The animal body is more than a machine. It requires fuel to enable it not only to work but also to live, even without working. A part of the food eaten must go to maintain the body, for while the inanimate machine is sent periodically to the repair-shop, the living machine must do its own repairing, day by day and minute by minute.
The adult animal lives, repairs waste, and does work; while the young animal does all these and more - it grows. For growth and repairs something else is needed beside starch and fat.
The muscles are the instruments of motion, and they must be nourished in order that they may have power. The nourishment is carried to them by the blood in which, as well as in muscular tissue, there is found a food element which we have not heretofore considered, namely, nitrogen. It has been proved that the use of the muscles and the brain sets free certain nitrogenous compounds which pass out of the system as such, and this loss must be supplied by the use of some kind of food which contains nitrogen. Starch and fat do not contain this element; therefore they cannot furnish it to the blood.
The American breakfast will probably include meat, fish, or eggs. These are examples of the nitrogenous food-stuffs. Nitrogenous food compounds are sometimes classed together under the name of proteins. These may be divided into proteids, gelatinoids, and extractives.
The proteids all resemble albumin, which is found nearly pure in the white of an egg. These in some form are never absent from animal and vegetable organisms. They are most abundant in animal flesh and in the blood. Other common articles of diet belonging to this group in addition to albumin, are the curd of milk (casein), the lean of animal flesh and fish and gluten of wheat, and the legumin of peas and beans. The proteids are the most important nitrogenous food materials. They build up and repair the muscles, tendons, cartilage, bones, and skin and supply the albumin of the blood and other fluids of the body.
The animal skeleton - horns, bones, cartilage, connective tissues, etc. - contains nitrogenous compounds which are converted by boiling into substances that form with water a jelly-like mass. These are known as the gelatinoids and are so named because of their resemblance to gelatin. Although somewhat similar to the proteids in composition they are not thought to be true flesh formers. However, they do help out the proteids in some unknown way.
The chief constituent of the connective tissues of meats is collagen. This is insoluble in cold water, but in hot water becomes soluble and yields gelatin. Collagen swells when heated and when treated with dilute acids. Steak increases in bulk when placed over the coals, and tough meat is rendered tender by soaking in vinegar. Meat a few days old is tough, for the collagen is dry and hard. In time it becomes softened by acids which are secreted by bacteria either in or on the meat; the meat thus becomes tender and easily masticated. Tannic acid has the opposite effect upon collagen, hardening and shrinking it. This effect is taken advantage of in tanning, and is the disadvantage of boiled tea as a beverage, since tea always contains a little of this tannic acid when freshly made and much more if the tea is boiled.
The last class of nitrogenous compounds are the extractives, so called because they are readily extracted by water from meat where they principally occur. The proteins of this class are thought to have little value as food, but they give the flavor to meats, etc., and are therefore of great importance. They are stimulants, somewhat of the nature of caffein of coffee and the thein of tea.