176 



PELVIS. 



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 he 

 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 

 tubcrosity 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. 



The Y-shaped cpipliysial coti/loid 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 margin of 

 the acetabulum (see fig. 110. Art. Marsu- 

 pialia) ; and one of a similar nature and 

 position is described by Geoffrey 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- 

 tebrae, and in the articular ends of some long 

 bones, as those forming the elbow-joint. 



From the position of the ilio-pcctineal 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 

 Monotremes, in a great state .of develop- 

 ment with the 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 psoce 

 parvcc 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 pubcs 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 Poiqmrfs ligament, and which is 



implanted upon the pubic spine.* In support 

 of this opinion, it may be stated that, in Pou- 

 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 upwards, 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 

 Jig. 99. <). 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 Jibro- 

 carlilages, 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 linea; 

 transversee, 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 jtoints, 

 often found permanently separated by an open- 

 ing in the lower pieces of the human sternum, 

 and in the bifurcated xiphoid appendage. 



We have seen that, while in the Kangaroos 

 and the leaping Rodents, Ruminants, and 

 Carnivora, the ischial and pubic symjihyscs are 

 largely developed and coalesced, and seem, in 

 many cases, to be compensatory for the weak- 

 ness of the sacral portion of the pelvis ; in the 

 Sloths and some others the ischial symphysis 

 is entirely wanting ; and in the Bats, Shrews, 

 Moles, and Birds, the elements of the pubic 

 symphysis also entirely fail. A congenital 

 and similar deficiency of this symphysis has 

 been observed in some human pelvic mal- 

 formations, from arrest of development. This 

 deficiency may be well compared to a like 

 congenital absence of the sternum, some in- 

 stances of which have been recorded. 



The loss of pelvic firmness consequent 

 upon this is compensated for, in great part, in 

 the animals just named, by the ossification 

 of another pelvic element, the sacro-sciatic. 



* Laurent and Owen consider thepo bones as 

 sesamoid trochlear ossicles. (See Art, Marsu- 

 ' 



