OSSEOUS SYSTEM. (Comp. Anat.) 
arrangement which is here convenient to permit 
of the extrusion of the egg through the pelvic 
cavity, and may be permitted in this race of 
animals in consequence of the prodigious con- 
solidation of the dorsal parts of the pelvis. 
In the Cetacea only the pubic elements of 
the pelvis are developed, both ilia and ischia 
being deficient, so that they are quite detached 
from the rest of the skeleton. In all other 
Mammalia they correspond both in position 
and general arrangement with what is found in 
the human subject. 
The marsupial hones are peculiar to the mar- 
supial division of Mammals. They are two 
triangular pieces articulated to the anterior sur- 
face of the pubic bones, and imbedded in the 
parietes of the abdomen behind the marsupial 
eee which they assist in supporting. It has 
4 asserted that rudiments of these bones 
may be traced even in the human subject in 
_the shape of minute cornicles sometimes at- 
tached to the pubis. 
__ The femur represents, in the posterior extre- 
-mity, the humerus of the anterior, articulating 
immediately with the pelvic arch, but modified 
in form according to the difference of its func- 
tion. In Fishes this element of the skeleton 
does not exist at all, the digital rays and tarsal 
bones of the ventral fin, the representative of 
‘the posterior extremities of other Vertebrata, 
_heing affixed immediately to the pelvic bone, 
which sustains it. In the Perennibranchiate 
Amphibia it is but very feebly developed when 
he hinder extremities are present, which is not 
always the case. In the Anourous Amphibia, 
owever, as, for example, in the Frog, it sud- 
denly assumes a very great importance in ac- 
ordance with the saltatory habits of those 
Batrachia. It exists also in all other quadru- 
pedal forms of the Reptilia, only modified in 
hape according to their different modes of 
_ In Birds the femur is short and strong, but 
‘presents no peculiarity requiring special notice. 
__ In the Mammalia, likewise, except in the 
Cetacea, it is invariably present, its size and 
Shape altering in the different tribes as their 
habits vary. 
_ The tibia, the principal bone of the leg in all 
/quadrupedal Vertebrata, does not exist in Fishes, 
where all the elements of the skeleton, usually 
interposed between the foot and the pelvis, are 
found to be deficient. 
_ In the Batrachian, Saurian, and Chelonian 
J eptiles it is invariably a bone of very consi- 
derable importance, whether it be united with 
the other bone of the leg, the fibula, or remain 
rate and distinct. 
n the feathered races this bone is of great 
strength, having to support the weight of the 
body in a very unfavourable position, and in 
il the Mammalia that are possessed of poste- 
i0r extremities it is necessarily present. 
_ The fibula, which in the hinder extremity re- 
presents the ulna of the anterior limb, like that 
one, isnot unfrequently very imperfectly deve- 
yped, especially where great strength is re- 
wired in this part of the limb, and mobility 
ecomes a secondary object. In Fishes it is 
843 
not yet developed. In the Batrachian Reptiles 
it exists, but is generally so completely anchy- 
losed to the tibia throughout its whole extent 
_as only to be distinguished from that bone by 
very accurate examination. In the Saurian 
Reptiles it is a distinct and very important bone, 
as is likewise the case in the Chelonians, al- 
though here the two bones of the leg are so 
firmly connected by ligaments that but little 
motion is permitted. 
The fibula of Birds is a mere rudiment, a 
slender splint appended to the external aspect 
of the tibia, distinct above, but inferiorly com- 
pletely lost, being gradually solidly united to 
the latter bone, with which it becomes com- 
pletely confused. 
In the Monotremata, notwithstanding the 
near relations that exist between these singular 
quadrupeds and the feathered races, the fibulz 
are very largely developed, as likewise in most 
of the unguiculate Quadrupeds; but in the un- 
gulate Mammalia the fibula is reduced to a 
mere rudiment attached to the outer side of the 
tibia. 
The tarsal bones are, in the posterior extres 
mity, the representatives of the carpus of the 
anterior, but from various circumstances are 
very considerably altered in form, and not un- 
frequently differ in number from the latter, even 
in the same animal, in consequence of the very 
different offices not unfrequently assigned to 
the two pairs of limbs. In Fishes they are 
very imperfectly developed, or confused with 
the other elements entering into the composition 
of the ventral fin. In Frogs and Toads, how- 
ever, they are very distinctly formed, being in 
these amphibious reptiles six in number ; but 
of these the two proximal ones, corresponding 
to the astragalus and os calcis, are remarkably 
elongated, and by the uninformed might easily 
be mistaken for the tibia and fibula. In Sau- 
rian and Chelonian Reptiles they present what 
may be called their normal or medium state of 
developement, the os calcis being here left pro- 
minent for the insertion of the extensor muscles 
of the foot. 
No tarsal bones are distinguishable in the 
adult bird, the few elements which in the 
young animal are developed by distinct points 
of ossification being rapidly confused with the 
metatarsal portion of the limb, so that both 
these divisions of the hinder extremity are here 
represented by a single piece,to which the 
appropriate name of tarso-metatarsal bone has 
consequently been applied. 
In all unguiculate Mammalia the tarsal 
bones are well developed and more or less 
resemble the human; but in the Ungulata, 
owing to the extreme length of the metatarsus 
or canon bone, they seem to occupy a position 
corresponding with that of the knee in other 
animals; and the remarkably prominent os 
calcis, to which the tendo Achillis is fixed, is 
well calculated to remind the anatomist of the 
olecranon of the ulna. 
The metatarsal bones are but a repetition 
of the metacarpal bones of the atlantal ex- 
tremity, and immediately support the digital 
phalanges of the foot, varying in number as the 
