350 HANDBOOK OF PHYSIOLOGY. 



to be that of keeping the skin moist and supple, and, by its oily nature* 

 of both hindering the evaporation from the surface, and guarding the 

 skin from the effects of the long-continued action of moisture. But 

 while it thus serves local purposes, its removal from the body entitles it 

 to be reckoned among the excretions of the skin; though the share it 

 has in the purifying of the blood cannot be discerned. 



(4.) The Skin is also an organ of Excretion, as it contains 

 Sweat Glands. The fluid secreted by the sweat-glands is usually 

 formed so gradually that the watery portion of it escapes by evaporation 

 as fast as it reaches the surface. But, during strong exercise, exposure 

 to great external warmth, in some diseases, and when evaporation is pre- 

 vented, the secretion becomes more sensible, and collects on the skin in 

 the form of drops of fluid. 



The perspiration, as the term is sometimes employed in physiology, 

 includes all that portion of the secretions and exudations from the skin 

 which passes off by evaporation ; the sweat includes that which may be 

 collected only in drops of fluid on the surface of the skin. The two 

 terms are, however, most often used synonymously ; and for distinction, 

 the former is called insensible perspiration ; the latter sensible perspira- 

 tion. The fluids are the same, except that the sweat is commonly min- 

 gled with various substances lying on the surface of the skin. The con- 

 tents of the sweat are, in part, matters capable of assuming the form of 

 vapor, such as carbonic acid and water, and in part, other matters which 

 are deposited on the skin, and mixed with the sebaceous secretion. 



Table of the Chemical Composition of Sweat. 



Water, . . , . . 995 



Solids : 



Organic Acids (formic, acetic, butyric, propionic, ) g 



caproic, caprylic), [ 



Salts, chiefly sodium chloride, 1.8 



Neutral fats and cholesterin, . . . . .7 



Extractives (including urea), with epithelium, .1.6 5 



1000 



The sweat is acolorless, slightly turbid fluid, alkaline, neutral or acid 

 in reaction, of a saltish taste, and peculiar characteristic odor. 



Of the several substances it contains, however, only the carbonic acid 

 and water need particular consideration. 



a. Watery vapor. The quantity of watery vapor excreted from the 

 skin is on an average between 1-J and 2 Ib. daily. This subject has been 

 very carefully investigated by Lavoisier and Sequin. The latter chemist 

 inclosed his body in an air-tight bag, with a mouth-piece. The bag 

 being closed by a strong band above, and the mouth-piece adjusted and 

 gummed to the skin around the mouth, he was weighed, and then re- 



