A STUDY OF THE SKIN AND THE KIDNEYS 239 



perspiration. This passes upward through the twisted duct 

 and oozes out through the pore upon the surface. Hence 

 the skin, in addition to its protective and sensory functions, 

 is also an important excretory organ. 



There are two kinds of perspiration, insensible and sen- 

 sible. In the former the sweat evaporates as rapidly as it 

 reaches the surface, and we are not conscious of perspiring. 

 That the process is going on, even when we feel cool, can be 

 demonstrated by placing the palm of the hand on a cool 

 mirror. By far the larger amount passes off in insensible 

 form. During exercise or on a hot day the perspiration 

 stands in drops on the surface of the body ; this condition 

 is known as sensible perspiration. 



Heat Regulation in the Body. The temperature of the 

 healthy human body, as we learned on p. 221, is 981 F. 

 This does not vary to any appreciable extent in winter or 

 summer, no matter how vigorously one may exercise. Yet 

 during exertion metabolism goes on much more rapidly, and 

 a great deal of heat is thereby caused. What then becomes 

 of this extra heat ? 



In fevers the temperature sometimes runs ten degrees 

 higher than the normal. At this time we know that the 

 skin is dry and parched, for the body is unable to perspire. 

 We may infer, then, that in health we keep cool by perspir- 

 ing, and such proves to be the case. During exercise the 

 heart beats with greater rapidity, and the heated blood is 

 driven more rapidly through the skin as well as through 

 other organs of the body. When it comes in contact with 

 the two and a half millions of sweat glands of the skin, a 

 great deal of water is given out, and this soon reaches the 

 surface and collects in drops. In evaporating this water 

 the body loses its surplus of heat, just as a stove loses 

 heat when it causes a pan of water to pass off into the air in 

 the form of vapor. By this automatic process our bodies 

 keep at an even temperature, whatever may be the condition 

 of the air or the degree of metabolism within us. 



