From the posterior part of the inner border of the iliac wing passes backward a thin plate of bone (g), along the external borders of the diamond-shaped sacral plate, from which it is marked by a distinct line of suture (c). It is continuous, posteriorly and externally, with the sacro-sciatic ossification, to be presently men tioned, from which it is also marked, especially in the Partridge and some other birds, by a raised line of demarcation (t). This thin plate is convex above and concave below, and enters into the formation of the pelvic cavity, being much hollowed in the Partridge and the Gal linacece generally (fig. 105. g), to receive the pelvic viscera. It seems to result from the ossification of the sacro-iliac oblique ligament, and to form a separate pelvic element which may be called the sacro-iliac, or ilio-sacral.
The ischia of birds (k) are long, strong, and divergent posteriorly ; and, from the perfora tion of the cotyloids, appear to be bifurcated at the anterior extremity. The inferior bifur cation is horizontal, coalesces with the ilium and pubis, and separates the acetabulum (i) from the obturator foramen (o). The superior . bifurcation is vertical in direction, separating the acetabulum (2) from the sacra sciatic foramen (/), and coalescing above, inter nally with the long sacral transverse process (b) and ilio-sacral bone (g), and anteriorly with the ilium in the ischio-sacral buttress (f), before mentioned, which it principally con tributes to supportand form, and which may be considered as the homologue of the ischio-sacral arch in the human pelvis, separated from the cotylo-sacral rib (e) by a thin plate of bone above, and by the perforated acetabulum below. The posterior extremity of the ischium is much elongated, and constitutes the bulk of the bone. Its inferior border is spread out into a broad thin plate, slightly prolonged into an anterior process (222), which seems to represent the ascending ramus of Mammals, from its frequently uniting with the pubis and forming the posterior boundary of an obturator foramen.
Its superior border is prolonged into a broad thin plate (h) hollowed out in the pelvic cavity, and which constitutes the sacro-sciatic pelvic element, being evidently formed by ossi fication of the sacro-sciatic ligaments, from its completing posteriorly the sacro-sciatic foramen (I), and coalescing with the sacro-iliac plate (g), before mentioned, and, behind it, with the sacrum. The posterior extremity of the ischium is prolonged generally into a thin angular spinous process (n). The Ischia in Birds generally form a right line with the ilia; but in the Birds of priy they constitute a remarkable exception, and make a very marked ilio-ischial angle in the reverse direc tion to that of Mammals generally', e. with
the retiring sides anterior (see fig. 107.).
The pubes of birds are generally long, slender, rib-like, and divergent, and are com posed of a single curved branch (p), having no angle, and never forming a true interpubic symphysis, though, in the Ostrich and Falco Fulvus, they are closely approximated at their posterior extremity, and form a sort of sym physis. The ilio-pubic angle is very large, from 155° to 160°, except in the birds of prey above alluded to ; and the pubes and ischia are generally almost parallel. Sometimes the posterior extremities of the pubes and ischia unite to• form complete elongated obturator foramina ; and they may be united also near their anterior extremities, forming a lesser an terior division of the foramina, as in the Cur sores (see fig. 106. k). Very often, the boun daries of the obturator openings are incom plete from the failure of this junction, and the foramina are wanting altogether ; or the an terior union and foramina only may he present, as in the Duck (fig. 104. r), from deficiency of the pubes posteriorly, or their entire approxi mation to the ischia. The pelvic cavity is in creased in size posteriorly, by the divergence of the pubes and ischia, and is capable of great enlargement by the yielding of their unfixed extremities. The ilio-pectineal emi nence is generally present, and often large in size, constituting a spinous process (q). The acetabula (i) are perforated and placed almost close to the borders of the sacrum, and generally much anterior to the centre of the whole pelvic length, that the points of support may be nearer the centre of gravity.
The bird's pelvis thus constitutes a firm, compact, immobile, box-like structure, de ficient inferiorly, affording a large and firm hold, by the elongated and strong ischia, for the extensor muscles of the leg ; and, by the large sacrum and ankylosed ilia, for those of the trunk, which is placed almost entirely in front of the supporting femora, and always more or less at an angle with them, except in the Grebes and Penguius. The centre of gravity is not, in birds, directly above these supports, as in the true erect position of man, but is placed considerably in advance of the femurs, and necessitates considerable flexion of the lower parts of the legs, and great length of toes, to kcep the centre of gravity within the base of support. The long pelvic muscles, the tendons of which reach to the toes, by a constant tendency to flex them, contribute mainly to preserve this semi-erect position, even during sleep, and independently- of the will of the animal.