4 8 ELEMENTS OF MAMMALIAN ANATOMY. 



groove. The groove becomes fainter distally and finally 

 disappears entirely. The chevron bones are the paired 

 ossicles projecting ventrad from the cranial ends of the 

 bodies of the vertebrae, from the second or third to the 

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

 tebrae the chevron bones unite ventrally in the middle line, 

 forming an arch. The last ten caudal vertebrae are scarcely 

 more than cylinders of bone representing the bodies of the 

 vertebrae. 



The number of vertebrae in the different species of mam- 

 mals 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 

 orang to thirty-six in the cetacean Delphinus. The ele- 

 ments composing the sacrum likewise vary from one in the 

 ape, Cercopithecus, to nine in some of the Edentata. The 

 human sacrum is composed of from four to six vertebrae. 

 The same is true of the gorilla, chimpanzee, and orang. 

 The caudal vertebrae vary greatly in number in the different 

 forms. In the adult human there are present only four or 

 five vertebrae, which form a single bone, the coccyx, while 

 in the embryo eight segments make up the caudal region. 

 Ossification takes place, however, in only five or six 

 segments. 



THE STERNUM. 



The sternum of the cat is composed of eight pieces, 

 called sternebrce, which lie in the median line on the ven- 

 tral side of the chest (Fig. 21). The sternum serves for 

 the attachment of the cartilaginous portions of nine pairs 

 of ribs. The first sternebra (Fig. 28), which ends craniad 

 in a laterally compressed pointed process, is the manu- 

 briurn. The six succeeding sternebrae form the body or 



