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 This aid makes the blood flow faster and relieves the heart of part of its work, so that it beats faster, just as a horse might trot faster if another horse helped to draw the load (Exp. 3). The pressure of a contracting muscle upon an artery does not aid the blood flow in the artery because the latter is destitute of valves.

How Breathing aids the Heart.—Breathing is a blood-pumping process as well as an air-renewing process. When the chest expands, blood is drawn into it. When the chest contracts, the flow of blood away from it is aided. As the chest expands, the downward pressure of a great, broad muscle, the diaphragm (Fig. 74) compresses the liver, stomach, and other abdominal organs, and forces the venous blood upward into the expanding chest, thus helping it on its way to the heart. But if the abdominal wall is weakened by tight lacing or by the pressure of belts and bands which support the clothing, the weak abdominal wall yields to the downward pressure of the diaphragm, and no compression of the liver or aid to the circulation will result.

relaxed (above), and contracted (below).

How the Blood Vessels are Controlled.—Evidently the blood vessels are not regulated by the will. We cannot voluntarily increase the beating of the heart, or cause it to slacken its action. Even an actor cannot cause his face to turn pale or to blush at will. This is because the tiny muscles in the walls of the blood vessels are involuntary muscles. They are controlled by nerves of the sympathetic system called vaso-*motors. They are not subject to the will (see Fig. 25). The nerve center which controls the blood vessels is located in the top of the spinal cord at the base of the brain. When cold air strikes the skin the nerves near the arteries are stimulated, the arteries in the skin contract, and the skin turns white. When the heat from a hot fire strikes the skin, the nerves are soothed, the arteries relax, and the face becomes red. When the stomach is filled with food, the heart beats faster and sends more blood to aid in digestion. When we run fast, the heart beats fast to supply more blood to the muscles, but it slows down as sleep comes on, that the body and brain may rest.

Parts of the Blood.—The blood which flows from a cut finger seems to be a bright red throughout. When a drop of it is looked at through a microscope, however, the