294 THE ANATOMY OF VEETEBEATED ANIMALS. 



the centre of which is always closed by fibrous tissue, so that, 

 in the dry skeleton, the bottom of the acetabulum is per- 

 forated by a wide foramen. An articular surface on the 

 ilium, on which the great trochanter of the femur plays, is 

 called the antitrochanter. In all ordinary birds, the ischium 

 (Fig. 86, Is.), which broadens towards its hinder end, extends 

 back, nearly parallel with the hinder part of the ilium, and 

 is united with it by ossification, posteriorly. The iliosciatic 

 interval is thus converted into a foramen. The pvibis (Pb.) 

 enters, by its dorsal or acetabular end, into the formation of 

 the acetabulum, and then passes backwards and downwards 

 as a comparatively slender, curved, bone, nearly parallel 

 with the ischium. It is vmited with its fellow only by 

 fibrous tissue. Neither the ischia, nor the pubes, unite 

 directly with the sacrum. Yery few birds present any im- 

 portant deviation from this stinicture of the pelvis. In 

 Tinamus, Casuarius, Dromceus, Apteryx, Dinornis, the ischium 

 is not united with the backward extension of the ilium by 

 bone. In Rhea, the ischia unite with one another beneath 

 the vertebral column, and the vertebi"se in this region are 

 very slender and imperfectly ossified. In Struthio, alone, 

 among birds, do the pubes unite in a median ventral sym- 

 physis. Another, not less remarkable circumstance, in the 

 ostrich, is, that the 31st to the 35th vertebi-se inclusively 

 (counting from the atlas) develope five lateral tuberosities. 

 The three middle tuberosities are large, and abut against 

 the pubis and the ischium. In these vertebrae, as in the 

 dorsal vertebrae of Chelonia, the neural arch of each ver- 

 tebra shifts forward, so that haK its base articulates with 

 the centrum of the next vertebra in front ; and the tube- 

 rosities in question are outgrowths, partly of the neural 

 arch, partly of the juxtaposed vertebral centra, between 

 which it is wedged. Hence, in young ostrtches, the face of 

 each tuberosity exhibits a triradiate suture. 



The upper articular head of the femiir is rounded, and its 

 axis is almost at right angles with the body of the bone ; a 

 structure which is not found in ordinary Beptilia, but exists 

 in the Iguanodon and other OrnitJioscelida. The shaft is re- 



