136 PHYSIOLOGY. 



spiration when it accumulates enough to be perceptible. 

 These are not two distinct kinds of sweat, but it is con- 

 venient to distinguish between the perceptible and the 

 imperceptible. Sweat varies greatly in its wateriness, and 

 hence in the relative amount of solid matter contained. 



The Amount of Perspiration. There is about one 

 quart in twenty-four hours. It varies with: 



1. Temperature, dryness, and rate of renewal of air. 



2. Condition of the blood; e.g. if watery from drinking 

 much water. 



3. Muscular exercise. 



4. Certain drugs some exciting perspiration, e.g. 

 camphor ; others diminishing it, e.g. belladonna. 



5. The nerves exercise great influence on the activity of 

 the cells of the gland. 



The Functions of the Skin. 



1 . Protective. 



2. Excretory. 



3. Absorptive. 



4. Sensory organ of touch. 



5. Heat-regulating. 



Next to its excretion, the heat regulation by the skin is 

 the most important for our present consideration. 



Regulation of the Temperature of the Body by the 

 Skin. It is a striking fact that, except in disease, the 

 temperature of the body varies only a little from 98.5 F. 

 in summer and winter, during exercise and rest. The rate 

 of heat production varies greatly. The rate of giving off 

 heat must therefore vary accordingly. 



The Body gives off Heat. In considering the regula- 

 tion of the body's temperature, we must bear in mind that 

 the body is surrounded by air almost always considerably 



