364 SYSTEMATIC ZOOLOGY. 



dips clown into the deeper layer (derma), forming a pit or 

 follicle from the bottom of which the hair grows, continual 

 additions being made at this point, commonly known as 

 the 'root. ' The hair itself is a solid column, varying con- 

 siderably in shape in different animals, from the delicate 

 fur of the fur-seal, to the bristles of the pig or the spines 

 of the porcupine. There are usually glands present which 

 open into the follicle and which secrete a fluid, the object 

 of which is to keep the hair moist; and besides, each 

 follicle is provided with muscles which serve to erect the 

 hair at times of fright (as in cats and dogs) or in cold 

 weather. 



Closely related to hair are nails, claws, hoofs, and horns.* 

 In fact these structures must be regarded as hairs united 

 throughout their length. At other times a similar con- 

 solidation of hair gives rise to protective scales covering 

 the body, as in the case of the pangolins (fig. 163). 



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 bear no free ribs, and, except in 

 three tropical species, they are constantly seven in number, 

 the long-necked giraffe and the short-necked whale having 

 the same number of cervicals. The thoracic 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 vertebrae 

 are those which unite with the pelvic bones. The caudal 

 vertebrae are found in the tail. In the whales only cervi- 

 cal and thoracic vertebrae can be distinguished, since the 



* Here are intended such horns as those of the cow, sheep, antelope, 

 and rhinoceros; the horns of the deer are true bone. 



