CHAP. XVI.] ELIMINATION. 179 



the metabolism of the body will be certain waste matters, and 

 we shall now proceed to describe the mechanism of the organs 

 by means of which these wastes are removed from the body. 



Elimination. In passing through the blood and tissues of the 

 body, the proteids, fats, and carbohydrates are transformed into 

 urea (or some closely allied product), 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, and some of the fats 

 taken as food contain phosphorus ; these elements are converted 

 by oxidation into phosphates and sulphates, and are excreted in, 

 that form in company with the other salts of the body. 



Broadly speaking, then, the waste products 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 kidneys. 

 Some part, it is true, leaves the body by the bowels, for, as we 

 have seen, the feces contain, besides undigested portions of food, 

 substances which have been secreted into the bowels, and are 

 therefore waste products ; but the amount of these is very small 

 and, except in diseased conditions, of no special importance. 



The waste matters discharged relatively by the lungs, skin, 

 and kidneys may be stated as follows : 



By the lungs : The greater part of the carbonic acid. 



A considerable quantity of water. 



By the skin : A variable but, on the whole, large quantity of 



water. 



A little carbonic acid. 



A small quantity of salts. 

 By the kidneys : All, or nearly all, the urea and allied bodies. 



The greater portion of the salts. 



A large amount of water. 



A very small quantity of carbonic acid. 



We have already studied the mechanism by means of which 

 the lungs rid the blood of carbonic acid and water, and it now 

 remains for us to consider the mechanism of the skin and kid- 

 neys. In the present chapter we shall devote ourselves to the 

 consideration of the kidneys, which secrete the urine, and the 

 other urinary organs, the ureters, bladder, and urethra, which 

 collect the urine arid conduct it to the outside of the body. 



