362 EXCRETION 



It is a very thin liquid, slightly heavier than water (Sp. 

 Gr. 1.004), and usually shows an alkaline reaction from 

 the dissolved salts present. These salts consist mainly 

 of alkaline phosphates and sulphates and sodium chloride. 

 The organic wastes (urea, uric acid, etc.), which are pro- 

 duced by the breaking down of the proteid parts of the 

 tissue cells, are present only as mere traces, though as the 

 result of muscular work the amount of urea thus thrown 

 off may become considerable. The principal waste given off 

 by the sweat glands, however, is water, and it is as a 

 remover of water from the body that the skin is espe- 

 cially important. The amount of water thus removed 

 varies greatly with conditions of temperature, and with 

 the mental and physical states. It has been calculated 

 that the average amount of sweat given off in twenty- 

 four hours by a normal person is between two and three 

 quarts. 



The sebaceous glands contribute an oily liquid that 

 sets in a cheesy mass upon exposiu-e to the air. It con- 

 tains fats, soaps, and some proteid materials. It varies 

 in quality in different parts of the body, that of the ear 

 constituting the ear wax. Its principal function is to 

 soften the skin and hairs, and protect the skin from exter- 

 nal moisture, while it also tends to prevent loss of heat 

 by evaporation. 



In some of the lower animals the skin also excretes car- 

 bon dioxide and takes in oxygen, thus acting as a respira- 

 tory organ. In man the amount thus given off by the 

 skin is small under ordinary conditions. In profuse per- 

 spiration, however, the amount of carbon dioxide removed in 

 this way becomes considerable. Under ordinary conditions 

 the removal of carbon dioxide takes place in the lungs. 



