766 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Excretory Functions of the Skin. The physiological activi- 

 ties of the skin are varied. It forms, in the first place, a sensory 

 surface covering the body, and interposed, as it were, between the 

 external world and the inner mechanism. Nerve fibers of pressure, 

 temperature, and pain are distributed over its surface, and by means 

 of these fibers reflexes of various kinds are effected which keep the 

 body adapted to changes in its environment. The physiology of 

 the skin from this standpoint is discussed in the section on special 

 senses. Again, the skin plays a part of immense value to the body 

 in regulating the body temperature. This regulation, which is 

 effected by variations in the blood supply or the sweat secretion, 

 is described at appropriate places in the sections on Nutrition and 

 Circulation. In the female, during the period of lactation, the mam- 

 mary glands, which must be reckoned among the organs of the 

 skin, form an important secretion, the milk; the physiology of this 

 gland is referred to in the section on Reproduction. In this section 

 we are concerned with the physiology of the skin from a different 

 standpoint, namely, as an excretory organ. The excretions of 

 the skin are formed in the sweat-glands and the sebaceous glands. 



Sweat. The sweat or perspiration is a secretion of the sweat 

 glands. These latter structures are found over the entire cutaneous 

 surface except in the deeper portions of the external auditory meatus, 

 the prepuce, and the glans penis. They are particularly abundant 

 upon the palms of the hands and the soles of the feet. Krause 

 estimates that their total number for the whole cutaneous surface 

 is about two millions. In man they are formed on the type of 

 simple tubular glands; the terminal portion contains the secretory 

 cells, and at this part the tube is usually coiled to make a more or 

 less compact knot, thus increasing the extent of the secreting sur- 

 face. The larger ducts have a thin, muscular coat of involuntary 

 tissue that may possibly be concerned in the ejection of the secre- 

 tion. The secretory cells in the terminal portion are columnar in 

 shape, possess a granular cytoplasm, and are arranged in a single 

 layer. The amount of secretion formed by these glands varies 

 greatly, being influenced by the condition of the atmosphere as re- 

 gards temperature and moisture, as well as by various physical and 

 psychical states, such as exercise and emotions. The average quan- 

 tity for twenty-four hours is said to vary between 700 and 900 gms., 

 although this amount may be doubled under certain conditions. 



According to an interesting paper by Schierbeck,* the average 

 quantity of sweat in twenty-four hours may amount to 2 to 3 liters 

 in a person clothed, and therefore with an average temperature 

 of 32 C. surrounding the skin. This author states that the amount 



*"Archiv f. Physiologic, " 1893, 116; see also Willebrand, "Skandi- 

 navisches Archiv f. Physiologic," 13, 337, 1902. 



