| 1. Inorganic food—Oxygen, salts. | ||||
| 2. Organic foods |  | Animal |  | Nitrogenous. |
| Non-nitrogenous. | ||||
| Vegetable | | Nitrogenous. | ||
| Non-nitrogenous. | ||||
| Or, as— | ||||
| 1. Solid foods | | Animal | | Nitrogenous. |
| Non-nitrogenous. | ||||
| Vegetable | | Nitrogenous. | ||
| Non-nitrogenous. | ||||
| 2. Liquid foods | | Water. | ||
| Milk and its products. | ||||
| Tea and similar beverages. | ||||
| Alcoholic beverages. | ||||
| 3. Gaseous foods—Air. | ||||
[CHAPTER III.]
THE VARIETIES OF FOOD.
Nitrogenous Animal Foods.—These are divided into two groups, the one containing gelatin, and the other all the proteid or albuminoid substances, which are taken in the flesh of various animals, and in milk and eggs.
Gelatin is obtainable from bones, and from connective tissue wherever found. Being easily digested, and absorbed, it has been very popular as an invalid’s food; but the fact that animals cannot sustain life on it without the addition of proteids proves that its value is limited. It is incapable of building tissues, but is a valuable proteid-saver, being able to save from metabolism half its weight of proteid, or twice as much as is spared by an equal weight of carbohydrate. Its utility in this direction is, however, limited, because of the dilute form in which it is taken in ordinary foods. It is useful for invalids, partly because it forms a bulk, and prevents the evil tendency to give their food in too concentrated a form; partly because it forms a source of easily metabolised material, and so prevents tissue-waste; and partly because it commonly contains phosphate of lime, derived from the bones forming the source of gelatin.
Gelatin as prepared for the table contains a considerable proportion of water; as little as one per cent. of gelatin in water will cause it to gelatinise on cooling. Isinglass obtained from the floating bladder of the sturgeon is an example of the purest kind of gelatin; glue is an inferior sort, made from bones, etc.
Gelatin is only a cheap food when obtained, for instance, from bones which cannot otherwise be utilised. When made from veal it is costly out of proportion to its dietetic value.
The Flesh of various animals is one of the main sources of our nitrogenous and fatty food. Meats may be divided into two kinds, viz., red meat and white meat. These gradually merge into one another. As common examples of red meats, we have beef, mutton, pork, game, wild fowl, and salmon.
The common fowl and turkey, most fishes, rabbits, crustaceans, and molluscs, are examples of white meat. As a rule white meats are more digestible than red, having more delicate fibres, and containing a smaller proportion of nitrogenous matter.
Flesh consists almost entirely of muscular tissue, of which there are two kinds, striped and unstriped.