THE SKIN 269 



may absorb liquid substances, these being taken up by the 

 blood and lymph vessels, and perform a respiratory func- 

 tion, throwing off carbon dioxide. But the most important 

 function of the skin, in addition to protection, is that of 

 serving as 



An Organ of Adaptation. Forming, as it does, the 

 boundary between the body and its physical environment, 

 the skin is perhaps the most important agent through 

 which the body is adapted to its immediate surroundings. 

 Evidence of this is found in the great variety of influences 

 which are able to affect the body through their action 

 upon the nerves in the skin, and in the changes which the 

 epidermis undergoes on exposure. The latter function 

 is especially marked in the lower animals, the coverings 

 of epidermal tissue (hair, scales, feathers, etc.) adapting 

 each species to the physical conditions under which 

 it lives. In man the most striking example of adapta- 

 tion through the skin is seen in the variations in the 

 quantity of blood circulating through it, corresponding 

 to the changes in the temperature outside of the body. 

 These variations are of great importance, having to do 

 with the 



Maintenance of the Normal Temperature. It is neces- 

 sary to the continuance of life that the temperature of the 

 body be kept at a nearly uniform degree, called the normal 

 temperature, which is about 98.6 F. The maintenance of 

 the normal temperature depends mainly upon four condi- 

 tions : the chemical changes at the cells, the circulation of 

 the blood, the nervous system, and the skin. The chemical 

 changes produce the heat, the blood in its circulation dis- 

 tributes the heat over the body, and the nervous system con- 

 trols the heat-producing and distributing processes (page 

 320). The skin is the chief means by which the body 



