CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 551 



The amount of perspiration given off is affected not only by 

 the condition of the atmosphere, but also by the circumstances of 

 the body. Thus it is influenced by the nature and quantity 

 of food eaten, by the amount of fluid drunk, by the character 

 of exercise taken, by the relative activity of the other excreting 

 organs, more particularly of the kidney, by mental conditions 

 and the like. Variations may also be induced by drugs and 

 by diseased conditions. How these various influences produce 

 their effects we shall study immediately. 



The fluid perspiration, or sweat, when collected, is found to 

 be a clear colourless fluid of a distinctly salt taste, with a strong 

 and distinctive odour varying according to the part of the body 

 from which it is taken. Besides accidental epidermic scales, it 

 contains no structural elements. 



Sweat, as a whole, is furnished partly by the sweat-glands 

 and partly by the sebaceous glands, for as we shall see the small 

 amount which simply transudes through the epidermis, apart 

 from the glands, may be neglected. Now the secretions from 

 these two kinds of glands differ widely in nature, and the charac- 

 ters of the sweat as a whole will vary according to the relative 

 proportion of the two kinds of secretion. The amount of secre- 

 tion of the sebaceous glands appears to be fairly constant, the 

 larger variations of the total sweat depending chiefly on the 

 varying activity of the sweat-glands. Hence when sweat is 

 scanty, the constituents of the sebum influence largely the charac- 

 ters of the sweat; when on the contrary the sweat is very abun- 

 dant, these may be disregarded and the sweat may be considered 

 as the product of the sweat-glands. 



We are not able, at present, to make a complete statement 

 as to what bodies occur exclusively in the sebum and what in the 

 secretion of the sweat-glands. The former consists very largely 

 of fats and fatty acids, and appears to contain some form or 

 forms of proteids; but we have reason to think that the sweat- 

 glands secrete in small quantity some forms of fat, and especially 

 volatile fatty acids. 



When sweat is scanty, the reaction is generally acid, but when 

 abundant, is alkaline; and when a portion of the skin is well 

 washed the sweat which is collected immediately afterwards is 

 usually alkaline. From this we may infer that the secretion of 

 the sweat-glands is naturally alkaline, but that when mixed 

 sweat is acid, the acidity is due to fatty (or other) acids of the 

 sebum. In the horse, which is singular among hair-covered 

 animals for its frequent profuse sweating, the sweat is said to be 

 always alkaline, and to contain a considerable quantity of some 

 form of proteid. 



Taking ordinary sweat, such as may be obtained by enclos- 

 ing the arm in a bag, we may say that, in man, the average 

 amount of solids is from 1 to 2 p. c., of which about two-thirds 



