KIDNEY AND SKIN AS EXCRETORY ORGANS. 767 



of sweat given off from the skin in the form of insensible perspira- 

 tion increases proportionately with the temperature until a certain 

 critical point is reached (about 33 C. in the person investigated), 

 when there is a marked increase in the water eliminated, the in- 

 crease being simultaneous with the formation of visible sweat. At 

 the same time there is a sudden increase in the C0 2 eliminated from 

 the skin. It is possible that the sudden increase in C0 2 is an in- 

 dication 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. Nor- 

 mally it is a very thin secretion of low specific gravity (1.004) and 

 an alkaline reaction, although when first secreted the reaction may 

 be acid owing to admixture with the sebaceous material. The 

 larger part of the inorganic salts consists of sodium chlorid. Small 

 quantities of the alkaline sulphates and phosphates are also pres- 

 ent. The organic constituents, though present in mere traces, are 

 quite varied in number. Urea, uric acid, creatinin, aromatic oxy- 

 acids, ethereal sulphates of phenol and skatol, 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 con- 

 siderable. Under pathological conditions involving a diminished 

 elimination of urea through the kidneys it has been observed that 

 the amount found in the sweat is markedly increased, so that crys- 

 tals of it may be deposited upon the skin. Under perfectly nor- 

 mal 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 formation of water. 



Secretory Fibers to the Sweat Glands. Definite experimental 

 proof of the existence of sweat nerves was first obtained by Goltz * 

 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 injury 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 

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



