FOODS. 113 



the carbon dioxide and water and ammonia (urea) which 

 animals excrete. 



As regards our own bodies the question might, indeed, 

 be apparently answered by saying that we get our proteids 

 from the flesh of the other animals which we eat. But, 

 then, we have to account for the possession of proteids by 

 those animals ; since they cannot make them from urea 

 and carbon dioxide and water any more than we can. The 

 animals whose flesh is used by us as food get their proteids from 

 plants, which are the great proteid formers of the world; 

 the most carnivorous animal really depends for its most 

 essential foods upon the vegetable kingdom; the fox that 

 devours a hare, in the long run lives on the proteids of 

 the herbs that the hare had previously eaten.* 



Non-Oxidizable Foods. Besides our oxidizable foods a 

 large number of necessary food materials are not oxidizable, 

 or at least are not oxidized in the body, Typical instances 

 are afforded by water and common saU. The use of these is 

 in great part physical: the water, for instance, dissolves ma- 

 terials in the alimentary canal, and carries the solutions 

 through its walls into the blood and lymph vessels, so that 

 they can be conveyed from place to place ; and it permits 

 interchanges by enabling the things it has dissolved to soak 

 through the walls of the vessels. The salines also influence 

 the solubility and chemical interchanges of other things 

 present with them. Serum albumen, one of the proteids 



Where does the proteid that a man eats in a piece of beef come 

 from? Explain. 



What foods are necessary in addition to oxidizable ? Give ex- 

 amples. What are their physical uses? 



* Some animals are known which contain chlorophyl, the green coloring 

 matter of plant leaves; and it has recently been proved that these animals, like 

 plants, can, when exposed to the action of light, live on the waste products of 

 other animals:. 



