400 PRACTICAL ZOOLOGY 



blubber, just beneath the skin, which prevents the escape of the 

 body heat. The more usual method of protection from the cold 

 is a thick covering of hair. 



Hair. — All mammals possess hairs and may be distinguished 

 from all other animals by these peculiar structures. The hairs 

 project out from pits in the skin, called hair follicles (Fig. 271). 

 The hair shaft (Sch) broadens at the base, extending around 

 a highly vascular papilla {HP) at the bottom of the pit. When 

 hairs are shed, new hairs usually arise to take their place. 

 Secretions from the sabaceous glands (HBD) keep the hairs 

 glossy. 



The two main types of hairs are (1) contour hairs which are 

 long and strong and (2) woolly hairs which are shorter and 

 constitute the under fur. In some animals the woolly hairs 

 have a rough surface, as in the sheep, which causes them to co- 

 here and gives them their felting quality. Certain of the 

 stronger hairs may be moved by muscular fibers (Fig. 271, Ap), 

 which are responsible for the erection of spines or the gristling 

 of the other hairs. 



The air spaces between the hairs prevent the escape of heat 

 since air is a bad conductor of heat. Besides protecting the 

 body from loss of heat the hairy covering also prevents to a 

 large extent injury due to blows. Human beings are almost 

 entirely covered by hair, except on the soles of the feet and palms 

 of the hands. This covering is of practically no service except 

 the thick growth on the head. 



Color. — As a rule mammals are not very highly colored, but 

 many of them are characterized by stripes, as in the zebra and 

 tiger, or spots, as in the leopard. The dull colors of mammals 

 and the stripes or spots are all supposed to aid in concealing the 

 animals amid their surrounding and thus to protect them 

 from their enemies. Animals like the Arctic fox that live in the 

 colder regions of the earth change color in the winter, becoming 

 white. This change is of advantage, since it renders them in- 

 conspicuous against the background of snow. 



