282 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 CO 2 . In some of the lower animals the frog, for ex- 

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

 inating CO 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 CO 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. 



H. BODY-METABOLISM ; NUTRITIVE VALUE OF THE FOOD-STUFFS. 



Determination of Total Metabolism. We have so far studied the 

 changes that the food-stuffs undergo during digestion, the form in which they 

 are absorbed into the blood, their history in the tissues to some extent, and the 

 final condition in which, after being decomposed in the body, they are eliminated 

 in the excreta. To ascertain the true nutritional value of the food-stuffs it 

 is of the utmost importance that we should have some means of estimating 

 accurately the kind and the amount of body-metabolism during a given period 

 in relation to the character of the diet used. Fortunately, this end may be 

 reached by a careful study of the excreta. The methods employed can readily 

 be understood in principle from a brief description. It has been made suf- 

 ficiently clear before this, perhaps, that by determining the total amount of the 

 nitrogenous excreta we can reckon back to the amount of proteid (or albu- 

 minoid) destroyed in the body. In the case of proteids or albuminoids which 

 undergo physiological oxidation all the nitrogen appears in the forms of urea, 

 uric acid, creatinin, xanthin, etc., which are eliminated mainly through the urine, 

 and may therefore be collected and determined. The following practical facts 

 are, however, to be borne in mind in this connection : The nitrogenous excre- 

 tion of the urine is mainly in the form of urea which can be estimated as such, 

 but it is much more accurate to determine the total nitrogen in the urine during 

 a given period, using some one of the approved methods for nitrogen-deter- 

 mination, and to calculate back from the amount of nitrogen to the amount of 



