THE SKELETON 



59 



tally and finally disappears entirely. The chevron bones are 

 the paired ossicles projecting ventrad from the cranial ends 

 of the bodies of the vertebra}, from the second or third to the 

 thirteenth vertebra. In the sixth, seventh, and eighth verte- 

 brae the chevron bones unite ventrally in the middle Hne, 

 forming an arch. The last ten caudal vertebrae are scarcely 

 more than cylinders of bone representing the bodies of the 

 vertebrae. 



Fig. 27. — Caudal Aspect of Fourth 

 Lumbar Vertebra. 



a. Accessory process; cr,' centrum 

 or body; pa, pedicle; tr, transverse 

 process; s, neural canal; sp, spinous 

 process; zg, the mammillary process 

 of the anterior articular process; am, 

 lamina; pz, articular process. 



Fig. 28. — Dorsal Aspect of the 

 Sacrum. 



fl. Floor of the neural canal; fr, 

 dorsal aperture of the intervertebral 

 foramen; //, lateral mass; pzg, poste- 

 rior articular process; sp, spinous 

 process; si, aurictilar surface for artic- 

 ulation with the ilium; zgt, tubercle 

 formed by fusion of the articular proc- 

 esses; zg, anterior articular processes. 



The number of vertebrae in the different species of mammals 

 varies widely. As a rule, there are seven elements in the 

 cervical region. The sea cow (Manatus) has only six cervical 

 vertebrae, while the three-toed sloth has nine. The number 

 of thoracico-lumbar varies from sixteen in the organ to thirty- 

 six in the cetacean Delphinus. The elements composing the 

 sacrum likewise vary from one in the ape, Cercopithecus, to 



