WHAT IS NERVE-FORGE ? 
.Ajsr 
ADDRESS 
BEFORE THE 
BIOLOGICAL SECTION 
OF 
THE AMERICAN ASSOCIATION 
FOR THE 
ADVANCEMENT OF SCIENCE 
AT THE 
BUFFALO MEETING, AUGUST, 1886. 
BY 
H. P. BOWDITCH, 
VICE-P RESIDENT, SECTION F. 
[From the Proceedings of the American Association for the advancement 
of Science, Vol. XXXV.] 
SALEM, MASS. 
PRINTED AT THE SALEM PRESS. 
1886. WIIA T IS NER VE-FORCE ? 
•A.2ST 
ADDRESS 
BEFORE THE 
BIOLOGICAL SECTION 
OF 
THE AMERICAN ASSOCIATION 
FOR THE 
ADVANCEMENT OF SCIENCE 
AT THE 
BUFFALO MEETING, AUGUST, 1886. 
H. P. BOWDITCH, 
BY 
VICE-PRESIDENT, SECTION F 
[From tlie Proceedings of the American association for the Advancement 
of Science, Vol. XXXV.] 
PRINTED AT THE SALEM PRESS. 
SALEM, MASS. 
1886. ADDRESS 
BY 
PROP. H. P. BOWDITCH, 
VICE PRESIDENT, SECTION F. 
WHAT IS NERVE-FORCE? 
Ever since I learned that the honor which had been conferred 
upon me of presiding over the section of biology involved the con- 
stitutional obligation of producing an opening address, I have 
been led to speculate upon the wisdom of the provision which 
compels a presiding officer thus to occupy the time of his section 
with formal remarks. 
In the early days of the Association it may have been found de- 
sii-able to stimulate the literary activity of the members, and 
to secure by a constitutional provision the presentation each year 
of a certain number of more or less carefully prepared papers; 
but now, when time can scarcely be found for the discussion of 
the really valuable papers awaiting consideration, a chairman 
would indeed show himself strangely insensible to the duties of 
his post who should, by perfunctory remarks of his own, long de- 
tain his section from the important work which has called them 
together. 
I would not of course be understood as speaking disparagingly 
of the addresses of former vice presidents in this and other sec- 
tions of the Association. It is doubtless true that many valuable 
contributions to science have found their way to the public 
through the medium of an annual address, but it may be fairly 
asked whether these same contributions would not have been 
made through some other channel if the writer had not found 
himself thus called upon to address his section. 
A worker in the field of science who has a message which he 4 
SECTION F. 
feels called upon to deliver to his brethren is sure to find some 
means of making himself heard ; nor can it be said that a volume 
of transactions, even of such an active association as our own, 
offers a particularly favorable medium for such a communication. 
In fact, the special journals of science, in the promptness of 
their appeai'ance and the extent of their circulation, have such an 
advantage over the transactions of an association as channels for 
the conveyance of scientific thought, that it is not uncommon to 
hear the exaggerated opinion expressed that an article might as 
well not be printed at all as buried in a volume of transactions. 
But whether volumes of transactions or special journals are 
the chosen medium of communication, it is clear that the literary 
activity of workers in science needs no stimulation at the present 
time. The constantly increasing difficulty which specialists, even 
in quite narrow fields, find in keeping themselves posted with 
regard to progress in their lines of work is a sufficient proof 
of the correctness of this statement. 
The accumulated literature in eveiy department of science is 
already so enormous in amount and is increasing at such a rapid 
rate that any association or individual undertaking to contribute 
thereto should do so only under a sense of grave moral respon- 
sibility. 
Under these circumstances is it not desirable for the members of 
this Association to consider whether the best interests of science 
would not be served by so amending the article of the constitution, 
which requires annual addresses by the vice presidents, as to make 
it permissive and not mandatory in its provisions ? 
Having thus entered my protest against a system which places 
me before you in the position of one who has got to say something 
rather than of one who has got something to say, I will proceed to 
comply with the regulation and will ask your attention for a few 
moments to some of the evidence which has been recently collected 
relating to one of the most important problems of physiology. 
A distinguished biologist has remarked with great truth that the 
study of the nervous system is the true field of battle for physiolo- 
gists, all other investigations, however interesting and important, 
being of the nature of skirmishes, preparatory for and leading up 
to the final conflict in which we must engage before we can hope to 
gain a position from which nature’s most mysterious processes are 
laid bare to our view. Of all the functions of the nervous system, ADDRESS BY H. P. BOWDITCH. 
5 
the one which at first sight would seem most accessible to investi- 
gation is that of the nerve fibre itself. What conception can we 
form of the physical or chemical changes which take place in those 
white glistening bands which are for us the only channels through 
which knowledge of the physical universe can be obtained and 
which also enable us to impress upon the world around us the evi- 
dence of our conscious personality? 
From the earliest times this problem has been earnestly discussed 
by physiologists but I do not intend to weary you with an account 
of the various crude theories which have been from time to time ad- 
vanced. With the discoveries of Du Bois Reymond, the hope arose 
that nerve activity might be explained as an electrical phenome- 
non and the attempts made to build up a satisfactory electrical 
theory of nervous action have been numerous and ingenious. The 
important facts which forbid the identification of nerve force with 
electricity are the absence of an insulating sheath on the nerve 
fibre, the slow rate at which the nerve force is transmitted, and 
the effect of a ligature on a nerve in preventing the passage of 
nerve force while not interfering with that of electricity. The 
electrical phenomena connected with the functional activity of 
nerves (action current, electrotonus) appear, therefore, to be sec- 
ondary in their character and not to constitute the essential process 
in nerve action. In this connection should be noted an experiment 
of d’Arsonval1 which shows how the electrical phenomena associ- 
ated with the activity of nerves may be imitated by purely physical 
means. This observer filled a glass tube of one or two millimeters 
interior diameter with drops of mercury alternating with drops of 
acidulated water, thus forming a series of capillary electrometers. 
The tube was closed at its two ends with rubber membranes and was 
provided with lateral openings by which its interior could be con- 
nected with electrical conductors. A blow upon one of the 
membranes caused an undulation of the liquid column which was 
propagated from one end to the other of the tube and was accom- 
panied by a wave of electrical oscillation which was propagated 
at the same rate. The phenomenon is, according to d’Arsonval, 
to be explained as follows : The blow upon the membrane changes 
the form of the surface of contact between the first two cylinders of 
mercury and acidulated water. This change of form is transmitted 
to the following cylinders with a rapidity dependent upon the na- 
IComptes rendus de la Soc. de Biologie, Apr. 3,1886. 6 
SECTION F. 
tnre of the fluid. But each of these changes of shape is accompanied 
by the production of an electric current (Lippmann’s phenomenon 
or variation of superficial tension) and the tube is therefore trav- 
ersed by an electric wave which necessarily has the same rate as 
the undulation of the liquid column. The analogy between this 
phenomenon and the wave-like propagation of the action current in 
nerves is sufficiently obvious. 
In studying the nature of nerve force two alternatives present 
themselves. We may conceive the impulse to be conducted through 
the nerve fibre by a series of retrograde chemical changes in\he 
successive molecules of the nerve substance, the change occurring 
in one portion of the fibre acting to produce a similar change in 
the neighboring portion. As this process is associated with the 
using up of organic material and the consequent discharge of po- 
tential energy in the successive portions of the nerve, the theory 
may be called the discharging hypothesis. The burning of a line 
of gunpowder may be taken as an example of this sort of action. 
On the other hand, we may conceive that the nerve force is 
transmitted from molecule to molecule by some sort of vibratory 
action as sound is transmitted through a stretched wire. As this 
theory does not involve the using up of any material but simply 
the transferring of motion, it may be called the kinetic hypothesis. 
Let us now enquire what evidence can be obtained in favor of 
one or the other of these hypotheses by the study of the changes 
which are associated with the activity of nerves. 
Inasmuch as the discharging hypothesis involves the destruction 
of organic material we may, if this theory be correct, reasonably 
expect to find in the active nerve fibre evidences of chemical de- 
composition and of heat production. Moreverif the organic sub- 
stances are used faster than they are replaced, or their products of 
decomposition removed, as would naturally be the case under con- 
stant stimulation, we may expect to observe a diminution of nerve 
action during the continuance of the stimulation : in other words 
we shall have the phenomena of fatigue. 
On the kinetic li3Tpothesis, on the other hand, we may expect to 
find an entire absence of chemical decomposition and fatigue and, 
if the moving particles are endowed with perfect elasticity, an 
absence also of heat production. 
We must therefore consider what results have been reached by 
the experimental study of these three subjects, viz,, the chemical ADDRESS BT H. P. BOWDITCH. 
7 
changes, the heat production and the fatigue of active nerve fibres, 
and ascertain whether these results are more favorable to a dis- 
charging or a kinetic theory of nerve action. 
Chemical Changes. 
The only functional chemical change of nerves for the existence 
of which an experimental proof has been offered is the change in 
the reaction to test paper. Just as the normally alkaline tissue of 
muscles becomes neutral or acid in activity, so, according to Funke2 
and Ranke3, do nerve fibres and the white substance of the spinal 
cord change in activity from an alkaline to an acid reaction. 
Liebreich4 and Heidenhain5, on the other hand, experimenting 
with a slightly different method, failed to get any evidence of the 
acidification of nerves in connection with functional activity. 
Hermann, in the resume of this subject given in his Handbook 
of Physiology (2,1, 139), gives his opinion that, while Funke’s 
statement as to the irritative acidification of nerves has not been 
disproved, yet the phenomenon is of so delicate a nature that it 
can be detected only by the most sensitive reagents. The phenom- 
enon must indeed be a delicate one since Ranke himself urges 
that the question should be decided by experiments on the spinal 
cord and should not depend upon the “doubtful results of tests 
applied to the nerve trunks.” Now since the cord contains gray 
as well as white substance and as the gray substance, according 
to Ranke himself, becomes more acid than the white in functional 
activity it is clear that an acid reaction of the white substance of 
the spinal cord may depend upon an acid formed in the gray and 
passing by diffusion into the white substance. This possibility, 
which is indeed admitted by Ranke, seems to deprive the experi- 
ments on the spinal cord of what little value they possessed as 
evidence of the production of acid in connection with the activity 
of nerve fibres. 
The other chemical changes which have occasionally been assert- 
ed to occur in active nerves rest on still weaker experimental 
evidence audit is therefore clear that chemical investigation gives 
us but little reason for maintaining a discharging in opposition 
to a kinetic theory of nerve action. 
2Archiv. fur Anat. und Physiologic, 1859, S. 835. 
BCentralblatt f. d.med. Wiss. 1868, 8. 769; 1869, S. 97. 
4Tagebl. d. Naturf. Vers, zu Frankfurt 1867, S. 73. 
iv, S. 218; Centralbl. f. d. med. Wiss. 1868, S. 833, 8 
SECTION F. 
Heat Production. 
The first experiments to test the heat production of active nerves 
were those of Helmholtz6 who, after studying the analogous phe- 
nomenon in muscles, extended his investigations to nerve fibres. 
He failed, however, when all sources of error were carefully avoid- 
ed, to obtain any evidence of heat production in connection with 
nervous activit}7, though his apparatus was capable of registering 
a change of temperature of 0.002° C. 
Similar negative results were obtained by Heidenhain7 who also 
experimented with the most delicate forms of thermo-electric ap- 
paratus. 
On the other hand Valentin,8 Oehl9 and Schiif10 maintain the 
affirmative side of the question, asserting that nerve fibres really 
are warmed by the passage of the nerve impulse. The statements of 
the latter observer are, however, rendered somewhat suspicious by 
the fact that he found the nerve fibre warmer at a point near than 
at one distant from the point irritated. This is explained by Schiff 
on the untenable hypothesis that the nerve impulse diminishes in 
intensity as it passes along the fibre. 
In summing up the evidence on this question it is important to 
bear in mind that the methods of research employed by Helmholtz 
and Heidenhain were, to say the least, as accurate and as delicate 
as those of the other observers and we may, therefore, safely agree 
with Hermann (Handbook 2,1, 143) that the question whether a 
nerve produces heat on stimulation must be regarded as not yet 
settled, but if heat production does occur it must be exceedingly 
slight. 
It seems, then, that the results of thermometric investigations 
speak no more positive!} 7 than those of chemical research in favor 
of a discharging rather than a kinetic theoiy of nerve action. 
Fatigue. 
Without entering upon the question whether deficiency of de- 
composable organic material or accumulation of its products of 
decomposition is the essential chemical condition of fatigue, it will 
be sufficient for our purpose to consider what evidence there is 
that a nerve fibre, kept constantly stimulated, becomes less capable 
7 Studien xv, S. 250. 8 Moleschott IJndersuch. ix, S. 2’5. 
9 Gaz. Med. de Paris 1886, p. 225. i° PflUger’s Archiv, IV, S. 230. 
6 Archiv. fiir Anat. und Physiologic, 1848, S. 158. ADDRESS BY H. P. BOWDITCH. 
9 
of performing its function, i. e., of transmitting the impulse to 
the organs with which it is connected. 
The evidence of the activity of a nerve may be either direct or 
indirect. The direct evidence consists in the occurrence of that 
change of the electrical condition known as the “ negative varia- 
tion” of Du Bois Reymond or the “action current” of Hermann. 
The latter writer quotes the former as authority for the statement 
that this phenomenon becomes less intense in successive repetitions 
of the experiment and regards this as evidence of the exhaustion 
of the'nerve fibre. Unfortunately Hermann does not refer to the 
exact passage which contains this statement and an examination 
of the chapter on the negative variation of nerves in Du Bois 
Re}7mond’s Untersuchungen fails to show any s}rstematic study of 
the effects of fatigue on this phenomenon. Indeed, the method 
employed by Du Bois Reymond seems poorly adapted for such a 
study and the diminishing intensity of the negative variation no- 
ticed by this observer11 may very possibly have depended upon 
causes not connected with the exhaustion of the nerve by its func- 
tional activity. 
The indirect evidence of the activity of a nerve consists in the 
effect which it produces upon the central and peripheral organs 
with which it is connected. Of these effects the contraction of a 
muscle is the one which is most conveniently observed, but the 
fact that a muscle is more readily exhausted than a nerve renders 
it impossible to study the fatigue of nerves in this way without 
some special modification of the experiment. 
Bernstein12 was the first to employ the muscular contraction in 
experiments on the exhaustion of nerves. This observer ap- 
plied a tetanic stimulation to the nerves of two nerve-muscle 
preparations through one of which, at a point on the nerve be- 
tween the place stimulated and the muscle, a constant current of 
electricity was sent. The stimulus was thus prevented from 
reaching the muscle in one preparation while in the other its pas- 
sage was unimpeded. While the latter muscle therefore was te- 
tanically contracted, the former remained at rest. After the 
contracting muscle had become entirety exhausted, the constant 
current through the other preparation was opened and the muscle, 
which had been previously at rest, immediately contracted, thus 
n Untersuchungen, n, 425. 12 Pfluger’s Archiv, XV, 289. 10 
SECTION F. 
Showing that the nerve had not become exhausted by a stimulation 
which had lasted long enough to exhaust the muscle. To deter- 
mine how long a stimulation was necessary in order to exhaust the 
nerve this method was not used because it was found that, after 
the prolonged passage of a constant current through the nerve, 
the opening of the current of itself caused a contraction (opening- 
tetanus) even without any stimulus applied above. 
By a somewhat modified method, however, Bernstein reached 
the conclusion that a nerve may be exhausted by 5-15' tetanic 
stimulation. 
The experiments of Bernstein have recently been repeated by 
Wedenskiil3 who, by using a feeble polarizing current and by 
frequently changing its direction, was able to avoid the opening 
tetanus which had led Bernstein to abandon this method of in- 
vestigation. Experimenting in this way Wedenskii was unable 
to find any evidence of the exhaustion of the nerve even after the 
tetanic stimulation had continued six hours. The opening of the 
constant current was invariably followed by a muscular contrac- 
tion., which was proved to be due to the tetanic stimulation ap- 
plied above by the fact that, when this stimulation was omitted, 
the opening of the constant current had no effect. 
Another method of stopping the passage of the stimulus 
through the nerve to the muscle is by poisoning the animal with 
curare. In the case of an animal poisoned by this drug, a con- 
tinued stimulation of a nerve should remain without effect upon 
the muscle connected with it as long as the animal continues 
under the influence of the poison, but as soon as the drug is suf- 
ficiently eliminated the muscle should begin to twitch. The use 
of the drug for this purpose was suggested by Wedenskii, but his 
experiments, which were made on frogs, do not seem to have been 
successful. 
Their failure may well be supposed to depend upon the slow and 
uncertain manner in which curare is eliminated by these animals, 
and a study of the subject upon warm-blooded animals seeming 
desirable, experiments were made upon cats in the laboratory of 
the Harvard Medical School.14 The animals were kept under 
the influence of a dose of curare just strong enough to prevent 
muscular contractions, while artificial respiration was maintained 
Centralblatt fur die mod. Wissenschaften, 1884, p. 65. 
14 Bowditch, Journal of Physiology, VI, 133. ADDRESS BY H P. BOWDITCII. 
11 
and tbe sciatic nerve constantly subjected to stimulation suffi- 
ciently intense to produce in unpoisoned animals a tetanic con- 
traction of the muscles. In this way it was found that stimulation 
of the nerve lasting from fi to 4 hours (the muscle being pre- 
vented from contracting by curare) did not exhaust the nerve, 
since on the elimination of the curare the muscle began to con- 
tract. 
It thus appeal’s that evidence of fatigue in nerves resulting from 
functional activity is as difficult to obtain as that of chemical 
change or of heat production. A nerve isolated from the body 
for experimental purposes will of course gradually lose its irrita- 
bility ; but this change, which was observed by Du Bois Reymond, 
as above mentioned, seems to be associated with the gradual death 
of the nerve due to its altered surroundings rather than to phy- 
siological fatigue. It is conceivable that the irritability of a nerve 
should depend upon its possessing a certain definite chemical 
composition constantly maintained by metabolic changes and yet 
that the irritation of the nerve should produce no change whatever 
in its composition. 
In support of this view an analogy may be drawn from the phys- 
iology of the muscular system. We find here that the power of the 
muscles to perform their function is intimatety associated with 
the amount of nitrogenous material undergoing decomposition in 
the body but the performance of a given amount of muscular work, 
if within physiological limits, does not affect the amount of ni- 
trogen excreted. In the case of muscles, to be sure, we have evi- 
dence of a considerable decomposition of non-nitrogenous material 
and also of heat production in connection with functional activity, 
but, if we limit our consideration to the nitrogenous element of 
muscular substance, the hypothesis above proposed for nerves finds 
its complete analogy in the muscular system. 
The connection between nerves and muscles is so close that the 
muscular fibre has been described as “ the contractile termination 
of the nerve.” If the hypothesis here suggested be correct, we 
have a possible explanation of this close connection, for we may 
conceive that the vibratory impulse, after traversing the nerve 
fibre, continues its course through the muscular substance, produc- 
ing in the nitrogenous portion of the muscle-molecule a kinetic 
change similar to, or identical with, that which occurs in the nerve, 
and, at the same time, setting up in the adjacent non-nitrogenous 12 
SECTION F. 
portion, an explosive decomposition, the result of which is mani- 
fested as a muscular contraction. 
Without, however, indulging in further speculation, let it suffice 
for the present to note the fact that investigations into the chemi- 
cal changes, the heat production and the fatigue of active nerves 
all lead to results more favorable to a kinetic than to a discharg- 
ing theory of nerve action. 
We may, therefore, reasonably hope that future researches, if 
directed on this line, will throw further light on this most myste- 
rious and interesting process.