332 THE VERTEBRATE SKELETON. 



VERTEBRAL COLUMN. 



The vertebral column of birds is readily divisible into a 

 very mobile cervical region, and an extremely rigid post- 

 cervical region. In most birds the vertebral centra are with- 

 out terminal epiphyses, but these structures are found in 

 Parrots. The cervical vertebrae are generally large and vary 

 in number from eight or nine to twenty -three in Swans. 

 Except in some extinct forms, such as Ichthyornis and Apa- 

 tornis, in which they are biconcave, the centra are charac- 

 terised by having saddle-shaped articulating surfaces, which 

 in front are concave from side to side and slightly convex 

 from above downwards, while posteriorly they are convex 

 from side to side and concave from above downwards. The 

 atlas is small and ring-like, and its centrum is fused with the 

 axis forming the odontoid process. Cervical ribs are often 

 well developed, and in some of the Ratitae they remain for a 

 long time distinct from the vertebrae. 



The thoracic vertebrae are distinguished from the cervical 

 by the fact that their true ribs are united to the sternum 

 by means of sternal ribs. This distinction, however, though 

 convenient, is somewhat arbitrary, as it has been shown that 

 in the fowl and gannet, two pairs of ribs which in the adult 

 are free from the sternum, are connected with it in the em- 

 bryo. When, as in the Swans, the thoracic vertebrae are 

 not all fused together, they generally have saddle-shaped 

 articulating surfaces, but sometimes, as in the Penguins, Auks 

 and Plovers, the centra are convex in front and concave be- 

 hind. The trunk vertebrae generally have well-marked neural 

 spines, while in the Divers the anterior ones have peculiar 

 bifurcating hypapophyses. 



The trunk vertebrae are not readily divisible into thoracic 

 and lumbar. There are two true sacral -vertebrae, but as 

 development proceeds a number of other vertebrae become 

 fused with the true sacrals, the whole forming a large com- 

 pound sacrum. These pseudosacral vertebrae generally include 



