THE VERTEBRAL COLUMN. 235 



one another, since the cervical vertebrae, as in the Crocodiles, bear 

 ribs, which unite with the transverse processes to form a foramen 

 transversarium. The neck is long and always freely movable, and 

 contains 9 to 23 (Swan) vertebrae. The shorter dorsal vertebras are 

 always less numerous ; they have superior and inferior spinous pro- 

 cesses and all bear ribs, to the ventral ends of which sterno-costal ^ 

 bones are articulated at an angle which projects backwards (fig. 650, 

 Stc). The sternocostals also articulate with the margin of the 

 sternum, and serve when they are extended to increase the distance 

 between the latter and the vertebral column. But, since the ribs 

 are firmly applied to one another by means of posterior processes 

 (processus uncinati), the movement of the sternocostal ribs must 

 necessarily affect the thorax as a whole, and dilate it (inspiration). 

 The sternum is a broad flat bone which covers not only the thorax 

 but a great part of the abdomen, and bears a projecting keel-like 

 crest which serves for the attachment of the muscles of flight 

 (Carinatce). The sternal crest is reduced or obsolete only when the 

 power of flight is feeble or entirely absent (Ratitce). 



The rib-bearing dorsal vertebrae are followed by a tolerably exten- 

 sive division of the vertebral column, which corresponds to the lumbar 

 and sacral regions, and which, by the fusion of a number of vertebrae 

 with each other, and with the long iliac bones of the pelvic girdle, 

 presents the characters of the sacrum. The sacrum is much elon- 

 gated, and includes sixteen to twenty and more vertebrae ; of these 

 a certain number can be shown to be lumbar (praesacral), and are 

 almost always preceded by two to three rib-bearing dorsal vertebrae. 

 Then follows the true sacrum ; it consists of two vertebrae, which are 

 equivalent to the sacral vertebrae of Lizards and Crocodiles, and 

 constitute by means of their transverse processes (with fused ribs) 

 the main support of the pelvis near the cup of the hip- joint 

 (acetabular vertebrce). Finally the true sacrum is followed by a 

 postsacral region, which is composed of from three to seven of the 

 anterior caudal vertebrae. The short caudal region, which succeeds 

 the postsacral, consists, as a rule, of from seven to eight movable 

 vertebrae, of which the last is represented by a vertical, laterally 

 compressed plate the pygostyle to which the muscles for the move- 

 ment of the steering feathers (rectrices) of the tail are attached. 

 This deep ploughshare-shaped terminal body is composed of from four 

 to six vertebrae, so that the reduction of the number of the caudal 

 vertebrae, as compared with that of. the Saururce (Archceopteryx), is 

 by no means so considerable. 



