We have seen that, while in the Kangaroos and the leaping Rodents, Ruminants, and Carnivora, the ischial and pubic symphyses 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.
These are represented in Alan and Alam malia generally by the sacro-sciatic ligaments, and appear to be ossified by extension from the epiphysis of the ischiadic tuberosity, and from the ischiadic spine. In Birds, they constitute an important element of the pelvis, and are separated, more or less, from the ischium and sacrum by a faintly,' marked suture, more evident, however, at the sacral extremity. Taken in a scientific point of view they represent two additional pelvic rib shafts, or pleurapophyses, of the two last sacral vertebrm, and are implanted on the sacrum, in the human pelvis, in the site of the two lower lateral epiphy sial plates of that hone, exactly as the ilia are articulated upon the three upper. Their extension to the coccyx would also seem to connect them with the elements of the first coccygeal vertebrm ; — and their attachment to the ischia would be, in this point of view, a repetition of the kind of union of the latter hones with the descending pubic rami, as the cartilages of the false ribs are connected consecutively to each other in the thorax.
The obturator and sacro- sciatic foramina, considered in this light, constitute simply consecutive and enlarged interchondrat and intercostal spaces respectively ; the lesser sa cro-sciatic foramen becoming, in the Sloths, Bats, and Birds, entirely obliterated. This manner of viewing them exptains the ap parently indifferent way in which the boun daries of these openings, especially the ob turator, are left incompleted, or entirely obliterated, in Birds and Reptiles. The cotyloid notch may be thus considered as the commencement of the obturator separation, and the formation of two ohturator open ings, as in the Ostrich, may be readily ex plained.
The obturator membrane may also be thus related to the anterior intercostal aponeuroses, and the membranous expansion of the upper border of the great sciatic ligaments (before mentioned as connected with the fascim cover ing the internal obturator and pyriformes mus cles), to the posterior intercostal ligaments.
The homologues of the liganzents of the sacro-iliac articulations are readily found in those connecting the head, neck, and tubercles of the thoracic ribs to the vertebrm.
The anterior and superior sacro-iliac figaments are evidently repetitions of the anterior or stel late costo-vertebral ; the superficial fibres of the posterior sacro-iliac, repetitions of the posterior costo-transverse ; the deep fibres, or inter osseous sacro-iliac, of the middle or interosseous costo-transverse ; and the ilio-lumbar and lumbo sacral ligaments, of the oblique or an'erior costo transverse ligaments of the ribs. The connec
tion of the last-named ligament with the trans verse process of the vertebra above, and with the iliac crest below, is very similar to the oblique direction and attachments of the homologous costo-transverse. The ossifica tion of these ligaments in Sloths, Bats, and especially in Birds, gives additional support to their otherwise feebly connected pelves.
The ligaments of the pubic symphysis find easily their homologues in the chondro-sternal lig,aments. The anterior and posterior peripheral inter-pubic liganients are repetitions of the similarly placed and constituted chondro sternal, which, like them, connect the hmma pophyses together across the interposed elements, as well as directly to the endo sternal bones and inter-pubic fibro-cartilage. The superior pubic ligament finds its homo logue in the inter-clavicular, and the sub-pubic, in the chondro-xiphoid and interchondral fibres.
It would be easy also to point out the mus cular homologues of the two regions ; but space will not perrnit more than to mention the evident ones of the external and internal oblique muscles with the similarly placed and directed intercostals ; of the levator ani with the diaphragm ; and of the pyriformes with the psow muscles. The internal obturator muscles would represent the triangularis sterni and transversalis abdominis ; am.I the external, the lesser pectoral and external oblique muscles.
The homologues of the pelvic bones with those of the shoulder are best seen in the Rep tiles. According to Duges, the pelvis of the Salamander has a close resemblance to the shoulder of the Cameleon.
The i/ium is generally considered to be the homologue of the scapula ; the pubis, of the clavicle ; and the isehium, of the coracoid bone. Meckel considers the body of the ischiunz to represent the spine of the scapul ; while, in the opinion of Oken, the pubis represents the acrontion process, and the marsupial bones the clavicles.
In the shoulder of the Cameleon, the sca pula is longer even than the ilium, and the furculm clavicles and coracoids are ankylosed together like the ischiutn and pubis. And, in the pelvis of the Crocadile, we have seen that the ischium excludes the pubis from the formation of the acetabulum by an apophysis which overhangs that cavity, somewhat as the human coracoid process intervenes between the glenoid cavity and the davicle.
The ascending branch of the ischium rnay, further, be taken to represent the epicoracod bones of the Monotremes, Lizards, Batrachians, and some Fishes, as the Cod, Carp, and Perch, In the Cock-fish, Snipe-fish, and Lancet-fish, these bones are joined in a kbid of symphysis, forming an independent arch behind the sca pular arch.
And those bones of the scapular arch of the Fish, which are considered by Owen to be the coracoids, (but by Aleckel, Agassiz, Geof froy, and Spix to be clavicles,) unite also in a median symphysis, presenting an homologous affinity to the ischial symphysis seen in the Reptiles,—the whole of these symphysial ele rnents being represented, combined, by the elongated ischio-pubic symphysis in many Mammalians, but especially in that of the Marsupials and Alonotremes.