660 BIRDS* 



vical, thoracic, lumbar, sacral, and caudal. The mobile 

 neck consists of fourteen cervical vertebrae ; from the third 

 to the twelfth these bear short ribs fused to the centra and 

 transverse processes; the thirteenth and fourteenth have 

 them free and well developed, but not reaching the sternum. 

 Of the thoracic vertebrae, namely, those whose ribs reach 

 the sternum, the anterior three are fused to one another, 

 while the fourth is free. The complex sacral region 'consists 

 of the fifth thoracic (with free ribs reaching the sternum), 

 five or six lumbars, two sacrals, and five caudals, all fused. 

 Lastly, there are six free caudals ending in a pygostyle or 

 ploughshare bone, a fusion of about four vertebrae (cf. 

 coccyx in man). This bone serves as a base for the 

 rectrices. 



A cervical vertebra shows on the anterior surface of the 

 centrum a distinctive curvature, described as saddle-shaped 

 or heteroccelous. It is concave from side to side, convex 

 from above downwards. Posteriorly the curvatures are, of 

 course, the reverse. 



The ribs have two heads a capitulum articulating with 

 a centrum, a tubercle articulating with a transverse process. 

 The ventral part of the rib, which reaches the sternum, is 

 called the sternal rib, and is joined at an angle to the dorsal 

 part, which articulates with a vertebra. In Birds the sternal 

 ribs are always bony; in Mammals they are usually cartila- 

 ginous. On the posterior surface of each of the first four 

 thoracic ribs there is an uncinate process, absent only in 

 the S. American screamers (Palamedeae). 



The skull has a rounded cranial cavity, large orbits, and 

 a narrow beak, which is mostly composed of the premaxillae. 

 All the bones are fixed except the quadrate, lower jaw, 

 columella, and hyoid. The surface is polished ; the sutures 

 are obliterated very early in life. 



The back part of the skull is formed by the basioccipital, 

 the two exoccipitals, and the supraoccipital, surrounding 

 the foramen magnum. The basioccipital forms most of the 

 single condyle. 



The roof of the skull is formed from the paired parietals, 

 frontals, and nasals, the last being small and in part super- 

 seded by the upward extension of the premaxillae. 



The line of the upper jaw consists of premaxilla, small 



