=The Nerve Impulse and the Contraction.=--Each nerve that goes to a muscle is composed of many fibers; the fibers soon separate and go to all parts of the muscle, _and each muscle fiber receives its nerve fiber_ (see Fig. 45). In the brain each fiber is stimulated at once, and all the fibers shorten and thicken together. This change is spoken of as contraction; but since the muscle does not become smaller, the word may be misleading. When the muscle shortens, it thickens in proportion and occupies as much space as it did when relaxed. =Where does Muscular Energy come from?=--_The nerve does not furnish the energy which the muscle uses when contracting. The muscle cells have already stored up energy from the food and oxygen brought to them by the blood_, and the process called oxidation sets free the energy. Activity of muscle may increase the carbon dioxid output fivefold. Mental work has practically no effect upon it. [Illustration: FIG. 45.--MOTOR NERVE FIBERS, ending among fibrils of voluntary muscle. Compare with Fig. 48.] =How a Muscle stays Contracted.=--The muscle relaxes at once after contraction; and in order to keep it contracted, nerve impulses must be sent in quick succession, causing in fact many contractions; the effect of this is sometimes visible, as the trembling of the muscle. Figure 47 shows an easy standing posture. =What causes Fatigue.=--Fatigue or exhaustion is due to the using up of the living material in the nerve cells and muscle cells by oxidation. Rest is necessary to give cells opportunity to repair themselves. Why is it less fatiguing to walk for an hour than to stand perfectly still for ten minutes? [Illustration: FIG. 46.--IMPROPER POSITION; causes spine to curve to side; raises one hip and shoulder above the other.] [Illustration: FIG. 47.--BEST POSITION; chest is free to expand, and weight is easily shifted from one foot to other.] =Degeneration of Muscles= begins with habitual disuse. We dare not furnish a substitute for the work of a muscle, if we wish the muscle to remain sound. A belt or a stay at the waist will cause the muscles of the trunk to become flabby and the abdomen to relax and protrude. =How Muscular Activity helps the Health.=--Life is change, stagnation is death. _Muscular activity uses up the food, gives a good appetite, and sets the digestive organs to work; it uses up the oxygen and sets the lungs to work; but most of all, every contraction of a muscle helps the blood to flow._ As a muscle contracts, it presses upon the veins and lymphatics, and, by this pressure, forces the blood and lymph along (Fig. 48). In any ordinary activity, dozens of muscles are being used. That the effect upon the circulation is very powerful, is shown by the rosy skin, deep breathing, and rapid heart beat. The many benefits of an active circulation of the blood and lymph will be discussed in the next chapter. See page 67.