THE EXCRETORY SYSTEM 201 



CHAPTER X 



THE EXCRETORY SYSTEM 



The process of excretion is an essential part of the activ- 

 ity of all living substance. The substances resulting from 

 the breaking down of living matter and the various mate- 

 rials taken into the organism which are never built up into 

 living substance give rise to many compounds no longer 

 useful which must be gotten rid of if the life of the organism 

 be maintained. Every cell of the body execretes as well as 

 assimilates and respires. A part of the waste is eliminated 

 in the form of carbon dioxide, which is thrown off from the 

 body through the organs of respiration. The solid products 

 of metabolism, however, cannot be disposed of in this way ? 

 and specialized organs are developed for their removal. In 

 excretion, as in respiration, we must distinguish between the 

 discharge of substances into the blood which takes place 

 throughout all parts of the organism, and the elimination of 

 these substances from the blood to the outside of the body. 

 The latter function is carried on by several organs. The 

 skin is to a certain extent an organ of excretion, although 

 little is known of its function in this respect among the 

 Amphibia. In higher forms in which sweat glands occur a 

 certain amount of salts and other substances is gotten rid of 

 by cutaneous excretion. The liver is an important excre- 

 tory organ, and the walls of the intestine also subserve the 

 same function. The most important organs of excretion, 

 however, are the kidneys, of whose structure and function 

 we shall give a short account. 



