This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
Since animals are essentially proteid consumers, 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 the carbon dioxide and water and ammonia (urea) which animals excrete.
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 proteids and other complex foods kept up? How is the question answered?
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.*