When we make a mixture of equal parts of blood and muscle and confine young rats to it they may grow in a fairly normal manner, whereas with either of these two tissues alone they will suffer disaster (47). Mixtures of blood, liver or kidney, muscle and bone may be so selected as to proportions as to make a very satisfactory diet (31). Here, again, the controlled experiment of the laboratory reveals the secret of success in the nutrition of a class of animals whose excellent physical development on a diet derived solely from animal tissues has often been interpreted as signifying that meat, in the general sense in which we use this term, is a complete food and one of exceptional value for promoting vigor. The meat-eating of the peoples of the temperate regions of America is limited essentially to the consumption of muscle tissue. In dietary properties this is the poorest part of the carcass. Those who take a varied diet in which cereals and potatoes form the most prominent sources of energy, are guided by their sense of taste to select muscle meats rather than other meats superior in dietary value.