"858 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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

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

 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 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 of sweat given off 

 from the skin in the form of insensible perspiration increases pro- 

 portionately 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 increase being 

 simultaneous with the formation of visible sweat. At the same 

 time there is a sudden increase in the CO2 eliminated from the 

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



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

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



