218 ANATOMY FOB, NUESES. [CHAP. XVIII. 



abundant it is alkaline. The chief normal constituents of sweat 

 are water, salts, fatty acids, and, some authorities state, a slight 

 amount of urea. In various forms of kidney disease urea may 

 be present in considerable quantity, the skin supplementing to 

 a certain extent the deficient work of the renal organs. 



Quantity of perspiration. Under ordinary circumstances, the 

 perspiration that we are continually throwing off evaporates 

 from the surface of the body without our becoming sensible of 

 it. This insensible perspiration, as it is called, usually amounts 

 to about a pint (0.473 litre) in the course of twenty-four hours. 

 The amount, however, varies to a very great extent with the 

 condition of the atmosphere ; the amount of exercise taken ; 

 the quantity of fluid drunk ; the action of the kidneys. Varia- 

 tions also occur under the influence of mental emotions, the 

 action of drugs, or are induced by certain diseased conditions. 

 When more sweat is poured upon the surface of the body than 

 can be removed at once by evaporation, it appears on the skin 

 in the form of scattered drops, and we then speak of it as sen- 

 sible perspiration. 



Less important functions of the skin. Besides being an impor- 

 tant excretory organ, the skin is to a slight extent an absorbing 

 organ. In the sound, healthy skin, it is doubtful whether 

 matters in solution can be absorbed through the epidermic 

 covering, but if the horny layers of the epidermis be removed 

 by blistering, or in any other manner, substances in solution 

 readily pass into the blood-vessels in the true skin. Oily sub- 

 stances, especially when well rubbed in, are absorbed without 

 removal of the epidermis. 



Oxygen in small amount is also taken in through the skin, 

 but this gain to the body is balanced by the carbon dioxide 

 which is thrown off. 



To sum up : the skin excretes a large amount of water and a 

 small amount of carbon dioxide and salts; it absorbs a small 

 amount of oxygen and, under certain conditions, oily substances 

 and watery solutions ; it is a protective organ and a tactile organ ; 

 it supports two appendages, viz. the hair and nails, and keeps 

 itself flexible, and the hair glossy, by the secretion of sebum. 



There is still another function of the skin to be considered 

 before closing this chapter, and that is the part it plays in regu- 

 lating the temperature of the body. 



