'52 



HUMAN PHYSIOLOGY 



CHAP. 



creting sweat. Usually the secretion evaporates as soon 

 as it reaches the surface of the skin, and you would not 

 know that the glands were at work at all. This we call 

 insensible perspiration. On very hot days or during 

 vigorous exercise" the amount may be so great as to 

 accumulate in drops that can be seen on the surface of 

 the skin. This we call sensible perspiration. About two 



pounds or one quart 

 of perspiration is pro- 

 duced on an average 

 every tiventy -four 

 hours, and a very much 

 greater amount on a 

 very hot day. The 

 purpose of tJie sweat 

 glands is to remove 

 from the blood or- 

 ganic wastes, salts, 

 and water; and hence 

 we call them true 

 organs of excretion. 

 They have still an- 

 other function more important than this. A large 

 amount of heat, about eighty per cent of all that leaves 

 the body, escapes through the skin. It is the perspira- 

 tion evaporating from the surface of the skin that cools 

 the body when there is danger of becoming overheated. 

 In this way the skin regulates the body temperature. 



The oil or sebaceous glands are situated near the 

 roots of the hairs and pour their secretions into the little 



FIG. 77. A piece of skin of the palm of 

 the hand, showing pores of the sweat 

 glands, and grooves between the papillae 

 (Newell). 



