THE RABBIT 321 



the odontoid process. The process projects into the ring of the 

 atlas, whose centrum it represents morphologically. The axis has 

 also a pair of foramina transversaria. 



The other five cervical vertebrae are similar to one another. They 

 all have foramina transversaria. Their transverse processes, like 

 those of the altas and the axis, are not homologous to those of the 

 other vertebrae, but each is equivalent to a transverse process plus 

 the rudiment of a rib. It is between this rudiment and the process 

 that the foramen transversarium is present. The seventh cervical 

 vertebra may lack this foramen. The vertebrarterial canal, which 

 is formed by the succession of the foramina transversaria on each 

 side, is the passageway of the vertebral artery and vein, which 

 supply the brain with blood. 



The thoracic vertebrae are distinguished by their very long 

 spinous processes and by the ribs which articulate with them. 

 Observe their zygapophyses. Twelve or thirteen pairs of ribs are 

 present, the dorsal portion of each being bone and the ventral 

 portion being cartilage. The seven anterior pairs of ribs articulate 

 separately with the sternum and are called true ribs ; the five or 

 six posterior pairs are not attached to the sternum and are called 

 false ribs. Of these latter the three anterior pairs are united with 

 one another ventrally ; the two or three posterior pairs are free at 

 their ventral ends and are called floating ribs. The capitulum, or 

 head, of the rib articulates with the centrum of its vertebra, and 

 also, in the case of the first ten ribs, with the centrum of the verte- 

 bra immediately in front of it. Each of the first nine ribs artic- 

 ulates also with the transverse process of its vertebra, by means 

 of an articular projection called the tuberculum, which is just be- 

 low the capitulum. 



The sternum, or breastbone, is a row of six bones which lie in 

 the ventral wall of the thorax and ^orm the ventral support of the 

 ribs. The foremost of these bones is the manubrium ; the hinder- 

 most is much elongated and is called the ensiform process, the 

 posterior part being cartilaginous. 



The lumbar vertebrae are larger than the thoracic, and are dis- 

 tinguished by the large size of their transverse processes, which 

 are directed forward and downward. 



