CHAPTEE III. 



THE ELIMINATION OF WASTE PKODUCTS. 



392. 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 however we cannot as yet satisfactorily do; and it will be 

 more convenient to study first the final products of the metabolism 

 of the body, and the manner in which they are eliminated, and 

 afterwards 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 into 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 conir- 

 bination or other, and some of the fats taken as food contain 

 phosphorus ; these elements ultimately undergo 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. These leave 

 the body by one or other of three main channels, the lungs, the 

 skin, and the kidney. Some part, it is true, leaves the body by 

 the bowels, for, as we have seen, the faeces contain, besides un- 

 digested portions of food, substances which have been secreted into 

 the bowel, and are therefore waste products; but the amount of 

 these is so small that they may be neglected. 



