MAMMALIA. 279 



The axis or second cervical vertebra has an odontoid peg (which 

 ossifies separately), projecting forwards above the centrum of the 

 atlas. 



A convex articular surface is present on the under side of the peg, which 

 plays upon a corresponding surface on the upper side of the atlas-centrum, 

 and there are also two large convexities on its base which fit into the two 

 shallow cups on the hind end of the atlas. The neural spine of the axis is 

 a prominent ridge which bifurcates behind. 



The last (seventh) cervical vertebra has a half-facet on the 

 side of the centrum, at its hinder end. 



The thoracic vertebrae are 12 (or 13) in number, and are 

 characterized by the possession of free thoracic ribs. 



They increase in size from before backwards, their centra, which are 

 thick from above downwards, elongating. The neural spines of the first 9 

 are slender and backwardly directed, that of the tenth is vertical, and those 

 of the remainder slope forwards like the lumbar spines. From the ninth 

 backwards metapophyses appear, stout processes directed upwards and for- 

 wards in the region of the pre-zygapophyses. The transverse processes of 

 the first 9 possess tubercular facets on the under side of their tips, and 

 capitular half-facets on the sides of their centra, one in front, the other 

 behind. The last 3 (or 4) have entire capitular facets on the sides of their 

 centra, situated near the front. 



The ribs are curved flattened rods, increasing in length up to 

 the sixth, and then shortening. Each consists of a bony vertebral 

 portion, possessing two articular processes (tubercle and capitulum), 

 and a much shorter sternal part of more or less ossified cartilage. 



The tubercles of the ribs articulate with the tubercular facets, and the 

 capitulum of each of the first 9 articulates with a capitular facet, formed in 

 part by the anterior half-facet of its own vertebra, in part by the posterior 

 half-facet of the preceding vertebra (the seventh cervical in the case of the 

 first rib). The last 3 (or 4) possess no tubercles, and their capitula arti- 

 culate with the corresponding facets on their own vertebra. The sternal 

 parts of the first 7 ribs unite distally with the sternum ; those of the remain- 

 ing ones do not. This is the distinction between " true " and " false " ribs. 



The lumbar vertebras are 7 (or 6) in number. They are large, 

 with elongated centra, laterally flattened neural spines, directed 

 forwards, and strong elongated transverse processes running down- 

 wards and forwards. 



They also possess large metapophyses, stout processes overhanging the 

 pre-zygapophyses, and small anapophyses, backwardly directed processes 

 given oft below the post-zygapophyses. The first two also have hypapo- 

 physes, unpaired processes running downwards from the under side of the 

 centra. 



