little 
more, than that of the mouth. It is clear that the tem- 
perature of the skin cannot be a constant one; it is affected 
both by the external conditions and by the variations 
of blood-flow in the superficial vessels. 
There are small but distinct changes of body tempera- 
ture during each day. The lowest figures are noted early 
in the morning before one has become active and when the 
sense of prostration is apt to be overpowering. A gentle 
upgrade is maintained until the maximum is reached in the 
late afternoon or early evening. The average extent of the 
rise is rather more than 1° F. It coincides suggestively 
with the temperamental change, which is to be observed 
from a prosaic and literal frame of mind to a condition of 
emotional instability. Compared with the morning, the 
evening is a feverish period. When there is actual fever 
the diurnal ascent of the temperature often adds consider- 
ably to the restlessness and discomfort of the patient as 
night approaches. 
After we have made allowance for such irregularities it 
remains true that the approximate constancy of the tem- 
perature is one of the most wonderful facts which the 
physiologist has to explain. Uniformity of temperature 
implies equality of heat production and heat loss. Our 
task, then, is to show how this equality is continued in the 
face of variable factors tending to disturb it. Artificial 
contrivances for keeping constant temperatures in cer- 
tain chambers may be considered with advantage before we 
attempt to analyze a mechanism so much more intricate 
than they. There are two principles on which incubators 
or thermostats may be operated: (1) The heat given to the 
system may be increased or diminished to keep pace 
with the heat lost; (2) a constant supply of heat may be 216 
NUTRITIONAL PHYSIOLOGY 
provided and adjustments made to favor its escape or to 
conserve it, according as the tendency is toward a rise or 
fall in temperature. 
The first principle is illustrated by those thermostats in 
which gas flames are automatically caused to rise when the 
apparatus begins to cool off and are cut down when it 
begins to be warmed above the intended level. The 
second way of securing the same result is exemplified by 
the common incubator used for hatching eggs, in which the 
heat is furnished by a lamp kept burning at the same height 
at all times, while a ventilator, opening and closing, dissi- 
pates or retains the heat as required. In the living 
body we can recognize the working of both these princi- 
ples, but it is rather surprising to find how much is ac- 
complished by the second without the aid of the first. 
In other words, the constant internal temperature is 
maintained much of the time by the promotion or the 
retarding of heat loss without any appeal to the tissues for 
metabolic support. 
Our practice of adapting our clothing to the season and 
to in-door and out-door life is an extension of the means 
which the organism itself employs for the same purpose. 
Extra clothing hinders the escape of heat from the body 
and makes possible the maintenance of the normal state 
with no increase of oxidation in spite of some degree of 
external cold. A race of naked savages must certainly 
vary the amount of their metabolism much more positively 
from summer to winter than we do with the habits of our 
civilization. Animals may enjoy the protection of heavier 
coats in cold weather, and they show, moreover, an instinct 
to assume those positions that reduce to a minimum their 
surface exposure. 
To explain in detail how changing circumstances are 
met, let us imagine a man placed successively in atmo- 
spheres of different temperatures. We will begin with a 
room at 68° F., a condition which we regard as agreeable 
and which we aim to produce when artificial heating is in 
use. If our subject has spent an hour in this room he has 
very likely had a metabolism of 100 Calories, and he has in THE MAINTENANCE OF THE BODY TEMPERATURE 
217 
the same time discharged by radiation, conduction, and 
evaporation a similar amount of heat. His blood is some 
30 degrees warmer than the air around him. Let him now 
take his place in a room where the thermometer stands at 
84° F. Suppose him to remain for an hour in this disagree- 
ably warm apartment. His temperature will be found to 
rise but little, perhaps not at all. Yet the change has re- 
duced the difference between his own temperature and that 
of his surroundings from about 30 degrees to about 15. 
The tendency of his environment to withdraw heat from 
his body must have been halved. What, then, has 
happened? Has his metabolism fallen to 50 Calories or has 
there been a readjustment to facilitate the removal of the 
full 100 Calories? The latter is found to be the case. 
The withdrawal of heat from the body is favored by two 
reflex changes. One of these consists in an increase in the 
amount of blood flowing through the skin and thus exposed 
to the cooling influence of the outside air. The other is 
seen in the breaking out of perspiration. The evaporation 
of the water thus brought to the surface cools the skin and 
the blood beneath it. The blood then mingles with that 
which has been passing through the deeper tissues and the 
rising temperature is checked. It is well to insist just here 
that the appearance of the skin gives little indication of the 
rate at which the sweat is being secreted. So long as evap- 
oration keeps up with the arrival of the water at the pores 
there will be no visible moisture. We notice the perspira- 
tion most when it is failing to accomplish its object, that is, 
when it accumulates instead of being vaporized. The 
evaporation of water from the respiratory passages is, of 
course, one means of removing heat, but with human 
beings this is not a quantity which increases with external 
warming. It does enter into the adaptive reaction in the 
case of animals which pant. 
Let us now transfer our subject to the unusual tempera- 
ture of 105° F. The radiation and conduction effects will 
now be reversed; the blood will tend to be warmed rather 
than cooled as it approaches the surface. The metabolism 
will continue unabated. The body is thus exposed to 218 
NUTRITIONAL PHYSIOLOGY 
warming from without, while it does not cease to heat itself 
from within. How can it escape a steady rise of its in- 
ternal temperature leading to prostration and death? 
Benjamin Franklin answered this question when he pointed 
out that the sole resource of the organism in such a situa- 
tion must be its power to evaporate water. To dispose of 
100 Calories in an hour by evaporation alone demands the 
secretion of about 200 grams of water in the same time, an 
amount which can readily be produced. 
When the air is warm the humidity has much to do with 
human power to endure the condition. If there is full 
saturation and a temperature as high as that of the blood 
the heat of metabolism will be pent in the body and heat- 
stroke is inevitable if there is not a prompt relief; 100 Calor- 
ies added to the average human body in an hour will raise 
its temperature by nearly 4° F. A second hour of such 
an upgrade could hardly be survived. Men may live and 
work for several hours on a stretch in dry air with the tem- 
perature around them as high as 130° F., but they cannot 
be active in saturated air at 90° F. The first of these 
conditions is realized in the stoke-holds of ocean steamers; 
the second, in certain deep mines. Everyone knows that 
the most trying weather we have to put up with is not 
that which makes the record for the mercury, but those 
days which are less warm, but which we describe as muggy 
or sticky. We are acceding to a correct instinct when we 
are relaxed and indolent under such circumstances. 
We may now return to the starting-point and submit our 
imaginary victim to temperatures lower than 68° F. If 
he is taken to a room where the thermometer is at 60° F. 
he will probably feel chilly. We are affected more by a 
slight change in the vicinity of 65° F. than in any other 
part of the scale. The fact is, apparently, that when the 
external temperature is cut down from 68° to 60° F. the 
skin temperature, on which our sensation depends, is re- 
duced by a good deal more than 8 degrees. This is due to 
the reduction in the volume of blood in the cutaneous 
vessels. It is the expression of the endeavor of the organ- 
ism to economize to the utmost its outgo of heat. Dis- THE MAINTENANCE OF THE BODY TEMPERATURE 
219 
comfort is permitted to develop as an incident of the adap- 
tation. The metabolism still remains about as it has been 
throughout the series of trials. 
If the room temperature is now lowered decidedly and 
no wraps are provided the body can no longer maintain 
itself by mere economy of heat loss. It must shift from 
the method of the incubator with a constant flame and 
an adjustable ventilator to the other form of regulation- 
that of the thermostat with variable flame. That is to 
say, the metabolism must be stimulated. A sign of this 
rallying on the part of the heat-producing tissues is seen 
in the onset of shivering. This is obviously a form of 
muscular exercise, and as such is attended with increase of 
metabolism. When we resist the impulse to shiver, as 
we sometimes do, we merely adopt another kind of con- 
tractile activity with its accompanying contribution of heat 
to the body. If we analyze the experience of being cold 
we find that we can recognize a disposition to muscular 
tenseness, while it is familiar enough that when the cold 
is severe we cannot keep from moving briskly and so sup- 
plying the necessary heat. It is desirable to reiterate that 
the muscles are not merely organs of movement, but our 
main reliance for heat production. Cold weather generally 
means large metabolism, but the connection is indirect; the 
increase is largely a special case of the rise always asso- 
ciated with exercise. So, too, the increase of appetite 
which is usual in winter is secondary to the greater use 
of the muscles. 
Humidity makes for discomfort in cold as well as in 
warm weather. It seems at first unreasonable to say that 
moisture can make us more sensitive to heat in summer 
and also to cold in winter, yet this is true. The climate of 
our Atlantic coast is notorious for the "penetrating" char- 
acter of its cold, and this in spite of the fact that the 
thermometer does not fall so low as it does a short distance 
inland. The paradox is easily explained. We have said 
that high humidity in hot weather interferes with our com- 
fort and efficiency by hindering free evaporation. In 220 
NUTRITIONAL PHYSIOLOGY 
winter it has no such influence, because even though the 
cold air is fully saturated it ceases to be so when it has been 
warmed by contact with the skin. Evaporation can, 
therefore, never be retarded seriously by moisture in cool 
air. What we notice in cold weather is the increased con- 
ducting power of air containing water vapor. Damp air 
may fairly be said to partake of the nature of the water 
that is in it; water feels colder to the hand than does air of 
the same temperature because it abstracts the heat more 
rapidly. Similarly, moist cold air takes heat more rap- 
idly than does dry cold air. This property is present 
just as surely in warm humid air, but it can affect us only 
when there is a wide difference in temperature between the 
skin and the surroundings. 
Temperature Maintenance During Exercise.-We have 
been discussing the ways in which the human body guards 
itself against changes of temperature which tend to impress 
themselves upon it from the outside. Another question 
is in regard to how it escapes the tendency to overheat 
itself when, during exercise, its metabolism is doubled, 
trebled, or even more strikingly augmented. Reflection 
shows that it employs the two reflexes on which it relies 
for defense against the heat of the warm room, namely, 
increased surface blood-flow and increased perspiration. 
These are rendered more efficient by the hastened circula- 
tion, a condition not produced in any great degree by ex- 
ternal heat without the activity. 
A supplementary factor exists in the deepened breathing 
which takes heat from the system, both in the act of warm- 
ing the respired air and in the process of saturating it. 
Still another factor can be recognized in the fanning effect 
of the movements of parts of the body or of the body as a 
whole. A man running brings the exposed portions of his 
skin constantly into contact with fresh volumes of air and 
slips away from the air which he has just warmed and 
saturated. The cooling of his blood is in this way con- 
siderably facilitated. If he is not progressing through 
space, but carrying on his activities in one place-for 
example, in sawing wood-his arms and trunk still make THE MAINTENANCE OF THE BODY TEMPERATURE 
221 
short excursions and exchange old air for new. No doubt 
an important factor is the displacement of the heated air 
from within the clothing. A breeze makes a considerable 
difference with the heat output of a man's body. 
Fever.-When the body temperature is found to rise 
above its normal level and to persist at an elevation which 
brings many ill consequences upon the subject, what shall 
we name as the cause of the disorder? Shall we say that 
the metabolism is excessive? Studies made upon fever 
patients show that their nitrogenous metabolism is often 
surprisingly high, an indication of rapid destruction of the 
tissues, but the total heat production is not impressively 
large. We shall more nearly express the facts if we say 
that there is interference with heat loss. Frequently we 
can see evidence of a withholding of the perspiration. 
Perhaps it is best to say that the central fact in fever is the 
setting up of a false standard by the nervous system to 
which for a time there is an adherence as strict as that 
obtaining in health for the normal one. This perverted 
action of the nervous system is brought about by the 
poisons in the circulation at such times. Transient fever 
may be brought on by very severe exercise; it is experienced 
by men who run Marathon races. In these cases the con- 
trolling centers are probably acting in the normal way, but 
cannot secure the complete removal of the extraordinary 
quantities of heat which are produced. 
Summary.-The maintenance of a nearly uniform body 
temperature is the result of a balance between heat evolved 
and heat dissipated. So long as the external conditions 
are not such as to cause shivering, muscular tension, or 
instinctive activity of some other form the organism regu- 
lates its temperature almost wholly by making adjust- 
ments to promote or to restrict the loss of heat. The 
wearing of clothing adapted to the.season makes it possible 
to minimize the demands made upon the muscles for extra 
heat. Decidedly low temperatures and exceptional ex- 
posure can be withstood only by calling upon the contrac- 
tile tissues for an increased heat production. 
Reference: Barbour, Physiological Reviews, i, 1921, 295. CHAPTER XXII 
THE HYGIENE OF NUTRITION 
It is convenient to make a division under this general 
heading between those factors not directly connected with 
the diet, which none the less deserve consideration, and 
those which do concern the choice of food. Probably too 
little attention is paid to the fact that disorders of digestion 
and nutrition frequently arise when the food eaten is above 
criticism, both as to quantity and kind. 
Nervous Conditions Affecting Digestion.-Enough has 
already been said to make it plain that the processes oc- 
curring in the alimentary canal are greatly subject to 
influences radiating from the brain. It is especially strik- 
ing that both the movements of the stomach and the secre- 
tion of the gastric juice may be inhibited as a result of 
disturbing circumstances. Intestinal movements may be 
modified in similar fashion. Emphasis has been placed on 
the dependence of the whole digestive process upon a good 
start. This can hardly be too strongly enforced. The 
good start can scarcely be secured if the mental state of the 
subject is not favorable to the enjoyment of his meal. 
Cannon has collected various instances of the suspension 
of digestion in consequence of disagreeable experiences, 
and it would be easy for almost anyone to add to his list. 
He tells us, for example, of the case of a woman whose 
stomach was emptied under the direction of a specialist 
in order to ascertain the degree of digestion undergone by a 
prescribed breakfast. The dinner of the night before was 
recovered and was found almost unaltered. Inquiry led 
to the discovery that the woman had passed a night of 
intense agitation as the result of misconduct on the part of 
222 223 
THE HYGIENE OF NUTRITION 
her husband. People who are seasick some hours after a 
meal often vomit undigested food. Apprehension of 
being sick has probably inhibited the gastric activities.1 
Just as a single occasion of painful emotion may lead to 
a passing digestive disturbance, so continued mental de- 
pression, worry, or grief may permanently impair the 
working of the tract and undermine the vigor and capacity 
of the sufferer. Homesickness is not to be regarded lightly 
as a cause of malnutrition. Companionship is a powerful 
promoter of assimilation. The attractive serving of food, 
a pleasant room, and good ventilation are of high import- 
ance. The lack of all these, so commonly faced by the 
lonely student or the young man making a start in a strange 
city, may be to some extent counteracted by the cultiva- 
tion of optimism and the mental discipline which makes it 
possible to detach one's self from sordid surroundings. 
Alcohol works to the same end, but is a perilous resource 
under these circumstances. 
Children are very often the victims of sharp attacks of 
indigestion. Their sicknesses, which are accepted with 
little surprise in many families, are almost always held to be 
due to injudicious eating. While this is a reasonable belief 
in many cases, it may be asked whether emotional causes 
of indigestion in children are considered as much as they 
should be. How common it is to see children made to cry 
while at the table by ill-timed rebukes. The quick temper 
and thoughtlessness of parents destroys the happiness of 
many a meal. Granting that instruction in table-manners 
ought to be given at suitable times, one may still protest 
against the downright rudeness of elders toward children 
1 Miller, Bergeim, and Hawk (Science, lii, 1920, 253) have'demon- 
strated the retarding of digestion which attended worry over an ex- 
amination. On the other hand, Holder, Smith, and Hawk (Science, 
li, 1920, 299) found that a subject under their observation success- 
fully assimila'ed a diet which had been rendered as repulsive as pos- 
sible. Various foods were minced, stirred up together, and black- 
ened, then eaten in the presence of a vile odor. The quality of the 
material was good, but the trial brought the volunteer to the verge 
of nausea. This shows the power of a strong constitution, but need 
not modify our general teaching. 224 
NUTRITIONAL PHYSIOLOGY 
when it is shown in the infliction of ridicule and humilia- 
tion. Consideration of others' feelings is the finest ele- 
ment in deportment. 
Moreover, it is probably fair to claim that children may 
be injured by being forced to eat food which they dislike. 
The aversions of early life are singularly strong. What 
passes for a foolish whim may be an instinctive loathing. 
There is an element of hypocrisy in the attitude of parents 
who are selecting precisely what they please to eat, while 
compelling little children to swallow food w'hich repels. 
To oblige a child to finish a plateful of food against its 
inclination may be crass brutality. Of course, children 
cannot be humored in the selection of eccentric diets, but 
they need not be made to eat when they would rather go 
hungry. There is little likelihood that they will refuse the 
staple articles. Occasionally a child may be encountered 
so wanting in appetite as to prefer other pastimes to eat- 
ing. Such an anomalous case is most difficult to deal with 
and some compulsion in the matter of feeding may be 
justified. 
Unhappiness may give rise to digestive difficulties which 
do not disappear with the removal of the first cause. It 
is not hard to show how this may be. Suppose that the 
power to digest and absorb food is lessened by central in- 
hibitions. A consequence is likely to be the accumulation 
of unabsorbed organic material in the colon and perhaps 
higher up as well. Bacterial decomposition will be fos- 
tered. Some of the products of such a process may be 
sufficiently like the normal products of enzyme action to 
play a part in nutrition, but others will probably prove 
distinctly detrimental. With the entrance into the circu- 
lation of such bodies there is originated what is known as 
auto-intoxication. (It has been urged by the authors of 
one of our best manuals of hygiene, "How to Live,"1 that 
the term ought to be reserved for disorders due to abnormal 
products of the body's own metabolism. There is obvious 
ground for the claim that a poisoning induced by the 
1 Fisher and Fisk, Funk & Wagnalls, New York, 1918. THE HYGIENE OF NUTRITION 
225 
activities of micro-organisms is not a "self" intoxication. 
But it will be hard to alter the common designation.) 
Long ago it was recognized that the reception into the 
system of bacterial products might be a cause of general 
ill health, of headache, and of somnolence. Within a few 
years the impression has gained ground that poisons from 
the colon have a much larger and more definite share in the 
development of disease. Much that goes by the name of 
rheumatism appears traceable to this source. Some of the 
toxic compounds seem to have the property of dissolving 
the red corpuscles of the blood, leading thus to anemia and 
the serious crippling of the energies which accompanies it. 
Nervous symptoms are among the most frequent nowadays 
referred to this condition, seemingly so remote from the 
brain. 
Physicians apply the term "vicious cycle" to a set of 
conditions in which the establishment of one tends to ac- 
centuate the others, and these, in their turn, add to the 
intensity of the first. We can see how a vicious cycle may 
become operative in the case of a person whose digestive 
abilities have been once reduced by mental depression. 
Auto-intoxication may be induced and one of the clearest 
results is a further depression of spirits. In this way a 
lasting injury may be done and the indigestion which be- 
gan as the effect of temporary unhappiness may be per- 
petuated in spite of the return of favorable circumstances. 
Auto-intoxication may come from errors of diet as well as 
from emotional causes, and a further discussion of it will 
be postponed. It is fair to say as a comment upon these 
paragraphs, now standing through successive editions, 
that less emphasis is placed upon auto-intoxication than 
was formerly the case (1924). 
Physical fatigue as well as mental may interfere with the 
progress of digestion. It is well that the appetite usually 
flags in times of exhaustion, so that one is in a measure 
insured against the tendency to overtax weakened organs. 
But when the fatigue is habitual there is an unfortunate 
dilemma; the body must have abundant food to support 226 
NUTRITIONAL PHYSIOLOGY 
the heavy labor and it is not well able to care for the food 
eaten. Loss of weight is common when a man is so situ- 
ated. Deprivation of sleep emphasizes this state of affairs, 
and by dulling both the appetite and the digestive capacity 
it proves for many people the surest means of reducing 
adipose tissue. 
Other possible causes of indigestion may be mentioned 
briefly. Taking cold is one of these. While the congestion 
and inflammation which so often follow exposure to drafts 
and dampness are most frequently centered in the mucous 
membranes of the nose and throat, a corresponding involve- 
ment of the alimentary canal is not rare. Diarrheal at- 
tacks are common in the spring and fall when the weather 
changes are erratic. They are probably to be classed as 
intestinal "colds." Another source of alimentary trouble 
is to be found in uncorrected defects of vision. While 
headaches are the most persistent symptoms of astig- 
matism and other ocular imperfections, indigestion is not 
uncommon, and its disappearance when glasses are worn 
seems sometimes almost magical. 
Quantity of Food.-The familiar teaching of the period- 
icals and popular books is that people generally eat too 
much and would be better in every respect if they should 
cut down their intake. Often the statement is reck- 
lessly made that we eat twice as much as we ought. When 
reduction in diet is recommended it is apparently as- 
sumed that the weight can be preserved and the same 
program of life followed in spite of the proposed re- 
trenchment. This turns out to be a fallacy, as will be 
shown. Another assumption freely made is that there are 
small and large eaters in the community quite apart from 
stature and occupation; some are believed to be econom- 
ical in constitution, while others are spendthrift. 
We hear it said that one individual "gets more of the 
goodness of his food" than another does of his. But this 
does not seem to be the case unless the comparison is 
extended to include victims of chronic diarrhea. People 
who are not obviously abnormal absorb a high and nearly THE HYGIENE OF NUTRITION 
227 
uniform proportion of their food. As to the spendthrift 
organizations mentioned above they are indeed to be 
found. The most striking examples are found among 
subjects with overactive thyroid glands. But some- 
thing of a like extravagance in metabolism is to be sus- 
pected in numerous individuals who are judged to be 
hearty eaters and who yet remain thin. Gulick1 has made 
a most instructive study of his own nutritional problem. 
He was reputed to be a man who could not gain weight. 
He had, however, a normal basal metabolism and could 
not, therefore, be rated as a hyperthyroid. By con- 
scientiously taking increased portions of food he succeeded 
in adding about 25 pounds in a little more than a year. 
The original ration represented about 2700 Cal. while 
at the height of his drive he reached 4100. In round num- 
bers he increased his mass 20 per cent, by raising his 
food intake 50 per cent. His basal metabolism did not 
rise. The extravagance inherent in this man's system 
appears when his rations are compared with his theoretical 
needs. At the time when he was consuming 2700 Cal. it 
appeared that 2250 should have sufficed. When his supply 
was 4100 it seemed as though 3200 ought to have provided 
for his maintenance and estimated activities. It would be 
of extreme interest to have corresponding figures for other 
cases. 
The simple fact is, as Lusk2 has pointed out, that any 
man whose weight is constant is eating just enough for 
that weight. Therefore, he cannot be said to overeat un- 
less he is overweight. If he weighs more than he should 
he cannot escape the reproach of eating too much. All 
the advocacy of reduction in rations should evidently be 
concentrated upon those who are definitely above the 
standard in avoirdupois. Whether the weights accepted 
as normal are really the best possible for health and 
efficiency may be open to discussion, but otherwise there 
are no uncertainties in the problem. 
1 American Journal of Physiology, Ixii, 1922, 371. 
2 Science, xlv, 1917, 345. 228 
NUTRITIONAL PHYSIOLOGY 
The metabolism or consumption of food in active life 
may be said to be separable into three fractions: First, 
we have the Basal quantity representing the bare neces- 
sities of the resting and fasting organism. Second, there is 
a rather small quantity, sometimes referred to as the 
Cost of Digestion,1 it stands for the expenditure, over and 
above the Basal, stimulated by the daily food. Third, 
there is the Work fraction which may vary within the 
widest limits. If the weight is to be maintained the net 
income of food must equal the sum of the three. 
If the income is reduced the Cost of Digestion is auto- 
matically lowered. But this is a small item; there must 
be a reduction of the Work fraction or a loss of weight, 
stored fuels being called upon to supplement the diet. 
The way in which the system reacts to an enforced cut 
in the ration is a matter of the greatest interest. We can 
conceive of two possibilities: there may be loss of weight 
with a full performance of routine work or there may be 
maintenance of weight with a discontinuance of a part of 
the daily activity. Instinct will probably impel toward the 
latter result. A compromise between the two courses is 
likely. 
During the war restriction of food supply was acute in 
many regions'and did not seem remote in the most favored 
lands. An experimental study of the effects produced by 
definite and severe restriction was carried out at the Car- 
negie Nutrition Laboratory2 and its main features must 
be recounted. The volunteers who underwent the hard- 
ships of this trial were students from the Internationa] 
Y. M, C. A. College at Springfield, Massachusetts. It will 
suffice to indicate the procedure followed with the principal 
squad, composed of twelve young men. 
First of all a general inquiry into their dietetic habits 
when free from compulsion was made. It appeared that 
they secured average allowances of from 3200 to 3600 
1 The term is ill chosen, for it has already been pointed out that 
the extra metabolism following the taking of food is not due to 
digestive activities. 
2 Carnegie Institution of Washington, Publication 280, 1918. THE HYGIENE OF NUTRITION 
229 
Calories per day. They were at once limited to about half 
this ration and the decline of weight was noted. This 
amounted eventually to something like 12 per cent, of the 
initial; that is, subjects who had weighed 150 pounds fell 
away to 132. The time necessary to effect this loss varied 
from three to ten weeks. The desired reduction of weight 
having been accomplished, the feeding was cautiously in- 
creased until the progressive fall was checked. About 
2000 Calories answered for maintenance at the new level 
which was held for two months, more or less. 
During this period of equilibrium all manner of observa- 
tions and tests of condition were made. Gymnasium 
determinations of muscular power and endurance showed 
no important change. The blood-pressure was low. This 
is of interest as confirming the common assumption that 
we can cut down blood-pressure when desirable by restrict- 
ing the diet. The heart-rate during rest in bed dropped 
below any previous experience of the subjects; often it 
was in the thirties. There was marked constipation, 
which was counteracted so far as possible by adding bran 
to the food. The resting metabolism fell not merely in 
proportion to the 12 per cent, loss of body weight, but 
much more. It was reduced by about 18 per cent, when 
referred to the unit of weight or surface. (This means 
that a man whose original basal metabolism was 1500 
Calories was found to have, after cutting down his weight 
by 12 per cent., not 1320 Calories, but about 1100 as his 
new standard.) Many of the bodily processes must have 
been slowed very decisively. 
The students were able to carry out their daily physical 
exercises and to keep their examination marks up to the 
level of their former grades. The most serious effects of 
this subnutrition were sensory and temperamental. The 
discomfort was intense. It was at all times hard to ex- 
clude thoughts of food, while to see others eating without 
restraint was torture. There was marked sensitiveness to 
cold. Muscular activity was successfully maintained, as 
has been said, but it was with a feeling of reluctance and 230 
NUTRITIONAL PHYSIOLOGY 
unwonted effort. It may be pointed out that the instinct 
to avoid work when short of food is biologically sound; 
it makes for conservation of resources and so for longer 
survival. Sexual impulses died away.1 The life of the 
individual had taken precedence over radial interests. 
Reproduction is appropriate to times of surplus supply, 
not to periods of dearth. 
These young men were of the highest character and of 
exceptional self-control. They were able to keep on with 
their allotted tasks while they doubtless economized effort 
in non-essentials. Given subjects having less of principle 
and discipline we find it easy to see that the situation 
might have led to dishonesty or, with a populace, to dis- 
order. This has been exemplified in Europe.2 But it is the 
testimony of witnesses that prolonged and extreme under- 
nutrition disposes at last to apathy rather than to tur- 
bulence. All activity is shunned, even though the hope of 
bettering intolerable conditions may be held out as an 
incentive. Observers abroad have noted the pathetic 
fact that the children have lost the impulse to play. 
Reduction of income is not always due to poverty or to 
actual shortage in the food supply of a community. It 
may be brought about by want of appetite. There are 
great numbers of people, particularly middle-aged and 
elderly women, who present a picture much like that of 
the Springfield squad save for the fact that they are not 
hungry. They eat lightly, are under weight, constipated, 
sensitive to cold, perform their routine tasks faithfully, but 
seem to have little enthusiasm for anything outside the 
habitual round. Their outlook is not particularly cheer- 
ful; at best they may be described as "resigned and 
patient"-often they are morose, bigoted, suspicious. 
They stand contrasted with another type characterized as 
"hearty and comfortable" which is certainly easier to live 
1 Miles, Journal of Nervous and Mental Diseases, xlix, 1919, 208. 
2 Lusk, Physiological Reviews, i, 1921, 523. For the skilful meet- 
ing of the emergency in Denmark, see Hindhede, Journal of the 
American Medical Association, Ixxiv, 1920, 381. THE HYGIENE OF NUTRITION 
231 
with. Few will now claim that the lowest metabolic level 
at which equilibrium is possible is in any sense the best 
standard to be sought. 
In many instances the underweight individual cannot 
arbitrarily increase his intake of food; his limited appetite 
forbids. This was not true of the members of the Spring- 
field groups, for their inclinations led them to resume full 
diet and rapidly to regain their lost weight when they 
were free to do so. The want of a proper appetite is the 
great impediment to successful nutrition; the possession of 
an appetite out of proportion to existing needs is the essential 
cause of obesity. Many people who definitely need to eat 
more and so to attain to a higher nutritional level find it 
hard to alter their habits. They are like those who have 
long lived in poverty and become committed to the 
severest frugality in all things; it is not easy for such to 
adopt a more liberal scale of living when increased means 
are provided. A difficulty like this stands in the way of 
restitution among the half-starved populations of Austria 
and Poland. There is an uneasy consciousness of the 
want of food, but the appetite itself has been enfeebled and 
the capacity to eat full rations with pleasure is regained 
only by slow degrees. 
What shall we say when we are assured that a fat man 
is a small eater?-or that a lean man consumes enough 
for two? We may concede that there are moderate varia- 
tions in constitution. It is an important consideration 
that the heavy man probably takes little exercise. The 
work fraction of his metabolism may be quite small. The 
lean man is often active, either in useful employment or 
in nervous unrest. Another point is this: that the reputed 
large eater is, in many cases, one who enjoys a large meal 
now and then. He may have a capacious stomach and give 
impressive demonstrations of his gastronomic prowess, 
but his average intake may not be especially large. He 
may be an irregular rather than a great eater. We often 
assume that our ancestors ate more than we do. Very 
frequently, no doubt, this was the case, for most of them 232 
NUTRITIONAL PHYSIOLOGY 
led more active lives. Aside from this, it may well be that 
we emphasize their feasts and forget their fasts. They 
had somewhat gross conceptions of what should con- 
stitute a banquet, but their festive affairs may not have 
been so numerous as we have supposed. 
It may be of interest to insert here a description of an 
eighteenth century dinner. The occasion was the ordina- 
tion of a minister at the Old South Church in Boston in 
February, 1761. A current newspaper gave the following 
account:1 "There were six tables that held one with 
another eighteen persons each, upon each table a good 
rich plumb pudding, a dish of boil'd pork and fowls, and 
a corned leg of pork with sauce proper for it, a leg of 
bacon, a piece of alamode beef, a leg of mutton with caper 
sauce, a roast line of veal, a roast turkey, a venison pastee, 
besides chess cakes and tarts, cheese and butter. They 
had the best of old cyder, one barrel of Lisbon wine, punch 
in plenty before and after dinner, made of old Barbados 
spirit." The impression is of an overhwelming and most 
unhygienic repast with an excessive amount of protein 
from animal sources. We must bear in mind that the 
season was winter and that many kinds of vegetable food 
were therefore not to be had. 
The Peculiarities of Protein.-While the advice to 
limit all forms of food is constantly heard, it is the use 
of much protein which is most vigorously condemned. 
The most active movement in favor of careful limitation 
has probably been that led by Chittenden some years ago.2 
He demonstrated that his own needs over a long period 
could be met by a protein allowance of about 35 grams a 
day. Under his supervision a number of athletes trained 
and performed with credit on a daily supply of 55 grams. 
These figures are striking indeed when it is noted that the 
protein consumed by college oarsmen just before this 
1 Alice Morse Earle, "The Sabbath in Puritan New England," 
Scribners, New York, 1892. 
2 "Physiological Economy in Nutrition," Stokes, New York, 
1904. THE HYGIENE OF NUTRITION 
233 
time had been from 133 to 174 grams per day.1 The 
Maine lumbermen referred to on page 195 included more 
than 150 grams in their hearty rations. We do not hear 
so much about the virtues of a minimal protein intake as 
we did formerly, but it is still maintained that excess has 
its dangers. 
The decisive effect of a large protein allowance upon the 
metabolism-an effect in the direction of a seemingly use- 
less increase-has been mentioned. Other peculiar prop- 
erties call for discussion. We may conveniently distin- 
guish the influences proceeding from an excess of protein 
in the intestine from those which are associated with an 
excess of nitrogenous metabolism. In regard to the first 
it may be said that protein far more than the non- 
nitrogenous foods is capable of generating toxic sub- 
stances and so of becoming a cause of true auto-intoxi- 
cation. An unabsorbed surplus of protein is, therefore, 
to be avoided. 
If, however, the absorption is as complete as can be 
desired, reasons can still be given for keeping the protein 
income of the body relatively low and depending largely 
upon carbohydrates and fats in preference to nitrogenous 
food. Emphasis has been placed elsewhere on the sim- 
plicity of the normal oxidation products formed from sugar 
and fats in contrast to the numerous and rather complex 
bodies which arise from the working over of the protein 
derivatives. Carbon clioxid and water are disposed of by 
the healthy system with apparent ease. The nitrogenous 
wastes require preliminary treatment by the liver and per- 
haps by other organs, and must then be removed from the 
blood by the kidneys. The principal one, urea, is not 
commonly a source of trouble, but the minor attendants, 
such as uric acid, are viewed with disfavor. 
One is naturally led to the opinion that high protein 
feeding must lay a burden upon the liver and the kidneys, 
but it is probably just to assume that what is a severe 
1 Atwater and Bryant, U. S. Department of Agriculture, Bulletin 
75, 1900. 234 
NUTRITIONAL PHYSIOLOGY 
tax for one person may not be so for another. The native 
efficiency of these organs is doubtless as variable as many 
other inherited qualities. It has been assumed that 
Oriental races with low protein consumption must be com- 
paratively free from kidney disease. Contrary to this 
impression, the limited statistics available seem to show a 
high prevalence of Bright's disease in vegetarian India. 
Nitrogenous by- or end-products, whether having their 
origin in the decompositions within the colon or in the 
metabolism, are presumably responsible for the premature 
fatigue of which we have spoken. An additional injury 
which may be laid to their charge must now be considered. 
This is the production in later life of arteriosclerosis. 
The saying is current among physicians that "a man is 
as old as his arteries." It is certainly a fact that the 
stiffening of these vessels is a chief cause of malnutrition 
and waning power in the tissues of the aged. If arterio- 
sclerosis sets in prematurely, other features of a senile 
decline may be expected. Metchnikoff has sought to 
establish a connection between the overconsumption of 
proteins and early loss of elasticity and adaptability in the 
human circulatory apparatus. Such a connection has 
long been believed to hold for alcohol. We seem brought 
to admit that this serious impairment of efficiency may 
spring from intemperance in eating as well as in drinking. 
The postponement of old age by frugality in feeding 
seems in a measure possible. Yet we must remember that 
self-denial in this respect may defeat its own end, since it 
may too greatly weaken the digestive capacity. An al- 
ternative to low diet, it has been suggested, may be found 
in the deliberate regulation of the bacterial conditions in 
the intestine. The claim is made, and seemingly with good 
reason, that a harmless type of fermentation may be en- 
couraged with the result that the undesirable decompo- 
sition may be prevented. This is the theory underlying 
the various modes of sour-milk treatment so much in 
vogue during the last five years. Lactic acid in moderate THE HYGIENE OF NUTRITIOxN 
235 
amounts appears to be quite devoid of danger to one's 
health. Its presence in the canal is hostile to the develop- 
ment of the organisms, which cause radical putrefaction 
of the proteins and definite auto-intoxication. A domi- 
nant acid fermentation may be secured either by the 
taking of sour milk (kephir, koumiss, matzoon, etc.) or by 
swallowing from time to time cultures of the lactic organ- 
isms, together with sugar for them to work on. 
Constipation.-The comparative harmlessness of even 
extreme constipation when associated with sparing feeding 
and active absorption has been granted. Such inaction 
of the intestine when it accompanies more liberal indul- 
gence in food must be unfortunate and must favor some 
phases of auto-intoxication. Many people resort period- 
ically to the use of cathartics when they feel slightly 
"under the weather," and enjoy a buoyant recovery of 
energy and ambition when the disturbance is over. It is 
natural to interpret such an experience as showing, first, 
the existence of a source of poisoning, and, second, its 
successful removal. But the relief experienced may be 
due in great part to the easing of mechanical tension 
which has been producing reflex and sensory disturbances 
(Alvarez).1 The wise man, however, will not be satisfied 
with knowing a way out of such disorders; he will aim to 
prevent their recurrence. The habit of depending upon 
laxatives instead of general hygiene is to be deplored. 
Irrigation of the colon to relieve from auto-intoxication is 
often resorted to as a part of medical treatment with good 
effects, but is not to be advised so long as attention to 
diet and exercise can be made to serve the need. 
Cases of constipation are of various types and the expert 
judgment of the physician is required to differentiate and 
1 How important this is has been made clear by Donaldson's 
experiments. Five volunteers refrained from defecating for four 
days. They were very uncomfortable and experienced the symp- 
toms usually attributed to auto-intoxication. But the same ill 
feelings could be reproduced in the absence of constipation by 
packing the rectum with cotton. Journal of the American Medical 
Association, Ixxviii, 1922, 884. 236 
NUTRITIONAL PHYSIOLOGY 
treat them.1 It is often pointed out that regular evacua- 
tions do not guarantee freedom from constipation. The 
advance of the intestinal contents may be slow. Sadler 
has said that "the movement may be like the Missouri 
Pacific train which appeared to be on schedule, but was, in 
fact, twenty-four hours late." Quite often progress as far 
as the region of the spleen is all that can be desired, while 
there is most unfortunate sluggishness in the reactions of 
the terminal part of the canal. Laxatives give little assist- 
ance to such subjects. Suggestion is often surprisingly 
helpful. Hertz tells us of a boy who had proved refractory 
toward many kinds of treatment whose difficulty was 
promptly relieved by a financial arrangement. He was to 
receive six pence for the fulfilment of his daily obligation 
and forfeit a shilling for each omission. After he had 
incurred one penalty we are told that he became a pattern 
of regularity-as the author pleasantly says, "he went on 
and made his fortune."2 
. Obesity.-The accumulation of adipose tissue in burden- 
some and disfiguring deposits is regarded as mirth provok- 
ing, but should be viewed with due appreciation of its 
seriousness.3 In the light of what has gone before, there 
is no escape from the conclusion that during the period of 
increasing weight the diet must have been in excess of 
the requirement. Yet when a person has once become 
notably stout he is often reputed to be a light eater. It 
is likely to be found that he cannot reduce his allowance of 
food without feeling quite uncomfortable. Various means 
may be resorted to in the attempt to abate the unwelcome 
condition, but frequently without success. Thus out- 
door exercise, which, of course, increases the metabolism 
and should destroy the body fat, may stimulate the ap- 
petite to an extent which fully corresponds with the oxida- 
tion, and so defeats its own purpose. 
1 Hertz, "Constipation and Allied Intestinal Disorders," Oxford 
University Press, 1909. 
2 Ibid., p. 259. 
3 Fisher and Fisk, "How to Live," p. 212. THE HYGIENE OF NUTRITION 
237 
Reduction of adipose tissue by fasting is possible, but 
not popular. Several other methods have been tried. 
The Banting system, formerly much in vogue, consisted 
essentially in a diet containing a maximum of protein. 
It was held that such a diet would keep the muscles and 
glands from losing substance, while not promoting the 
formation of fat. We have seen, however, that fat forma- 
tion from protein is at least theoretically possible. The 
success of the Banting treatment probably depended upon 
two factors: first, foods rich in protein are satiating, and 
an unconscious cutting down of income is therefore likely; 
second, the specific dynamic effect of so much protein 
would be expected to increase the metabolism. Restric- 
tion of water drinking is often recommended. Laxatives 
are sometimes used, presumably to hinder absorption, 
or to create an uneasy state in which loss of appetite may 
be expected. A procedure which seems rational is the 
selection of a rather bulky but not highly nutritious diet, 
including fruit and the coarser vegetables. By this means 
hunger can be fairly appeased, while the actual quantity of 
food entering the circulation is not sufficient to maintain 
carbon equilibrium. 
Carbohydrates and Fats Compared.-In the discussion 
contained in the foregoing chapters the impression has 
probably been conveyed that carbohydrates and fats are 
freely interchangeable. One statement has been made 
which qualifies this conception, namely, that fat cannot 
completely replace carbohydrate in a diet containing the 
ordinary allowance of protein (p. 150). The substitution 
leads to acidosis. There is ordinarily no tendency to let 
the fat crowd out the cheaper starches and sugars. The 
opposite condition is of more practical interest; in Central 
Europe during the war fats were hard to obtain and 
average diets were correspondingly ill-balanced. 
It had been assumed that carbohydrates could quite 
successfully take the place of fats in human food. But 
the attempt to push this substitution as far as possible 
brought out the serious drawbacks of a fat-free ration. 238 
NUTRITIONAL PHYSIOLOGY 
Starling1 has summarized them somewhat as follows: 
The bulk of the diet becomes burdensome. This bulk is 
due in part to water and in part to cellulose. In systems 
weakened by hardship irritation of the canal and edema 
of the tissues may result. The food is excessively sub- 
ject to fermentation. The stomach unloads its contents 
too rapidly and premature hunger and faintness are 
experienced. There may be definite failure to secure the 
vitamins associated with fats, although the use of greens 
tends to prevent this. 
1 "The Feeding of Nations," Longmans, London, 1919, Chap. IV. CHAPTER XXIII 
THE HYGIENE OF NUTRITION (Continued) 
WATER; MEAT; SUGAR 
Water.-There are particular foods which call for indi- 
vidual notice; one of these is water. The fact is already 
familiar that this compound so abundant in nature is also 
•the largest item in the income of the human body. It is 
an essential part of all the tissues, and its percentage in 
their make-up cannot be materially reduced while life 
continues. It has been described before as an important 
vehicle of excretion, and we have seen that when it evapo- 
rates it often provides for the removal of heat, which would 
otherwise accumulate in the body to its hurt. When the 
discharge of water is unusually gieat the feeling of thirst 
is roused and dictates a renewal of the stored supply. 
We can vary considerably our practice of water drinking. 
Hence this is a matter often dealt with by writers on 
hygiene. The common teaching is to the effect that one 
can hardly drink too much water unless it be at mealtimes. 
The beneficial results supposed to accrue from free drink- 
ing are assumed to include the avoidance of constipation 
and the promotion of the elimination of dissolved waste by 
the kidneys and possibly by the liver. There seem to be 
no reasons for doubting the soundness of these popular 
ideas. The drinking of a great deal of w'ater is an excel- 
lent habit, and consumption of tea, coffee, and other 
beverages in which water is the principal constituent has 
this in its favor-it leads people to take much more water 
than they would otherwise. 
It is probably correct to say that the duties of the kid- 
neys are made lighter when we give them more water to 
239 240 
NUTRITIONAL PHYSIOLOGY 
excrete. This may be contrary to the general impression, 
for the temptation is to judge the work of a gland by the 
volume of its output. But in the light of facts which 
cannot be discussed here we are led to believe that con- 
centration rather than sheer amount of secretion is what 
puts the tubules of the kidneys to the severest test. They 
act at the greatest disadvantage when required to excrete 
a maximum of solids in a minimum of water. The urine 
almost always has a concentration much higher than that 
of the blood from which it is derived, and it is fair to assume 
that the separation of the two fluids would be made easier 
if the difference of concentration could be lessened. 
Water drinking is the natural way to secure this result. 
"Water," says Osler, "is, after all, the great diuretic." 
While teachers of hygiene are well agreed upon the value 
of water in liberal quantities as conducing to health, 
there has been much said against its free use with meals. 
We can hardly question the impression that many cases of 
indigestion have been benefited by the prohibition of water 
at the table. An approach to Fletcherism is favored 
when the saliva is not aided by swallows of water. Slower 
eating is likely, and slower eating may be expected to 
satisfy the appetite with a smaller actual intake. The 
idea that the digestive juices are seriously diluted by water 
taken with the food does not seem to be well founded. 
Laboratory experiments show that dilution of the fresh 
secretions is at least as likely to increase as to diminish 
their activity. It must be borne in mind that the dilution 
of a liquid containing a fixed amount of enzyme does not 
reduce the quantity of the enzyme, but only makes it act 
in a larger volume of the mixture. 
Very cold water swallowed rapidly may chill the mucous 
membrane of the stomach and possibly retard the prog- 
gress of gastric secretion. Digestion itself may also be 
slowed if the contents of the stomach are cooled. But 
bacterial fermentation should apparently be delayed just 
as definitely as the normal hydrolysis, and we ought not 
to make too much of these possibilities. Hawk, at the 
University of Illinois, once made a number of trials which THE HYGIENE OF NUTRITION 
241 
are entirely favorable to the practice of drinking all 
the water that one chooses with meals. He has shown 
that the fecal nitrogen is lower when water is taken in 
large volume than when it is forbidden. This fact he holds 
to indicate more complete digestion and more thorough ab- 
sorption. His subjects were in the best of health, and his 
results do not contradict those of physicians who have found 
the restriction of water beneficial in particular cases. 
The opinion is commonly held that drinking much water 
favors increase of weight. To a limited extent such in- 
crease of weight may result from actual retention of water. 
We can see that a definite addition to the adipose 
tissue may proceed from the tendency to eat more food 
when water is taken freely. Perhaps the converse of this 
form of statement is more accurate; namely, that weight 
is lost when water is restricted and- the quantity of solid 
food unconsciously diminished. Sometimes an erroneous 
inference may have been drawn from the fact that stout 
people drink a great deal of water. This is in part a 
consequence rather than a cause of their condition. Sub- 
cutaneous fat is a hindrance to the escape of heat from the 
body, and its presence during warm weather necessitates 
an unusual amount of perspiration. This, in turn, pro- 
duces thirst. 
Meat.-Much that is written tends to create the im- 
pression that meat is entirely unlike any other food. Its 
peculiarities are constantly exaggerated.1 When Erasmus 
Darwin excused himself from Lenten abstinence on the 
ground that "all flesh is grass" he perverted Scripture, but 
uttered a physiologic truth. Meat has two distinctive 
characters: it is very rich in proteins and in extractives. 
It is not any richer in protein than are dried peas and 
beans, but while these vegetables contain a large propor- 
tion of carbohydrates, lean meat contains these bodies 
very scantily. A diet of lean meat, therefore, comes near 
to being a straight protein diet. A plate of beans is 
equivalent in composition to a plate of meat and potato. 
1 Lusk, American Journal of Public Health, xiv, 1924, 297. 242 
NUTRITIONAL PHYSIOLOGY 
Vegetarianism has sometimes been advocated upon 
humanitarian grounds and sometimes because of its sup- 
posed favorable effect upon health. The idea that the 
destruction of animal life for human nutrition is morally 
wrong cannot be discussed here. It is true that not many 
people would kill animals for their own use if there were no 
other way to obtain meat. It is equally true that so many 
of the noblest and gentlest people that ever lived have 
enjoyed meat and fish that we cannot well credit the claim 
that flesh foods cause deterioration of character. Atten- 
tion has often been called to the exceptional kindliness of 
the Esquimaux, a carnivorous race. 
A diet containing much meat is naturally a high protein 
diet, and is, accordingly, subject to the drawbacks already 
mentioned in this connection. These have been seen to 
include an unprofitable spurring of the metabolism-more 
particularly objectionable in warm weather-and the 
menace of auto-intoxication. The typical proteins of meat 
are probably not better nor worse than other proteins in 
these relations. There is, however, greater, likelihood of 
overconsumption of proteins when meat is the source be- 
cause of its very attractiveness. Men, especially those who 
spend money freely, are certainly prone to such indiscre- 
tions, while women set an example of temperance in this as 
in so many other practices. So far as the proteins eaten 
are destined to be reconstructed into those of the blood 
and tissues, it may be asserted that meat proteins are pecu- 
liarly well suited to the purpose. Their molecular corre- 
spondence with the pattern to be imitated is close. But 
the demand for amino-acids for this use seems to be 
small. 
The real individuality of meat is owing to the presence 
in it of the secondary substance which we call extractives. 
These give it its odor and, in conjunction with mineral 
salts, its flavor. Their absolute amount is not large, but 
they are of much interest. They have been described by 
writers holding meat in disfavor as waste-products of 
animal life. The characterization seems broadly to be a THE HYGIENE OF NUTRITION 
243 
justifiable one, though our disgust at the notion is not 
necessarily well founded. The extractives of meat cer- 
tainly originated during the lifetime of the animal through 
the breaking down of its proteins, and were destined for 
ultimate excretion, either unchanged or after some altera- 
tion. It is hard to escape the conclusion that our pleasure 
in the taste of meat is due to compounds nearly akin to 
those of the urine. 
Furthermore, it is plain that the opponents of meat are 
correct in making the point that when we receive these ex- 
tractives we are simply adding to the duties of our own kid- 
neys. When we eat 50 grams of protein in beans we have 
later to excrete about 8 grams of nitrogen in urea and other 
forms. When we eat 50 grams of protein in beefsteak we 
must subsequently excrete the same quantity of nitrogen 
plus that of the extractive bodies. The addition is not a 
large one, but it is partly in the form of uric acid and per- 
haps other bodies less tractable than urea. The advisa- 
bility of limiting meat in rheumatism and related conditions 
has long been a tradition. Considering that our ancestors 
ate a great deal of meat one wonders whether this habit 
affected their health in any specific way. Eighteenth 
century literature certainly suggests an occurrence of gout 
and calculus beyond what is now familiar. Yet we are 
told that the Esquimaux, the champion meat-eaters of the 
world, escape these troubles. 
While the extractives may be held to account for some of 
the possible ill effects of meat, they are also the source of its 
especial virtue. Palatability itself is of great hygienic 
worth, and these substances confer qualities which for 
most people cannot be equaled apart from meat. The 
promotion of gastric secretion in the normal subject and 
its establishment in the invalid are most surely secured by 
means of these same extractives. Aside from their favor- 
able influence upon the stomach, they are probably mild 
stimulants in the same sense in which coffee and tea can 
be called so. 
Some kinds of meat are well known to occasion indiges- 244 
NUTRITIONAL PHYSIOLOGY 
tion. Pork and veal are particularly feared While we 
may not know the reason why these foods so often disagree 
with people, it seems probable that texture is an important 
consideration. In both these meats the fiber is fine, and 
fat is intimately mingled with the lean. A close blending 
of fat with nitrogenous matter appears to give a fabric 
which it is hard to digest. The same principle is illus- 
trated by fat-soaked fried foods. Under the cover of the 
fat thorough-going bacterial decomposition of the proteins 
may be accomplished with the final release of highly poi- 
sonous products. Attacks of acute indigestion resulting 
from this cause are much like the so-called ptomain- 
poisoning. But it is best to reserve the term for those 
cases in which the harmful bacterial change had taken 
place in the food before it was eaten. 
Texture is an exceedingly weighty factor in determining 
the ease or difficulty of digestion for any food. It is this 
which makes the recognized difference between new and 
old bread and between the upper and the under crust of a 
pie. Dry bread or pastry takes up the saliva as blotting- 
paper would, and starch digestion is immediately begun 
even at the center of the morsel. Saturated food material 
does not imbibe the digestive juice in this way. Osler has 
said, with his usual picturesqueness, that "pie north of 
Mason and Dixon's line and hot bread south of it have 
done more harm than alcohol." Fats are best cared for 
when emulsified if liquid, and when of a flaky or crystalline 
character if solid. This last quality is realized in good 
bacon. 
Sugar.-The reader will have noted that the starch, 
which is usually the most abundant compound in the daily 
income of man, is converted to a sugar before it is admitted 
to the circulation. The question arises why it is not 
equally well to eat sugar altogether in place of starch. This 
is the actual habit during the period of milk feeding in in- 
fancy. The enzymes that act upon starch are not freely 
furnished until some months after birth. In later life, 
however, starch comes to be the main reliance of the race. THE HYGIENE OF NUTRITION 
245 
Experience has shown that cane-sugar is often productive 
of indigestion. Can we find definite reasons for the 
superiority of starch to sugar? 
The fact has previously been mentioned that much sugar 
causes alimentary glycosuria, while this is never produced 
in strictly normal subjects by the freest eating of starch. 
Here we have a clue to the difference between the two 
classes of carbohydrates. The glycosuria is not in itself a 
serious matter, but it shows that the solubility of sugar and 
the slight changes requisite for its digestion lead to its very 
rapid absorption. The digestion of starch is a more grad- 
ual and protracted process, and the resulting glucose is not 
formed so swiftly as to raise the concentration of the in- 
testinal contents appreciably, nor delivered to the blood 
so abruptly as to increase markedly the percentage of sugar 
in circulation. It seems safe to assume that the storage of 
glycogen is effected more smoothly and easily after the 
ingestion of starch than after the taking at one time of a 
large quantity of sugar. 
Highly soluble bodies of low or moderate molecular weight 
are said to confer on their concentrated solutions the prop- 
erty of high osmotic pressure. This is not the place to 
discuss what is meant by the expression. For practical 
purposes it can be said that concentrated solutions take 
water from tissues with which they may be brought in 
contact. Hence an irritant effect is to be expected. This 
is exemplified by the action of strong salt solutions in 
producing vomiting. Everyone who is fond of candy 
knows that it can be eaten until a point is reached at 
which an uneasy sensation of satiety verging on nausea 
is developed. This is relieved by drinking water, which, of 
course, lowers the concentration of the syrupy gastric 
contents and so lessens the irritation. Hard candy, dis- 
solving gradually in a large quantity of saliva, must pro- 
duce a less concentrated and irritating solution than that 
resulting from the rapid "melting" of soft bonbons. 
Cane-sugar is much sweeter than sugar of milk or the 
other sugars which arise in the course of digestion. It is, 246 
NUTRITIONAL PHYSIOLOGY 
therefore, cloying and its free use may blunt the appetite 
for other foods. It is said also to be more disturbing to the 
stomach than the others. These two properties perhaps 
account sufficiently for the ill effects which are attributed 
to the increasing consumption of candy in this country. 
Still it is probable that the evils resulting have been much 
exaggerated. There is, however, no question that the 
constant eating of candy threatens to damage the teeth, 
and the indirect impairment of the digestive powers may be 
serious. 
The relation between sugar and the decay of the teeth 
is apparently quite simple. Sugar is prone to ferment, 
a change brought about by bacteria which are inevitably 
present in the mouth. The chief product of such bacterial 
decomposition is lactic acid. This acid attacks the lime- 
salts of the teeth at points where the enamel has been 
chipped or worn away, or where it fails to meet the gum. 
The dissolving of the lime-salts leaves a soft and perishable 
organic structure which readily undergoes true decay. 
Every tiny deposit of sugar in the crevices of the teeth may 
soon become a focus of acid production and a center of 
disintegration. The popular impression that plain sugar 
is not so hurtful as sugar mingled with other substances in 
candy has this basis: pure sugar is so readily dissolved by 
the saliva that it is not likely to remain long clinging to the 
teeth. On the other hand, sugar which is mixed with fatty 
materials like chocolate may be sealed into crannies or 
retained under the edge of the gum with unfortunate effect. 
One who is bound to eat much candy should be willing to 
exercise unusual care to free the teeth from the remains, 
and may, in spite of his pains, have periodical days of 
reckoning at the dentist's. 
There is much testimony to the effect that free indulg- 
ence in sugar is bad for the complexion. When we con- 
sider this tradition we must remember that pimples and 
other gross blemishes are directly due to infections from 
without. If their development is conditioned by feeding, 
the relation must be indirect and referable to changes in 247 
THE HYGIENE OF NUTRITION 
the resistance of the tissues. It is a suggestive fact that in 
diabetes we have a great increase in susceptibility to sur- 
face infections. Perhaps an increased concentration of 
sugar in the blood has this influence in all cases. 
Cane-sugar is not to be thought of as a relish or ac- 
cessory, valuable chiefly to make possible the prepara- 
tion of numerous tasty dishes. It is true that the lack 
of sugar in war-time galled us most as it limited the 
variety of our fare. But, beyond this, sugar has a very 
substantial position in the American diet. It furnishes, 
when obtainable, at least one-eighth of our supply of 
Calories. Its replacement by other food is a problem of 
great magnitude. Many people think of candy and other 
sweet food as "unsubstantial." This is far from the truth. 
Confections, ice-cream sodas, and other extra foods con- 
tribute largely to the ration in many cases. (A four-ounce 
cake of sweet chocolate may have a value of 600 Calories.1) 
Glucose.-The glucose of commerce is made from corn- 
starch by treatment with weak hydrochloric acid, which is 
afterward neutralized by sodium carbonate, leaving a 
small percentage of sodium chlorid in the product. The 
resulting substance contains a considerable quantity of the 
dextrins-bodies intermediate between starch and sugar 
-some maltose, and dextrose to the extent of about 
50 per cent, of the whole. Such a mixture is hardly to be 
distinguished from that which exists at a certain stage in 
the salivary or pancreatic digestion of starch. It is hard 
to account for the intense popular prejudice constantly 
manifested against this food product. The only possible 
objection to its free use is connected with its tendency to 
induce glycosuria, an effect secured somewhat more read- 
ily with glucose than with cane-sugar. During the short- 
age of cane-sugar which prevailed in France in wartime 
glucose was sweetened by adding a little saccharin and 
made a fair substitute. 
1 C. G. and F. G. Benedict, Boston Medical and Surgical Journal, 
1918, clxxix, 153; 1919, clxxxi, 415; 1921, clxxxiv, 436. 248 
NUTRITIONAL PHYSIOLOGY 
Food Accessories.-It is perhaps unnecessary to enlarge 
upon the service of those compounds which are classed 
under this head. The double value of the extractives of 
meat which favor digestion both because of the flavors 
which they develop and because of their direct action upon 
the stomach wall has been sufficiently emphasized. The 
various condiments are believed to have a similar signifi- 
cance. So far as they season the food so as to make it 
more acceptable, they must evidently promote secretion. 
Their power to call forth the gastric juice by a purely local 
effect is not so well established, but they are known to in- 
crease the blood-flow in the mucous membrane, which 
must help to sustain the local activities. 
Tea and Coffee.-These beverages owe what limited 
food value they have to the cream and sugar usually 
mixed with them. They give pleasure by their aroma, but 
they are given a peculiar position among articles of diet by 
the presence in them of the compound caffein, which is 
distinctly a drug. It is a stimulant to the heart, the kid- 
neys, and the central nervous system. It is chemically 
related to uric acid, but is not known to yield this incon- 
venient waste-product in the human body. Individual 
susceptibility to the action of caffein varies greatly. 
Where one person notices little or no reaction after a cup of 
coffee, another is exhilarated to a marked degree and hours 
later may find himself lying sleepless with tense or trem- 
bling muscles, a dry, burning skin, and a mind feverishly 
active. Often it is found that a more protracted disturb- 
ance follows the taking of coffee with cream than is caused 
by black coffee. 
It is too much to claim that the use of tea and coffee is 
altogether to be condemned. Many people, nevertheless, 
are better without them. For all who find themselves 
strongly stimulated it is the part of wisdom to limit the 
employment of these decoctions to real emergencies when 
uncommon demands are made upon the endurance and 
when for a time hygienic considerations have to be ig- 
nored. If young people will postpone the formation of the THE HYGIENE OF NUTRITION 
249 
habit they will have one more resource when the pressure 
of mature life becomes severe.1 
Chocolate and its derivative, cocoa, may be regarded as 
having somewhat similar properties. There is a measure 
of drug action, though it is less pronounced than in the 
case of coffee. Here the active principle is theobromin, 
nearly related to caffein. Chocolate is more than a food 
accessory, being exceedingly nutritious. A chocolate 
habit is easily formed, but, aside from threatening damage 
to the teeth, it is comparatively innocent. 
Mineral Salts.-These compounds have been referred 
to in an earlier chapter as forming an essential part of the 
tissues. Hence they must be supplied in sufficient quan- 
tity and variety during the period of growth. The danger 
of failing to do this is only occasional, though there are 
certain cases of malnutrition in which the central difficulty 
seems to be the lack of such constituents in the body of the 
child. The actual trouble is probably with the assimilation 
rather than with the diet. Thus, in rickets there is an evi- 
dent deficiency in the quantity of lime-salts incorporated 
into the developing skeleton, but it is not often true that 
there is any shortage in the amount offered in the food.2 
When the full stature is reached the need of a continued 
salt income is less marked, though there is reason to believe 
that the demand always exists. A certain loss in the urine 
and through the skin seems bound to occur, though the 
usual excretion of salts is far greater than the bare mini- 
mum, and appears to indicate a needless excess in the in- 
come. The salts of the diet have much to do with its 
palatability and so deserve a place with the organic con- 
diments.3 An unusual quantity of saline matter may be 
supposed to impose a hard task upon the kidneys, and is 
known to aggravate any dropsical tendency that may be 
present. 
1 See Hollingworth, "Caffein and Mental and Motor Efficiency," 
Archives of Psychology, The Science Press, New York, 1912. 
2 Lusk, "Elements of the Science of Nutrition," The W. B. Saun- 
ders Company, Philadelphia, 1917, 358-360. 
3 Job vi, 6. 250 
NUTRITIONAL PHYSIOLOGY 
Sodium chlorid is the one salt which we take pains to 
secure. We are inclined to think of it as a mere relish, 
but it has been shown that it has a deeper significance. 
The Austrian physiologist Bunge has found that it is 
sought both by animals and men whose food is largely or 
entirely vegetable. It is repugnant to those that are ap- 
proximately carnivorous. Bunge points out that vegetable 
foods are generally rich in compounds of potassium and 
relatively deficient in those of sodium. He has demon- 
strated that when an excess of potassium salts is eaten 
the kidneys discharge the foreign material promptly, and 
in doing so let slip a good deal of the sodium chlorid from 
the blood. Accordingly, it is inevitable that the steady 
consumption of foods rich in potassium1 should create a 
demand for sodium. The seeking of common salt to meet 
the need is a singular illustration of the almost unerring 
working of instinct. 
Stefansson has furnished an amusing instance of the dis- 
like for salt which is to be relied on in those who live entirely 
upon a diet of flesh. When he first went among the Esqui- 
maux he found himself in danger of losing his provisions 
at a ruinous rate. Policy required that he offer refresh- 
ments to all comers and their appetites were appalling. 
The embarrassment was happily relieved by the simple 
measure of salting the food merely to the degree which he 
himself found agreeable. The visitors thenceforth were 
content with very modest portions. 
The Acid-base Balance.-If the constituents of a ration 
are mixed and completely burned the residual ash may be 
either acid or alkaline. The proteins leave behind them 
definite quantities of sulphuric and phosphoric acids. 
These may be dominant in the mixture or they may be 
more than offset by the alkalies furnished by the com- 
bustion of certain foods. Most fruits and vegetables leave 
alkaline remains. So does milk. It is to be noted that a 
food may have an initial acidity, as in the case of the 
apple or the orange, and yet contribute at last to the al- 
1 Potatoes have this character. 251 
THE HYGIENE OF NUTRITION 
kaline side. This is equally the case with milk which sours 
so intensely under bacterial influence. Lactic acid and 
most of the fruit acids burn and literally "vanish into thin 
air"-be it outside the body or in the metabolism. An 
exception, within the body, is afforded by benzoic acid 
which the tissues fail to oxidize. It remains like the sul- 
phuric acid from the proteins to be dealt with by the 
kidney. Hence cranberries, plums, and prunes which con- 
tain benzoic acid add to the task of acid excretion which 
the body has usually to perform. 
If the ash of the diet is acid the urine must be so. It is 
ordinarily on this side of neutrality. There is evidence 
that an excessively acid urine is undesirable; it does not so 
surely bear away the minor waste-products like uric acid. 
Hence it is commonly taught that a neutral or alkaline 
residue is a characteristic to be sought in making up a 
ration.1 
Food Values.-American householders are now reading 
and hearing a great deal about economy in selection and 
the conservation of food. The subject is too large to be 
dealt with here. References to sources of reliable informa- 
tion may be found in the back of this book. Just a word 
may be ventured regarding the errors of judgment into 
which it is easy to fall. The fact which needs always to be 
borne in mind is that the water and the innutritious part 
of any article of diet must be deducted to obtain a true 
estimate of its potentiality. How greatly these compo- 
nents may reduce nutritive worth and how delusive mere 
bulk can be will be appreciated after considering the fol- 
lowing equivalents (in fuel value): 
1 lb. butter = 1.1 lb. bacon = 1.8 lb. oatmeal or cheese 
= 2.1 lb. flour or sugar = 4.2 lbs. lean meat = 8.4 lbs. 
potatoes = 11.2 lbs. milk = 15.4 lbs. onions or oysters = 
16.8 lbs. squash = 19.6 lbs. skimmed milk = 42 lbs. lettuce 
or celery.. 
1 Lusk, loc. cit., 361. CHAPTER XXIV 
FOOD POISONING 
It is probably safe to say that within the lifetime of 
the average individual critical illnesses occur which one 
is tempted to regard as cases of food poisoning. Yet 
there is a good deal of doubt concerning the accuracy 
of the term in many of these instances. In any attack of 
indigestion the element of poisoning may be present; 
but the poison has usually been generated within the ali- 
mentary tract by an unfortunate type of decomposition and 
the food may have been perfectly wholesome when it was 
eaten. We should, so far as possible, limit the designation 
to cases where the food has been clearly at fault, and not the 
temporary conditions prevailing in the canal. 
As regards the food itself, we should distinguish between 
that which is truly poisonous and that which is the bearer 
of infection. Infected food may cause typhoid or scarlet 
fever, but it is clear that these diseases are in a different 
class from the genuine poisonings. Nevertheless, it is hard 
to draw the line with certainty. Simple diarrheal attacks 
which ensue after a particular food has been eaten may 
be due either to organisms introduced with it or to the toxic 
character of the food itself. The impression seems to 
prevail that the first possibility is much more common than 
the second. Food that really deserves to be termed poison- 
ous should remain so even though sterile. On the other 
hand, the sterilization of poisonous food by heating to the 
boiling-point or higher may decompose and render harm- 
less certain of the toxic substances as well as destroy the 
micro-organisms. 
Our pure food laws are framed to guard consumers 
252 FOOD POISONING 
253 
against "decomposed" food products. Most people need 
to be reminded that a great deal of decomposition is 
perfectly consistent with wholesome quality. In fact, 
there are cases in which decomposition is deliberately 
encouraged. Many cheeses testify by appearance, con- 
sistency, and odor to the advanced stage which the process 
has attained. They rarely cause trouble. A consider- 
able degree of similar change is often tolerated in meat. 
Our feeling in regard to the matter seems largely deter- 
mined by tradition and prejudice; we object intensely to 
the earliest signs of such deterioration in eggs. 
Idiosyncrasy.-In any discussion of this kind allowance 
must be made for individual peculiarities. Many people 
have found by bitter experience that they must avoid cer- 
tain foods. Some cannot eat lobster, others are seriously 
affected by strawberries. Still others must do without 
butter, a curious fact, inasmuch as this is a milk product 
and everyone must have begun life upon a milk diet. A 
severe reaction toward potato has been recorded. In a 
case where this had been observed, the victim, a highly 
intelligent woman, expressed the belief that her repeated 
sicknesses were due to mental suggestion, her ingrained fear 
of potato. She told a friend that she wished to be tested 
by being given some potato in an unrecognizable form. 
The experiment was soon tried, a sponge cake being made 
with potato flour and eaten by the unsuspecting subject. 
A sharp and exhausting illness promptly followed. 
There are rather frequent instances of idiosyncrasy 
toward eggs. A most striking one may be described. A 
baby of nine months was given for the first time a little 
poached egg. It swallowed one spoonful, .spat out a 
second, and smeared some upon its face and fingers. 
Almost immediately vomiting set in, diarrhea followed very 
shortly, and wherever the egg had touched the skin there 
was a swelling and eruption. Some weeks later the child 
was again made sick when it ate a crumb of frosting which 
it had found on the floor. It is an interesting fact that a 
normal tolerance for egg was secured by giving minute 254 
NUTRITIONAL PHYSIOLOGY 
quantities from day to day and gradually increasing the 
amount. 
True food poisoning has until recently been due in 
most cases to animal products. Meat and fish have been 
most often implicated, while milk and ice-cream have 
occasionally figured in the accounts. Generally the foods 
responsible have been rich in protein. It would be natural 
to expect similar poisonous decompositions among the 
legumes, and, in fact, a rapidly increasing number of 
instances are being recorded in which beans appear to be 
concerned. Certain foods not at all rich in protein- 
for example, tomatoes-have recently been inculpated. 
Before we take up the subject of animal food poisoning 
attention should be called to the possibility of dangerous 
properties which may develop in cereals. 
Ergotism.-This is a form of disorder which has been 
rather common in Europe. While it cannot be said to 
threaten our own people upon any extensive scale it may 
fairly stand as an example of its class. The grain which 
is involved is rye. When this has been damaged by a 
certain microscopic fungus it becomes definitely poisonous 
and continues to be so even after cooking. Persons who 
consume this cereal may develop the most various symp- 
toms. Sometimes a chronic condition is induced, in which 
there is a slow decline due to degeneration within the 
central nervous system. Sometimes acute attacks occur 
with convulsions, or the chief effect may be a strange inter- 
ference with the circulation leading to gangrene of the 
extremities. 
Poisoning by Meat and Fish.-We may evidently dis- 
tinguish between poisonous meats which were rendered 
so during the life of the animal and those which have be- 
come dangerous owing to postmortem changes. Instances 
of the first class are not now common. There is a remote 
possibility that an animal may eat something which can 
make its flesh unwholesome. Certain cases of sickness 
attributed to the meat of partridges have been referred 
to the supposed inclusion of laurel in the diet of the 255 
FOOD POISONING 
birds when hard pressed for food. Animals suffering from 
various diseases may prove to have poisonous properties 
if their flesh is marketed and eaten. The practice of 
"emergency slaughter"-killing and dressing for con- 
sumption animals which are near to natural death-is 
happily rare. Not many years ago ghastly examples of 
this criminal procedure were recorded and the victims 
were numerous. One is reminded of the thrifty counsel 
given to the Hebrews in Deuteronomy xiv, 21. 
When meat which was originally wholesome becomes 
poisonous because of changes wrought in it by micro- 
organisms we have the conditions for ptomain-poisoning. 
There is no doubt that the term has been misused and made 
to cover many cases of acute indigestion in which the 
trouble had been less with the food than with the alimen- 
tary tract of the sufferer. We are not usually warranted 
in designating as ptomain-poisoning solitary reactions on 
the part of individuals when many other persons have eaten 
a certain food without ill effects. The multiple cases are 
much more likely to deserve the designation, although even 
here, as already noted, it is hard to draw the line between 
true poisoning and the transmission of germs capable of 
mischief. Many authorities now urge that the term be 
discarded. 
The standard picture of ptomain-poisoning is, apparently, 
that given long ago by Ballard (Wellbeck epidemic, cited 
by Osler. Ham was the food responsible). The average 
time between eating the food and experiencing the first 
symptoms was about twenty-four hours. It has often 
been shorter. The onset was generally sudden, often 
absolutely without warning, though in other cases there 
were premonitory signs, such as languor and fleeting pangs 
in the abdomen. Sometimes the initial experience was of 
faintness and vertigo, sometimes of chilliness. Within a 
short time the abdominal pain became most severe, pro- 
ducing prostration and cold sweating. Diarrhea and 
vomiting shortly set in. Muscular weakness rapidly 
became extreme; headache and thirst were common. The 256 
NUTRITIONAL PHYSIOLOGY 
temperature rose, in some subjects, to 104° F. The pulse 
was fast and feeble. Death from exhaustion sometimes 
occurs in such cases, but in the great majority speedy 
recovery takes place. It is favored, no doubt, by the 
thoroughness with which the system has eliminated the 
poison. 
There have been particular outbreaks in which the 
symptoms were quite unlike those just described. There 
are even types of poisoning attended with few signs of 
alimentary disturbance, the chief effects being upon the 
central nervous system. This has been characteristic of 
the occurrences of mussel-poisoning, which have been re- 
ported from Germany. In these the symptoms are those 
of profound collapse with numbness, dilated pupils, and 
rapid, failing heart. The effects resemble quite closely 
those produced by atropin. 
Botulism.-By this name is known a type of food poison- 
ing which is not particularly common, but which is remark- 
ably clear cut in its manifestations. The clinical picture 
has been familiar ever since 1820, when it was first recog- 
nized in the cases of persons made ill by sausage. Since 
then there have been occasional records of similar prostra- 
tions until a total of several hundred has been reached. 
Sausage meat has by no means been the sole offender. 
Canned string beans have been frequently implicated. 
A specific organism, Bacillus botulinus, which does its 
work in the absence of oxygen, is believed to develop the 
intensely poisonous property. 
The victims of botulism have no fever and no acute 
pain. The central fact is the abolition of function of 
certain nerve-centers, especially those of the cranial 
division of the system. As a result there are disorders of 
vision, disturbances of equilibrium, and, subjectivly, 
distressing vertigo. Impaired control of the muscles 
about the throat may result in difficulty with speech and 
with swallowing. All the secretions are diminished, and 
there is stubborn constipation. Death supervenes in as 
much as 60 per cent, of the cases, usually between the FOOD POISONING 
257 
fourth and the eighth day. Botulism is, therefore, far 
more mortal than ordinary food poisoning. Recovery is 
halting and protracted. Permanent effects, such as local 
paralysis of either sensory or motor order, may remain. 
The liability of sausage meat to undergo poisonous de- 
composition reminds us that any food substance is far 
more subject to the action of micro-organisms when it is 
finely divided. A large piece of meat may be quite sterile 
internally and the agents of disintegration will make but 
slow progress working from the surface. When it is 
minced they are immediately introduced into every part, 
and at the same time the natural barriers to their spread, 
the partitions of connective tissue, are broken down. 
Hamburg steak illustrates the same condition. 
Attention should be called to a common source of error 
in fixing upon the food to be held accountable for an 
apparent case of poisoning. Laymen, and often practi- 
tioners also, unhesitatingly place the blame upon some 
article eaten at the previous meal. Particularly is this 
the tendency when some food is conspicuous in material 
vomited. We must consider that the real cause of the 
trouble may have been received into the system many 
hours before the onset of the acute symptoms, so that one 
or two entirely wholesome meals may have been enjoyed 
in the period of incubation. With botulism the delay 
may extend over two days. 
When we deal with any statistics of food poisoning 
we must keep our sense of proportion by noting how small 
-one might almost say infinitesimal-is the fraction of 
a product which even falls under suspicion. Thus Geiger1 
points out that canned salmon has been implicated more 
often than any other single food in a list of recent out- 
breaks, but that nevertheless for every can which seemed 
to cause trouble two and a half million cans were innocent. 
Metals, Preservatives, and Adulterants.-Much indi- 
gestion and general ill health is popularly attributed to 
these causes. There is little doubt that they have been 
1 Journal of the American Medical Association, 1923, Ixxxi, 1275. 258 
NUTRITIONAL PHYSIOLOGY 
overemphasized. The technic of the canning industry 
has been greatly improved. Articles which tend to dis- 
solve much tin are placed in enameled containers. Tin 
in such quantities as may conceivably enter other canned 
goods has not proved demonstrably harmful. Lead from 
solder is now excluded by the use of a new type of seam 
sealed only on the outside. 
Copper has been considerably feared as a possible poison, 
but there is no certainty that it has often figured in this 
way. It was formerly used quite generally to intensify 
the color of canned peas and other green vegetables, but it 
has fallen into disfavor and been largely replaced by or- 
ganic materials, such as chlorophyll. Metal from cooking 
utensils may enter the food in small quantities; it is not at 
all likely to be a source of trouble. Many cases of severe 
sickness following the eating of ice-cream have been as- 
cribed to the dissolved metal of the freezer. An extended 
study of poisonous ice-cream, conducted by Vaughan and 
Novy, has made it probable that the toxic substances are 
organic rather than metallic. 
There is no doubt that ice-cream may be intensely 
poisonous. The investigators just mentioned have tested 
samples of it which would act as almost instantaneous 
emetics, producing no other effect because not retained 
long enough. Inasmuch as a spoonful of such ice-cream is 
more active than an equal quantity of pure zinc sulphate 
it has been inferred that the metal cannot possibly be the 
agent concerned. Still, it is conceivable that obscure 
compounds formed by the union of metals with organic 
bodies may be much more toxic than the simple and fa- 
miliar salts. 
There is a widespread prejudice against the use of anti- 
septic substances for the preservation of foods. Our 
laws carefully restrict the practice. While our preference 
for fresh and natural food is undoubtedly wholesome, it is 
hard to prove that material harm has been done by such 
preservatives. If the choice must be made between foods 
have undergone a prolonged and gradual decompo- FOOD POISONING 
259 
sition and foods of the same age kept by the aid of antisep- 
tics in minute quantities, the latter must in many cases be 
the safer. The mention of benzoate of soda may be pro- 
ductive of disgust, but the same ingredient is native in 
certain foods, such as cranberries. The common adulter- 
ants are not likely to endanger health; a distinction has to 
be drawn between alterations in food which make it in- 
jurious and those which are merely fraudulent. 
We shall do well to refer once more to the possible 
inadequacy of certain diets (Chapter XVI). Such dis- 
orders as beriberi and scurvy might be regarded as 
chronic food poisonings, but it is better to place them in a 
separate class. True poisoning is due to the presence of 
some agent and not to a lack of a specific body. Polished 
rice is not poisonous, it is simply insufficient by itself. 
One reason why foods that have been kept for an unusually 
long time are not to be recommended is that some of the 
valuable compounds originally present may have disin- 
tegrated; it may not be just to call such foods poisonous, 
but their nutritive properties may have suffered some im- 
pairment.1 
1 See Jordan, "Food Poisoning," University of Chicago Press, 
1917. CHAPTER XXV 
ALCOHOL 
Alcohol occupies a peculiar position among the con- 
stituents of the diet of mankind.1 A perfectly dispassion- 
ate estimate of its values and its drawbacks is arrived at 
with difficulty. Most discussions of the subject are frankly 
partisan and, therefore, partial. Alcohol affects the 
human system in many ways, and it is possible to select for 
emphasis either those aspects of its action which are detri- 
mental or those which are favorable. In this way a writer 
may be entirely accurate in all his affirmations and yet fail 
to be just to a complex question because of what he leaves 
unsaid. One cannot properly approach such an analysis 
of the effects of alcohol without first learning the extent 
of its use the world over. In our owrn environment there 
is a feeling of hostility toward the intoxicant which would 
excite wonder in many countries enjoying an advanced 
civilization. Intemperance is deplored by thoughtful 
people everywhere, but the demand for total abstinence 
is sectional, though probably extending steadily. 
The older literature abounds in the praise of wine. The 
Bible itself has many appreciative references to its potency 
as a comforter. It has also eloquent passages which con- 
demn its abuse and records of abstinence on the part of 
certain sects or guilds among the Hebrews. No better 
physiologic distinction has ever been drawn than in the 
verse which places "wine which maketh glad the heart of 
1 This chapter should, perhaps, be recast to read in the past tense. 
The discussion which it contains may be regarded as closed. But 
it has seemed unnecessary to modify it. If the influence of alcohol 
is now a matter of history it may still be of interest to recall the 
time when it was a living issue. 
260 ALCOHOL 
261 
man" in comparison with "bread which strengtheneth 
man's heart." To "make glad" is to minister to feeling, 
to "strengthen" is to confer power which can be demon- 
strated to an observer-an objective instead of a subjective 
result. We cannot point to many great men in the his- 
tory of nations who have entirely avoided the use of al- 
cohol. 
Here in America there was little concerted protest 
against the use of alcoholic drinks until the nineteenth 
century. The Puritans, with all their restriction of recre- 
ation and self-indulgence, were singularly tolerant of hard 
cider and Jamaica rum. This laxity extended to all classes 
of society. About one hundred years ago Lyman Beecher 
described the immoderate drinking which was a feature of 
an ordination to the ministry in a Connecticut parsonage, 
host and guests being clergymen. Beecher himself be- 
came a vigorous leader of the movement for temperance, 
which was not until some years later an agitation for total 
abstinence. A New Hampshire town-meeting (Frances- 
town) voted in 1819 that the unseemly consumption of 
liquor at funerals be discouraged. 
Edward Everett Hale has told us in his "A New Eng- 
land Boyhood" of the common practice of serving wine 
at children's parties about the year 1830. He also 
tells us that when the "Franklin Medals" were annually 
awarded to Boston schoolboys, an entertainment and ban- 
quet was provided from which the youths departed in a 
tipsy condition. The utter impropriety of these proceed- 
ings shows us in an impressive manner how far we have 
moved from the standpoint of that age. The offenses of 
the time appear the more aggravated when we reflect that 
the liquors used were largely of the strongest type. 
The world has long had one conspicuous example of con- 
sistent abstinence on the part of a great population. This 
is afforded by the Mahometan peoples. It is said that the 
Lascar sailors who visit our ports can be allowed shore 
leave with implicit confidence that they will return to the 
ship as sober as when they left it. A seaman who erred 262 
NUTRITIONAL PHYSIOLOGY 
in this respect and was remonstrated with by his captain 
is reported to have excused himself on the ground that 
he had embraced Christianity. 
Something will be said of alcohol under each of five 
heads. We shall proceed to consider it as a relish, a food, 
a drug, a cerebral alterative, and as a poison. While 
such a treatment is most convenient, it must be recognized 
that alcohol can scarcely exert a single influence unmixed 
with the others. One of the five aspects named may be 
particularly prominent for the moment, but traces at least 
of the others are to be looked for. It is this intricacy of 
action which makes it so hard to weigh the facts with 
equity. Another disturbing consideration is found in the 
very unequal susceptibility of different persons to the 
temporary effect of alcohol as well as to its habit-forming 
property. 
Alcohol as a Relish.-It must be sufficiently clear from 
what has gone before that anything that adds to the zest 
and pleasure of a meal may be expected to promote diges- 
tion. The only exception to this rule may be looked 
for when the relish in some way interferes with the di- 
gestive process. Alcohol or, more correctly, alcoholic 
beverages may certainly be held to enhance the enjoy- 
ment of dining, and must, therefore, favor the digestion 
and absorption of food. It is often claimed in rebuttal 
that alcohol retards the action of enzymes. This is doubt- 
less true of high concentrations, but no such mixtures can 
possibly be made to exist in the stomach. Rapid dilu- 
tion by the juices and rapid absorption of the alcohol 
through the lining membrane combine to bring down its 
percentage to a level which cannot hinder the normal 
hydrolysis. 
The appeal of wines and cocktails may be said to be 
due quite as much to the minor substances which they 
contain as to the alcohol. Pure dilute alcohol would 
not be attractive to many people. At the same time the 
extractives conferring flavor and fragrance do not seem 
to make a complete beverage when the alcohol is removed. 263 
ALCOHOL. 
With the frank admission that the finer alcoholic drinks 
may be promoters of digestion, we may fairly couple 
certain qualifying statements. First, such stimulation 
is unnecessary for those whose health is what it should be. 
Second, the employment of such means to spur the ap- 
petite leads readily to overeating. It is also to be ob- 
served that the use of alcoholic relishes gives no sanction 
to drinking apart from meals. 
Alcohol in the stomach has the effect of increasing the 
local blood-flow, and it is well established that absorp- 
tion is thereby promoted. Whether it takes its place 
with the extractives or meat as a chemical agent to 
excite gastric secretion is not so certain. It is hard to 
discriminate between the influence which it exercises 
through the central nervous system and that which it may 
exert upon the stomach wall in a more direct manner. 
The maximum favorable effect upon the digestion is pro- 
duced by a small quantity of alcohol. Larger quantities 
are notoriously apt to nauseate and to precipitate dis- 
graceful scenes, all too common in connection with elab- 
orate banquets. 
Alcohol as a Food.-There has been a great deal of 
opposition to the claim that alcohol can be reckoned a 
food. It has seemed to many of its antagonists that the 
admission weakens their position, but this is not neces- 
sarily the case. To say that alcohol may be a food is not 
to deny that it is a dangerous one. Professor Atwater 
was roundly censured by leaders of the total abstinence 
movement for a publication which is really a powerful 
tract in favor of their position. He had said that alcohol 
might serve as a food, and all his earnest warnings against 
it were regarded as discounted. His experimental work 
on the subject is entirely conclusive. 
The fact has been reiterated that the chief use of food 
is to undergo oxidation with release of energy. Alcohol 
in considerable amounts may be oxidized to carbon dioxid 
and water in the human system. When it is thus oxidized 
the heat value of each gram is about 7 Calories. There 264 
NUTRITIONAL PHYSIOLOGY 
seems to be no provision for the retention of alcohol against 
a future time of need nor for its conversion into glycogen 
or fat. Its oxidation is bound to take place with little 
delay, and it is in this respect not so adaptable to the 
changing demands of the organism as is carbohydrate. 
Nevertheless we must grant that it may take its place 
in the diet as a substitute for other non-nitrogenous 
foods. Atwater's experiments, as well as many parallel 
studies, have made this evident. 
A subject was brought into equilibrium on a ration 
without alcohol. It was then found possible to withdraw 
carbohydrates and fats to a certain extent and to replace 
with alcohol in isodynamic amounts without disturbing 
the equilibrium. The well-known fattening effect of 
moderate drinking may be explained as due in part to the 
whetting of appetite and in part to the sparing of car- 
bohydrate and fat by the alcohol which is utilized in their 
stead. Fifty years ago the same fact was recognized by 
George Henry Lewes, in his interesting "Physiology 
of Common Life." He tells us how a convention of total 
abstainers once gathered in the city of Frankfort, in Ger- 
many, and how the cooks in the hotel in which the delegates 
lodged were put to it as never before to supply the pastry 
and pudding ordered by these unfamiliar patrons. The 
guests for whom the table had heretofore been set were 
accustomed to supply with alcohol a want which the tee- 
totalers met with carbohydrate. 
How far the substitution of alcohol for other non- 
nitrogenous foods may be carried has been much debated. 
If it is given too freely its oxidation is incomplete and, 
what is of more practical importance, the cerebral effect 
becomes prominent. To make the utmost use of alcohol 
as a nutrient it must be taken in small quantities and 
frequently. There seems to be a general agreement that 
from 50 to 75 grams of alcohol in twenty-four hours is 
about as much as can be allowed to an adult without un- 
toward reactions. If we assume the larger amount to be 
permissible, it follows that alcohol may furnish as much as ALCOHOL 
265 
500 calories, or about one-fifth of the day's total. The 
requirement may be translated into terms of various bev- 
erages: whisky, something less than | pint; sherry or port, 
1 pint; champagne, 1 quart; beer, 3 quarts. 
Meltzer has said that "alcohol in health is mostly a 
curse and in sickness mostly a blessing." Its peculiar 
merits in illness are connected with the fact of its appetizing 
character and with the circumstance that it requires no 
digestion. In this it resembles glucose. When alcohol is 
given to a patient no call is made upon the digestive glands. 
Hence it may be tolerated and absorbed when most foods 
would remain undigested. So far as it can be introduced 
into the circulation at such a time it becomes a source of 
heat and spares the dwindling stores of the system, but 
there is an obvious tendency on the part of physicians to 
restrict the use of alcohol in sickness. Statistics from rep- 
resentative hospitals show a marked shrinkage in the con- 
sumption of liquors during the last few years. 
Alcohol as a Drug.-Certain properties of alcohol may be 
conveniently brought under this head, although no sharp 
line of demarcation can be drawn between these qualities 
and others to be dealt with later. The drug effect is 
obtainable from rather large single doses in contrast to the 
nutritive effect which was said above to be best secured 
by small amounts given at intervals. The most striking 
reaction of the system to alcohol in doses which may be 
regarded as having the drug effect is manifested by the cir- 
culatory apparatus. The taking of a glass or two of wine, 
especially on an empty stomach, will usually cause increased 
heart action and a flushing of the skin, accompanied by the 
subjective impression of warmth. This bringing of more 
blood to the surface of the body may be expected to lessen 
the volume of blood passing through the internal organs. 
The chief value of alcohol as a drug is connected with 
this tendency to abate internal congestions. The central 
fact in the process which we call "taking cold" is an excess 
of blood in the mucous membranes. This may be con- 
tinued for a time without ill consequences, but is always a 266 
NUTRITIONAL PHYSIOLOGY 
menacing condition, lowering the local resistance to in- 
fection and so inviting disease. Alcohol has been much 
used to "break up" incipient colds and with very good 
success. Its influence upon the distribution of the blood 
is not unlike that of quinin and some other drugs; it is also 
similar to the action of hot applications to the skin. 
The supposed warming power of alcohol needs critical 
examination. It is a fact not generally recognized that we 
have no reliable sensations indicative of the true body 
temperature. We realize only the surface conditions. If 
a thermos flask and an ordinary bottle are both filled with 
hot water we know that the second will be much warmer to 
the touch. But this is precisely because it is losing heat. A 
man who is out in the cold may produce a sense of comfort 
by taking alcohol, which will send more blood into the 
cutaneous vessels, but this pleasant glow is the sign that 
heat is passing from the body to its surroundings. It may 
be purchased at the cost of an expenditure which is im - 
prudent. Arctic explorers seem well agreed that depend- 
ence on alcohol during exposure to intense cold is unwise, 
and that it is better to suffer a greater degree of discomfort 
than to rely upon its delusive support. This must be most 
distinctly the case when the hardships are to be borne for 
an indefinite period. It is more rational to give alcohol 
after exposure to wet and cold than during the trial. 
Afterward it may help to readjust the circulation and to 
ward off possible evil results. 
Alcohol as a Cerebral Alterative.-After all, the main 
reason why humanity clings to alcohol and is with so 
much difficulty won over to abstinence, is found in its 
singular influence upon temperament. This is at the 
root of its social employment. The rather awkward 
term, "cerebral alterative," has been chosen to avoid the 
more familiar but questionable name of stimulant. Much 
discussion has been carried on concerning the right of al- 
cohol to this designation. Stimulant and narcotic are 
opposing terms. Alcohol in large quantities is clearly a 
narcotic; whether it is invariably so is a subject of lively 267 
ALCOHOL 
debate. It might be supposed that there would be no 
difficulty in deciding. Observation of men slightly af- 
fected by wine shows them to be animated and talkative; 
the natural verdict would be that they exemplify stimula- 
tion. Yet all the results of taking alcohol can be explained 
upon the theory that it is a narcotic. To show how this 
may be it is only necessary to point out that many opera- 
tions of the nervous system are normally inhibitory in 
nature. When a reticent man becomes garrulous, it need 
not be inferred that he is stimulated in the best sense of 
the word; it may be more accurate to say that an agent 
which is essentially depressing in its influence has attacked 
first of all the inhibitory centers. Loss of self-conscious- 
ness is an obvious feature of the reaction, and loss of self- 
respect is reached by an easy transition. 
A genuine stimulant should be an aid to application. 
Alcohol, on the contrary, is hostile to perseverance. Our 
word dissipation, which we use for intemperance, is a very 
suggestive one. Scattering rather than concentration is 
the essence of the mental state produced. A keen thinker 
has said that we tacitly contrast alcohol with coffee, a 
recognized stimulant, when we acknowledge the difference 
between the feelings with which we should view the use of 
one and the other by the engineer of a limited train. We 
instinctively feel that coffee will favor his unswerving at- 
tention to duty and that alcohol will make him less reli- 
able. Alcohol in small quantities leads to inconsequence 
in thinking and is a handicap to any steady pursuit. 
On the other hand, the facile changes in the currents of 
mental life, the lightness and the unexpectedness of one's 
remarks, may promote social ease and the power to be 
entertaining. If alcohol is the foe of application it is also 
the foe of prosiness. 
Too much emphasis can scarcely be brought to bear 
on the wide disparity between what a man thinks that 
alcohol does for him and what an impartial study shows 
that it really does. That "wine is a mocker" is a shrewd 
observation. The subjective impression is often of an ex- 268 
NUTRITIONAL PHYSIOLOGY 
altation of capacity which objective testing fails utterly 
to confirm. A subject does certain problems before taking 
a drink of whisky and comparable ones afterward. He 
says that the second task was done with greater speed and 
with a nonchalant confidence in his results. The watch 
says that he was slower, and checking up the work shows 
that the errors were more numerous. It is evident that his 
judgment of his own performances is unreliable. This is 
true also of manual operations. The recent tests made 
upon German type-setters have shown that speed and 
accuracy are both made to suffer when alcohol has been 
taken even in very limited amounts. Here, again, the 
subjects have an impression of their own superior accom- 
plishment under alcohol, which turns out to be erroneous. 
More disastrous in its consequences is the fatuous assur- 
ance of the intoxicated chauffeur. 
It has been freely admitted above that a little alcohol 
may promote sociability, but there can be no question 
that the reputation which alcohol possesses of bringing out 
the best wit and humor of which men are capable is largely 
unfounded. The reason is not far to seek. This reputation 
rests on the reports of men who were themselves influenced 
by the same agent which was working upon the nervous 
systems of the speakers to whom they were listening. Their 
reminiscences are to be taken with more than a grain of 
salt. The auditor who is "vinously exalted"-to use a 
phrase of Holmes-is an exceedingly lenient critic. He 
applauds with delight sallies which a neighbor, who has 
turned down his glass, perceives to be inane, if not in bad 
taste. 
The justification of the social use of alcohol must be 
based on its power to produce this singular state of mind. 
It removes the consciousness of fatigue and the feeling 
of care. The attention is limited to the present moment 
and immediate interests. The faculty of discrimination 
is dulled, and with the consequent lowering of esthetic 
and intellectual ideals there comes a bland self-satisfaction 
and a naive admiration of one's fellows. A vigorous ALCOHOL 
269 
writer has called this process "drugging for delectation." 
Can such an artifice be defended? It is most difficult to 
answer this question with entire justice to both sides. 
Perhaps it may be impossible to answer it in sweeping 
fashion for all men. One who is cynical and pessimistic 
by nature may really view his affairs more justly and judge 
his neighbors more equitably while under the influence of 
wine. This may be true of other temperaments, the neu- 
rasthenic, for instance. But the optimist-and may wre 
not say the normal individual?-is not likely to be bettered 
by such an agent. Increased buoyancy and good humor 
in such subjects means silliness. If it is true, as is claimed, 
that gatherings of total abstainers are comparatively dole- 
ful, the lesson may be, not that alcohol is necessary to good 
fellowship, but rather that the average nervous system 
is below par. We doubt whether a man ought to rest 
content with any lower measure of health than that which 
will insure the social virtues without chemical aid. 
With advancing age it may be unreasonable to demand 
so high a standard. As infirmities increase there must 
usually come a time when comfort rather than efficiency 
is to be sought. When it is clear that this time has 
arrived there is much to be said in favor of the more or 
less regular, moderate use of alcohol. It is a great 
anodyne. Granting this, we may also point out that the 
beneficent effect in age will be more surely obtained by 
those who have not exhausted the consolations of alcohol 
in earlier years. 
Alcohol as a Poison.-It seems hardly necessary to en- 
large upon the poisonous properties of alcohol. That these 
have been ridiculously exaggerated is obvious; that they 
are very real is equally clear. The spectacle of drunkenness 
and the shame and misery that attend it are too familiar. 
No one who begins to use alcohol can be quite sure that he 
will continue within bounds. The temperate lives of his 
relatives cannot be held to prove him secure. Suscepti- 
bility to the tendency to increase the indulgence is found 
again and again in young men of clean heredity and fine 270 
NUTRITIONAL PHYSIOLOGY 
gifts. Hence the only absolute safety is in total abstinence 
Yet the chances do not favor the ruin of the average man 
who adds alcohol to his diet. 
Aside from the habit-forming property, it is becoming 
more and more widely recognized that alcohol often im- 
pairs the health of men who cannot be charged with in- 
temperance. It predisposes to diseases of the heart, 
liver, and kidneys. It notoriously lessens the chance of 
survival when the user contracts pneumonia. It makes 
him an unfavorable subject for surgical operations. By 
hastening the development of arteriosclerosis it shortens 
the period of active and effective life. Insurance exam- 
iners are glad when they can record of an applicant that 
he is a total abstainer.1 
We often hear people say that it is "bad" liquor that is 
harmful. This is often said where the reference is to the 
non-descript, illicit beverages sold where prohibition is 
supposed to obtain. There may be some ground for the 
claim that cheap and crude liquors are worse than those 
that are more choice. But the impression probably rests 
chiefly upon the sort of men who use the two kinds. Ex- 
pensive and delicate liquors are taken by men of discrim- 
ination, who usually exercise self-control. The prime ob- 
ject is not intoxication. Open a wine-cellar to the repro- 
bate who drinks anything he can buy or beg, and he will 
soon be as abject as though he were limited to his usual 
sources of supply. 
The arguments advanced for prohibition on economic 
grounds have considerable weight and especially in time of 
war. The amount of grain diverted to the manufacture of 
liquor is absolutely large. But the brewers and distillers 
show that it is small compared with the total of the cereal 
crops and they make much of the value of their by-products 
for feeding stock. There are better reasons than the eco- 
nomic for encouraging abstinence in a national emergency. 
At such a time we should forego entertainment if the sacri- 
fice makes us more efficient. If hardship is involved it is 
such as rightminded people should rejoice to accept. What 
1 Series of articles by Fisk in The Atlantic Monthly, 1916, 1917. ALCOHOL 
271 
more stimulating assurance could be given to our soldiers 
than the knowledge that the civil population had cheer- 
fully submitted to a restriction required of the men in 
khaki? 
Enough has been said to show how various are the 
aspects of alcohol. It has been easy to treat them sep- 
arately in the preceding paragraphs, but no such separa- 
tion is possible in practice. The undoubted value of the 
alcoholic relish, its occasional merit as a significant part 
of the ration, and even its virtue as a drug cannot be util- 
ized without some experience of its cerebral effect and the 
risk, not always remote, of forming a habit. The hygienic 
ideal to be striven for is a robustness of life w'hich shall 
make alcohol superfluous as relish, food, or drug, and a 
cheerful, active mind which needs no artificial aid to keep 
it hopeful and sympathetic. The attainment may not be 
an easy task. Grief and worry and overwork may be 
added to an original depression of temperament, but the 
use of alcohol is never more unsafe than when sorrows are 
the excuse, and never so selfish and cowardly as when the 
motive is to shun responsibilities that ought to be faced. 
Men do not often see the sinister suggestion in the high 
spirits of one who has forgotten his cares for an evening by 
the most moderate indulgence. They fail to see that the 
banishment of the sense of pressing duties is the very 
characteristic of the drunkard when, developed to a logical 
extreme, it makes him indifferent to every obligation of 
conscience and of love. 
The man who has never come to set a high value upon 
alcohol must stand amazed at the resistance now exerted 
against the enforcement of prohibition. Every voter has 
sworn to support the Constitution of the United States. 
Every federal officer has assumed additional and solemn 
obligations to do so. Yet responsible citizens, with an 
air of pride rather than shame, involve themselves in 
perjury and come near to treason. We learn of officials 
corrupted, we read tales of hardships endured by the 272 
NUTRITIONAL PHYSIOLOGY 
purveyors of liquor, men whose physical fortitude and 
resourcefulness would be admirable in a legitimate pur- 
suit. There is one obvious conclusion: the desire to 
drink is so strong that for multitudes it overrides ah moral 
restraint. Expressed in terms of money it is productive 
of the vast funds constantly available to suborn the 
guardians of the frontier and to compensate the hierarchy 
of the bootleggers for all risks and losses. 
This is not primarily an ethical treatment of the sub- 
ject. But the existing conditions seem to the writer to 
place alcohol in a worse light than ever before. The 
urgency of the demand is startling to imagine. Consider 
that the taste is an acquired one. The youth given liquor 
for the first time does not sincerely say, "This is some- 
thing I have always needed," but rather, "This is not 
nearly so good as I was led to expect." If the generation 
of drinkers could pass without making disciples of the 
young it is hard to believe that there would be any further 
desire for alcoholic beverages. 
Reference: "Alcohol: Its Action on the Human Organism," by a 
committee of British men of science, Longmans, New York, 1918. CHAPTER XXVI 
INTERNAL SECRETION 
In an early chapter of this book it was stated that there 
are two ways in which one organ of the body may exert 
an influence upon another. The more familiar and more 
studied method has been through the nervous connections 
which are maintained between all organs and the brain 
and cord. The existence of such ties provides for the 
possibility of reflex action. By this means any part of the 
organism may modify the behavior of any other part, not 
by affecting it directly, but by stimulating or inhibiting 
the centers which preside over the second organ. Some 
account of the work of the central nervous system is to 
be given hereafter. Before this is undertaken it is well to 
pay some attention to the other way in which co-ordination 
is promoted; namely, by the transfer of chemical products 
from place to place through the agency of the circulation. 
This is the subject usually covered by the term internal 
secretion. The word hormone1 has been used elsewhere to 
denote an active substance generated in one place, but 
destined to take effect in another. The secretin produced 
in the walls of the upper part of the small intestine and 
carried thence to the pancreas and the other digestive 
glands to excite them to pour out their juices is an example. 
So also is the contribution made by the pancreas to the 
blood, which proves to be so important to the utilization of 
sugar. In this case it seems to be chiefly in the muscles 
that the hormone is valuable. Other instances of similar 
1 The root-meaning of hormone is "an excitant." It has been 
urged by Shaefer, "The Endocrine Organs," Longmans, New York, 
1916, that internal secretions may be depressants as well as stimu- 
lating agents. He proposes to call those which are inhibitory 
"chalones." The term is not yet in general use. 
273 274 
NUTRITIONAL PHYSIOLOGY 
interaction can now be given, and there is reason to an- 
ticipate that the list of internal secretions will soon be 
made longer than it is at present. 
There are several organs once regarded as insignificant 
which are now recognized as vital to the welfare of the 
whole system. Among these the thyroid gland has at- 
tracted particular interest. This is a small bilobed mass of 
tissue situated in front of the trachea below the larynx. 
It is one of several organs sometimes called ductless glands. 
The term is more appropriate in this case than in some 
others, since the microscope shows that the arrangement 
of the cells is distinctly glandular. They surround small 
recesses such as in typical glands would be in communica- 
tion with an outlet or duct. Here, however, these cavities 
are blind. They are seen to contain a viscid material, the 
so-called colloid substance, which is evidently the product 
of the secreting cells. Since there is no channel leading to 
the exterior, the only possibility is that the distinctive 
secretion shall enter the circulation either directly or by 
way of the lymph. 
The thyroid gland is frequently enlarged and then gives 
rise to the disfiguring swelling known as a goiter. Such 
enlargements are not necessarily attended with general 
disturbances of health, yet in many cases there are symp- 
toms which can be referred to an excess of the active 
product. Palpitation, breathlessness, extreme nervous- 
ness, and marked loss of weight are likely to be observed. 
The same manifestations follow the giving of overdoses of 
thyroid extract. This has been employed for the correc- 
tion of obesity, but it seems unwise to resort to a drug 
so powerful and far reaching in its action for the treat- 
ment of this condition. 
Just as there may be too much of the thyroid material 
for the good of the subject, so there may be a serious de- 
ficiency. The relative failure of the gland to function as 
it should is the cause of a definite disease in human sub- 
jects, and its removal from dogs is followed by a decline 
if not by death. Young and growing animals are most 
seriously affected. All the facts which have been gathered INTERNAL SECRETION 
275 
support the belief that development and general well- 
being depend to a considerable extent on the normal 
functioning of the thyroid. Very recently it has been 
shown that what has been called the thyroid is, in reality, 
a compound structure. In addition to the type of tissue 
which forms the main mass of the organ in man, there are 
four nodules of a different sort, the parathyroids. These 
undoubtedly have a chemistry of their own and a distinc- 
tive relation to the economy of the body. For a statement 
of their peculiar importance reference must be had to 
larger works on physiology. 
The full measure of the influence which radiates from 
the thyroid can be appreciated by considering the condi- 
tion known as cretinism. This is the term used to de- 
scribe the state of individuals in whom the thyroid has 
never performed its proper work. These subjects remain 
for years in a condition of arrested progress, both physical 
and mental. They are uncouth dwarfs with large heads, 
slack-walled abdomens, and feeble limbs. If they survive 
to the age of twenty or thirty they will scarcely have ad- 
vanced beyond the stage reached at four or five. That 
the lack of the thyroid is actually responsible for these 
shocking cases is now abundantly proved. The demonstra- 
tion is found in the happy circumstance that great im- 
provement follows the judicious feeding of thyroid prepa- 
rations to cretins. The material is obtained from calves 
or sheep. Its administration for a few months often re- 
sults in transforming a repulsive cretin into a presentable 
child with a prospect of at least a moderate mental de- 
velopment. 
In the dog it is possible to graft an extra thyroid into 
the abdominal cavity and then to remove the original 
gland, when the second organ will assume the functions 
necessary to the preservation of health. This was an 
important discovery, since it removed the ground for a 
prevalent opinion that the service of the thyroid was 
limited to the reflexes which it was supposed to originate. 
It had been thought that the gland affected the nutrition 
and general health by sending impulses along the nerves 276 
NUTRITIONAL PHYSIOLOGY 
leading from it to the centers. A gland substituted for the 
native one and placed in a remote part of the body could 
not be in connection with the old afferent pathways, and 
whatever favorable effect it might have must be chemical 
in its origin. 
There has been the same discussion as to whether the 
reproductive organs exercise their well-recognized influence 
on growth by nervous or by chemical means. The strange 
modifications of the type which are produced as a result 
of castration are familiar. The differences in build and 
temperament between the unruly bull and the tranquil ox 
illustrate the consequences. Such peculiarities might be 
referred to reflex changes due to the removal of sources 
of stimuli, but the present tendency is to regard them as 
due chiefly to the loss of active internal secretions. The 
case of the pancreas reminds us that an organ may well give 
rise simultaneously to products which are permanently sep- 
arated from the blood and to others which return to it. 
In 1889 created a furore among people 
given to premature acceptance of extravagant claims when 
he announced that great rejuvenating virtues could be 
demonstrated to exist in the extracts of animal reproduc- 
tive glands. His so-called "Elixir of Life" was a pulp 
formed from crushed testes of sheep. At some peril 
of infection he injected this unsterilized mixture under his 
skin and into the bodies of other aged volunteers. A cer- 
tain degree of stimulation was noted, but it has usually 
been referred to suggestion. 
Since animals and men are at their best physically and 
intellectually when the reproductive organs are active, 
the expectation entertained was not wholly unreasonable. 
Yet it was not to be supposed that their decline was re- 
sponsible for all the losses incident to age. No exterxsive 
use of such extracts has followed the pioneer work of 
Brown-Sequard. But the principle has become familiar 
of late in connection with the grafting of. testicles-the 
"interstitial glands" of the daily press. Preparations from 
the female-ovarian extracts-have a. good standing in INTERNAL SECRETION 
277 
modern medicine. Given after surgical removal of the 
ovaries they greatly relieve many of the symptoms which 
commonly annoy the patient. During the progress of preg- 
nancy we witness the most marvellous adjustment of vari- 
ous parts of the body to meet new requirements. The 
changes in the uterus, the mammary glands, etc., seem 
to be due to the influence of hormones disseminated by the 
ovaries and the embryo itself. 
The adrenal bodies are also numbered among the organs 
which help to maintain the system in normal working order. 
These are two inconspicuous structures placed one above 
each kidney. Their microscopic features are obscure and 
do not indicate a glandular organization. From the ad- 
renal bodies can be obtained a powerful drug-like com- 
pound, adrenin, which the living cells of these organs may 
be supposed to deliver, continually or occasionally, to the 
passing blood. Its presence in the circulation is evidently 
a necessity. Destruction of the adrenals-which some- 
times results from local tuberculosis-produces the sin- 
gular fatal disturbance known as "Addison's bronze dis- 
ease." An odd feature of this condition is the dark pig- 
mentation of the skin, occuring sometimes uniformly and 
sometimes in patches. More important, however, is the 
steady loss of vigor affecting all the contractile tissues and 
leading to death. 
Adrenin injected into the blood-stream of an animal 
shows its most marked effect in the intense contraction of 
the small blood-vessels which is produced. This property 
has made the substance valuable to surgeons, since it can 
be used to check bleeding from cut surfaces. In like man- 
ner it can reduce congestion in inflamed tissues, for ex- 
ample, in a blood-shot eye. These artificial uses of adrenin 
can all be connected with its natural service, which is 
largely in the direction of a reinforcement of contractility. 
Cannon has recently called attention to this fact in an 
unexpected and interesting relation. 
He has proved by delicate tests that the blood of an 
animal receives additional adrenin during a terrifying 278 
NUTRITIONAL PHYSIOLOGY 
experience. For example, he has found the active com- 
pound increased in the blood of a cat which has been kept 
under restraint in the presence of a barking dog. There- 
fore it must be concluded that one of the many bodily 
accompaniments of an emotional outbreak is a stimulation 
of the adrenal bodies to unusual activity. The value of 
this reaction has just been made clear through the further 
researches of the same investigator. It appears that 
extra resistance to fatigue is conferred upon the skeletal 
muscles when adrenin is sent to them. The heart is stim- 
ulated, the blood is directed to the muscles rather than the 
viscera, and extra sugar appears in the circulation. Thus, 
in an emergency that might call either for flight or conflict 
the animal is prepared for a maximum output of energy. 
We have here a scientific conception of the "strength of 
desperation."1 
The influence of other organs than those mentioned upon 
the welfare of the organism as a whole is constantly being 
studied. A great deal of attention is being given to the 
very small but distinctly compound structure, called the 
hypophysis, which is lodged beneath the brain in a hollow 
of one of the cranial bones. Its relation to the course of the 
metabolism is far from simple, but it seems to be clear that 
it is an indispensable contributor to the circulating medium. 
Certain abnormalities of the hypophysis may be respon- 
sible for overgrowth even to gigantic stature. Opposite 
conditions may restrain growth and be responsible for the 
state of some dwarfs. Perverted action of this organ in 
the adult may strangely alter the features through changing 
the shape of the bones of the face. A much larger and 
more conspicuous affair, anatomically speaking, is the 
thymus of young animals, a mass of cells lying back of the 
breast-bone. It is the "neck-sweetbread" of the market. 
Since it is prominent during growth the inference is natural 
that it ministers in some way to the process of develop- 
ment. It almost vanishes at maturity. It is not yet 
1 Cannon, "Bodily Changes in Pain, Hunger, Fear, and Rage," 
Appleton, New York, 1920. INTERNAL SECRETION 
279 
possible to make very definite statements about the thy- 
mus. Removal does not seriously hinder the progress of 
the growing animal. 
Here and there in the body are firm kernels of tissue, 
spoken of as lymphatic glands or, better, as lymph-nodes. 
These may be discussed among the producers of internal 
secretion, though it is probable that this description does 
not cover all their activities. They are found especially in 
the neck, the armpits, the groins, and in the mesentery. 
Wherever placed, each is set upon the route of the lymph 
as it comes from some region toward the chest. Thus the 
lymph-nodes of the neck must be passed by the lymph 
that has had its origin in the head, while those of the 
mesentery intercept that which has come from the in- 
testine. Microscopic study shows that the lymph has to 
take a tortuous course among the cells of the nodes. 
Looked at from a mechanical standpoint these bodies are 
obstructions in the path. They are also suggestive of 
filters. Their own cells are continually becoming detached 
and drifting away in the lymph, a phenomenon which is a 
type of internal secretion made visible. They are believed 
to add substances in solution as well as cells to the passing 
fluid. 
There is good ground for the view that the lymph- 
nodes are a defense against the spread of infection. When 
a boil exists upon the arm the nodes above the place where 
the bacteria are working so destructively are usually ob- 
served to be enlarged and tender. The lymph which is 
returning from the seat of the trouble is bearing the pro- 
ducts of the suppuration, if not the infecting organisms 
themselves. Apparently this polluted lymph is more or 
less successfully disinfected before it is allowed to pass on 
into the thorax to merge with the blood in the veins. The 
lymph-nodes of the mesentery stand as outworks of the 
bodily fortifications against the entrance of microbic 
invaders from the intestine. When overpowered in their 
struggle the lymphatic glands themselves become foci of 
infection, as in the familiar form of tuberculosis known as 
scrofula. 280 
NUTRITIONAL PHYSIOLOGY 
One large organ, which from its anatomic relations has 
been called a ductless gland and which might be expected 
to have an internal secretion, has failed to give satisfac- 
tory evidence of such a function. This is the spleen, which 
is placed below the diaphragm to the left of the stomach. 
It remains an enigma to physiologists. Its blood-supply 
is large and its frequent changes of volume, contractions, 
and dilations alternating in a slow rhythm, give a strong 
suggestion of some well-marked action in progress. But 
it has never been shown that an animal is affected in any 
characteristic way by the loss of the spleen, provided the 
immediate effects of the severe operation are survived. 
Certain enzymes have been extracted from the tissue of the 
spleen, which may have to do with the metabolism of those 
peculiar proteins which yield uric acid. We may not be 
warranted in asserting that the spleen has no useful ser- 
vice to perform, but we can say that other organs appear 
to be able to make good its deficiency. 
In a few cases of anemia improvement has followed 
removal of the spleen. The suggestion is that the organ 
normally destroys red corpuscles and that the extent of 
this destruction may become excessive. CHAPTER XXVII 
THE NERVOUS SYSTEM 
In the introductory chapter it was said in substance 
that the one word which most nearly covers the work of the 
nervous system is the word co-ordination. This state- 
ment arouses in one the impulse to protest that it leaves 
out of account the relations subsisting between the nervous 
system and the states of consciousness which are of the 
most immediate interest to us all. The physiologist, being 
human, sympathizes with such a protest, but he must 
continue to treat his material for the most part from an 
external point of observation. This is not for want of 
respect for the psychologic method; it is rather with frank 
recognition of the vastness of the realm in which that 
method is applicable. It is because he must defer to 
experts in that field that he will not enter upon it as an 
amateur. 
Most readers need to be told with the utmost emphasis 
and with frequent reiteration that consciousness is the 
accompaniment of an excessively small share of the mani- 
fold reactions of the nervous system. In the words of 
President Hall, it is "a little candle burning in one room of 
the mind's museum." All that occurs from first to last in 
the life history of a fish or a frog can be explained as reflex 
adjustment, without the assumption of self-knowledge or 
conscious purpose on the part of the animal. That was a 
weird and fascinating picture which duBois Reymond once 
drew of a world precisely like our own, save that its in- 
habitants were unconscious. In such a world an artist 
without will or pleasure in his work might create a faultless 
statue because his inherited nervous mechanism and the 
281 282 
NUTRITIONAL PHYSIOLOGY 
existence of materials, tools, and a model, made the result 
inevitable. 
In the previous discussion of reflex action (Chapter V) 
the conception was developed that the nervous system con- 
sists of pathways capable of transmitting energy in the 
form of "nerve-impulses" to and from its central portion. 
That central part is represented in the higher forms by 
the brain and the spinal cord. The afferent or incoming 
paths begin in localities where external influences or stim- 
uli can be brought to bear. The nerve-endings which lie 
thus exposed are called "receptors." They may be simple 
terminal twigs or bulbs subject to stimulation by pressure 
or temperature. They may be connected with elaborate 
organs like the eye and the ear. The eye is a device for 
converting the energy of certain ether-waves into the im- 
pulses that traverse the optic nerve. The ear is, at least 
in part, a device for transforming the energy of certain 
air-waves into impulses that run along the auditory 
nerve. 
The impulses which arrive within the brain or the cord 
may cause the immediate or delayed return flow of im- 
pulses along the efferent or centrifugal paths of the system. 
These lead in most cases to the contractile tissues which 
give objective expression to animal life. It must not be 
forgotten that the efferent branches of the nervous system 
extend also to various glands and may modify secretion as 
well as contraction. It will be recalled that efferent im- 
pulses may initiate the flow of tears, of the saliva, the 
gastric juice, and the adrenal principle. Other instances 
are almost equally clear. 
In animals of all grades responses of the reflex type are 
the constant duty of the nervous equipment. Those forms 
which we regard as highest in the scale are distinguished by 
a second property, that of the modification of the re- 
sponses as a result of individual experience. By this is 
meant the capacity for training, forming habits, or learn- 
ing to profit by the past. It is what makes different mem- 
bers of the same species vary in their behavior. It is what THE NERVOUS SYSTEM 
283 
makes an older member superior to a younger one in his 
powers of adjustment and maintenance. This is, of course, 
exemplified in the fullest degree by human beings. The 
prolonged period of growth and progress must indicate 
a plasticity in the arrangements of the nervous elements 
that is hardly ever entirely outlived. 
Descartes in the seventeenth century grasped the two 
properties with his usual keenness. He pictured the reflex 
process with quaint symbolism, but with essential correct- 
ness. He likened a path of afferent transmission to a cord 
that is pulled. A valve is opened in the central organ and 
a fluid released to flow back along the nerve. Reaching 
a group of muscles this fluid wakes them to action. Des- 
cartes had used different analogies for the afferent and the 
efferent impulses, making one a mechanical twitch and the 
other a spurt of liquid; we now believe them to be nearly 
or quite identical in character. The same writer recog- 
nized the ability of the higher nervous systems to retain 
impressions, and likened the registration of a memory to 
the imprint of a seal upon wax. 
Physiologically, the particular mark of the highly de- 
veloped nervous axis is this capacity for recording indi- 
vidual and not merely racial experience. Anatomically, 
the advance in organization is signalized by an increasing 
predominance of the brain over the cord and of the 
"fore-brain" over the parts behind. A frog will carry out 
many complex reactions when the entire brain has been 
destroyed. That is to say that in the frog the cord is ade- 
quate for a great deal of reflex action. In a human para- 
lytic whose injury has removed the lower half of the spinal 
cord from functional union with the brain, the reflexes 
which can be obtained from the lower extremities are 
few. In other words, the cord in man has become less 
important as an organ for correlating afferent with 
efferent impulses and more important as the largest of all 
nerves, the highway to and from the brain. We shall now 
set forth in some detail certain of the adaptive changes 
which are constantly occurring through the mediation of 284 
NUTRITIONAL PHYSIOLOGY 
the brain quite apart from attention or desire on the part 
of the subject. 
This kind of subconscious control of organs is well illus- 
trated in the case of the respiratory center. About a 
hundred years ago it was found by French workers that 
cutting across the nervous axis at the point where it leaves 
the skull-that is, where the brain passes into the cord- 
instantly stops the breathing. A similar cut a little higher 
up does not have this immediately fatal effect. It was in- 
ferred that the part of the brain next adjoining the cord 
must contain the special cells which stand in charge of the 
respiratory muscles. This section of the brain, just within 
the skull, is called the medulla. Later experiments have 
confirmed the early belief that the breathing is governed 
from this region. The muscles employed are not in them- 
selves automatic. Every contraction which they make is 
referable to a metabolic change in the cells of the medulla. 
So there is a fundamental difference between the breathing 
movements and the beating of the heart. The former 
are due to central causes; the latter, to an innate quality 
of the contractile substance. 
The respiratory center is frequently involved in reflex 
action. In fact, it is easy to convince one's self that hardly 
any considerable reflex occurs without some disturbance of 
the breathing as an incident. Any sort of shock will in- 
fallibly change the depth, regularity, or some other feature 
of the movements. Outcries following such shocks are 
merely respiratory reflexes, but the center is not prompted 
to each successive discharge by afferent impulses; it shows 
us the possibility of another means of regulation. This 
is through the influence upon the nerve-cells of the 
chemical composition of the blood and lymph in the 
vicinity. When exercise is taken the breathing is involun- 
tarily deepened. The cause of this adjustment is found in 
the increase of carbon dioxid in the circulation. The 
center is remarkably sensitive to any rise in the percentage 
of this gas. Conversely, it is temporarily paralyzed by a 
reduction of the circulating carbon dioxid to an unusually THE NERVOUS SYSTEM 
285 
low level. Variations of the oxygen of the blood, unless 
extreme, affect its action surprisingly little. 
An important work of the nervous system and one which 
is quite overlooked by the layman consists in the regula- 
tion of blood-flow. In our previous description of the 
plan of the circulation only its mechanical principles were 
discussed: the heart was spoken of as a force-pump and the 
vessels as elastic tubes. This is a proper presentation so 
far as it goes, but we must now proceed to show that the 
whole circulatory apparatus is living, and being so has the 
biologic characteristic of adaptation. The changes by 
which the ever-varying requirements of the body are met 
are of two kinds, changes in the force and frequency of the 
heart-beat and changes in the caliber of the small arteries 
and veins. The former are brought about by the cardiac 
nerves; the latter, by a department of the nervous mechan- 
ism which we call the vasomotor system. 
It is a matter of familiar experience that the heart-rate 
is subject to striking variations. From an average of per- 
haps 65 per minute, when one is lying in comfortable re- 
laxation, it can be driven to 150 or more, when one is 
hurrying up a grade. Such a change under the influence of 
muscular activity is evidently purposeful. The working 
tissues need a swifter current of blood to supply them with 
oxygen and to bear away their waste. The lungs must be 
visited at shorter intervals to keep normal the gaseous com- 
position of the blood. Muscular activity is not to be sup- 
ported by increased breathing alone; it is equally necessary 
that there shall be acceleration of the circulation. As the 
quick, deep breathing of one who is taking exercise be- 
speaks a governing center for the muscles employed, so 
the rapid beating of his heart suggests a central control of 
that organ. 
Experimental proof of the central regulation of the heart 
is ample and detailed. Branches of the nervous system 
reach it from two distinct sources and are contrasted in 
their effect upon it. One set of fibers is said to be inhib- 
itory, the other to have an accelerator action. The terms 286 
NUTRITIONAL PHYSIOLOGY 
would seem to explain themselves. The inhibitory fibers 
restrain the heart much of the time from beating at the 
rate which it would exhibit if nervous regulation were 
entirely withdrawn. An exaggerated inhibitory influence 
may weaken its working to the point where the circulation 
becomes quite inadequate and faintness is produced. In 
laboratory trials actual arrest of the heart of a dog may be 
caused, though the beat is always resumed within a minute, 
so that death cannot be made to result . We ought not to 
lay much stress upon these extreme possibilities. It 
would be absurd to say that the function of the inhibitory 
cardiac nerves is to stop the heart; that could never be for 
the best good of the animal. We can say with more reason 
that their service is to economize the strength of the heart 
and to provide a reserve for emergencies. This has been 
described as a "brake action." 
The accelerator nerves of the heart have a stimulating 
effect which extends to the vigor as well as to the rate of 
its beating. When we observe a specific increase of heart 
action we can hardly say whether it has been produced by 
positive accelerator influence or by the abatement of the 
habitual inhibitory control. It may, of course, represent 
the combined result of both. The government of organs 
by means of the balanced action of two opposing sets of 
nerves is repeatedly met with in the body. It can be de- 
monstrated for the stomach and for other sections of the 
alimentary canal. A singular example is afforded by the 
nervous relations of the iris, the colored ring surrounding 
the black pupil of the eye. Stimulation of one nerve causes 
contraction of the pupil, while widening or dilatation fol- 
lows the application of stimuli to an entirely different 
strand of fibers. 
The heart may quicken the circulation by beating at a 
more rapid rate and with increased power, but it cannot 
send the blood to a particular part of the body at the ex- 
pense of another part. The total blood-flow thus depends 
upon the heart acting under the balanced sway of inhib- 
itory and accelerator nerves, but the distribution of blood, THE NERVOUS SYSTEM 
287 
the favoring of organs which need it, is the work of the 
vasomotor system. The walls of the microscopic vessels, 
those which adjoin the capillaries in particular, are provided 
with muscular elements of the same general order as those 
which produce the movements of the digestive tract. 
These contractile elements are connected with the terminal 
branches of certain nerves. Hence the anatomic study of 
the tissues involved, even when unsupported by physio- 
logic experiments, makes clear the possibility that the 
centers when acting reflexly or otherwise may influence 
the diameter of the blood-vessels and the volume of the 
circulation in any or all regions of the body. 
Physiology reinforces anatomy at this point. For sixty 
years it has been certainly known that nervous regulation 
of the blood-flow is a fact. Much earlier than this the 
power was assumed to exist, "Ubi stimulus, ibi affluxus," 
they said, meaning that where there is activity there is an 
increase of blood. The paling and the flushing of the skin, 
occurring either in response to external changes of tempera- 
ture or emotional conditions, are strongly suggestive of a 
central command of the vessels. This has been taken for 
granted in our own treatment of the matter of the main- 
tenance of a normal body temperature. Much as the heart 
is reached by impulses which inhibit it and by others which 
spur it to a greater expenditure of energy, the small arteries 
and veins are subject to antagonistic influences. We speak 
of a vasoconstrictor effect when we mean a reinforcement 
of the existing tone, and we use the word vasodilator in 
the opposite sense, that is, to characterize changes in 
which the degree of contraction is lessened, with the result 
that the blood finds its way in greater quantity through the 
affected vessels. 
The difference between the cardiac and the vasomotor 
factors in the control of the circulation can be made plain 
by means of an illustration borrowed from the laundry. 
Suppose that a supply-pipe runs along above a row of 
tubs and bears a faucet for each. Suppose, also, that there 
is a stop-cock on the pipe before it reaches the tubs. Fi- 288 
NUTRITIONAL PHYSIOLOGY 
nally, to make the correspondence closer, assume that the 
faucets can never be shut tight (for it is not likely that the 
blood-vessels can be). Now, by manipulating the cock 
(//) we can increase or diminish the outflow into all the 
tubs, but we cannot in this way hasten the filling of one 
more than another. This is obviously the type of cardiac 
regulation in the living system. The heart is appealed to 
Fig. 23.-The supply to all the tubs is controlled at the " shut 
off " (H), while the share flowing into each compartment is regulated 
by its own faucet (V). 
when there is a general and widespread demand for more 
blood per unit of time, muscular activity being by far the 
most important occasion. 
The faucets over the several tubs are the symbols of 
vasomotor equipment. By using them we can change the 
share of the total stream which shall enter any compart- 
ment. If our only desire is to fill one tub, we can open 
widely that faucet and close the others as far as possible. 
The vasomotor system actually operates in that way to the THE NERVOUS SYSTEM 
289 
extent that a dilation in one large area seems often to be 
offset by a compensatory constriction in another. We 
count on this reciprocal relation in our medical and hy- 
gienic practice. For instance, we think that if we can en- 
courage blood-flow at the surface we shall reduce it in the 
deeper parts. This is often an object, since congestions 
are important features of many disorders. We antici- 
pate this balanced reaction when we make use of hot ap- 
plications and when we give alcohol for a cold. 
No one is likely to overestimate the services rendered 
by the vasomotor system. It has both a general and a 
local action. In the first sense it maintains a certain 
average state of contraction in the arteries-the arterial 
tone-which keeps up to a desirable level the pressure in 
the arterial trunks. It is this pressure which guarantees 
the prompt and sufficient supply of blood wherever the 
paths are opened. The principle is the same that is ob- 
served in the mains distributing water through the streets 
of a city. The pressure must be great enough to keep up 
the supply on the high ground as well as the low. Leakage 
or wasteful use of water will lower the pressure and the 
failure will be noted first on the hills. An unusual dila- 
tion of many blood-vessels, such as probably accompan- 
ies an attack of indigestion where the abdominal organs are 
engorged, may lessen the pressure and the volume of blood 
passing through the brain with the consequence that there 
is faintness. Partial relief is found in lying down because 
the factor of gravity is eliminated and the head given an 
equal chance with the rest of the body. 
We notice the failure of the vasomotor system to do its 
duty in cases like the above. We ought also to appreciate 
how remarkable it is that the adjustments usually occur 
with such smoothness and success. It is really a wonder- 
ful thing that we can rear up the elongated human form 
from a horizontal to a vertical position without entirely 
deranging the circulatory mechanism. That the blood 
does not distend the vessels below the heart and forsake 
those above must be due in a great measure to vasomotor 290 
NUTRITIONAL PHYSIOLOGY 
correction. Doubtless, when one rises to his feet in the 
morning one is saved from falling in a faint by the prompt- 
ness with which the arterial tone in the lower extremities 
and abdomen is raised by the nervous system to fight 
back the threatened excess of blood. It is easy to see 
what would happen in a lifeless model under similar 
conditions. 
While our vasomotor reactions are usually executed 
without our conscious attention, it has always to be 
borne in mind that close ties exist between the higher and 
the lower regions of the nervous fabric. Emotions register 
themselves in changes of blood-flow. The obedience of 
the vasomotor center to hypnotic suggestion is almost 
unlimited. It would appear that this division of the ner- 
vous system is the chief mediator in the correction of the 
many ills which unquestionably yield to Christian Science 
and kindred ministrations. A normal circulation goes 
far to insure normal nutrition and irritability in each part, 
and these states are the fundamentals of health. Fur- 
thermore, the feeling of health-which is not always the 
same thing as the possession of health-must depend, in 
the absence of more positive sensations, on the character 
of the cerebral circulation. Assuming the blood to be 
chemically normal a moderate change in the amount 
passing through the brain may make all the difference 
between the sense of power and the feeling of utter help- 
lessness. 
Beside dictating the character of the breathing and 
determining the volume and distribution of the blood- 
flow, the lower part of the brain exercises control of the 
alimentary canal and, in varying degree, of the glands. 
To mention these facts is merely to recapitulate what has 
already been pointed out. It must be recalled that the 
government of the glands is partly through direct com- 
munication with their secreting cells and partly through 
vasomotor regulation of their blood-supply. In many 
cases of sustained glandular activity both features exist 
at the same time. CHAPTER XXVIII 
THE NERVOUS SYSTEM-ITS HIGHER WORK 
" Our wills are ours, we know not how. . . . " 
In the last chapter emphasis was constantly placed on 
the subconscious nature of the multifarious adjustments 
which are each moment secured through the action of the 
central nervous system. We are accustomed to think 
that the use of our sense-organs and the employment of 
our skeletal muscles are activities with which consciousness 
is far more closely concerned. While this is broadly 
true, we need to recognize that a great part of these re- 
actions also goes on without our notice and beyond the 
reach of our intervention. This is readily admitted of the 
breathing. It will be found almost equally characteristic 
of the maintenance of the balance at rest and during 
locomotion. 
The ability to stand is dependent on the occurrence of 
inconspicuous but indispensable reflexes which check each 
swaying movement of the body as it threatens a fall. 
When one walks the attention seems for the most part to 
be detached from the elaborate muscular performance and 
to be given to other matters. The contact of the feet 
with the ground, the gliding of one joint surface on another, 
the shifting of stresses from one muscle or tendon to a 
neighbor-these local changes become in a regular se- 
quence the source of impulses which ascend to the brain 
and evoke appropriate responses. It has bean generally 
believed that the division known as the cerebellum has a 
peculiar importance in this connection. 
The position of the thinker himself with reference to the 
great afferent and efferent departments of his nervous 
291 292 
NUTRITIONAL PHYSIOLOGY 
system may be likened to that of the general in command 
of a great army. From his headquarters he can see but 
a small proportion of his troops. Their line of battle 
stretches for miles beyond his sight. He issues the order 
for a general advance. His aides ride to the right and left, 
bearing the word to the commanders of corps and divisions. 
The simple act of the general has been followed by a train 
of events which becomes each moment more difficult to 
trace. The original order is transmitted to officers of 
lower rank and interpreted by them in conformity with 
local needs. When the private soldiers are at last put in 
motion it is at the word of colonels and captains. The 
impossibility of having the voice of the commander-in- 
chief the source of guidance in each company is perfectly 
evident. It is not merely that he is too far removed from 
most of his men, but that there are too many problems 
arising at one time. The minor ones must be solved at 
the discretion of his subordinates, 
So in the living body the will to walk-which seems as 
simple as the dictation of the first order from headquarters 
-is promptly followed by the action of many nervous 
mechanisms whose function is to distribute impulses and to 
apply them in helpful sequence. We have no sense of the 
subdivision which is involved. We cannot analyze the 
groups of muscular movements which take place. Yet 
it is plain that there is an apportionment of stimulation, 
more to this muscle and less to that, without which the 
effective result could not be secured. How utterly we 
should fail in the attempt to regulate the part taken by 
each of a hundred co-operating muscles by giving attention 
to each in its turn! The efforts necessitated would be like 
those of the general deprived of his staff and messengers 
who should seek to ride swiftly from point to point in the 
endeavor to direct all his soldiers by his spoken word. 
The comparison we have been using can be made to 
serve even further. We can find in it a place for the af- 
ferent side of nervous action. We have said that reflex 
elements can be found in almost any elaborate movement. THE NERVOUS SYSTEM-ITS HIGHER WORK 
293 
In walking, for example, the assumption of a given 
position by the body insures the return to the centers of 
impulses which cause the next appropriate change. We 
do not consciously resolve to take each step. The very 
idea is painful. One step accomplished makes the taking 
of another the natural sequel. Turning to our analogy, 
we readily see that when the grand advance is under way 
the continuance of the march will probably be intrusted 
to the direction of lower officers. Difficulties encountered 
they will report to the general if this seems necessary; 
minor obstacles they will meet upon their own responsi- 
bility. Thus when we walk we may be aware of gutters 
to be crossed, but we do not notice at all the incessant 
slight adjustments which are made for the lesser inequal- 
ities of the path. 
The human brain contains at birth connections which 
make possible the execution of a moderate number of 
useful reflexes. Among these are sucking, winking, cough- 
ing, sneezing, and vomiting. Of course, there are also the 
requisite mechanisms for the government of breathing, 
the circulation, and the digestive tract. Such a brain 
appears to differ from that of one of the lower animals 
chiefly in its capacity for continued development. Loeb 
has shown that all that is acquired in the experiences which 
come to the child may be covered by the term "associative 
memory." The expression as used by him does not refer 
to the power to review past experience as a conscious 
process, but only to the power to acquire new reactions to 
environmental conditions. An early gain in this direction 
is the attainment of the ability to reach after and grasp an 
object which has stimulated the visual department of the 
nervous system. This seems to signify the opening of a 
pathway in the brain from the region receiving impulses 
from the retina to the region from which impulses go out to 
the muscles controlling the hand. 
In similar fashion we can picture the acquirement of one 
accomplishment after another. All imitative action must 
be made possible by the establishment of bonds between 294 
NUTRITIONAL PHYSIOLOGY 
receiving and discharging stations in the surface gray 
matter of the cerebrum. As these connections are formed 
the conduct of the individual under given circumstances 
becomes predictable; in other words, we have here the 
physical basis of mannerisms and habits. It is even per- 
missible to say that the foundations of character are defined 
in the direction which these association ties are found to 
take. All of education, so long as it is viewed from with- 
out and not from within, consists in the cultivation of 
response to varied influences. There can be no doubt that 
the anatomic accompaniment consists in the multipli- 
cation of brain pathways. 
During the last hundred years the question has been 
much debated whether particular powers have definitely 
localized registration in the structure of the brain. In the 
first half of the nineteenth century the phrenologists at- 
tracted much favorable attention by their claims of very 
precise subdivision of the brain into "organs of different 
faculties." (Note the frequent references to these concep- 
tions in "Jane Eyre.") The notion of "bumps" is still cur- 
rent, though it is usually referred to in a jocular spirit. 
The literature of phrenology is very large, and it is prob- 
able that it would repay a critical review, but the advocates 
of the cult went much farther than the facts warranted and 
their conceptions fell into disrepute. By 1850 the reaction 
had carried scientific opinion to the belief that there is 
little localization in the brain. 
Since then a great many experimental studies, together 
with observations of the consequences of injuries suffered 
by human brains, have united to encourage the view that 
there is some degree of localization. The new doctrines 
have not followed the old traditions at all. Where the 
phrenologists sought to refer mental functions to particular 
regions of the brain surface, modern students have looked 
with more success for the central representation of bodily 
processes. They have shown that a certain area stands in 
definite relation with the skeletal muscles, that another 
area is the place for the reception of the visual impulses, 
and so on. In most respects the human brain has been THE NERVOUS SYSTEM-ITS HIGHER WORK 
295 
found to be organized in a manner closely corresponding 
with what is traceable in the brain of the ape and hinted 
at in the brain of the dog. 
Yet the distinctive accomplishments in which man ex- 
cels the lower animals must be coupled in some way with 
his cerebral equipment. One or two of these distinguishing 
features seem to have quite definite positions. This is 
especially clear in the case of the reputed "speech center." 
The belief is commonly held that the mechanisms which 
are correlated with such acquired powers as speech, 
reading, and writing are restricted to one-half of the 
brain, usually the left. The right hand, which in most 
subjects is so much superior in skill, is governed from the 
left side of the cerebrum. A charming account of the 
apparent relations between the human brain and human 
capacities is given in W. H. Thomson's "Brain and Per- 
sonality." The claims made for precise localization in 
that book are regarded by many conservative writers as 
extreme. It is certainly fair to say that at present the 
most learned interpreters of brain physiology are placing 
emphasis on the development of paths between centers 
rather than on the organization of the centers themselves. 
The characteristic of early life is the ease and freedom 
with which these paths are opened and the comparative 
frequency with which they are changed. During the long 
period of mature efficiency they are less subject to multi- 
plication and are used with increasing regularity and with 
rarer deviations. The man is becoming a creature of 
habit and acquiring "ruts." In old age, as has been finely 
said, the nervous system, instead of holding a prophecy 
of what may be, contains a record of the past. It is a 
fascinating fancy-though it is nothing more-that a 
physician of surpassing insight might look upon the warp 
and woof of fibers in a dead brain and tell us of the tastes, 
talents, and pursuits of its former possessor. When we 
walk through the rooms of a deserted house we can tell 
by the worn places on the floors and thresholds and by the 
grimy edges of the doors just where the tenants came and 296 
NUTRITIONAL PHYSIOLOGY 
went most often. The lifeless brain must bear a more 
subtle registry of the same order. 
Afferent fibers reach the brain from all parts of the body. 
Many of these have had their origin within its tissues, 
where they are normally stimulated by conditions that 
belong to the organism itself rather than to its environ- 
ment. The impulses that enter the brain from such 
sources are most of the time serving their purpose in pro- 
moting subconscious adaptive reflexes. When they affect 
consciousness it is to bring to the attention the so-called 
"general sensations"-those feelings which we refer to 
states of the organs. Such are hunger and thirst, many 
kinds of pain, satiety, nausea, faintness, fatigue, and the 
like. The majority of these general sensations seem to 
signify conditions that need to be rectified and they are 
mostly unpleasant. 
Contrasted with these are the "special sensations," 
which are referred to causes acting upon the afferent ap- 
paratus from outside. Various terminal structures are 
developed at or near the surface of the body which serve 
to transmute different forms of environmental energy 
into nerve-impulses. Such structures are called sense 
organs or "receptors." A very suggestive distinction has 
been made between those receptors which are affected only 
by the literal contact of the stimulating substance, and the 
"distance-receptors," which respond to forms of energy 
radiating from places more or less remote. The nerve- 
endings in the skin which are acted upon by pressure and by 
temperature changes are of the first class. So are those in 
the tongue on which various dissolved substances take 
effect, giving the conscious subject sensations of taste. 
The olfactory endings high up in the nasal cavities are 
reckoned by most writers to belong to the same order. 
The ear is a distance receptor. The energy which sets 
its intricate mechanism into vibration may have originated 
as far away as the thunder-cloud hanging near the horizon. 
The possession of an ear greatly extends the compass of the 
environment which can exert directing influences on the 
conduct of an animal. If this is true of the ear, how shall THE NERVOUS SYSTEM-ITS HIGHER WORK 
297 
we estimate the widening of environment that comes 
with the addition to the receptor system of an eye! Our 
ability to see the stars means nothing less than this: that 
the reactions of the organism so endowed may be modified 
by energy proceeding from the incomprehensible distances 
of the stellar universe. 
Throughout this book we have held steadily to the point 
of view defined in its very first paragraphs: that all living 
things are transformers of energy, and engaged so long as 
they live in reacting according to the principles of mechan- 
ics and chemistry in response to external changes. A pres- 
entation in this spirit provokes resentment and protest 
from many readers. It seems to leave out of account all 
that is instinctively held to be highest and finest in human 
life. To this remonstrance we are glad to give place. 
The scientist is, after all, a man, and no scientist was ever 
so ruthlessly logical as to convince himself that his friend 
was no more than a reflex mechanism. The impression 
that the study of science deprives one of the philosophic 
outlook and of the conviction of moral responsibility 
belongs to the earlier stages of the student's experience. 
Later it is seen that no incentive to right conduct and no 
worthy consolation is to be taken away. 
A few years ago a brilliant astronomer-Percival Lowell 
-was concluding a course of lectures in which he had traced 
the long story of planetary evolution. He had pictured 
the ages of formative process, the slow condensation and 
cooling of the globe, the gradual approach to conditions 
suitable for organic life. He had sketched the brief 
flourishing of that life, the remorseless chilling of the 
planet, and its frigid and sterile old age. His hearers were 
weighed down with the appalling sense of futility and insig- 
nificance. At the very last he asked abruptly: Which, 
after all, is the greater-these awful ranges of time and 
reaches of space, or the mind of man which comprehends 
and ponders them? INDEX 
Absorption, cell activities and, 
133 
chemical changes during, 137, 
138 
definition of, 45 
dialysis and, 132 
effect of concentration upon, 
134 
of condiments upon, 134 
in small intestine, 134-137 
of alcohol, 30, 133 
Acid-base balance, 250 
Acidosis, 151, 153, 237 
Adipose tissue, 140 
Adrenal bodies, internal secre- 
tion of, 278 
Adrenin, 277 
Adulterants, poisoning from, 257 
Alcohol, 30, 260 
absorption of, 133 
as cerebral alterative, 266 
as drug, 265 
as food, 263 
as poison, 269 
as relish, 262 
historical, 260, 261 
Alimentary canal, 54 
glycosuria, 145 
Alveoli of glands, 44 
Amino-acids, "deaminization" of, 
162 
from gelatin, 156 
from proteins, 95, 154, 155 
in blood, 162 
Amino-aeids in nutrition, 161, 
162 
surplus acids, 159, 162 
Amylopsin in pancreatic juice, 94 
Anemia, removal of spleen in, 280 
Animals as transformers of en- 
ergy, 192 
Antiperistalsis in colon, 103 
Antirachitic vitamin, 170 
Antiscorbutics, 169 
Antiseptic substances for preser- 
vation of foods, 258 
Antrum, 70, 75 
Appendix, vermiform, 58 
Appetite, hunger and, differentia- 
tion, 72, 73 
Assimilation, definition of, 14 
Auricle, 120, 124 
Auto-intoxication, 224, 233, 235 
Bacillus botulinus, 256 
Basal metabolism, 203 
and work, 207 
age and, 204 
ductless glands and, 204 
feeding and, 205 
sex and, 204 
Beriberi, 168 
Bile, description of, 97 
pigments in, 97 
salts in, 98 
waste products of, 97 
Blood, amino-acids in, 162 
as carrier, 109 
299 300 
INDEX 
Blood as equalizer of tempera- 
ture, 110 
coagulation of, 117 
corpuscles in, 112 
plasma of, 114 
proteins, 161 
in nutrition, 161 
sugar in, reduction by insulin, 
153 
Blood-flow, character of, 126 
in arteries, 126 
in veins, 126 
intermittency of, 127-129 
regulation of, 285-290 
velocity of, 129 
Blood-pressure, 126, 127 
Body cavity, definition of, 55, 56 
composition of, 22-27 
temperature, changes in, 215, 
221 
of metabolism in relation 
to perspiration and, 
216-219 
humidity and, 218, 219 
maintenance of, 214-221 
during exercise, 220 
Botulism, 256 
Brain, localization of function in, 
294, 295 
Breathing, 175, 176 
Calorie, definition of, 192 
Calorimetry, 197 
direct, 199 
indirect, 200 
Cane-sugar in diet, 247 
Canned goods, poisoning from, 
258 
Capillaries, 19, 120 
Carbohydrates and fat compared, 
237 
in blood, 116, 144 
Carbohydrates in body, 25 
in diet, 24 
in fasting, 185 
in liver, 142 
metabolism of, 142-152 
Carbon dioxid, elimination of, 
173-177 
in expired air, 176, 177 
from protein, carbohydrate, 
and fat, 186 
as stimulus to respiratory 
center, 177, 284 
maximum excretion of, 213 
minimum excretion of, 213 
retention, 189 
Cardia, 57 
Cardiac sphincter, effect of acid 
upon, 71 
regulation of, 70 
Cecum, 58, 102 
Cells, 12, 15 
Central resistance, 52 
Cereals, poisoning from, 254 
Chalones, 273 
Chocolate, value of, in diet, 249 
Cholesterin in bile, 98 
Chyme, 75, 77 
Circulation, portal, 124 
pulmonary, 122 
systemic, 120-122 
Coagulation of blood, 117 
fibrin and, 117 
thrombin and, 118 
Coffee, food value of, 248 
Colon, 58 
absorption in, 134, 137 
antiperistalsis in, 103 
movements of, 103 
peristalsis in, 90, 103, 104 
Connective tissue, 37, 156 
Constipation, 235, 236 
Contraction, 14, 36-42 INDEX 
301 
Contraction of stomach, sensa- 
tion of hunger and, 72 
Co-ordination, 17, 281 
Copper, poisoning from, 258 
Corpuscles in blood, 112 
red, 112 
white, 114 
Creatinin from protein, 165 
Deaminization, 162 
Decomposition of food, 253 
Deficiency disease, 167, 168 
Dextrose from amino-acids, 163 
Diabetes, cause of, 149, 150 
complications in, in fatal cases, 
150 
insulin in treatment of, 152, 
153 
light thrown on protein metab- 
olism by, 163, 164 
Diabetic and non-diabetic per- 
sons, differentiation, 151 
Dialysis and absorption, 132 
definition of, 132 
Diastatic enzymes, 32, 68, 93, 94 
Digestion, children and, 223 
cooking an aid in, 30 
definition of, 28 
effect of fatigue upon, 225 
of nervous condition upon, 
222, 225 
gastric, 87 
intracellular, 34 
peptic, 87 
quantity of food and, 226 
salivary, 68 
tryptic, 94, 95 
Digestive juices, action of, 31-35 
gastric juice, 80-89 
intestinal, 96 
pancreatic juice, 93-95 
saliva, 63-65, 67 
Disaccharids, 26, 96 
Diuretics, action of, 181 
Ductless glands, 45, 273 
Duodenum, 57 
Eggs, idiosyncrasy to, 253 
Elimination of carbon dioxid, 
173-177 
of nitrogen, 178-182 
Endogenous metabolism, 165 
Energy, animals as transformers 
of, 192 
from various foods, 192-194 
of metabolism, 194 
of muscular contraction, 37, 38 
plants and, 20-22 
source of, for work, 203-211 
transformation of, 197-199 
Enterokinase in intestinal juice, 
95 
Enzyme action, energy change 
in, 35 
equilibrium, 33 
Enzymes, action of acids and al- 
kalies upon, 34 
amylopsin, 94 
definition of, 32 
diastatic, 32, 68, 93, 94 
erepsin, 97, 138 
gastric lipase, 89 
invertase, 96 
lactase, 96 
lipase (pancreatic), 94 
lipolytic, 89, 94 
maltase, 96 
pepsin, 88 
proteolytic, 88, 94, 96 
ptyalin, 68 
relation to temperature, 34 
rennin, 87 
steapsin, 94 
thrombin, 118 302 
INDEX 
Enzymes, trypsin, 94 
Equilibrium in weight, 189 
nitrogen, 188 
Erepsin in intestinal juice, 97 
Ergotism, 254 
Esophagus, peristalsis in, 67 
sensibility of, 67 
Excretion, maximum, of carbon, 
213 
of nitrogen, 213 
minimum, of carbon, 213 
of nitrogen, 213 
Exogenous metabolism, 165 
Extractives, drawbacks of, in 
diet, 242, 243 
exciters of gastric secretion, 85 
in diet, 26 
Fasting, 161 
carbohydrate in, 185 
fat in, 185 
metabolism in, 150, 151 
Fat and carbohydrates com- 
pared, 237 
definition of, 25 
derived from carbohydrates 
and fats, 140, 146, 147 
distribution of, in body, 25, 140 
in diet, 26 
in plasma, 116 
substituted for glycogen, 148 
"Faulhorn experiment," 208 
Feces as bearers of wastes, 182, 
183 
composition of, 106 
Fever, 221 
Fibrin in clotting, 117 
Fish, poisoning by, 254 
Food accessories in diet, 248 
action of bacteria upon, 31 
antiseptic substances for pres- 
ervation of, 258 
Food, decomposition of, 253 
idiosyncrasy to, 253 
infected, 252 
poisoning, 252 
purpose of, 13, 139 
values, 251 
Fuel value, definition of, 192 
of carbohydrates, 193 
of fats, 193 
of hydrogen and carbon, 192, 
193 
of proteins, 193, 194 
Fundus, definition of, 70 
function of, 72 
movements in, 72, 79 
Gastric digestion, 87 
juice, 80-89 
acid of, 81, 82 
• Spallanzani and, 80 
lipase, 89 
secretion excited by different 
means, 83-86 
Gelatin, defective protein, 156 
nutritive value, 156, 157 
Glands, ductless, 45, 273 
nervous control of, 42, 83 
work of, 42, 44 
Glucose, 142 
in diet, 247 
Glycogen from protein, 164 
from sugar in blood, 142 
Glycosuria, alimentary, 145 
emotional, 181 
Habit, 53, 294 
Haustral churning, 103 
Heart, anatomy of, 120 
beat of, 123 
nervous control of, 285, 286 
valves in, 123 
Hormone, action of, 149, 150 INDEX 
303 
Hormone, definition of, 45 
root-meaning, 273 
secreted by pancreas, 149, 150 
Humidity and body temperature, 
218, 219 
Hunger, appetite and, differentia- 
tion, 72, 73 
contraction of stomach and, 72 
Hydrolysis. See Digestion. 
Hydrolytic cleavages, 30 
Hypophysis, abnormalities of, 278 
Ice-cream, poisoning from, 258 
Idiosyncrasy to eggs, 253 
to food, 253 
to potatoes, 253 
Ileocecal valve, 62, 103 
Iletin, 152 
Ileum, 57 
Income, 13 
Infected food, 252 
Inhibition, 62, 67, 103 
Insulin, 152 
Internal secretion, definition of, 
44, 273 
of adrenal bodies, 277 
of pancreas, 149, 153 
of reproductive organs, 276 
of thyroid, 274, 275 
Intestinal juices, 96 
enterokinase in, 95 
enzymes in, 96 
Intestine. See Small intestine or 
Colon. 
Invertase in intestinal juice, 96 
Irreciprocal permeability, 137 
Islands of Langerhans, 152 
Jejunum, 57, 91 
Kidneys as organs of excretion, 
178, 182 
Kidneys, blood-supply of, 178 
work of, 239 
Lactase in intestinal juice, 96 
Langerhans, Islands of, 152 
Large intestine, bacterial action 
in, 107, 224 
effect of cellulose upon, 106 
Larynx, 65 
Lead, poisoning from, 258 
Lipase, definition of, 32 
in gastric juice, 89 
in pancreatic juice, 94 
Lipolytic enzymes, 89, 94 
Liver, deaminization of proteins 
by, 162 
secretion of bile by, 97 
storage of glycogen in, 142 
Lymph, composition and use of, 
19 
in villi, 134, 135 
movement of, 131 
Lymphatics, 19, 118 
Lymph-nodes, action of, 279 
Malnutrition, 169 
Maltase in intestinal juice, 96 
Maltose, 69 
Mastication, 63 
Meat, drawbacks of, as food, 242, 
243 
in diet, 241-244 
poisoning, 254, 256 
Mechanism, 11 
Mesentery, 60, 61 
Metabolism and body tempera- 
ture, 221 
and work, 207 
basal, 203 
calculation of, 183-191 
carbohydrate, 142-152 304 
INDEX 
Metabolism, carbohydrate, rela- 
tion of pancreas to, 152, 153 
endogenous, 165 
energy of, 192-201 
exogenous, 165 
fasting and, 151 
fats, 140, 141 
feeding and, 205 
modifications of, by age and 
sex, 204 
by cold, 210 
by ductless glands, 204 
by mental state, 211, 212 
by muscular work, 203-210 
nuclear, 167 
total daily, 194 
Metals, poisoning from, 257 
Milk, acid fermentations of, in 
stomach, 83 
proteins in, 157 
sour, 234 
Mineral salts in diet, 249, 250 
Mouth, 54, 64 
Mucin, 65 
Multiple neuritis, 167 
Muscles, connection of, with 
' nerves, 39, 40 
efficiency, 38 
properties, 36-42 
skeletal, 36 
Muscular contraction, 14, 36-42 
tone, 41 
Mussel-poisoning, 256 
Nervous system, 17, 18, 46, 53, 
281-297 
Neuritis, multiple, 167 
Nitrogen, calculation of, 184 
elimination of, 178-182 
from protein, 184 
in feces, 106 
in urine, 180 
Nitrogen, maximum excretion of, 
213 
minimum excretion of, 213 
retention of, 189 
Nitrogenous equilibrium, 188 
metabolism, 154-171 
Nuclear metabolism, 166 
Nutrition, amino-acids in, 161 
bacteria and, 224, 225, 234 
bloo l proteins in, 162 
hygiene of, 222-250 
Obesity, 236 
Oils, 25 
Outgo, 183-191 
Oxygen carried by blood, 113, 175 
Pancreas, internal secretion of, 
149, 153 
relation to carbohydrate met- 
abolism and diabetes, 151, 
152 
Pancreatic juice, 93-95 
cause of secretion, 93 
enzymes in, 93-95 
Parotid gland, 56 
Pepsin in gastric secretion, 88 
Peptic digestion, 87-89 
Peptones, 88, 95 
Peristalsis in defecation, 105 
in esophagus, 67 
in intestine, 90, 103,104, 105 
Peritoneum, 60, 61 
Permeable membranes, 132 
Perspiration and body tempera- 
ture, 216-219, 220 
as carrier of waste, 181 
composition of, 181 
Pharynx, 56 
Phosphorus in proteins, 166 
Plants, energy and, 20-22 
Plasma, blood, 114 INDEX 
305 
Plasma, carbohydrates in, 116 
composition of, 114-117 
fats in, 116 
proteins in, 115 
urea in, 116 
Poisoning, food, 252 
from adulterants, 257 
from canned goods, 258 
from cereals, 254 
from copper, 258 
from fish, 254 
from ice-cream, 258 
from lead, 258 
from metals, 257 
from preservatives, 257 
meat, 254, 256 
mussel, 256 
ptomain, 255 
Portal circulation, 124 
Potatoes, idiosyncrasy to, 253 
Preservation of foods, antiseptic 
substances for, 258 
Preservatives, poisoning from, 
257 
Prohibition, 270 
Protein, 23, 24 
blood, 161 
defective, 155-159 
dextrose from, 163 
extent of synthesis, 162 
in body, 23 
in diet, 26 
in plasma, 115 
peculiarities of, 232, 233 
perfect, 155-158 
phosphorus in, 166 
place of synthesis, 161 
synthesized by animals, 154, 
155 
from amino-acids, 157-162 
Proteolytic enzymes, 88, 94, 96 
Protoplasm, 13 
Psychic secretion, 83, 84 
Ptomain-poisoning, 255 
Ptomains, 31 
Ptyalin, 68 
Pulmonary circulation, 122 
Purins, 166 
Pyloric sphincter, effect of acid 
upon, 77 
regulation of, 77 
Pylorus, 57 
Rations, changes in, 205 
effect of sex, 204 
reduction of, 226-232 
Receptive relaxation of stomach, 
73 
Receptors, 282, 296, 297 
Rectal feeding, 107 
Rectum, 104, 105 
Red corpuscles, 111 
description of, 111-113 
function of, 113 
hemoglobin, 112 
origin, 114 
Reflex action, 46-53 
Rennin in gastric secretion, 87 
Reproductive organs, internal 
secretion of, 276 
Respiration, 13, 14, 173-178 
Respiratory center, 284 
quotient, definition of, 186 
variations in, 187 
Rhythmic segmentation, 91 
Rickets, 170 
Saliva, 63-65, 68 
mucin in, 65 
ptyalin in, 68 
Salivary digestion in stomach, 
68, 69 
glands, 56, 57 
Scurvy, 168 306 
INDEX 
Secretin, 93 
Secretions, 42-45 
Secretory nerves, 42 
Sensations, general, 296 
special, 296 
Sensibility of esophagus, 67 
of stomach, 74 
Shivering, 219 
Skeletal muscles, 36 
Skin as organ of excretion, 182 
Small intestine, absorption in, 
134-137 
digestion in, 99 
peristalsis in, 90 
rhythmic segmentation in, 91 
structure, 57, 58, 62 
villi, 134 
Sour milk, 234 
Specific dynamic effect of protein, 
206 
Spleen, function of, 280 
removal of, in anemia, 280 
Starch, hydrolysis of, 68, 69, 94 
Starvation, 190 
Steapsin in pancreatic juice, 94 
Stomach, absorption from, 133, 
134 
antrum of, 70, 75 
cardia of, 57 
contraction of, sensation of 
hunger and, 72 
fundus of, 70-72, 79 
glands of, 81 
movements of, 70-79 
nervous control of, 76 
peristalsis in, 75 
pylorus of, 57 
receptive relaxation of, 73 
salivary digestion in, 68, 69 
sensibility of, 74 
share of digestion, 88, 89 
x-ray studies of, 74-78 
Sublingual gland, 57 
Submaxillary gland, 57 
Sugar absorbed from stomach, 
134 
and teeth, 246 
in diet, 244-246 
in urine, 144 
Sunlight in rickets, 170 
Swallowing, 65-67 
Systemic circulation, 120 
Tea, food value of, 248 
Tendon, 37 
Thirst, 74 
Thrombin in coagulation, 118 
Thyroid gland, 274 
and cretinism, 275 
and goiter, 274 
secretion of, 274-276 
Transformation of energy, 197- 
199 
Transverse band, 70 
Trypsin, activation of, 94 
in pancreatic juice, 94 
Trypsinogen, 95 
Tryptic digestion, 94, 95 
Urea from amino-aeids, 163, 178 
in plasma, 116 
in urine, 178 
Uric acid from purins, 166 
in urine, 180 
properties of, 180, 243 
Urine, action of salts upon, 249 
albumin in, 181 
composition of, 178-181 
secretion of, by kidneys, 178- 
182 
sugar in, 144 
urea in, 178 
uric acid in, 180 INDEX 
307 
Ventricle, 120, 123, 124 
Vermiform appendix, 58 
"Vicious cycle," 225 
Villi, 134 
in absorption, 134 
Vitalism, 11 
Vitamins, 167 
Vomiting, 78 
Waste products in bile, 97 
in blood, 116 
in feces, 105, 106, 182 
Water as metabolic product, 173 
drinking, excretion and, 240 
meals and, 240, 241 
nutrition and, 239-241 
effect of temperature upon 
elimination of, 217, 218 
elimination of, by kidneys, 
176 
White corpuscles, 114 
Work and metabolism, 207 
x-ray studies of stomach, 74-78