384 MANUAL OF PHYSIOLOGY. 



bathes the skin, being produced more rapidly than it can be 

 evaporated. The quantity of secretion necessary to become sen- 

 sible varies with the dryness and heat of the air, that is, with the 

 rapidity with which evaporation takes place. It happens, how- 

 ever, that the very circumstances which tend to assist evaporation 

 also promote the secretion of sweat. Indeed, the effect of great 

 heat and dryness of the air is to increase the. cutaneous secretion 

 more rapidly than they increase the capability of evaporation, 

 and therefore when the air is hot and dry and evaporation is going 

 on very actively, we have the secretion of sweat made sensible to 

 our feelings. When dampness is associated with warmth of the 

 atmosphere the sweat collects in large quantities on the skin, for 

 the heat, as we shall see hereafter, aids the secretion, and the 

 damp air impedes the evaporation. 



The quantity of perspiration given off is considerable, but the 

 wide limits within which the amount may vary render an attempt 

 to express an average in numbers useless. The amount will de- 

 pend on (1) the temperature of the air, (2) the quantity and 

 quality of fluids imbibed, (3) the amount of heat generated in the 

 body, and it therefore varies directly with muscular exercise. 

 The amount that becomes perceptible to our senses depends on 

 the impediments to evaporation that may exist, as well as on the 

 amount of fluid produced. 



The chemical composition of sweat varies with the amount se- 

 creted. When collected as a fluid by inclosing a part of the body 

 in an impervious sac, it is found to have about 2 per cent, of solid 

 matters, the greater quantity of which is made up of inorganic 

 salts, sodium chloride being by far the most abundant. It also 

 contains some epithelial debris, traces of neutral fats, and several 

 volatile and fatty acids (butyric, proprionic, caproic), to which it 

 owes its peculiar smell. It is said to contain urea, but this has 

 been denied, and since all the nitrogenous income is accounted 

 for in the urea excreted by the kidneys, it is probable that the 

 cutaneous elimination of urea is minimal, if not exclusively patho- 

 logical. It is also said to contain salts of ammonia, and it affords 

 a means of escape to many drugs. In certain parts of the body, 

 especially in some individuals, it contains a considerable amount 



