VERTEBRAL COLUMN, RIBS, AND STERNUM 307 



trunk- and tail-regions) is now divisible into regions corre- 

 sponding to neck, thorax, sacrum, and tail. In the amniotes 

 the first vertebra (the atlas) becomes detached from its centrum, 

 which becomes attached to the second vertebra or axis, and 

 forms its odontoid peg. There are therefore two vertebrae 

 specially modified in connexion with the neck, and a variable 

 number of normal cervical vertebrae which differ from the 

 thoracic in that their ribs are short and do not reach the 

 sternum. The vertebrae between the thoracic (whose ribs 

 reach the sternum) and the sacral are the lumbar. In primitive 

 forms the sacrum affects only one vertebra, to the ribs or 

 transverse processes of which the ilia are attached. In higher 

 forms, and especially in birds, there are several sacral vertebrae. 



The vertebrae of land-animals bear facets by means of which 

 they articulate with one another, and so enable the vertebral 

 column to bend with considerable flexibility without 

 diminishing its strength. These facets are the pre- and 

 postzygapophyses. In some groups such as the lizard and 

 snakes, additional facets may be developed. The faces of the 

 centra are either flat or slightly concave or convex, but in the 

 birds a special saddle-like shape has been developed, which 

 allows of very great flexibility. 



In the mammals, the number of cervical vertebrae is seven 

 in all species with only three exceptions. These are the 

 Edentates Bradypus which has nine, and a species of Choloepus 

 which has six or seven, and the Sirenian Manatus which has 

 six. 



Ribs are extensions of the basiventrals, and they may be of 

 two kinds. Those which pass just on the outside of the 

 splanchnocoelic cavity are pleural or ventral ribs, and they 

 occur in Dipnoi. " True " or dorsal ribs pass in the horizontal 

 septum which separates the myotomes into dorsal and ventral 

 portions, and they occur in Selachii and in the Tetrapods. 

 Both kinds of ribs are present in Polypterus and some Teleosts. 



In the land- vertebrates, the ribs primitively articulate with 

 the vertebrae by a broad head which touches the centrum and 

 the neural arch. These holocephalous ribs as they are termed 

 are present in Labyrinthodonts, Cotylosauria, and Sphenodon. 



