CHAPTER III. 

 THE ELIMINATION OF WASTE PRODUCTS. 



319. 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 combination 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 fyeces contain, besides 

 undigested 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. 



The lungs serve as the channel for the discharge of the 

 greater part of the carbonic acid, and a considerable quantity of 

 water ; this discharge we have just studied. Through the skin 

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