CHEMISTRY OF DIGESTION AND NUTRITION. 281 



Functions of the Skin. The physiological activities 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-fibres of pressure, temperature, and pain are distributed over its sur- 

 face, and by means of these fibres 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 Cutaneous Sensations. 

 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 Animal Heat, Circulation, and Secretion. In the female, during 

 the period of lactation, the mammary glands, which must be reckoned among 

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

 of this gland is described in the sections on Secretion and Reproduction. In this 

 section we are concerned with the physiology of the skin from a different stand- 

 point namely, as an excretory organ. The excretions of the skin are formed 

 in the sweat-glands and the sebaceous glands. The sweat-glands are distrib- 

 uted more or less thickly over the entire surface of the body, with the excep- 

 tion of the prepuce and glans penis, while the sebaceous glands, usually in con- 

 nection with the hairs, are also found everywhere except upon the palms of 

 the hands and the soles of the feet. 



Sweat. Sweat, or perspiration, which is the secretion of the sweat-glands, 

 is a colorless liquid with a peculiar odor and a salty taste. Its specific gravity 

 is given at 1004, and in man it usually has an acid reaction. As can readily be 

 understood, the quantity secreted in twenty-four hours varies greatly, the secre- 

 tion being influenced by variations in temperature, by exercise, and by psychical 

 and pathological conditions ; an average estimate places the daily secretion at 

 from 700 to 900 grams. Chemically, the secretion consists of water and inor- 

 ganic salts, traces of fats, fatty acid, cholesterin, and urea. Of the inorganic 

 salts, NaCl is by far the most abundant : it occurs in quantities varying from 

 2 to 3.5 parts per thousand. The elements of the sweat which are of import- 

 ance from an excretory standpoint are water, inorganic salts, and urea or related 

 nitrogenous compounds. As was said above, sweat constitutes the second in 

 importance of the three main channels through which water is lost from the 

 body. The quantity, eliminated in the sweat is to a certain extent inversely 

 proportional to that secreted by the kidneys ; but the physiological value of 

 the secretion of water by the sweat-glands seems to lie not so much in the fact 

 that it is necessary in maintaining the water-equilibrium of the blood and tis- 

 sues as in the important part it takes in controlling the heat-loss from the skin : 

 the greater the evaporation of sweat, the greater the loss of heat. The urea is 

 described as occurring in traces. As far as it occurs, it represents, of course, so 

 much proteid destroyed, but usually in calculating the proteid loss of the body 

 this element has been neglected. Argutinsky demonstrated, however, that in 

 special cases namely, during periods of unusual muscular work or after vapor- 

 baths the total weight of nitrogen eliminated by the skin may be of consider- 



