342 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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, eholesterin, and urea. Of the inorganic 

 salts, Nad 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- 

 able importance, amounting to as much as 0.7 to 0.8 gram. Under ordinary 

 circumstances the excretion of urea and related compounds through the skin 

 must be regarded as of very subsidiary importance, but the amount may be 

 increased markedly under pathological conditions. 



Sebaceous Secretion. — The sebaceous secretion is an oily, semi-liquid 

 material, the quantity of which cannot be estimated even approximately. 

 Chemically, it consists of water and salts, albumin and epithelium, fats and 

 fatty acids. Its excretory importance in connection with the metabolism of 

 the body must be slight. Its chief physiological value must be sought in 

 its effect upon the hairs, which are kept oiled and pliant by the secretion. 

 Moreover, it forms a thin, oily layer over most of the surface of the 

 skin ; and we may suppose that this layer of oil is of value in two 

 ways — in preventing too great a loss of water through the skin, and in 

 offering an obstacle to the absorption of aqueous solutions brought into 

 contact with the skin. 



Excretion of C0 2 . — In some of the lower animals — the frog, for ex- 

 ample — the skin takes an important part in the respiratory exchanges, elim- 

 inating C0 2 and absorbing O. In man, and presumably in the mammalia 

 generally, it has been ascertained that changes of this kind are very slight. 

 Estimates of the amount of C0 2 given off from the skin of man during 

 twenty-four hours vary greatly, but the amount is small, and is certainly less 

 than one one-hundredth part of the amount given off' through the lungs. 



