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148 GENERAL ORNITHOLOGY. 
ischium, and pubis, which have independent ossific centres, but become firmly consolidated 
together to form the haunch-bone or os innominatum. Each of these bones unites with the 
other two, somewhere near the middle of the whole affair, at a ring-like structure called the 
acetabulum (Lat., a vinegar-cruet, fig. 56, a; fig. 60, arrow ac), which all three consequently 
contribute to the formation of, and which is the socket for the head of the thigh-bone (femur, 
p- 119). When free ribs issue from under cover of the pelvis, they are commonly anchylosed 
with the ilia; and all the abortive pleurapophyses of the lumbar-and uro-sacral vertebree have 
likewise iliac anchylosis, as explained in treating of the sacrum (p. 140). As a whole, the pelvis 
varies like the sternum in relative length, breadth, and degree of convexity ; and especially in 
the configuration of its posterior border; but few zodlogical characters are derived from this 
structure. 
Viewed from below, the pelvis is seen to be much hollowed or excavated for the lodgment 
of the kidneys, and cross-cut into compartments by the sacral rafters; the series of sacral 
bodies forming a ridge-pole along the middle line. Above, the series of sacral spinous pro- 
cesses represent the ridge-pole; anteriorly, the somewhat spoon-shaped iliac bones are 
applied, concavity outward, to the dorso-lumbars ; posteriorly, in the middle line, is a more or 
less flattened horizontal expansion, and laterally are the more expanded sides of the ischiac roof, 
finished along the eaves and behind by the slender pubie bone, which commonly projects 
backward, and inclines toward its fellow of the opposite side. The most prominent formation 
of the side wall of the pelvis is the thick-lipped smooth articular ring, the acetabulum, con- 
verted in the natural state into a cup by a membrane. 
The postero-superior segment of the rim is promi- 
nent, to form the antitrochanter (Gr. dvri, anti, 
against ; tpoxavrnp, trochanter of the femur) against 
which the shoulder of the femur abuts when the 
head is in the ring. . 
Fig. 61. — Pelvis of ag grouse, showing 
three distinct bones. Il, Is, P, ilium, ischium, 5 : 2 
pubis. In front of former a at vertebra pro- It is normal to recent Carinate birds to have 
trudes. (Dr. R. W. Shufeldt, U. 8. A.) the ischium fused with the ilium, however distinct the 
pubis may remain; but to Cretaceous birds (even the carinate Ichthyornis), and the existing 
Ratite, to have both ischium and pubis distinct in most of their extent. 
The ium (Lat. tliwm, haunch-bone ; pl. ila ; adj..ihac ; figs. 56, I; 60, 61, Il) is the 
median, most anterior and longest of the haunch-bones, and the only one which patente in ad- 
vance of the acetabulum. Such anterior prolongation of this bone is the specialty of the avian 
pelvis: it commonly overlies one or more ribs, and is often overreached by the end of the scapula. 
It is longest and narrowest and flattest in some of the lower swimmers; the reverse among the 
highest birds. Its relations and connections have been sufficiently indicated. The bone is 
almost always separated from its fellow by the sacrum, though the approximation may be 
very close over the back of the Pe along the middle line. 
The Ischium (Gr. icyioy, ischion, the haunch-bone; pl. ischia ; adj. ischiadic, ischiatic, 
better ischiac; figs. 56, 60, 61, Is) lies entirely post-acetabular, or behind the socket which it 
contributes to form, and composes most of the’ side-wall of the pelvis thence to the end. It is 
generally a thin, plate-like bone. Among Cretaceous birds and existing Ratite it only unites 
with the ilium at and just behind the acetabulum, whence a deep ilio-ischiac fissure between 
the two exists, as in the young grouse, fig. 61; but in ordinary adult birds this fissure is con- 
verted into a fenestra or window of large size, just behind the acetabulum, by union of the two 
bones behind it. This vacuity, whether a notch or a hole, corresponds to the ‘ sacro-sciatic 
notch” of human anatomy (fig. 56, im). The ischia of opposite sides are distinct, except in 
Rhea. 
