98 ELEMENTS OF COMPARATIVE ZOO LOOT. 



In fact these structures must be regarded as hairs united 

 throughout their length. At other times a similar consoli- 

 dation of hair gives rise to protective scales covering the 

 body, as in the case of the pangolins. 



The bodies of the vertebrae usually have flat faces, and 

 the vertebral column in most forms can be divided into five 

 regions cervical, thoracic, lumbar, sacral, and caudal. 

 The cervical vertebrae occur in the neck; they bear no ribs, 

 and, except in three rare forms, they are constantly seven 

 in number, the long-necked giraffe and the short-necked 

 whale having the same number of these bones. The tho- 

 racic vertebrae are more variable in number. They bear ribs, 

 some of which extend downward and unite with the breast- 

 bone or sternum. Between the thoracic and pelvic regions 

 occur the ribless lumbar vertebrae, while the sacral verte- 

 brae are those which unite with the pelvic bones. The cau- 

 dal vertebrae are found in the tail. In the whales only 

 cervical and thoracic vertebrae can be distinguished, since 

 the absence of a pelvis in these forms allows no line to be 

 drawn between lumbar, sacral, and caudal regions. 



In the skull there is a tendency for bones which are dis- 

 tinct in the fishes and reptiles to fuse with each other, so 

 that the number of distinct elements is considerably re- 

 duced. The skull is borne on the first cervical vertebra, 

 upon which it slides by means of two rounded surfaces or 

 condyles. The lower jaw articulates directly with the 

 skull, and is never suspended by a quadrate bone, as in the 

 forms already studied. 



The fore limbs are always present; the hind limbs are 

 absent in the whales and manatees, being represented in a 

 few forms by one or two bones imbedded in the muscles of 

 the trunk. Except in the Monotremes (p. 102), the cora- 

 coid does not occur as a distinct bone, but as a small prom- 



