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.
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.