The first use Nature makes of pro-teids in the body is in the actual adding to or increasing of body-tissue. When an emaciated young man from the city goes to work on a farm and gains twenty pounds, the cells of his muscles have actually increased in size and number. This requires proteids, which can be obtained only from the nitrogenous material in food. The growth during early-life is due to an actual increase in the size of all the organs of the body, and is merely an accumulation of proteid substance.

Proteids as tissue-builders.

The second use of proteids, and the one which, in matured life, is of more importance than those already referred to, is in the formation of the various nitrogenous products which are produced in connection with the different processes of the body and which are destroyed by the function of life. For example, the pepsin of the gastric juice is a nitrogenous substance which can be formed only from proteids. All digestive enzyms and other substances in the muscles, nerves, and in the various organs throughout the body are of a nitrogenous nature, and in their formation and use a certain amount of proteid material is consumed. When the digestive enzyms are formed from proteids, they consume more than their own weight of proteid material.

Proteids form the nitrogenous part of the body.

The third form in which proteids may be consumed in the body is in the actual replacement of worn-out cells. The skin, the hair, and the mucous or lining membranes of the body-cavities are constantly being cast off on the external surface, new cells being formed underneath. When cells within the interior of the body have become injured, or have passed their usefulness, they are removed by the phagocytes or white blood-cor-puscles, and must be replaced by other cells. In the case of bacterial infections, as tumors, boils, or contagious diseases, the bacteria feed upon the proteids of the blood. The white blood corpuscles are destroyed in the conflict, or effort to remove the intruders, and all these substances must be replaced by proteids from food.

Proteids replace worn-out cells.