258 ANATOMY AND PHYSIOLOGY 



Its most important function is to secrete perspiration. Per- 

 spiration is a clear watery fluid consisting of a solution of certain 

 waste products of metabolism (tissue waste), in other words, water 

 and solids. It is acid in reaction; saline to taste. The quantity 

 excreted by a healthy active person in twenty-four hours has been 

 estimated as one quart. 



Although the amount of solids in the perspiration is small, it 

 is enough to relieve the system if the kidneys are disabled, or to 

 embarrass it if retained. Urea is one of the substances contained 

 in perspiration, and in diseased conditions of the kidneys the skin 

 is able to excrete an increased quantity of urea. 



The removal of tissue waste is, however, not the only important 

 use of perspiration. By bringing water to the surface of the skin, 

 it is a most efficient agent for regulating the temperature of the body. 

 Muscle exercise, for example, which increases heat production, is 

 accompanied by increased activity of the sweat glands, and the 

 consequent evaporation oj water carries off the excess of heat 

 generated in the muscles. Again, high temperature of the sur- 

 rounding atmosphere causes dilatation of the cutaneous vessels, 

 and more perspiration and consequent evaporation. Conversely, 

 a cold atmosphere stimulates the cutaneous vessels to contract, 

 and stops the production of sensible perspiration. All of this 

 activity is, of course, a response to the controlling force of nerve 

 centers which regulate blood supply, and stimulate secretory 

 action. 



Clinical note. From these facts one may understand why it is so impor- 

 tant to conserve the surface temperature of a patient with nephritis, or with 

 diminution of urine from any cause, by the use of blankets and warm clothing; 

 and to increase it oftentimes by the use of hot baths, packs, etc. 



In health the quantity of perspiration is modified by the die- 

 tary, particularly by the amount of liquid taken, and the kind of 

 liquid; also by the character of clothing, the season of the year, 

 temperature of the atmosphere, etc. 



It may be noted that the activities of the skin and kidneys 

 alternate with change oj season; in summer when the skin is active 

 the urine is scanty. In winter, when the skin is inactive the urine 

 is free. 



Drugs which increase the production of perspiration are called 

 diaphoretics. Aconite and sweet spirits of nitre are familiar exam- 



