258 UNCONSCIOUS NERVOUS OPERATIONS 



373. Structure of the Skin. It will be remembered that 

 the skin is composed of two layers: the epidermis, com- 

 posed of many layers of cells; and the dermis, or true 

 skin, in which are found the papillae, blood vessels, and 

 nerves, with the end organs for touch and glands of differ- 

 ent sorts (Fig. 49, p. 83). 



374. Perspiration. The excretion of the skin is called 

 perspiration, or sweat, and consists of water, a little dis- 

 solved salt, and some fat. When the perspiration is evapo- 

 rated from the skin as fast as it is secreted, it is called 

 insensible perspiration; but if the quantity is larger, so that 

 it collects upon the surface, we call it sensible perspiration. 



When the amount of sweat produced is scanty, it is acid 

 in chemical composition; but when the discharge is profuse, 

 it is alkaline. This difference is understood to be due to 

 the mixture of the products of the sebaceous glands with 

 those of the sweat glands. The former are extremely 

 minute glands pouring their secretions into the hair 

 follicles. Their product is acid and fatty, and is constant, 

 or nearly so, in quantity, while that of the sweat glands is 

 alkaline and variable in amount. 



375. The Sweat Glands are very abundant over the whole 

 skin. They consist of coiled tubes lying deep in the der- 

 mis, and the duct of each reaches the surface by a cork- 

 screwlike channel. 



376. The Nerves controlling the Sweat Glands are of two or 

 three different sets. Those affecting the blood circulation, 

 vasomotor, diminish the secretion of sweat by narrowing 

 the size of the blood vessels, and increase it by dilating 

 them. The special secretory fibers, when stimulated, 

 cause production of perspiration. Still other nerve fibers 

 supply the plain muscle fibers of the glands and regulate 

 the expulsion of the fluid. All these nerve fibers are 



