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 salt. The use of these is in great part physical: the water, for instance, dissolves materials 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 which is carried in the blood all over the body to supply albuminous material to the tissues, is, for example, insoluble in pure water, but dissolves readily if a small quantity of common salt be present. Besides such uses, the non-oxidizable foods have probably others, as what may be called machinery formers. In the lime salts, which give their hardness to the bones and teeth, we have an example of such an employment of them; and to a less extent the same may be true of other tissues. The body is a self-building and self-repairing machine, and the material for this building and repair, as well as the fuel or oxidizable foods which yield the energy the machine expends, must be supplied in the food. While experience shows us that even for machinery construction oxidizable matters are largely needed, it is nevertheless a gain to replace such substances by non-oxidizable material when possible; just as, if practicable, it would be advantageous to construct an engine out of a substance which would not rust, although other conditions determine the selection of iron for building the greater part of it.

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 examples. 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 auimals.