=Arrangement of Voluntary Muscles.=--_Circular_ muscles, called _sphincter muscles_, are found around the mouth and eyes. Muscles that extend straight along the limb either bend it and are called _flexors_, or straighten it and are called _extensors_. Most of 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. 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. [Illustration: Illustrated Study of Muscular Function Draw a dotted line from each function mentioned on margin to the muscle or muscles having that function. Bows the head? Straightens the elbow? Straightens the fingers? Swings leg outward? Bends the knee? Straightens the knee? Crosses the leg? Straightens toes? Draws shoulder back? Lifts the whole arm outward and upward? Draws whole arm downward and forward? Bends the elbow? Bends the fingers? Raises the body on the toes? Raises toes? FIG. 44.--SUPERFICIAL MUSCLES AFTER THE STATUE OF “THE DIGGER” (Lami).]