Page:Popular Science Monthly Volume 3.djvu/459

 in their vital work, the leading characteristic of each kind is so marked as to warrant the classification which Liebig has formulated.

It is understood that the structures of the body are in a state of continual change, so that atoms which are present at one hour may be gone the next, and, when gone, the structures will be so far wasted, unless the process of waste be accompanied by renewal. But the renewing substance must be of the same nature as that wasted, so that bone shall be renewed by bone and flesh by flesh; and hence, while the body is always changing, it is always the same. This is the duty assigned to food—to supply to each part of the body the very same kind of material that is lost by waste.

As foods must have the same composition as the body, or supply such other materials as by vital action may be transformed into the substances of the body, it is desirable to gain a general idea of what these substances are.

The following is a summary statement of the principal materials of which the body is composed:

Flesh in its fresh state contains water, fat, fibrine, albumen, and gelatine, besides compounds of lime, phosphorus, soda, potash, magnesia, silica, and iron, and certain extractives.

Blood has a composition similar in elements to that of flesh.

Bone is composed of cartilage, gelatine, fat, and salts of lime, magnesia, soda, and potash, combined with phosphoric and other acids.

Cartilage consists of chondrine, which is like gelatine in composition, with salts of soda, potash, lime, phosphorus, magnesia, sulphur, and iron.

The brain is composed of water, albumen, fat, phosphoric acid, osmazome, and salts.

The liver consists of water, fat, and albumen, with phosphoric and other acids in conjunction with soda, lime, potash, and iron.

The lungs are formed of a substance resembling gelatine, albumen, a substance analogous to caseine, fibrine, various fatty and organic acids, cholesterine, with salts of soda, and iron and water.

Bile consists of water, fat, resin, sugar, fatty and organic acids, cholesterine, and salts of potash, soda, and iron.

Hence it is requisite that the body should be provided with salts of potash, soda, lime, magnesia, sulphur, iron, and manganese, as well as sulphuric, hydrochloric, phosphoric, and fluoric acids and water; also nearly all the fat which it consumes daily, and probably all the nitrogenous substances which it requires, and which are closely allied in composition, as albumen, fibrine, gelatine, and chondrine. It can produce sugar rapidly and largely, and fat slowly and sparely, from other substances; also lactic, acetic, and various organic acids, and peculiar extractive matters.

So great an array of mysterious substances might well prevent us from feeding ourselves or others if the selection of food depended solely upon our knowledge and judgment; but it is not so, for,