550 MAMMALS. 



The first vertebra or atlas is ring-like, its neural canal 

 being very large, its centrum unrepresented unless by the 

 odontoid process which fuses to the second vertebra. The 

 ring is divided transversely by a ligament, through the upper 

 part the spinal cord passes, into the lower the odontoid 

 process projects. The transverse processes are very broad ; 

 the articular surfaces for the two condyles of the skull are 

 large and deep. 



The second vertebra or axis has a broad flat centrum pro- 

 duced in front in the odontoid process. The neural spine 

 forms a prominent crest, the transverse processes are small, 

 the anterior articular surfaces are large. 



A typical lumbar vertebra will show the centrum and its 

 epiphyses, the neural arch and neural spine, the transverse 

 processes, the anterior and posterior articular processes or 

 zygapophyses, the median ventral hypapophysis, the small 

 anapophyses from the neural arch below the posterior 

 zygapophyses, below the anapophyses the posterior inter- 

 vertebral notches — passages through which the spinal nerves 

 pass out, and anteriorly a similar pair of notches. There are 

 twelve or thirteen pairs of ribs which support the wall of the 

 thorax, and aid in the mechanism of respiration. The first 

 seven pairs articulate with the breast-bone, the eighth and 

 ninth are connected to the ribs in front, the others are free. 

 Any one of the first seven or more typical ribs consists of 

 two parts, a vertebral portion articulating with a vertebra, an 

 imperfectly ossified sternal portion connecting the end of 

 the vertebral portion with the sternum. Each of the first 

 nine ribs has a double head — the capitulum articulating 

 with the centrum of the corresponding vertebra, and partly 

 with that of the one in front, the tubercle articulating with 

 the transverse process of the corresponding vertebra. The 

 posterior ribs have no tubercles, and the capitular articula- 

 tions are restricted to the corresponding vertebrae. 



The sternum is a narrow jointed plate, with a large keeled 

 praesternum or manubrium, then five segments composing 

 the mesosternum, then a posterior xiphisternum ending in 

 cartilage. 



The Skull. — The skull consists, as in all the higher Verte- 

 brates, of two sets of bones, — cartilage-bones preformed in 

 the cartilage of the original gristly brain-box and its asso- 



