One of these facts, abundantly substantiated by common observation, is that there are many examples of animals subsisting entirely upon food derived from the tissues of other animals. The wolf, lion and tiger are familiar examples among mammals, and the eagle, owl and vulture are examples among birds. Nowhere do we find more remarkable physical development than among this class of animals. It is obvious, therefore, that the tissues of one animal contain everything which is essential for the nutrition of another. Those who accepted the old ideas regarding the necessary components of an adequate diet saw no reason for distinguishing between different types of carnivorous diets. Actually there are decided differences in the dietary properties of muscle meats as contrasted with the glandular tissues. Muscle meats are very similar in their dietary properties to the cereal grains, and in order to make them complete it is necessary to supplement them with respect to calcium, sodium chlorid and all three of the uncharacterized "vitamins" associated with the causation of the deficiency diseases. The flesh-eating of the carnivorous animals is a very different thing from that of civilized people in the temperate zones. For some unknown reason the latter have developed a special liking for muscle cuts of meat, as steaks, chops, etc., whereas the carnivorous among men and animals eat blood, glandular structures and bone marrow, fat and an appreciable amount of bone substance in preference to muscle tissue. Miss Simmonds and the author have found that suitable proportions of these tissues form very-satisfactory diets for the rat, just as they do for the wild car-nivorae. Most people in the past have been satisfied with inaccurate and incomplete descriptions of food habits. With the greater knowledge we now possess, it is necessary to be more specific and accurate in all discussions of nutrition and diet (4).