One example of the action of the liver in rendering substances harmless, is its oxidation of _alcohol_. From one to three ounces of alcohol a day may be oxidized and made harmless in the liver, varying according to the individual and to the condition, at different times, in the same person. If the limit of one to three ounces is exceeded, the excess is not oxidized and intoxication results. This is the reason one may become intoxicated at one time when the same amount of liquor would not appreciably affect one at another. The _muscles_ play an important part in the use of foods. Most of the heat is generated in them by the action of the oxygen in the blood upon the sugar and fats, liberating their latent heat. This heat is liberated during every moment of the twenty-four hours whether one is asleep or awake. Of course, more is liberated during exercise, since the movement of the muscles sets all tissues into activity and the blood circulates more strongly, bringing a greater supply of oxygen to them. It is always well during active exercise to stop frequently and fully inflate the _lungs_, not only to bring more oxygen to the blood, but to change the residual air and in the inflation to exercise the lung tissue more freely, bringing a better supply of nourishment to it. We forget that the lung tissue as well as every other tissue of the body needs exercise and a full supply of nourishment. One should form the habit of breathing fully and deeply—otherwise the liberated carbon dioxid will cause an increased pressure throughout the blood stream, particularly about the heart and in the head. This pressure is relieved when the excess of carbon dioxid has been thrown off by the lungs. Much dull headache is due to the retention of carbon dioxid resulting from shallow breathing. Nature makes the effort to throw off this excess of carbon dioxid by forcing one to breathe more rapidly while running or taking unusual exercise. A certain amount of protein is constantly oxidized in muscular action also, being broken down into carbon dioxid, water, and a number of nitrogenous mid-products. The carbon dioxid and water are thrown off by the lungs, and the partially oxidized nitrogenous waste is carried to the liver, where it is further oxidized and prepared for excretion through the kidneys, lungs, skin, and intestines. Through their stimulant action, the _nerves_ aid in oxidizing food materials. During periods of rest, food materials are also stored in the nerve cells. 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. When one is continuously using an excess of nerve activity, all reserve food material, stored in the nerve cells, is used and the nerves become undernourished. The result is seen in neurasthenic conditions of various kinds.