GENERAL STRUCTURE. 35 



as vibrissae. Their roots are provided with delicate nerve- 

 endings of touch, so that the animal may find its way 

 with ease through dark narrow passages. There are a 

 few long hairs above the eyes, forming the eyebrows, but 

 no eyelashes are present. 



Most of the hairs are inserted obliquely into the skin, 

 but when angered the cat can erect them by the con- 

 traction of a small muscle passing from the skin to the 

 hair-bulb. 



On some mammals the hairy covering is partial and 

 limited to particular regions ; in others, as the hippopota- 

 mus and the Sirenia, it is very scanty, but scattered over 

 the whole surface; while in the Cetacea it is reduced to a 

 few small bristles about the mouth. 



Some kinds of hair, as those of the mane and tail of 

 the horse, are shed and renewed annually. Most mam- 

 mals have a long hairy coat in winter which gives place 

 in spring to a short coat. The Arctic fox, hare, ermine, 

 and numerous other animals of the colder regions undergo 

 a complete change of color in the two seasons, being 

 white in winter and brown or gray in summer. By this 

 protective coloration they escape many of their enemies. 



