The coccygeal pieces are, in the human subject and the higher Simice, the aborted continuations of the sacral vertebrm, gradually diminishing into their most permanent and principal elements,—the bodies of the ver tebrw. The upper ones show also rudimen tary transverse processes, and the first pre sents well marked articular processes to join with those of the sacrum, or sacral horns. Dr. Knox considers the 1st coccygeal ver tebra to be the representative, in man, of a class of vertebrm distinct both from the sacral and remaining coccygeal.
In Man these bones are placed verYobliquely, to support the pelvic viscera ; but in the lower mammals, the Birds, and especially the Reptiles and Fishes, they are placed in the line of the vertebral column, and are developed as caudal vertebrm, in adaptation for their various uses in propulsion or prehension, &c. In many, they present not only a complete neural arch and spine, enclosing the caudal continuation of the spinal chord, but also an anterior or hcemal arch and spine, to enclose_and protect the arteries of the tail.
The ilia evidently consist of the shafts of the three or four sacral ribs, coalesced into one mass of bone on each side, and constitute the homologues of the shafts of the thoracic ribs, termed by Owen the " pleurapophyses." The descending branch or body of the ischium is considered by the same writer also to form one of these plenrapophyses. The develop ment of the whole of the ischium, as well as the pubis, from a single and separate centre ; and the connection of both these bones to the iliuni in the coty-loid cavity, seems, however, to place the whole of each bone in the same relative position, and to class them both with the chondro-sternal elements, or " hcemapo physes" of the thoracic ribs. Professor Owen considers that the pubes and the ascend ing rami only of the ischia are the homo logues of the rib cartilages, — an arrangement that would make a separation of the ischium, which its mode of development from a single centre would hardly justify. The bent and hook-like form of the Mammalian ischia finds a very close counterpart in the cartilages of the eighth ribs, in Man, especially when these are ossified, as often occurs ; and the manner of its junction with the pubes in the human subject is exactly similar to that of the eighth cartilage with the seventh, before it reached the sternum. In many of the lower AIam malia, especially in the AIarsupials and in the Reptiles, we have seen that the ischial ribs reach each other, and are connected, like the pubic, in the median line. The formation
of a pubic apophysis or descending ramus to meet the ascending ischium, is a disposition which finds a counterpart in a similar apo physis from the acetabular end of the ischium to support the pubes, which we have seen in the pelvis of the Crocodiles, excluding the latter bone from participating in the formation of the acetabulum, exactly as the ischia in man and some animals are excluded from the formation of the median symphysis. The greatest extent of this ischio-pubic coales cence is seen in the Batrachians and the saltatory Carnivora, Ruminants, and Marsu pials ; its entire absence in some of the Che Ionian and in the Saurian reptiles. In the Birds and Bats it is often present even where there is no median symphysis.
The iliac element has been seen to be largely developed in its shaft, and placed very ob liquely on the lumbar vertebrm in quadrupeds characterised chiefly by saltatory quadrupedal progression, and requiring long hold for the great muscles of the hip, as the Carnivora, the Deer tribe, the Alonkeys, the Horse, and the Frog, — while it is contracted to a remark able degree in the Walrus and Seal, which approach in their habits the Cetaceans and Fishes, in whom the iliac element of the pels is is the first to disappear altogether. Its altu e see elongated behind the sacrum in those animals whose pseudo-sedentary habits require a long leverage for the muscles of the back arising from the iliac crest, such as some of the Rodentia and the Kangaroos—and its alw, on the contrary, to be e.zpanded in those requiring support to the abdominal viscera, either from their size, as in the Pachydermata, or from the erect or semierect position, as in Alan and the Sloths.
The ischiadic element has been seen to be adapted — by its large development and direct line with the ilia, for saltatory progression, re quiring a long leverage for the flexor muscles of the leg, as in the C'arnivora, the Deer tribe, the Rodents, the Kangaroos, and the Birds ;— or, for the support of the sacrum, by its angu larity with the ilium, and by its elongated tube rosities, in the Ox, Hippopotamus, and some others, and by ankylosis with the sacrum, as in the Sloths, Bats, and Birds ; — or for the support of a carapace, as in the Arrnadilloes ; —or for the true ischial sedentary support of the body, as in the Apes and Monkeys.