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 the voluntary muscles are arranged in pairs and cause motion in opposite directions; they are said to be antagonists. The biceps (Fig. 43) bends the arm. Its antagonist is the triceps on the back of the arm. By feeling them swell and harden as they shorten, locate on your own body the muscles mentioned in Fig. 44.

How a Muscle grows Stronger; its Blood Supply.—Nature has provided that any part of the body shall receive more blood when it is working than when it is resting. When it works the hardest, the blood tubes expand the most and its blood supply is greatest. So whenever a muscle is used a great deal, an unusual amount of material is carried to it by the blood, the cells enlarge and multiply, and the muscle grows. The walls of the capillaries are so thin that the food which is in the blood readily passes from them to the muscle. Because of the oxidation taking place, a working muscle is warmer than one at rest. By use a muscle grows large, firm, and of a darker red; by disuse, it becomes small, flabby, and pale. But if muscles are worked too constantly, especially in youth, their cells do not have time to assimilate food and oxygen, and their growth is stunted.

Unless the meal has been a very light one, vigorous exercise should not be taken after eating, as the blood will be drawn from the food tube to the muscles and the secretion of the digestive fluids will be hindered. Persons whose entire circulation is weak may find that light exercise after a meal, such as walking slowly, may help circulation and digestion.

Why the Muscles work in Harmony.—When a boy throws a stone, almost every part of the body is concerned in the action. His arms, his legs, his eyes, the breathing, the beating of the heart, are all modified to assist in the effort.