CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 729 



increase of the secretion, and it is possible that mere dryness of 

 the air has a similar effect. 



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 con- 

 ditions. 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 by glands, 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 characters of 

 the sweat as a whole will vary according to the relative proportion 

 of the two kinds of secretion. The secretion of the sebaceous 

 glands appears to be relatively 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 characters of the sweat ; when on 

 the contrary the sweat is very abundant, 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, and that when mixed 

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

 sebum. 



Taking ordinary sweat, such as may be obtained by enclosing 

 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 consist of 

 organic substances. The chief normal constituents are: (1) Sodium 



F. ii. 47 



