This storage of glycogen in the muscles and in the liver is a wise provision of Nature. It is a reserve to be called upon whenever the expenditure of heat and energy exceeds the amount supplied in any day’s rations. * * * * * [Sidenote: Work of the Nerves] The nerves oxidize food materials, but not to any great extent, excepting during nervous activity. During periods of rest, food materials are stored in the nerve cells in grandular form. They represent concentrated nerve foods and are the result of anabolic processes. During nervous activity they are oxidized and carried away through the blood and the lymph. This oxidation of the food, stored in the nerves, creates nervous energy and heat. The energy liberated by the nerves resembles electrical energy. Where one subjects himself continuously to an excess of nervous activity, all reserve food material, stored in the nerve cells, is used and the result is a trying nerve tension. Such individuals need plenty of easily digested food. * * * * * [Sidenote: Work of the Lungs] 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 capillary walls of the lungs. The oxygen is absorbed into the capillaries through the thin air sacs in the walls of the lungs. * * * * * [Sidenote: Work of the Kidneys] 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. The blood passes through them in a transverse branch from the abdominal aorta. In its circuit urea, uric acid, urates, sulphuric acid, sulphates and sodium phosphates pass from the blood with the water and 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 and the lungs is impure. The above substances cannot be thrown off from the lungs. They are the products of oxidation of proteins, partly of the living tissues and partly those broken down direct as they are supplied in the foods, in excess of the needs of the system. Interference in the action of the kidneys results in a hoarding of these substances in the blood, and may produce an intoxicated condition known as uremic poisoning. Water in abundance and diuretic fruits and vegetables, which increase the activity of the kidneys, should be taken where uremia is indicated. (Foods which cause a free flow of urine are called diuretic foods.) [Sidenote: Work of the Skin]