112 THE HUMAN BODY. 



ety of them it can turn it into other varieties, but it cannot 

 make proteids from things which are not proteids. Hence 

 these albuminous or proteid substances are an essential 

 article of diet. 



The Limited Constructive Power of the Animal Bodyo 

 From what has been said above it is clear that our bodies 

 are, on the whole, destructive rather than constructive in 

 relation to the outer world. They require for their nutri- 

 tion very complex chemical compounds (starch, sugar, fat, 

 proteids), build these up into living tissues, and then 

 oxidize the tissues and return the carbon, hydrogen, and 

 nitrogen, which were received from outside in the form of 

 complicated chemical molecules, to the outer world in the 

 form of much simpler chemical compounds, namely, carbon 

 dioxide, water, and urea. None of these latter substances 

 is capable of nourishing an animal ; it cannot from them 

 alone build up its tissues or set free energy. 



How Plants Supply Food for Animals, and Animals 

 Food for Plants. Since animals are essentially proteid con- 

 sumers, and destroyers also of other complex substances, 

 as starch and sugar, the question naturally suggests itself, 

 How is it, if animals are constantly consuming these 

 things, that the supply of them is kept up? For example, 

 the supply of proteids ; they cannot be made artificially 

 by any process known to us. The answer is, that animals 

 live on the things which plants make, and plants live on 



Do our bodies on the whole build up or break down chemical 

 compounds? What class of compounds do they require for their 

 nutrition? What do they do with these compounds? What simple 

 compounds does the body return to the outer world? Can these 

 compounds feed any animal? 



What facts suggest the question, How is the supply of pro- 

 teids and other complex foods kept up? How is the question 

 answered ? 



