198 ELEMENTARY PHYSIOLOGY ^ less. 



8. The Composition and Quantity of Sweat.— The 

 sweat glands have the function of forming a fluid, the 

 sweat, which is passed out on to the surface of tlie body. 

 This fluid is composed chiefly of water containing a small 

 amount (1-2 per cent.) of solid matter in solution, chiefly 

 sodium chloride. 



In its normal state the sweat, as poured out from the 

 proper sweat-glands, is alkaline ; but ordinarily, as it 

 collects upon the skin, it is mixed with the fatty secretion 

 of the sebaceous glands, and then is frequently acid. In 

 addition it contains scales of the external layers of the 

 epidermis, which are constantly being shed. 



Pro. 62. — Section of the Skin showing the Roots or the Hairs and 

 THE Sebaceous Glands. 



o, epidermis ; 6, muscle of c the liair sheath, on the left hand. 



Under ordinary conditions the sweat is evaporated from 

 the surface of the skin as fast as it is .secreted ; in this 

 case it is frequently spoken of as inseiisible perspiration. 

 But when violent exercise is taken, or under some kind 

 of mental emotion, or when the body is exposed to a 

 hot and moist atmosphere, the sweat is secreted faster 

 than it evapor;ites : the perspiration then becomes sensible, 

 that is it appears in the form of scattered drops on the 

 surface of the body. 



The quantity of sweat, or .sensible perspiration, and 

 also the total amount of both sensible and insensible 

 perspiration, vary immensely, according to the temi)era- 

 ture and other conditions of the air, and according to the 



