Pro′teids, a large group of organic compounds found in animals and plants. They form the chief constituents of blood, muscle, nerves, glands and other organs of animals, and play an important part in plant-life. Although much work has been done in attempting to learn their exact chemical constitution, little definite is yet known. They are amorphous, and contain about 54 parts carbon, 7 hydrogen, 16 nitrogen, 21 oxygen and from 1 to 1½ sulphur. They differ widely in solubility and in their decomposition-products. Food-stuffs have been divided into two divisions: heat-producing and tissue-forming, the amyloids and fats constituting the former and proteids the latter division. This, however, is very misleading if it implies that the oxidation of proteids does not develop heat and that amyloids and fats subserve only the production of heat. Proteids are tissue-formers, but they also are heat-producers because they are competent by chemical metamorphosis in the body to give rise to amyloids. The following is one of the several classifications that have been made of proteids:
I. Albumens, soluble in water: 1. Serum; albumen; 2. Egg albumen.
II. Globulins, insoluble in water, soluble in very dilute acids and alkalies, soluble in dilute solutions of sodic chloride and other neutral salts: 1. Myosin; 2. Globulin; 3. Fibrinogen; 4. Vitellin.
III. Derived albumens, insoluble in water and solutions of sodic chloride, soluble in dilute acids and alkalies; 1. Acid albumen; 2. Alkali albumen or albuminate casein.
IV. Fibrin, insoluble in water, sparingly soluble in dilute acids and alkalies and in neutral saline solutions.
V. Coagulated albuminoids,
VI. Amyloid substance or lardacein.
VII. Peptones, produced by action of the gastric juice on all albuminoids.