THE FUNCTIONS OP THE SKIN. 237 



fluids from its glands. Contrary to general belief, the glands 

 of the skin do not excrete the waste substances of the body, or 

 at least do so only to a very limited degree. Their functions are : 

 to regulate the internal heat of the body (sweat glands) ; to lubri- 

 cate its surface and hairs (sebaceous glands) ; and to provide the 

 best form of nourishment for the newborn animal (mammary 

 glands). 



The Sweat Glands. These are simple coiled tubular struct- 

 ures, found practically everywhere in the cutaneous tissue of the 

 body, being especially numerous in certain parts, as in the palms 

 of the hands and the soles of the feet. The excreting cells line 

 the lower portions of the tubules, and are composed of granular, 

 columnar epithelium. The glands are richly supplied with nerve 

 fibers. 



The amount of sweat given off in a day varies greatly, since 

 it is influenced by many things, as heat, moisture, exercise, cloth- 

 ing, etc. (see p. 135). The perspiration of which we are uncon- 

 scious amounts to a considerable number of grams (700 to 900 

 grams) in a day. Although it is very difficult to obtain pure 

 sweat unmixed with the secretions of the other glands of the 

 skin, we know that it consists for the most part of water, having 

 a specific gravity of about 1.004. The salty taste is due to inor- 

 ganic salts and to the impurities which the sweat dissolves on the 

 surface of the skin. There is only a trace of urea and related 

 substances, and probably the sweat glands never aid the kidneys 

 in the excretion of these bodies. 



The most important function of the sweat glands is to control 

 the temperature of the body by regulating the rate of its heat 

 loss. Dry air is a poor conductor of heat, and to vaporize water 

 requires a large amount of heat. As the water of the sweat is 

 evaporated, the body loses heat rapidly. This principle is practi- 

 cally applied by the housewives of tropical countries. The water 

 is placed in porous pots and the rapid evaporation on the out- 

 side of the pot cools the water within. 



The secretion of sweat, like the secretion of saliva, is under 

 the control of the central nervous system, as can be demonstrated 

 by electrically exciting the nerves supplying the paw of a cat or 



