422 EXCRETION 



But during strong exercise, exposure to great external warmth, in some 

 diseases, and when evaporation is prevented, 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, in- 

 cludes all that portion of the secretions and exudations from the skin which 

 are thrown on the surface by the sweat glands. As a matter of fact, this is 

 mingled 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 secretions. 



The secretion of the sebaceous glands and hair follicles consists of cast- 

 off epithelium cells, with nuclei and granules, together with an oily material 

 and extractive matter. In certain parts, also, it is mixed with a peculiar 

 odorous principle, which contains caproic, butyric, and other fatty acids. 

 It is similar in composition to the unctuous coating, or vernix caseosa, which 

 is formed on the body of the fetus while in the uterus, and which contains 

 ordinary fat and cholesterol esters with fatty acids. This sebaceous secre- 

 tion serves the purpose 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 

 listed among the excretions of the skin. 



CHEMICAL COMPOSITION OF SWEAT. 



Water 995 



Solids: 5 



Organic acids (formic, acetic, butyric, pro- 



pionic, caproic, caprylic) 0.9 



Salts, chiefly sodium chloride 1.8 



Neutral fats and cholesterol 0.7 



Extractives (including urea), with epithelium i .6 



The sweat is a colorless, 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 carbon dioxide 

 and water need particular consideration. 



. The quantity of water vapor excreted from the skin is, on an average, 

 between 750 and 1,000 c.c. daily. This subject has been very carefully 

 investigated by Lavoisier and Sequin. The latter chemist enclosed his 

 body in an air-tight bag provided with a mouthpiece. The bag was closed 

 by a strong band above, and the mouthpiece adjusted and gummed to the 

 skin around the mouth. He was weighed, then remained quiet for several 



