PELVIS. 



167 



marked ilio-ischial angle in the reverse direc- 

 tion to that of Mammals generally, i. e. with 

 the retiring sides anterior (see j%/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. A-). 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 be 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 (</). 

 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 Penguins. 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 keep 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. 



The pelvis of the Cursores (fig. 106.) ap- 

 proaches most nearly in the massiveness of 

 the bones to the Mammalian type, as well as, 

 in the Ostrich, in the formation of a pubic 

 symphysis. 



The sacrum (a) is very long and narrow, 

 and is composed, according to Cuvier's 

 tables, of twenty pieces in the Ostrich and 

 of nineteen in the Emu and Australian Casso- 



wary. The spinous and transverse processes 

 are distinct, and coalesced only at their ex- 



106. 



Dorsal view of the pelvis of the Ostrich : <*, coa- 

 lesced sacral spines ; 6, ilium ; c, c 1 , sacro-iliac plate ; 

 d, sacral chink ; e, ischium ; f, ischio-sacral buttress ; 

 g, pubis ; A, symphysis ; i, acetabulum ; A, anterior 

 ischio-pubic suture ; /, anterior obturator opening ; 

 77i, ilio-pectineal spine ; o, posterior or greater ob- 

 turator hole. 



tremities (a), in the Ostrich (the former being 

 the only part of the sacrum appearing dorsally), 

 presenting another close approximation to the 

 Mammalian condition. 



The coccyx is straight, and composed of 

 seven pieces, which are perforated for the 

 termination of the spinal marrow, and end in 

 a conical bone. In the Rhea or American 

 Ostrich, boih the sacral and coccygeal bones 

 are much atrophied. 



The ilia (b) are comparatively very short, 

 especially in the Rhea. The alse are thick, 

 short, and little curved, and lie close to each 

 other at the upper half of their inner borders, 

 by which they are ankylosed to the sacral 

 spines, whose coalesced extremities are seen 

 between them, forming a tent-like eminence 

 above the anterior sacral vertebrae, and sup- 

 ported by their spinous processes in the 

 manner of tent poles. Anteriorly, they overlap 

 the posterior ribs ; and posteriorly, they are 

 prolonged on the sides of the sacrum into a 

 distinct and prominent ilio-sacral element. 



This is an elongated piece of bone, with 

 a superior (c) and a lateral (c') surface, 



H 4 



