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 As the boy wills to throw the stone, nerve impulses are sent to all the organs that can assist, and they are excited to just the amount of action needed.

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.

, 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