The muscle tissue of an animal consists of highly specialized substance the chief function of which is to do mechanical work through contraction. In this process it derives the energy from glucose or from fat. It is, in addition, a storage organ for glycogen, or so-called animal starch, and for fat. It contains but little of cellular structures which are the seat of chemical activity, such as are exhibited by the glandular organs. Chemical analysis shows muscle to consist for the most part, aside from the reserve food-stuffs, of water, protein and salts. The glandular organs yield relatively much nucleic acid while the muscle yields but little. The inorganic content of muscle resembles that of the seeds of plants rather than that of the leaves. Its ash is highly acid, whereas that of leaves is highly alkaline. Muscle is, however, very poor in magnesium as compared with any vegetable food materials, is rich in phosphorus, and contains much more iron. Since it is also very poor in fat-soluble A, it does not supplement the seeds in any appreciable degree except with respect to the protein factor.

It follows, therefore, that we should not expect to secure growth and normal nutrition with mixtures of seeds and muscle meats. Experimental trials have shown that this is the case. Such mixtures require supplementing with respect to calcium, sodium and chlorin, just as do seed mixtures alone. In most instances such mixtures will also require to be supplemented with fat-soluble A before they will constitute diets which are satisfactory for normal growth and for the support of females during gestation and nursing of the young. It is doubtless possible that with yellow maize or millet seed in the diet in very liberal amount, this factor may be of sufficient quality to permit over a considerable period of time of apparently normal functioning.

It seems highly probable, however, that even with diets of this special selection, animals would be benefited by a more liberal intake of fat-soluble A. Meat and seed diets can be supplemented with liberal amounts of the edible leaves, so as to make good their dietary deficiencies to an extent which will bring about approximately normal nutrition provided the right selection of seeds and leaves is made. Our knowledge in this direction is still very fragmentary.

The glandular organs are not superior to the muscle tissues in their mineral content. They are vastly more satisfactory as sources of protein, fat-soluble A, water-soluble B and water-soluble C.