MEMOIRS 
LEAD BEFORE TIIE BOSTON SOCIETY OF NATURAL HISTORY. 
I. The Significance of Bone Structure. 
By Tiiomas Dwigiit, M. D. 
Parkman Professor of Anatomy at Harvard University. 
Bead March 3, 1880. 
‘It is not easy to determine who first appreciated the fact that the spongy tissue of 
hone is not a meaningless tangle but an architectural structure. The late Professor 
Jeffries Wyman read a paper "On the Cancellated Structure of some of the Pones of 
the Human Body” before this Society on Xov. 7, 1849. lie stated that there is a ref- 
erence to the matter in Ward’s "Outlines of Human Osteology,” which was published 
in London in 1838. He, however, gave the credit of first calling attention to the sub- 
ject to Bourgery and Jacob (1835). Professor Wyman wrote as follows: "The cancelli 
of such bones as assist in supporting the weight of the body are arranged either in the 
direction of that weight or in such a manner as to support and brace those cancelli which 
are in that direction. In a mechanical point of view, they may be regarded in nearly all 
these bones as a series of 'studs’ and 'braces.’"1 lie thought that the arrangement in 
some of the human bones is characteristic and has a definite relation to the erect position. 
"As a rule the strength of the bone seems to be obtained in other mammals at the ex- 
pense of its lightness by giving greater thickness and density to the outer shell, as well 
as by stouter cancelli with smaller areoke.” He found but slight traces of a corre- 
sponding plan in other animals; a conclusion that seems surprising* on the part of so 
acute an observer. 
..Professor Humphry drew attention to the mechanical advantage gained by the ar- 
rangement of the cancelli in his treatise on the human skeleton published in 1858. 
Professor Hermann v. Meyer2 of Zurich announced this discovery anew in 18(57 and is 
generally looked up to as the first to grasp the idea. II(' certainly deserves the credit of 
having studied it more thoroughly than any of his predecessors. He thought that the. 
cancellated tissue could be divided into two types, that fitted to resist pressure in one 
and that fitted to resist it in many. The discussion was taken up with great 
interest by Herman anatomists. 
‘Boston Jamnal of Natural History, Vol. vx. 
MEMOIRS BOSTON SOC. NAT. IIIST., VOL. IV. 1 
Mleichert und DuBois lteyilbnd’s Arcliiv, 18G7.' 2 
THOMAS DWIGHT ON THE 
Dr. Julius Wolff1 applied the principles of mathematics more rigidly than had been 
heretofore done to the study of the bony plates. At his request Professor Culmann 
caused to be drawn the stress lines of a crane of the general shape of the upper end 
of the femur and found a surprising resemblance between the stress lines and the tra- 
beculae. Dr. Wolff concluded that from our knowledge of the needs of a hone we 
can predict its internal structure and also that the converse of the proposition is true. 
He writes: "And thus we come to the view which, when we have once grasped it, seems 
so natural and self-evident, that the plan on which the hone is built is the only possible 
one.” I make no comment on these conclusions as the discussion of this and allied ques- 
tions is the purpose of the present paper. 
Wolfermamr wrote shortly after in the same sense and included in his observations 
the bone§ of many animals. Aot, content with admitting the relation between the me- 
chanical needs of the bone and its structure, he went so far as to assert that the former 
are actually the cause of the latter. 
Aeby3 maintained that the arrangement of the laminae of a hone depends on its move- 
ments. The laminae, according to this theory, are parallel when the axes of two con- 
nected hones remain so, but if one of these hones is flexed on the other by turning on one 
axis, the plates converge in the planes perpendicular to the axis. If the bones move on 
more than one axis there is a general convergence. Moreover, all these types show near 
joints a system of slightly developed transverse plates. At points of muscular insertion 
the muscle represents the axis of a bone. 
Merkel,4 in a paper on the femur, expressed some doubt as to the teleological arrange- 
ment of the plates, basing his objection on the striking similarity of the plan in the 
human heel bone which rests on the ground and that of the quadruped which projects 
in the air. 
Bardeleben5 published a monograph on the spinal column in man and animals which he 
argued is to be regarded as a trestle-work. 
Langerhans6 and Wagstaffe7made many sections of human bones and each concurred 
in the prevailing view. 
Ogston8 started the theory that plates developed from articular cartilage are perpen- 
dicular to the surface of the bone, and those from the periosteum parallel to it. 
Professor Humphry9 contraverted this, maintaining that the arrangement is not due 
to development, but rather has "relation to the line of weight and the direction of the 
forces to he resisted.” 
In 1882, v. Meyer10 classified more accurately than he had done the different kinds of 
cancellated tissue. lie recognized a round-meshed structure fitted to resist pressure 
in all directions. This generally presents in the middle what he calls an intermediate 
spongy portion, made of thicker plates and larger spaces, which adds to the strength. 
Then there are the short bones that receive pressure in two opposed directions and con- 
'Virchow’s Arcliiv, Band l, 1870. 
2B.eichert und Du Bois Reymond’s Arcliiv, 1872. 
3G'entralblatt fiir med. Wissensch , 1873. 
4Virchow’s Arcliiv, Band lix, 1874. 
5Beitriige zur Anatomic der Wiqbelsaule, Jena, 1874. 
6Virchow’s Arcliiv, Band lxi, 1874. 
7St. Thomas’ Hospital Reports, 1875. 
8Journal of Anat. and Bhys., Vol. xrr, 1878. 
9Ditto, Vol. xnr, 1878. 
l0Beitr;ige zu Biologie. (Bisclioff’s Jubilaum.) SIGNIFICANCE OF BONE STRUCTURE. 
3 
sequently have a system in the main longitudinal. Finally, there are the shafts of the 
long bones, the solid walls of which break up into plates near their ends. Moreover 
there are plates near the ends of bones which he thinks must be considered continuations 
of tendons. 
Roux1 of Breslau, in a paper in which he analyzes the internal structure of an anchy- 
losed knee, makes a further classification which, though very good, is rather long to be 
given here especially as it is more minute than the purposes of this paper require.2 
That the internal structure of bone shows a well-planned architecture suited to the 
mechanical requirements of the part is an axiom so generally accepted that we may take 
it as a starting-point, although, as will appear later, it requires considerable modification. 
There are, indeed, many aspects to the question, which is one of no common difficulty. 
Although some of the most thorough of the authors mentioned above have referred to 
the mechanical requirements in the widest sense, it seems that many, if not most, writers 
have looked upon bones merely as weight-bearing appliances. Considered in this light 
alone the question is not a simple one. Is the arrangement of the plates that requisite 
when the bone is in the position of the greatest strain? If not, it is clear that there 
must be what builders call a large factor of safety, that is, that much more material 
must be disposed along the stress lines than would otherwise be necessary. Another 
point not to be lost sight of is that, besides the actual weight a bone may have to bear, 
it must be able to resist compression against the next bone to which it is subjected by 
muscular action. 
But if we look on bones as not merely adapted to weight-bearing but as serving also 
for the origin and insertion of muscles, do we find the plan of the bone modified for 
these requirements? There seems no possibility of doubting it. The femur of a quad- 
ruped inclines downward and forward. If we suppose the upper part to be made of 
the least amount of bone sufficient to bear the weight, it is clear that the shaft at the 
neck would be thicker from before backward than transversely, as the strain comes 
in the former direction. But the contrary occurs: the greatest breadth is transverse. 
Clearly then there are other factors than the purely static needs of the bone. If this 
shape be for the purpose of resisting any strain it must be that of the muscles. Although, 
as I shall undertake to show later, certain structures occur that are in no way teleolog- 
ical, it is hard to believe that so important a feature as this widening of the femur should 
be purposeless, and we are almost forced to hold that either it is to give greater surface 
for muscular origin and insertion, or that it is to resist lateral tension from muscular 
action, or that the two reasons coexist. 
The precise part played by muscles is extremely difficult to determine. With them 
it is proper to associate ligaments, there being no doubt that what is a ligament in some 
animals is a muscle in others and, moreover, it seems a matter of no consequence whether 
the strain is applied to a bone by the pull of a muscle on a tendon or less directly by 
making tense a ligament. There seems no doubt that a factor in the shape of bones, 
besides weight-bearing, is muscular attachment. Clearly a large muscle must have a 
'Beitriige zur Morphologic der functionellen Aupassung. 
Arcliiv fiir Anat. und Entwick., 1885. 
2It is not thought necessary to discuss what has been writ- 
ten on the growth of bone nor on}the arrangement of the 
cancelli in pathological conditions. 4 
THOMAS DWIGHT ON THE 
sufficiently extended surface of origin, and again it is necessary for leverage that many 
muscles, at least, should be attached to projections. We find, therefore, that the whole 
shape of a hone is determined so as to meet other than the static requirements. As for 
processes like the trochanters no one denies that they serve for muscular attachment. 
I have elsewhere1 published some notes on the structure of bone beneath processes for 
the insertion of muscles and ligaments. In some cases, notably when the process is 
very small, like the anterior inter-trochanteric ridge of the femur, the shaft is continued 
beneath a layer of cancellated tissue. When the process is larger the surface of the 
shaft seems to go round it rather than under it. The shaft is sometimes, but by no 
means usually, found continued under the tubercle of the human radius and the coronoid 
process of the ulna. It is sometimes represented by plates recalling the process which 
Bigelow2 has named the "true neck of the femur” which, when very well developed, 
appears to represent the posterior wall of the neck continued beneath the lesser tro- 
chanter. As a rule, however, there is no well-marked separation of the cancellated 
tissue of any of the larger processes from that of the main bone beneath. The third 
trochanter of the horse (fig. 1) is a striking instance. One would be inclined to look 
for a continuation of the shaft beneath it to support the weight, but such is not the case. 
It being then admitted that the shape of bones shows provision both for a sufficient 
extent of muscular origin and for tendinous insertions it remains to see what internal 
arrangement of the laminae is found in the processes. Of course in cases where what 
would be the shape of the bone, were static needs the only ones, has been modified to 
admit of a larger space for muscular origin, it follows that the internal structure must 
also of necessity be modified; but an important and difficult question is whether the 
internal structure shows any special arrangement to resist the pull of the muscles and 
whether any of the bony plates are continuous with tendinous or ligamentous insertions. 
I am strongly inclined from the study both of sections of dry bone, and of bones decal- 
cified with the soft parts unremoved, to reply to both questions in the negative. Lig- 
aments and tendons are, I believe, never inserted at right angles to the surface of a bone. 
They spread out over it, being inseparably united with the periosteum and in some cases 
with cartilages. Thus the strain is diffused over a larger surface. It is very remarkable 
to what thin surfaces and to what weakly supported ones powerful muscles are attached; 
familiar instances are the tuberosities of the humerus and the trochanters of the femur 
in man. The eancelli inside of these processes are light and seem arranged to support 
the delicate shell of bone and not to resist muscular action. In some few places, as for 
instance at the back and underside of the os calcis, there are series of plates that seem 
to represent fibres from the tendo Achillis and the plantar ligaments, but I have not been 
able to satisfy myself of any continuity in decalcified specimens. Be that as it may, 
such systems are exceptional. 
To sum up to this point it appears that the shape of the bone depends not solely on 
its needs as an organ of support but also on its needs as a fixed point for the origin and 
insertion of muscles. Further, that although this modification in shape must necessarily 
demand a change of the internal architecture, yet this change is only what is requisite 
'Remarks on the Position of the Femur and on its so- 
called “True Neck.” Journal of Anatomy and Physiology, 
Vol. ix, 1875. 
2 The Hip (Philadelphia, Henry C. Lea) 1869; also The 
Boston Medical and Surgical Journal, 1875. SIGNIFICANCE OF BONE STRUCTURE. 
5 
for the support of the bone under the changed conditions and that the strain of the 
muscles being diffused over its surface the internal structure shows no special adaptation 
to meet it. Finally, that, in some few cases, there are apparent exceptions to this last rule. 
We have so far considered an individual hone as if it were simply a piece of a ma- 
chine which, to fulfil its function, must have a certain size and shape and a certain power 
of resistance. Other and broader questions now present themselves. 
We know that certain classes and orders of animals have a characteristic structure of 
bone of which the most striking instances are the slender bones of fishes and the ex- 
panded hones of birds. The same is true of certain orders of mammals; but we do not 
know just how far such characteristic structures can be ascribed to genera or to species. 
That these peculiarities are in general advantageous may be considered as accepted, but 
are they always so? Do they extend to every bone of the skeleton? Are useless or 
rudimentary structures found in bone? Do particular bones show a similarity to corre- 
sponding bones in other animals, and if so, is the structure in each case solely teleologi- 
cal? In fine is there ground for Wolff’s assertion that a bone is built on the only 
possible plan? 
It was the writer’s intention originally to limit the discussion to mammalian bones, and 
this plan has been followed in the main, but he has been led in one or two instances to 
refer to those of other classes. 
Let us now pass in review*the proximal ends of the humerus and femur of a number 
of mammals, beginning with man, and having compared the bones of the two extremities 
in the same species together, then compare each with the corresponding bones of other 
animals. 
Man. These bones have been so often described, that a summary sketch shall suffice. 
In the humerus (fig. 2) the shaft is thin, the spongy tissue light and its plan very indis- 
tinct. It is denser in the head where it is of the round-meshed pattern. It is very light 
in the greater tuberosity. Sometimes a system of plates running up into the head from 
the inner side of the shaft can be made out. The femur (fig. 3) is of a much heavier 
make, the shaft is much thicker and the plan distinct. In brief, this consists of decussa- 
ting plates from either side of the shaft, of a very well marked series, running from the 
lower side of the neck into the dense, round-meshed head, of a series of arches from 
the outer shaft which meet and perhaps pass through this series, and finally of the light 
tissue in the trochanter major with a few plates parallel to its surface. The important 
differences between the bones are the rudimentary neck of the humerus, its much lighter 
structure and indistinct plan. The teleological significance is evident. 
Apes. The humerus of the chimpanzee (fig. 4) might easily be mistaken for a small 
human one, its chief difference being a greater density of structure. The femur (fig. 5) 
is on the same plan as in man. The tissue of the head is very dense. Its centre is al- 
most solid bone. The bones are larger in the gorilla. The humerus (fig. G) is a little 
broader and the series of plates running to the head from the inner side of the shaft is 
more defined. The femur (fig. T) is very like the human one, but below the trochanter 
the shaft is convex. The head is very dense. In both these animals the femur is much 
denser and heavier than the humerus, though the difference is less than in man. It is 
hard to say whether the difference between the two is greater in the chimpanzee or the 
gorilla. In the mandrill the humerus (fig. 8) is broader, the plates from the shaft show 6 
THOMAS DWIGHT ON THE 
a decussation and the series from the inner side to the head is more distinct. In short, 
it is more like a femur. The bones of monkeys differ from those of man and the higher 
apes by their greater lightness. The cavity in the shaft is relatively larger and reaches 
nearer the ends. 
Carnivora. I have examined the hones of the dog, lion, otter and sea otter. ISTo de- 
tailed description is necessary. The humerus and femur are more alike than in man and 
the apes, the neck being relatively longer in the humerus and shorter in the femur than 
in the latter order. There is the same general plan in humerus and femur. The head 
of the latter shows a somewhat greater solidity than in the former. This difference was 
especially marked in the dog and hardly perceptible in the sea otter. This animal was 
chosen in order to ascertain whether the bones present any resemblance to those of the 
seal. Unfortunately, I could examine but a single sea otter and the bones proved imma- 
ture and consequently unsatisfactory, for typical structure is usually evident only after 
the union of the epiphyses. 
Pinnipedia. The bones of the seal are very interesting. The section of the hu- 
merus (fig. £)) passes through the head and the inner1 tuberosity. It shows plates leav- 
ing both sides of the shaft and tending towards the middle. Others pass into the head 
from below. The line of the epiphysis of the head is well marked and probably the line 
obliquely cutting off the tuberosity marks the epiphysis also. The head is of a light, 
round-meshed structure. The greater part of the tuberosity is taken up by plates, ra- 
diating from two solid masses, situated, respectively, on the upper and outer border, which, 
meeting at its outer extremity, form with the attachedbase a triangle, inside of which is 
some light, spongy tissue. The femur (fig. 10) is no means very different, but less 
distinct. It shows a head of no greater solidity than the humerus. In both bones we 
find a structure differing from any that we have yet seen, consisting of broad, relatively 
thick plates with large spaces between them, which appears to be a generic peculiarity. 
This description is made from bones of the P. vitulina, but the figure of the femur is 
taken from the P. Groenlandica which corresponds strikingly. The humerus in Otaria 
jubata shows a denser structure in the head and the characteristic structure of the seal is 
less evident, though there is an approach to it. As above stated, the bones of a young 
•sea otter showed no trace of this peculiarity. 
Cetacea. I have examined bones of small whales fifteen or eighteen feet long, 
probably the Globicephalus melas, and also of porpoises. The section (fig. 11) embraces 
the whole humerus. The denser, spongy tissue of the head is distinguishable. Plates 
from the thick inner side run into it, and then there is a beautifully marked decussation 
at the upper end of the bone of plates from either side. The highest ones from the 
outer side run to the top of the bone. The tuberosity is of lighter texture. The thick- 
ening of the solid parts on each side near the middle of the bone is even more marked 
in a similar section of a young porpoise. The structure is very dense, and consists of 
thin plates arranged very closely together. It seems characteristic. 
Ungulata. I have examined bones of the horse, moose, gazelle, caribou, sheep, goat 
and hartebeest (an African antelope). The four first-mentioned species differ from the 
last-mentioned three in having the cancellated tissue prolonged much further into the 
1 This section through the inner tuberosity shows, I 
think, a greater resemblance to the rest of the series than 
one through the outer, owing probably to the curious for- 
mation of the humerus in this animal. SIGNIFICANCE OF BONE STRUCTURE. 
7 
shaft of the bone. The moose (figs. 12 and 13) is a good example of the former type, 
and indeed it is an excellent specimen of the structure in the quadruped. The internal 
structure is as similar as the difference in shape between the humerus and femur allows. 
There is very little difference in density between the heads of these bones in the same 
animal. I have been able to cut only the femur of JRangifer tarandus, the caribou. The 
structure of the head is very dense, decidedly more so than in the femur of the moose. 
The plates in the-neck are very delicate and near together, giving the bone rather 
a characteristic appearance. The bones of the licirtebeest (figs. 14 and 15) are very strik- 
ing. The walls of the shaft are very thin in proportion to its diameter, and there is no 
cancellated tissue below the very ends. This is pretty dense in the heads and lighter in 
the tuberosity and trochanter. A series of plates can be traced from the inner side of 
the shaft into the head. Both bones, but especially the humerus, present large plates 
and bars running through empty spaces, giving very much the appearance of a bird’s 
bone. As it was known that this animal had died in a menagerie, it seems possible that 
the bones are pathological, but both the sheep and the goat show in a less degree the thin 
shaft and the absence of cancellated tissue in parts where it is found in the other un- 
gulata that I have examined. 
Marsupialia. In the kangaroo the humerus is much smaller than the femur, and the 
plan not very clear. The femur (fig. 16) shows the usual plan; but these bones present 
one remarkable peculiarity. In the bones hitherto described the surface of the head is 
always of spongy tissue and the line of the epiphysis when visible cuts off a considerable 
portion of the head. In the femur of the kangaroo the line of the epiphysis separates a 
crust of solid bone covering the head. This crust seems precisely as compact as the 
bone of the shaft. In the humerus there is a similar arrangement, only the outer portion 
of the crust shows a number of small holes, an approach to spongy structure. Anxious 
to know if this is a marsupial peculiarity, I examined the bones of the op>ossum and 
found precisely the same hard cap constituted by the epiphysis in the femur, but in the 
humerus the epiphysis consists of spongy tissue. 
To study the general plan of the heads of these bones we should begin with a quad- 
ruped. We find in the humerus and femur a system of plates passing off from the outer 
and inner walls of the shaft and forming Gothic arches at the top of the bone. The 
head is of dense, spongy tissue, of the round-meshed pattern, into which runs a series 
of plates from the underside of the neck. The great tuberosity and trochanter are made 
of looser tissue. In the former, the plan is less certain; in the latter, it is in the main, 
(as seen in frontal plane) a rectangular network of vertical and horizontal plates. Ex- 
ternally there may be a series parallel with the surface. In most of the animals men- 
tioned, this general description will apply to both humerus and femur. The head of the 
humerus \t of about the same consistency as that of the femur in ungulata, in which the 
weight is about equally distributed between the bones, and in the seal, in which neither 
bone bears any weight. In the carnivora, in which the anterior extremity is more pre- 
hensile, the head is lighter and this difference is still more evident in apes and man. In 
the apes we find a lengthening of the neck in the femur and a shortening of that of the 
humerus, Avith a corresponding change in the internal structure. 8 
THOMAS DWIGHT ON THE 
Reserving till later a discussion of several questions, we may make from this series of 
specimens at least the general conclusion that these homologous bones of the two ex- 
tremities of the same animal correspond closely in structure when their function is nearly 
the same, and that when their function is different their structure differs also but that 
the main features of the common plan can still be recognized. 
The study of the calcaneum and the olecranon is very instructive. A longitudinal sec- 
tion of the human heel bone (fig. 17) may he described as follows: There is a thickening, 
from which plates radiate, situated at the lower border of the upper articular surface. 
Some go forward to the plate that rests against the cuboid, others downward, others back- 
ward and downward, the latter being joined by other plates arising above the point 
mentioned. There is then a series of plates acting as ties passing from the posterior 
towards the anterior surface describing a curve with a downward convexity. These 
plates come very near to one another at the under surface of the bone. The whole 
structure may very aptly be compared to a triangular rafter even to the appearance 
of a vacant space in the middle at the neutral point. Besides these systems there is 
often seen a series of plates parallel to the posterior surface held to be continuous 
with the fibres of the tendon. 
This structure is so admirably adapted to the upright position that it seems very sur- 
prising that a strikingly similar arrangement should be found in the os calcis of quad- 
rupeds in which its position is utterly different, the hind end pointing upward in the air 
instead of resting on the ground. Merkel uses this as an argument against the teleo- 
logical significance of the internal structure of bone, and it long seemed to me an al- 
most insuperable difficulty; but in fact, rightly interpreted, it is a strong argument 
on the other side. Let us suppose a man in the act of running with one foot in the air 
and the whole weight of the body resting on the toes of the other. The heel is raised 
and the foot is in the position of a quadruped’s. Most of the weight is, of course, 
transmitted through the leg to the os calcis and it may be divided into two components, 
one running forward to the cuboid, the other backward to the tuberosities of the bone. 
These no longer rest on the ground and the pressure conveyed to that end of the bone 
is resisted by the tense tendo Achillis, so that the conditions are essentially the same in 
running and in standing; only of course there is a far greater strain in the former posi- 
tion which is the one normal to quadrupeds. 
The bone in the chimpanzee (fig. 18) is very like the human one but the series of 
plates that runs backward does not appear to reach the inferior surface of the bone. 
In the plantigrade bear (fig. 19) the bone is relatively longer, but the principal new 
features it presents are the greater thickness of the upper and lower surfaces. In the 
lion (fig. 20), horse, deer and gazelle the bone is more elongated, but the same general 
description will apply to all. In the seal (fig. 21) the bone is not used to bear the weight 
of the animal but for purposes of propulsion in swimming. The contraction of the mus- 
cles presses it against the astragalus and the line of pressure divides as in the other 
animals. "We find, therefore, an essentially similar plan of internal structure. The thick 
lower border is seen to be composed of many of the laminae of the inferior system which 
meet at this place. The structure of the bone is lighter than in most of the other ani- 
mals mentioned, but the characters of tlia seal are less distinct than in the long bones 
or the vertebrae. SIGNIFICANCE OF BONE STRUCTURE. 
9 
At first sight it would seem that in man there is little in common between the olecra- 
non and the calcaneum. It is possible, however, to recognize in the very delicate spongy 
tissue of the human olecranon (fig. 22) two systems that pass upward crossing each other 
in a series of graceful curves. The best marked comes from the posterior border, the 
other from the articular surface of the great sigmoid cavity. In the gorilla we have the 
same thing only the scale is larger and the plates stronger. The plan is such as we 
find in many other prominences. 
Now in the lion we have an example of the arrangement common in quadrupeds in 
which the olecranon, pointing upward just like the os calcis, has a strong tendon im- 
planted at the extremity in the same way and transmits the weight to the shaft of the 
bone as the os calcis transmits it to the tarsus. The teleological relations are strikingly 
similar. The main difference is in the point last mentioned, that in the os calcis there is 
a series of plates running downward and forward against the surface for the cuboid and 
that in the ulna the pressure is transmitted through the shaft. In the lion (fig. 23), 
which may do for a type, there are found in the elongated olecranon, two series of plates 
crossing one another at the back strikingly resembling the calcaneum, and we find that the 
compact substance of the back of the shaft is formed by the aggregation of many distinct 
plates. In the bear the shape of the olecranon is peculiar. It is pointed and quite dif- 
ferent from the rather human heel bone but the internal structure shows a decussation 
of plates. In some of the ungulata the shape as well as the internal structure is very 
similar. I think that in many cases it would be very difficult to tell whether a section, 
which one could not compare with others, were through the end of the olecranon or the 
end of the calcaneum. 
This series shows, therefore, a similarity of structure coincident with similarity of 
function in non-liomologous bones and the presence of the general plan throughout the 
series. 
We shall next examine the vertebrae and more particularly their bodies. The essen- 
tial features of the body of a mammalian vertebra are a series of longitudinal plates 
running from one end to the other and a series of weaker plates crossing them at right 
angles especially developed near the ends. The surfaces of the pedicles are almost 
always thick. They give off plates that in sagittal sections are sometimes seen joining 
the transverse system and in other places describing curves and joining the longitudinal. 
Sections parallel to the intervertebral discs show at times series of loops from the pedi- 
cles across the body decussating with similar ones from the opposite side.1 These are 
probably formed in part by the cut edges of longitudinal plates. The chief system in 
all mammals is the longitudinal one. 
In man (fig. 24) one is struck by the lightness of the whole structure. The plates 
are nearer together at the ends and wider apart at the centre, where they are thicker. 
This may serve for a centre of support and also form the walls of the vascular canals. 
Something may be seen of the oblique systems, but they seem too weak to be of much use. 
The same general description may apply to the lion (fig. 25) and the dog; but the 
plates are nearer together and the formation is denser, there being little space in the 
middle. In the horse (fig. 26) again the essential system is a dense longitudinal one. 
1 The reader will find much that is interesting and valuable in Bardeleben’s monograph on the spine already referred to 
MEMOIRS BOSTON SOC. NAT. HIST., VOL. IV. 2 10 
THOMAS DWIGHT ON THE 
The longitudinal arrangement is evident in the seal (fig. 27), but transverse sections 
of the lumbar vertebrae show the plates diverging from the compact bone on the inner 
side of the pedicle with remarkable clearness. There is also a set running horizontally 
inward from the lower surface of the transverse process which arises from the body of 
the vertebra. These vertebrae present another peculiarity: to wit, the great thickness 
of the epiphyses at the ends of the bodies which consist of cancellae arranged longitu- 
dinally. This description applies to the Groenlandica as well as to the P. foetida. In 
one or two dorsal vertebrae of the Otaria, which I had an opportunity to examine, the 
longitudinal system was clear and the transverse indistinct. In all these there were the 
thick plates and large spaces which are characteristic of the seal tribe. 
The vertebrae of cetaceans show the characteristic structure of many thin plates very 
close together. The arrangement is clearly longitudinal with tubular spaces. In the 
dorsal region the structure is very dense and heavy, the middle of the body in a small 
whale being almost solid bone. The structure is lighter in the lumbar region and in the 
caudal lighter still. The systems of horizontal plates from the transverse processes can 
be traced but a short distance in the body. The lumbar vertebra of the whale (fig. 28) 
has one peculiarity that I have seen in neither carnivora nor ungulata, namely, a solid 
crust of bone surrounding the body. The upper one is continuous with the compact 
tissue of the pedicles, the lower with that of the transverse processes. In the dorsal region 
this solid shell is found only at the upper side of the vertebra bounding the canal. I 
have not found this in either the dorsal or lumbar region of the porpoise. 
I have had the opportunity of cutting but one bone of the Sirenia, one of the anterior 
lumbo-cauda] vertebrae of the manatee (fig. 29). The structure is very whale-like. It 
has the same plate surrounding the body as in the same region in globicephalas; 
the same decussation of the plates from the transverse process on each side; a substantia 
intermedia and at the lowest part a structure of coarser plates and larger spaces. 
Above this on either side of the middle the structure is denser than in the lumbar ver- 
tebra of the whale. 
Summing up our observations on the bodies of the mammalian vertebrae we find that 
the plan is in the main a longitudinal one. In man, the structure is the lightest of all; 
in whales and the manatee the heaviest. There can hardly be a doubt that the pressure 
to which the bodies of the vertebrae are constantly subjected is in an antero-posterior 
direction being due to the compression they exercise one on another. In quadrupeds 
the dorso-lumbar region may be compared to a bridge,1 one end of which is supported 
on piers (to wit, the femurs), the other being suspended (from the scapulae) sometimes 
only by muscles, sometimes having the additional support of a clavicle. The thorax is 
suspended by the serratus, the neck by the levator anguli scapulae and therefore the tho- 
racic spine is supported through the ribs. The pieces of the spine are connected by 
strong longitudinal ligaments; many and powerful muscles, by their contraction, press 
the vertebrae together, hence the longitudinal arrangement of the plates, for it is neces- 
sary to the safety of the arch that it should be compressed. In man, the arrangement 
is different. The spine is placed on end and really requires less strength than in the 
quadruped. 
1 Compare Lucae; Robbe und Otter. Abhandlungen des Senckenb. naturf. Ges., Bd. viii and ix. SIGNIFICANCE OF BONE STRUCTURE. 
11 
In whales the conditions are not quite the same; nevertheless the spine is not straight 
but bent in the thoracic region. The shape of the bodies of the vertebrae is not such 
that this could be done without the restraining force of ligaments and the vast power 
of the muscles, mast pull the vertebrae together; hence, here also the longitudinal 
arrangement. 
In the seal the plan is less modified. It has been stated that the pressure is trans- 
mitted to the thoracic portion of the spine through the ribs and very probably this oc- 
curs through the tubercle that articulates with the transverse process much more than 
through the head of the rib. Hence the pedicles connecting the transverse processes 
with the body must be strong. Bat this is not the only reason; the pedicles have also 
to transmit the strain, probably very considerable, occasioned by the pressure on the 
articular processes in various movements. Thus the firm walls which the pedicles us- 
ually present are what might be expected, and it is natural enough that plates should 
pass off from these points of support through the vertebral bodies both lengthwise and 
crosswise. Still, as a rale, the work done by these systems does not seem important. 
Before approaching some other questions, it may be interesting to take a cursory view 
of the vertebrae of other classes. 
In the osseous fishes (fig. 30) the body is doubly concave. The end plates are sup- 
ported by others running antero-posteriorly which appear as radiating lines in a trans- 
verse section. These are connected by occasional transverse plates which in some places 
are so developed as to suggest concentric rings. 
In the alligator (fig. 31), the plan bears a certain resemblance to that of the fish. 
There is a general radiating arrangement with mostly large interspaces. Near the up- 
per part of the body, i. e., just below the canal, the structure is thicker. The cancellous 
tissue is very dense in the rounded end. Longitudinal sections show plates running 
from before backward and also near the side, strong plates running downward and 
backward from the anterior articular processes, and others downward and forward from 
the posterior ones. There is a beautiful strong system of plates with large interspaces 
in the arches and roots of the transverse processes that is very characteristic. 
In the great Galapagos tortoise a cervical vertebra shows a plan of rather thick plates 
and large interspaces. The osseous tissue is much less dense than in the alligator. 
The vertebra cut being the only cervical one in the series, it is not worth while to discuss 
the arrangement in detail as it cannot well be compared with the others. Suffice it to 
say that the plates are disposed chiefly longitudinally, and that the general appearance 
is not in the least like that of the alligator. 
The vertebra of the snake is curious, presenting more solid substance than any verte- 
bra I am acquainted with. Figs. 33 and 34 represent two frontal sections made respec- 
tively near the back and front of the body of a vertebra of a pithon. Fig. 34 is just 
behind the hollowed anterior end. The circular portion of spongy tissue is just back of 
the cup and is inclosed by an envelope of compact bone. The latter is found also at 
the lowest part of the body and in the articular processes which receive the posterior 
ones of the vertebra in front. The other section shows almost exclusively solid tissue 
both in the body and in the posterior articular processes. Fig. 32 shows a horizontal 
section through the body. The spongy tissue on the convex head is well shown; that in 12 
THOMAS DWIGHT ON THE 
the cup is seen better at another level. We see here the section of a median vertical 
plate extending from the cup of compact tissue at the front, back to the head where it 
breaks up into trabeculae. There is a band of spongy tissue running from the articular 
surface for the head of the rib obliquely backward and inward. It may be that elastic- 
ity is gained by the constituent parts of the ball and socket joint at the ends of the 
bone being made of spongy tissue, which transmits the pressure to deeper solid bone. 
The articular processes that fit into one another are remarkably solid while the articular 
tubercle for the head of the rib is cancellated. Fig. 35, showing a frontal section of a 
dorsal vertebra of a bald eagle, makes a description superfluou s. There is a general re- 
semblance to the dorsal vertebra of an alligator that is very striking. If we compare 
this highly specialized type with the vertebra of a peacock that flies but little, at the 
first glance the difference appears very slight, but with a little study the wonderful 
lightness of the plates in the eagle becomes apparent. Let us now put together the 
conclusions that seem justified by this series of studies of bones. 
We started by accepting as established, that the structure of bone both external and 
internal, is, in general, correlated with the strain it has to resist, but we found that its 
external shape is modified so as to give room for the attachment of muscles. The in- 
ternal structure is not modified to resist the pull of the muscles, except in some few 
instances, and the fibres of tendon are not continuous with the fibres of decalcified bone. 
We find also that the strength of the osseous structure increases with the strain on 
the bone. Thus the trabeculae of the human humerus are lighter than those of the fe- 
mur, while in the ungulata they are nearly equal, and the lumbar vertebrae of the whale 
are intermediate in density to the dorsal and caudal ones. 
We find a marked correspondence of structure between homologous parts of bones 
having nearly the same function, as the proximal extremities of the humerus and femur 
of the ungulata; and where the function is diverse there is a corresponding difference of 
structure, as shown by the shorter neck of the humerus and the longer neck of the 
femur in man and the apes, the traces of the common plan being recognizable. 
We find a similar correspondence between non-homologous bones having a nearly 
similar function, as between the olecranon and the end of the calcaneum in many quad- 
rupeds. 
The study of sections of different bones in the same animal, and my observations, are 
by no means limited to the parts discussed in this paper, shows that certain classes, or- 
ders and genera have a characteristic type. The main characteristics of the bones of 
fishes and birds are too familiar to require mention, and I have not given enough study 
to reptilian bones to discuss them in detail. Among mammals we find a characteristic 
structure in the pinnipedia, and another in the cetacea. We find a considerable differ- 
ence among the ungulata. These features are more marked in some bones of the same 
skeleton than in others. Beside these peculiarities of the skeleton as a whole, there are oc- 
casional appearances that are characteristic of certain animals. Are these teleological? 
To consider first the outer shape of the bones, it is hard to believe that the needs for 
support and motion should be so much alike in the horse and rhinoceros and so different 
from those of the even-toed ungulata, that the former should require a third trochanter, SIGNIFICANCE OF BONE STRUCTURE. 
13 
ancl that the latter should not. It is very problematical whether the snpra-condyloid 
foramen of the humerus is of advantage to its possessors, and if so, that it would not 
be equally useful in species that do not possess it. Why is there a supra-condyloid for- 
amen in the humerus of the seal and none in that of the sea lion? The solid epiphysis 
of the head of the femur in the kangaroo is not easily accounted for. If it is said that 
it is to resist the shock of the forcible extension of the femur in jumping, one might ask 
what explains its appearance, in a less developed form indeed, in the little used anterior 
extremities, or why it is found in the femur of the opossum. To say that this structure 
is the result of heredity, gives no explanation of its original appearance. If it is useful 
one would expect to find it in other leaping animals. 
The occurrence of apparently useless rudimentary structures in other systems is so 
well recognized that there is no a priori reason why they should not occur in bones. 
Professor Heiberg1 of Christiania has the merit of showing that the lines on the lower 
end of the human femur, erroneously supposed to be due to the pressure of the semi- 
lunar cartilages, are of this class, being, in fact, the representatives of the more marked 
separations of the patellar and the two condyloid surfaces in many animals. It is not 
unlikely that many features of the internal structure of bone may have this significance. 
It is curious that the two most striking peculiarities of the texture of bone that we 
have noticed in mammals, are those of the seal on one hand, and whale tribes and the 
manatee on the other. These aquatic animals resemble one another in the thick layer 
of fat under the skin and in the flipper-like anterior extremities, though these are rather 
rudimentary in the whale and highly specialized in the seal, the former having relatively 
weak muscles and the latter strong ones;, but the bones, or at least the larger ones, 
differ radically in these two types. Each departs from the ordinary plan in an opposite 
direction, the seal having thick plates and large interspaces; the whales and manatee, 
very numerous thin plates crowded together. It may be urged that the purpose of the 
heavy cetacean bone is to resist pressure at great depths, but though this need may ex- 
ist for the larger whales, it probably does not for several species, and certainly not for 
the manatee. The bone being then apparently unnecessarily massive in the whale, the 
architectural plan of the internal tissue seems doubly unnecessary, yet we have seen it 
beautifully marked in the whale’s humerus. In the manatee, especially, there is a strong 
suggestion of a degenerating type. On the other hand, the structure of the bones of 
the seal implies at once strength and lightness. 
It appears, therefore, evident that so far from the actual structure of the bone being 
the only possible one, it in many cases presents useless features, and that certainly there 
must be some determining factor besides teleology. Is it impossible to hold that the 
vertebrae of an alligator, for instance, could not answer their purpose equally well if the 
internal structure were more on the plan of mammalian vertebrae? Are there not pecul- 
iarities of race that in all cases, at least, do not answer any definite purpose? It seems 
to me that there can be no reasonable doubt on the matter. It may be urged that we 
lack the knowledge to decide whether any given organism considered as a whole is or is 
1 Archiv. ftir Anat. und Entwickelungsgeschichte, 1883. 14 
THOMAS DWIGHT ON THE 
not as good as its nature admits. It may be urged that an essential element of per- 
fection is the due proportion of parts and faculties, and that it is impossible for us to 
say that greater development in some particular direction would be for the welfare of 
the individual. I have no desire to dispute the great truth underlying these propositions, 
but in view of rudimentary organs alone, it would appear that at least in some subordi- 
nate details useless structures occur, and we have no means of deciding what limits to 
assign to the action of the causes producing them. It is certain that these appearances 
are not due to chance; there must be some determining cause modifying the structure 
in this direction. It is customary now to quote rudimentary organs and anatomical 
anomalies as evidences of descent, but it seems to me very improperly, occurring as 
many of them do quite out of the line of inheritance. This criticism applies with great- 
est force to anomalies, but rudimentary organs seem phenomena of the same general 
class. What principle then shall account for them? We must turn to homologies. 
We see throughout vertebrates a general plan; and though great modifications occur, 
the plan persists. ISTo liberties, so to speak, are taken with it beyond a certain point. 
There for instance, more than two eyes, or one mouth or four pairs of limbs. 
Is there reason to be sure that none of these, or analogous modifications, might not be 
for the benefit of the individual? Yet, I think every student of natural history would 
look on the suggestion that they will ever occur as chimerical. 
The principle of homology has a restraining influence on variations of structure, both 
in quasi-accidental instances, as in anomalies, and in transformation, of species (if that 
occurs) by restricting changes within the limits of the general plan. 
To condense further these, deductions, it appears that the internal structure of any 
particular bone may show evidence of three factors: first, that of teleology; second, that 
of homology; third, that of correlation to the structure of other bones of the same ani- 
mal. The relative prominence of these factors varies greatly. For example, in the hu- 
merus of the whale the first is of little moment and so is the third in the heel bone of 
the seal. Thus, while we find provision for the fitness of the part, we find also some- 
times apparently useless structures, sometimes apparently evidences of degeneration, 
but throughout are more or less distinct marks of harmony with other parts, and of 
homology with other forms. How has this been accomplished? Clearly the crude 
notion that accidental, purposeless, external forces should be sufficient to change by slow 
degrees one such organism into another of a different species is untenable. The doc- 
trine of chances alone shows it to be impossible. There is, moreover, the unanswerable 
argument of the inevitable uselessness of incipient structures. Where we see the need, 
we see the structure to meet it already perfect. We see also the combination of ho- 
mology with teleology. Whatever, therefore,the share of evolution maybe in the pro- 
duction of species, it is not one of chance. The changes must be for the most part 
comparatively sudden, and, therefore, due to an implanted, internal force acting in pre- 
determined directions. On the theory of external accidental forces, the preservation of 
homology is incomprehensible. The action, however, of this internal force, is, no doubt, 
modified by accidental secondary causes, which may produce degenerative as well as 
progressive changes. SIGNIFICANCE OF BONE STRUCTURE. 
15 
In conclusion, I wish to express my sincere thanks to the authorities of the Boston 
Society of Natural History and of the Museum of Comparative Zoology at Cambridge, 
for their kindness in allowing me to cut valuable bones. I am indebted for many of the 
best sections to the skill of Dr. S. J. Mixter, Assistant Demonstrator at the Harvard 
Medical School. The illustrations are phototypes by the Lewis Company made from 
photographs taken in every case from the actual specimen. 
DESCRIPTION OF PLATES I-III. 
Fig. 1. Femur of horse, cut through third trochanter. 
“ 2. Humerus of man. 
“ 3. Femur of man. 
“ 4. Humerus of chimpanzee. 
“ 5. Femur of chimpanzee. 
“ 6. Humerus of gorilla. 
“ 7. Femur of gorilla. 
“ 8. Humerus of mandrill. 
“ 9. Humerus of seal (Phoca vitulina). 
“ 10. Femur of seal (Phoca groenlandica). 
“ 11. Humerus of whale (Globicephalus melas?). 
“ 12. Humerus of moose. 
“ 13. Femur of moose. 
“ 14. Humerus of hartebeest. 
“ 15. Femur of hartebeest. 
“ 16. Femur of kangaroo. 
“ 17. Calcaneum of man. 
“ 18. “ “ chimpanzee. 
“ 19. “ “ bear. 
“ 20. “ “ lion. 
“ 21. “ “ seal. 
“ 22. Olecranon of man. 
“ 23. “ “ lion. 
“ 24. Vertebra of man. 
“ 25. “ “ lion. 
“ 26. “ “ horse. 
“ 27. “ “ seal (Phoca foetida). 
“ 28. “ “ whale (Globicephalus melas?). 
“ 29. “ “ manatee. 
“30. “ “ horse mackerel. 
“ 31. “ “ alligator. 
“ 32. “ “ pithon (horizontal). 
“ 33. “ “ “ (frontal, near posterior end of vertebra). 
“ 34. “ “ “ (frontal, near anterior end of vertebra). 
“35. “ “ bald eagle. Memoirs Boston Soc. Nat. Hist. Vol. IV. 
Plate I. 
THE LEWIS CO. BOSTON. 
DWIGHT, STRUCTURE OF BONE. Memoirs Boston Soc. Nat. Hist. Vol. IV, 
Plate 2 
THE LEWIS CO. BOSTON. 
DWIGHT, STRUCTURE OF BONE. Memoirs Boston Soc. Nat. Hist. Vol. IV. 
Plate 3. 
THE LEWIS CO. BOSTON. 
DWIGHT, STRUCTURE OF BONE.