KIDNEV AND SKIN AS SECRETORY GLANDS. 859 



tion of greater metabolism in the sweat glands in connection with 

 the formation of visible sweat. 



Composition of the Secretion. — The precise chemical composition 

 ■of sweat is difficult to determine, owing to the fact that as usually 

 obtained it is liable to be mixed with the sebaceous secretion. 

 Normally it is a very thin secretion of low specific gravity (1.004). 

 The reaction for human sweat when produced in quantities that 

 can be collected, as after the action of heat or muscular work, is 

 usually distinctly acid. The figures given show a variation in 

 hydrogen-ion concentration between pH 7.5 and pH 5.1. It is 

 noteworthy that the sweat produced by heat is somewhat more 

 acid than that resulting from muscular work.* The larger part 

 of the inorganic salts consists of sodium chlorid. Small quan- 

 tities of the alkaline sulphates and phosphates are also present. 

 The organic constituents, though present in mere traces, are 

 quite varied in number. Lrea, uric acid, creatinm, aromatic ox\- 

 acids, ethereal sulphates of phenol and skatol, serin (oxyaminopro- 

 pionic acid), and albumin, are said to occur when the sweating is 

 profuse. Argutinsky has shown that after the action of vapor 

 baths, and as the result of muscular work, the amount of urea 

 eliminated in this secretion may be considerable. Under patho- 

 logical conditions involving a diminished elimination of urea through 

 the kidneys it has been oliserved that the amount found in the 

 sweat is markedly increased, so that crystals of it may be deposited 

 upon the skin. Under perfectly normal conditions, however, it 

 is obvious that the organic constituents are of minor importance. 

 The main fact to be considered in the secretion of sweat is the form- 

 ation of water. 



Secretory Fibers to the Sweat Glands. — Definite experimental 

 proof of the existence of sweat ner^•es was first obtained by Goltzf 

 in some experiments upon stimulation of the sciatic nerve in cats. 

 In the cat and dog, in which sweat glands occur on the balls of the 

 feet, the presence of sweat nerves may be demonstrated with great 

 ease. Electrical stimulation of the peripheral end of the divided 

 sciatic nerve, if sufficiently strong, will cause visible drops of sweat 

 to form on the hairless skin of the balls of the feet. When the 

 electrodes are kept at the same spot on the nerve and the stimula- 

 tion is maintained the secretion soon ceases; but this effect seems 

 to be due to a temporary injur}- of some kind to the nerve fibers 

 at the point of stimulation, and not to a genuine fatigue of the 

 sweat glands or the sweat fibers, since moving the electrodes to a 

 new point on the nerve farther toward the periphery calls forth a 

 new secretion. The secretion so formed is thin and limpid, and 

 ihas an alkaline reaction. The anatomical course of these fibers 



* Talbert, "American Journal of Physiology," 50, 433, 1919. 

 t "Archiv f. die gesammte Physiologie," 11, 71, 1875. 



