EXCRETION 



187 



8. The secretion of the perspiration, like the secretion of 

 the gastric juice, is under the control of the nervous system. 

 When the nerves going to the sweat glands of a given 

 area of skin are cut or otherwise injured, the secretion of 

 perspiration ceases over that area; and the appearance of 

 cold beads of perspiration as the result of fright shows 

 how events taking place in the nervous 



system may excite these glands to activity 

 apart from the presence of their usual 

 stimuli the application of heat to the 

 skin and the liberation of heat within the 

 body by muscular and other activities. 

 The distinction should be made between 

 the so-called " sensible " and " insensible " 

 perspiration, the latter name being given 

 to the perspiration the water of which 

 evaporates as fast as secreted ; the former 

 to that which does not evaporate so rapidly 

 and hence remains for a time on the sur- 

 face of the skin. When the water evapo- 

 rates, the dissolved solids (salts, urea, and 

 other compounds) remain behind on the 

 skin. 



9. Value of profuse perspiration in the 

 care of the skin. While the skin is not 



.-, p ,. ,-, Note the coiled form 



primarily an organ of excretion, the per- of the tube in the 

 spiration contains a certain amount of 

 waste substances and salts, which are left 

 by the evaporation of the water upon the surface and, to 

 some extent, in the mouths of the ducts of the sweat glands ; 

 this is especially the case when evaporation takes place about 

 as rapidly as the perspiration is discharged. When the secre- 

 tion of perspiration is more abundant, as during muscular 

 work, or at very high temperatures, or, in general, where it 

 does not evaporate as rapidly as discharged, the accumulation 



FIG. 86. Sweat gland 

 (slightly magnified) 



subcutaneous tissue. 



