CHAP. IIL] ELIMINATION OF WASTE PRODUCTS. 695 



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 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 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 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 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, 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 acid, and to 

 contain a considerable quantity of some form of proteid. These 

 features are probably due to the large admixture of sebum from 

 the numerous sebaceous glands connected with the hairs. 



