Sauropsida: Class Aves 



515 



Fig. 401. Cervical verte- 

 bra of a bird, showing the 

 saddle-shaped articular sur- 

 face (a/) on the centrum 

 (c); (cr) cervical rib; (nc) 

 neural canal; (ns) neural 

 spine; (poz, prz) post- and 

 prezygapophyses; (»c) verte- 

 brarterial canal. (Courtesy, 

 Kingsley: "Comparative 

 Anatomy of Vertebrates," 

 Philadelphia, The Blakiston 

 Company.) 



brae varies with the length of the neck — in the common pigeon, 14, 

 including atlas and axis; in the swan, 25; in some birds, only 9. The 

 extreme flexibility of the bird's neck is due to a peculiar type of joint. 

 Usually the posterior surface of a bird's 

 centrum is saddle-shaped (Fig. 404), being 

 concave in the sagittal plane and convex in 

 the frontal plane. The anterior surface of 

 the centrum next behind is shaped to fit 

 this saddle-like surface. Such centra are 

 called heterocoelous. The cervical verte- 

 brae, except atlas and axis, bear short ribs, 

 most of which are fused to the vertebrae 

 (Figs. 401, 404). 



In all of the postcervical regions of the 

 bird's vertebral column, there is more or 

 less extensive loss of mobility, resulting 

 from fusion of successive vertebrae. The 

 corresponding regions of different birds 

 show much variation as to the number of 

 vertebrae and the extent and nature of the 

 fusion. Between the last cervical and the 

 beginning of the sacral region, there are commonly 4 or 5 vertebrae, 

 but in some birds fewer or several more, most of them usually immov- 

 ably fused together. 



The sacral region (Fig. 401) includes a varying, but always large, 

 number of vertebrae: in the domestic fowl (Gallus), 14 or 15; in many 

 birds, more — 15 to 23. In the embryonic development of the vertebral 

 column, the first connection with the pelvic girdle is usually made by 

 2 vertebrae whose short ribs become joined to the ilia. Somewhat later, 

 additional vertebrae, some anterior and some posterior to the primary 

 two, become connected with the girdle. This fact, together with the 

 numerical relations between corresponding regions of the columns of 

 reptiles and birds, indicates that the primary 2 sacral vertebrae of 

 the bird correspond to the usual 2 sacral vertebrae of reptiles, and 

 therefore the remaining sacral vertebrae of the bird correspond to 

 vertebrae of the posterior trunk (lumbar) region and anterior caudal 

 (postsacral) region of reptiles. Applying this interpretation to the 

 sacrum of the pigeon, it consists of the hindmost thoracic vertebra, 

 6 lumbar vertebrae, the 2 primary sacrals, and the first 5 caudal 

 vertebrae, a total of 14. The term synsacrum, applied to the avian 

 sacrum, expresses the idea that it has appropriated to itself regions of 

 the column anterior and posterior to a "primary" sacrum. The tail 

 of modern birds is a mere stump serving for insertion of the long 



