CHAPTER III. 

 SECRETION BY THE SKIN. 



WE have traced the food from the alimentary canal into the blood, 

 and, did the state of our knowledge permit, the natural course of 

 our study would be to trace the food from the blood into the 

 tissues, and then to follow the products of the activity of the 

 tissues back into the blood and so out of the body. This how- 

 ever we cannot as yet satisfactorily do ; and it will be more con- 

 venient to study first the final products of the metabolism of the 

 body, and the manner in which they are eliminated, and after- 

 wards to return to the discussion of the intervening steps. 



Our food consists of certain food-stuffs, viz. proteids, fats and 

 carbohydrates, of various salts, and of water. In their passage 

 through the blood and tissues of the body, the proteids, fats and 

 carbohydrates are converted unto urea (or some closely allied body), 

 carbonic acid and water, the nitrogen of the urea being furnished 

 by the proteids alone. Many of the proteids contain sulphur, and 

 also have phosphorus attached to them in some combination or 

 other, and some of the fats taken as food contain phosphorus ; 

 these elements ultimately suffer oxidation into phosphates and 

 sulphates, and leave the body in that form in company with the 

 other salts. 



Broadly speaking then, the waste products of the animal 

 economy are urea, carbonic acid, salts and water. Of these a large 



