=How Muscular Exercise aids the Heart.=--_When a muscle contracts_, it hardens and presses upon a vein which goes through the muscle, and _the blood is pressed out of the vein_ (see Fig. 58). The blood cannot go toward the capillaries, for the valves fill and close when it starts that way; so it must all go out toward the heart. _When the muscle relaxes_, the blood that has been pressed forward cannot go back because of the valves, but the valves nearer the capillaries open, and _the veins are filled from the capillaries_ (Fig. 53). When the muscle contracts again, the same effect on the blood movement is repeated. We see, therefore, that every contracting muscle converts into a pump the vein running through it, and when a person works or exercises, many little pumps are working all over the body, aiding the heart in its function. 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. [Illustration: FIG. 59.--THE VENTRICLES OF A DOG’S HEART relaxed (above), and contracted (below).]