THE EXCRETORY SYSTEM 225 



showed it to be the series of changes by which simple inorganic 

 matter is built up into complex organic matter, and all the 

 consequent changes by which it is broken down again into 

 simple elements. 



With the foregoing study of kidney excretion, the story of 

 metabolism is nearly complete. It begins with the life of 

 plants that take carbon, oxygen, hydrogen and nitrogen from 

 simple gaseous and mineral sources, and build them up into 

 proteid, carbohydrate and fatty compounds. Man, by eat- 

 ing plants, converts these into animal proteids, carbohydrates 

 and fats. To be sure, some of his foods come directly from 

 other animals, but even these come originally from plants. 

 After performing various functions as animal tissue in the 

 body, food is broken down and excreted. Much of it leaves 

 the body as carbon dioxid gas and water, reduced thus to the 

 condition in which plants originally obtained it. Part of the 

 food leaves the body as urea and part of it remains in 

 the body. Later, the urea secreted as well as the tissues of the 

 dead body undergo a further set of decomposition changes. 

 They are attacked by a host of microscopic organisms (bacteria 

 and molds) causing further and more complete breaking down. 

 These processes are called fermentation, decay and putrefaction; 

 but it should be remembered that these are simply names 

 for the final steps in the decomposition of food materials. 

 At last they are brought once more into the same condition 

 as that from which they started, so that a new generation 

 of plants can feed upon them. The whole process is a cycle, 

 the same materials circulating around in an endless succession 

 from soil and air to plants, from plants to animals, from 

 animals back again to the soil or air, partially through the 

 agency of bacteria, to start again on their round of usefulness 



DISEASES OF THE EXCRETORY ORGANS 



Excretion of the wastes of the body is absolutely necessary. 

 If anything interferes with the action of the kidneys, the 



