SERIAL HOMOLOGIES OF TIIE PELVIC BONES AND LIGAMENTS.
The sacrunz, according to Professor Owen, is to be considered as the centrum of the pelvic vertebral elements. The ankylosed bodies of the sacral vertebrm, as well as their coalesced spinous and articular processes, are sufficiently evident as the representatives of those components of the neural arch in the typical vertebra.* The lateral masses of the sacrum which support the ilia are, however, made up of two elements coalesced together, as is shown in the manner of their develop ment, before desciibed, viz., first, of the true transverse processes, or " durpophyses," con stituting the external row of tubercles seen on the posterior surface of the sacrum, and which are ossified, like those of the true vertebrm, by extension from the same points of ossification as the laminm, and spinous and articular pro cesses ; and, secondly, of the six characteristic sacral ossific points, three on each side of the three upper sacral bodies, which are placed on the anterior surface of and below the former, between the sacral foramina, as before de scribed.
These ossific points, as shown in prepara tions exhibited to the British Association, in 1837, by Mr. Carlyle, were four in number on each side, and very distinct from the true transverse processes ; and they were con sidered by him to represent the necks and heads of four sacral ribs on each side. Upon the truncated extremities of these three or four sacral ribs the auricular facets are sup ported. They appear to be similar to the anterior roots of the cervical transverse pro cesses, upon the last of which is occasionally developed, in the human subject, a short costal process. In the nomenclature of Professor Owen, they may be considered the sacral " pa rapophyses," but differ from these processes as seen in the rib-bearing vertebrm of the Croco dile,in being developed by separate and distinct centres. Blainville remarks that the four upper sacral vertebrm, scientifically considered, com pose the whole of the true sacral elements ; and that the fifth, which he calls "subsacral," is an ankylosed coccygeal vertebra. But the lateral
epiphysial plates of bone before described — the upper of which forms the auricular facet, opposite the three first, and the lower, the sides of the two last sacral vertebrm— would seem to connect these vertebrm more particu larly together, and to be the coales«,d serial homologues of the epiphyses forming the ar ticular facets on the tubercles of the ribs. In the Saurian reptiles these sacral ribs, two in number on each side, are very distinctly analysed, and have been before mentioned as intervening between the sacrum and the ilia. The anterior of these ribs in the Saurians are said by 141r. Carlyle, to be articulated to the bodies of the last dorsal and first sacral ver tebrm, as well as to the intervertebral sub stance between them% and the posterior, to the laq sacral and first caudal vertebrm, and to their intervertebral substance —affording an exact homologue to the true ribs. The ilia in the human fcetus, and for some years after birth, are connected to two only of the sacral vertebrm ; but, in the adult state, they join three sacral ribs, corresponding, as we have seen, to the three upper sacral vertebrm. Mr. Carly le has seen thetn, in the negro, join four sacral ribs, as above stated.
The sacral element of the pelvis we have found to be most largely developed in, Man, and its angle with the spinal column most marked, in adaptation to his destiny for the erect and bipedal posture. In Birds, also, of bipedal and semi-erect position of body, it is also large, and, in a less degree, in animals whose climbing or sedentary habits require an habitual upright direction of the vertebral column, such as the Apes, Bears, and Sloths ; while it is much contracted, both in breadth and length, in most of the animals of true quadrupedal progression. Its spinous pro cesses are long, and sometimes united in a crest, in the springing animals, and those re quiring long leverage for the muscles of the back and thigh arising therefrom.