The nerves as well as other tissues require protein to renew their substance as well as fats and carbohydrates for their energy. The vasomotor nerves influence digestion to a marked extent by regulating the blood pressure in the digestive organs and the consequent rate of speed with which digestion and absorption take place. They speed up or slow down the movements of the alimentary canal, thus aiding and preventing the admixture of the food with the digestive juices. By acting on the glands, they aid or prevent the secretions from being formed and poured out. They thus materially affect digestion. The vasomotor nerve centers are in the medulla oblongata. The _lungs_ absorb oxygen and eliminate carbon dioxid. They occasionally throw off a very little organic material. The carbon dioxid is carried to the lungs from the tissues through the venous stream and diffused through the walls of capillaries in the lungs. The oxygen is absorbed in the thin air sacs in the capillary walls. If the lungs are cramped by a faulty position of the body, by excess of fat, or by tight clothing, they cannot expand to their fullest extent. The blood is thus imperfectly aërated and oxygenized and is not freed of its waste. The lung tissue is imperfectly exercised, sufficient blood not being brought to the lung cells to insure their strength. The cramping of the lungs is due largely to incorrect habits of standing and sitting. The _kidneys_ do not absorb as do the lungs, neither do they perform any anabolic work as does the liver, nor catabolic work as the muscles, nerves, and the liver. They simply throw off waste matter. As the blood passes through them urea, uric acid, urates, sulphates, and sodium phosphates pass from it and with the water are thrown from the system; hence the kidneys are purifying organs, as are the lungs. The blood returning from the kidneys through the veins is pure, just as the blood in the pulmonary vein is pure, while that in the arteries to the kidneys is impure. Interference with the action of the kidneys results in an excess of these substances in the blood and may produce a condition of intoxication known as uremic poisoning. The _skin_, by pressure on the capillaries, controls, to some extent, their dilatation, and thus prevents an excessive loss of fluid. When a portion of the skin is removed by accident, as after burns, drops of moisture may be seen gathering on the denuded surface and may result in considerable loss if the denuded surface is large. The skin is a protective covering. We are constantly surrounded with bacteria, dirt, etc., and the skin prevents their absorption. It contains glands which secrete a fluid fat. This keeps the skin soft and flexible, preventing it from becoming too dry. The skin also prevents the underlying tissues from injury through abrasions or contact with foreign substances, as in various industries.