The pubic element has been likewise seen to be in a direct line with the iliac shaft in Alan wily, destined to the truly erect posture; —and in those animals formed for quadrupedal pro gression to be short and placed at a more or less marked ilio-pubic angle, so as to be out of the way of the approximated femurs, in their semi-flexed and angular movements on the pelvis ;—but in those habitually requiring a semi-erect position, as the Sloths, to be longer and less angular. It is also unusually long and oblique In the Seal tribe, with re ference, probably, to a tapering extremity and fish-like outline.
The iliac crest and its posterior spines and tuberosity are the hypertrophied, coalesced, and spread-out tubercles of the sacral ribs, a homologue which will be made more evident by the consideration.of the homologues of the pelvic ligaments. The epipleural spines of Fishes, and the costal appendages of Birds, show the serial homology of these processes.
Wm Y-shaped epiphysial cotyloid bone of M Serres has been considered by some to represent a marsupial bone ; but, seeing that in the marsupials themselves these Y-shaped bones and the real marsupial bones also, co exist, this opinion cannot be considered as tenable.
In the immature Potoroo, as described by Owen, there is another epiphysial cotyloid bone forming part of the anterior maron of the acetabulum (see fig. 110. Art. Illarsu pialia) ; and one of a similar nature and position is described by Geoffroy St. Hilaire as present in the acetabula of the Rabbit, and is considered by him to be a rudimental mar supial bone. There seems, however, greater reason to suppose that both these cotyloid epiphyses are rather of the nature of those complementary ossific points which are seen in the opposed articular surfaces of the bodies of the true vertebrae, and also in the sacral ver tebrm, and in the articular ends of some long bones,—as those forming the elbow-joint.
From the position of the ilio-pectineal emi nence or spine at the junction of the ilium and pubes opposite to the superior limb of the cotyloid Y-shaped bone, it would seem as if this process were connected with it by a con tinuation of its ossification upwards. As this spine is coexistent, as seen particularly in the Alonotremes in a great state ,of develop ment with ;he marsupial bone, the opinion that it represents that bone, seems to be quite untenable. We have seen it most de veloped in those animals whose posture or structure requires largely-developed psom parvm muscles, as in the Marsupials and Monotremes ; the tendons of those muscles being implanted upon them, presenting a close similarity to the implantation of the anterior scaleni muscles into the scalene tubercle of the first rib, to which this eminence would be thus homologically related.
The marsupial bones, being developed in the tendons of the external oblique muscle, present the greatest homology, both in position and office, with the spines of the plebes in Man and some animals • and it would appear, from the manner in which the cremaster muscles play over them, as if they were formed by an os sification of that part of the tendon which is• called, in human anatomy, the external pillar of the ring, or Poupart's ligantent, and which is implanted upon the pubic spine.* In support of this opinion, it may be stated that, in l'ou part's ligament near the spine of the pubis, cornicles, similar to those in the stylo-hyoid ligament, are said to have been found in the human subject. The cartilages upon the pubic plate of the Cameleon, before men tioned, are also significantly homologous to these cornicles and to the marsupial bones, as well as that upon the anterior- pubic angle in the Salamander, and considered by Duges as having a marsupial character.
The epiphysial plates, forming the articular surfaces of the pubic symphysis in man, are analogous with those which form the auri cular sacral facets.
In the immature Potoroo, there is a tri angular wedge of bone inserted, with its apex forwards, between the pubic bones posteriorly; and, in the adult Kangaroo, we have seen that a single V-shaped epiphysis is placed with the apex upvtards, between the ischial bones at the lower part of the ischio-pubic sym physis, and forming a prominent vertical median ridge on its anterior aspect (see fig. 99. c). These epiphyses appear to result from ossification, by independent centres, of the inter-pubic and inter-ischial fibro-car tilages, and to constitute, in these animals, a serial homology with the central ossific points of the sternum and xiphoid appendix ; and they may be considered as represented in the human subject by the inter-pubic fibro cartilages, which are continued along the ab dominal walls to their sternal homologues by the linea alba ; as the pubic and ischial ele ments themselves are represented by the linem transversa, and their cotyloid junction by the external border of the aponeurosis of the ex ternal oblique muscle and the linea semilunaris. The two cartilaginous plates of the pubic sym physis would seem to be the homological re presentatives of the double lateral ossific points, often found permanently separated by an open ing in the lower pieces of the human sternum, and in the bifurcated xiphoid appendage.