792 SPECIAL PHYSIOLOGY. 



as from the sweat glands. The perspiration is said to be insensible 

 when no visible moisture is discernible on the skin, and sensible when 

 it is so discernible ; but there is no real difference between them ; in 

 the former case the fluid part evaporates as fast as it exudes from the 

 orifices of the sweat glands, whilst, in the latter, it remains for a mo- 

 ment or so, in minute transparent colorless drops. 



The sweat usually contains about 97.5 of water and 2.5 of solid 

 matter, but sometimes less than 1 per cent, of the latter. The organic 

 constituents are little more than half of this, and are composed chiefly 

 of fat, which is probably almost entirely derived from an admixture of 

 the secretion of the sebaceous glands ; but the palms of the hands and 

 the soles of the feet are more or less greasy, although no sebaceous 

 follicles exist in that part of the skin. Besides this, the organic mat- 

 ters of the perspiration contain an albuminoid substance, the nature of 

 which is unknown, and acids which give it an acid reaction, by some 

 supposed to be lactic acid, but now usually regarded as a mixture of a 

 peculiar nitrogenous acid, named sudoric, with the volatile acetic, 

 metacetonic, formic, and butyric acids, together with the fatty caprylic 

 and caproic acids. Some of these acids are combined with alkalies. 

 Almost one-fourth of the solid matter is urea, the total daily quantity 

 having been estimated at about 150 grains, which would yield about 

 seventy grains of nitrogen. This urea is easily decomposed, and gives 

 rise to ammoniacal salts, such as were described by Berzelius, for no 

 ammonia is found in perfectly fresh perspiration. The inorganic mat- 

 ters are chiefly common salt and chloride of potassium, phosphate of 

 soda, and traces of earthy phosphates, and iron. On burning the total 

 solids, some sulphates are formed, indicating the presence of sulphur 

 in some combination, probably with the organic matter. A certain 

 quantity of epidermic cells and extraneous substances also occur in 

 the residue. The odor of the perspiration depends partly on the vola- 

 tile acetic, formic, and various fatty acids, but also perhaps on special, 

 but unknown, volatile odorous substances. Some of the odor may be 

 due to decomposing urea. In certain diseases, in which the excretion 

 from the kidneys is seriously diminished, or altogether suppressed, as 

 in Bright's disease and cholera, when the urea and uric acid are re- 

 tained in the blood, large quantities are frequently excreted by the 

 skin, probably chiefly by the sudoriferous glands. Besides the above- 

 named substances, alcohol in small quantity, sugar, albumen, biliary 

 matters, and other substances, have been found in the perspiration. 



The uses of the perspiration are twofold : first, to get rid of a cer- 

 tain quantity of water from the system ; and secondly, to eliminate 

 from the body certain special products of chemical metamorphosis. 



Many attempts have been made to determine the average quantity 

 of fluid exhaled by the skin, under ordinary circumstances, in twenty- 

 four hours, and the variable quantities which are given off under dif- 

 ferent conditions. In the earlier experiments, the losses, by exhala- 

 tion both from the skin and the lungs, were confounded, the body 

 being weighed together with the food and drink taken in twenty-four 

 hours ; at the end of that time, the weight of the body was again taken, 

 and also that of the intestinal and renal excreta ; the difference in 



