~Construction of Proteins.~--In plant structure the building up of the proteins is accomplished by the plants from inorganic substances existing in the soil and air; but in the animal body this is not possible, because the construction of the tissues requires the use of other proteins--the most available ones being found in food. Each animal (or species) forms the proteins characteristic of its own tissues,--while the proteins of food are similar to those found in the body, they cannot be utilized in their original form, but must be split into simpler substances from which the cells of the various tissues throughout the body may select those particularly adapted for their purpose. These transformed substances are known as amino acids, the production of which is a result of digestion in the digestive tract. There are about seventeen of these acids entering into the construction of the common proteins. One scientist has likened these units to letters of the alphabet, which, being combined, spell many proteins. When a protein contains all of the essential units, it may be said to be "complete," the best example of which may be seen in milk, eggs, and meat. When a protein lacks some of the essential elements, or letters of the protein alphabet, it is said to be incomplete. Gelatin is the best example of this type of protein, but the cereals and beans must likewise be supplemented by other substances; milk being the one most generally used for this purpose. For the purpose of building young tissues, and maintaining those already mature, it is logical to use foods containing the foodstuffs in their best form; that is, those that not only contain the complete protein, but also the requisite mineral salts and vitamines. Foods lacking in some of these respects become adequate when supplemented by these foods which can supply the missing constituents; hence, the use of such incomplete protein foods need not necessarily be abandoned, for, as in the case of cereals, the foods are both economical and palatable, and, when used in addition to milk, furnish valuable adjuncts to the dietary. ~Classification of Proteins.~--A brief description of some of the more important proteins with which we are chiefly concerned will serve to simplify the formulation of a diet. Those assuming the most important position in nutrition and food are ~globulins~, ~albumens~, ~nucleoproteins~, ~phosphoproteins~, ~hemoglobins~, and ~derived proteins~ such as proteoses and peptones. The albumens and globulins associated together occur in the tissues of both animals and plants. The albumens are richer in sulphur than the globulins and are found more abundantly in the animal fluids, such as the blood, while the globulins predominate in the more solid tissues of animals and in plants. The close association of these two proteins is particularly noticeable in the blood and cells. They have different characteristics, however.