232 ADVANCED PHYSIOLOGY 



follicle, a tiny canal of epidermal cells extending down into 

 the dermis; Figs. 113 and 114. The lower end of this follicle 

 is enlarged and projecting upward into it is a papilla on which 

 the hair grows. Cells of this papilla are constantly multiply- 

 ing and turning into hair substance and thus the hair length- 

 ens. After a time (in the scalp, perhaps three years), the top 

 of the papilla ceases to form new cells, the hair dies and falls 

 out. Before this occurs, however, a new papilla has been 

 budded off the side of the old one and from it a new hair starts. 

 When a hair is pulled out the epidermal cells lining the 

 follicle may come away also, but the papilla usually remains 

 uninjured and a new hair soon grows from it. The straight- 

 ness or curliness of hair depends on its shape: kinky hair is 

 flat or oval in cross section; wavy hair is oval; while straight 

 hair is round and rod-like. 



Emptying into each hair follicle is usually an oil or sebaceous 

 gland; Fig. 114. These glands secrete a substance which 

 moistens the surface of the hair and keeps it soft and flexible- 

 To the lower end of the follicle in most hair-bearing animals 

 is fastened a slender muscle, its other end being attached to 

 the lower layers of the epidermis; Fig. 114. The shortening 

 of this muscle pulls the follicle into a more vertical position, 

 making the hair "stand up straight," as a cat's fur frequently 

 does, for example. 



All epidermal structures are disturbed and cast off to a 

 greater or less extent in the case of some fevers, and not in- 

 frequently the hair falls out after a long sickness. 



Hair itself is not sensitive, though if it is stiff one may feel a 

 very slight touch upon it because of the presence of nerves in the 

 dermis at its base. The stiff hairs around the nostrils of a cat 

 are thus sense organs of touch. Over most of the human body, 

 hair remains so short as to be almost invisible and of only 

 slight protective value. The tendency for hair to fall out and 

 leave the head bald is probably due in part to the bad habit of 

 vvearing hot, heavy hats with stiff rims which bind the scalp. 



