22 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 



wide arch over the Tipper part of the acetabulum (am.),* the 

 ceutre of which is always occupied by membrane. In the 

 majority of birds the ischium (is.) becomes broader posteriorly 

 and extends backwards nearly parallel with the ilium, both 

 bones uniting posteriorly. The vacuity which occurs between 

 these two bones anteriorly corresponds to a notch in the mamma- 

 lian pelvis, known as the ischio-sciatic notch ; this interval in 

 most birds forms a foramen. 



The pubis (pb.) at its anterior extremity forms part of the 

 socket for the head of the femur (am), and from thence is 

 continued backwards and downwards as a slender curved bone 

 running" approximately parallel to the ischium ; in general it 

 is united only by ligament with its fellow of the opposite side. 

 The ischium and pubis are entirely shut out by the ilium from 

 any contact with the sacrum. 



Such is the general arrangement of the pelvic bones in birds, 

 though there are a few exceptions. In the Tinamous, and in 

 many of the Ratitse, the ischium is not united posteriorly by 

 bone with the ilium, and the ischio-sciatic interval consequently 

 forms a notch in place of a foramen. In Efiea the ischia unite 

 in a median symphysis beneath the sacral vertebrae, which latter 

 are only very imperfectly ossified. In the Ostrich (Struthio), 

 alone among birds, the pubes unite in a median ventral symphysis. 

 The general form of the pelvis of the Ratitae, and specially the 

 anterior projection of the ilium and the backward extension 

 and slender form of the ischium and pubis, shew many points of 

 affinity to the pelvis of certain extinct orders of reptiles (e.g., 

 Dinosauria), and indicate the close relationship which exists 

 between these two classes of vertebrates. 



The Hind-limb. — We now come to the consideration of the 

 hind-limb of birds, which, as we shall see immediately, presents 

 some very remarkable points of difference from the hind-limb 

 of the mammalia. The first segment of the limb forming the 

 thigh-bone or femurii (pi. I. f. and pi. IV., fig. 3) has the same 

 composition in birds as in mammals. The upper articular head 

 (h) which articulates with the acetabulum of the pelvis is round- 

 ed, and its axis is placed nearly at right angles to the long 

 axis of the shaft. The centre of the head presents a slight 

 rough depression (l.t.), indicating the point of attachment 

 of the round ligament {ligamentum teres). The shaft is 

 generally stout, and in its natural position lies nearly horizon- 

 tally ; it is terminated inferiorly by two condyles (i.e. & e.c.) 

 which are elongated antero-posteriorly. A very characteristic 



* Acetabulum, a vinegar-cup, — applied in anatony to the socket for the head of 

 the femur, 

 t Femur, the thigh — applied to the bone of the thigh. 



