KIDNEYS, SKIN, AND GENERAL EXCRETION. 407 



brain itself. The existence of such higher sweat centers of 

 the brain was just pointed out in the familiar experience of 

 everybody that strong emotions, especially great anxiety, at 

 once shows itself in an increased activity of these glands. 

 There are certain drugs which have a very specific effect 

 on these glands. Thus, pilocarpine will stimulate the 

 glands to active secretion, while, on the other hand, the 

 administration of atropine more or less completely checks it. 



2. Composition of the Sweat. The secretion of these 

 glands, or the sweat as it is called, has a composition not 

 yet well determined. It is difficult to get the fluid free 

 from a sebaceous admixture. It seems to consist, however, 

 of water, common salt and traces of a number of alkaline 

 salts. The most important organic constituent of sweat is 

 the urea. This becomes especially plentiful when for some 

 reason the function of the kidneys has been impaired. But 

 even when these are normally discharging their duty there 

 is a larger proportion of urea in the sweat than could be 

 accounted for by simple filtration, pointing to the fact that 

 the cells which make up the tube of the sweat glands in an 

 active physiological way pick up the urea from the blood 

 and eliminate it from the body in the perspiration. In ad- 

 dition to this urea fine chemical analyses have shown the 

 presence in minute quantities of some of the other sub- 

 stances found in the secretion of the kidneys. For such 

 reason the skin has been called an excretory tissue in ad- 

 dition to a protective one and classed physiologically with 

 the kidneys and lungs. It was formerly believed that the 

 death which soon followed the varnishing of the skin of an 

 animal, a procedure which stopped up all the 'pores of 

 the skin, was due to the fact that this excretory function of 

 the skin had been stopped. Such an explanation is, how- 

 ever, wrong, later experiments proving that the death of 

 such a varnished animal results from the increased loss of 

 heat which the varnished surface of the body radiates 

 away. 



