OSSEOUS SYSTEM. (CoMp. ANAT.) 



843 



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 

 pouch, which they assist in supporting. It has 

 been 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, 

 being affixed immediately to the pelvic bone, 

 which sustains it. In the Perennibranchiate 

 Amphibia it is but very feebly developed when 

 the hinder extremities are present, which is not 

 always the case. In the Anourous Amphibia, 

 however, as, for example, in the t'rog, it sud- 

 denly assumes a very great importance in ac- 

 cordance with the saltatory habits of those 

 Batrachia. It exists also in all other quadru- 

 pedal forms of the Reptilia, only modified in 

 shape according to their different modes of 

 life. 



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 

 Reptiles 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 

 separate and distinct. 



In the feathered races this bone is of great 

 strength, having to support the weight of the 

 body in a very unfavourable position, and in 

 all the Mammalia that are possessed of poste- 

 rior extremities it is necessarily present. 



Thejibula, which in the hinder extremity re- 

 presents the ulna of the anterior limb, like that 

 bone, is not un frequently very imperfectly deve- 

 loped, especially where great strength is re- 

 quired in this part of the limb, and mobility 

 becomes a secondary object. In Fishes it is 



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 fibulae 

 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 extre- 

 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 .4 chillis is fixed, is 

 well calculated to remind the anatomist of the 

 olecranon of the ulna. 



The metatarsal bones are but a repetition 

 of the metucarpal bones of the atlantal ex- 

 tremity, and immediately support the digital 

 phalanges of the foot, varying in number as the 



