The pronounced deficiencies of muscle tissue as a food-stuff naturally suggest the question as to the reason for the success of the nutrition of the strictly carnivorous animals. This explanation is found in the selection these animals make of the parts of their prey. The carnivorae are all fond of blood, and it is their custom to open the jugular veins of a freshly killed animal and suck blood as long as it will flow. After blood, the glandular organs appeal most strongly to them in temperate and tropical regions, but in the Arctic the second choice, or even the first may be fat, depending on the previous state of nutrition of the animal. If it has recently ingested liberal amounts of protein it may take only fat from the victim serving as its next repast. This is said to happen frequently in the case of polar bears. A seal is often found by explorers, stripped of its fat but otherwise untouched. The demands for energy are so great in these bears as to make it necessary to consume fat frequently and in large amounts. Since the protein metabolism is of much smaller magnitude, meat of either the glandular or muscle type may be taken at much longer intervals.

When the American Indians subsisted on the bison of the great plains, bone marrow was regarded as a morsel of the greatest delicacy, and this is also true to-day of the inhabitants of Arctic America. So far as possible blood is either used fresh or is preserved for making soup. Most of the Asiatic, and some of the European peoples have no abhorrence for blood as a food. In China blood is regularly sold as food, and in Europe blood sausage is a common article of diet.