the object of the other parts of the circulation is merely to move the blood continually through the capillaries.
The effect of gravity is to retard the flow in certain parts of the
body and aid the flow in other parts, according to the position of the
body (Exp. 2).
Fainting is usually due to lack of blood in the brain, which in turn results from a weakening of the heart beat. Since the brain cannot work without fresh blood, fainting is accompanied by unconsciousness. Recovery from fainting is aided by loosening the clothing at the neck and by placing the head of the patient a little lower than the body so that the weight of the blood may aid the flow to the brain. Dashing a little cold water in the face shocks the nerves and arouses the heart to stronger beats.
The veins have valves placed frequently along their
course (Fig. 58). These valves are pockets made by a
fold in the inner coat of the wall
of the vein. When a boy places
his hand in his pocket, the pocket
swells out; but if he rubs his hand
on the outside of the pocket from
the bottom toward the top, it flattens
down. So with the action of
the blood upon the valves in the
veins. (Repeat Exp. 6 in class.)
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Fig. 58.—Valves in Veins. (Jegi.)
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.
