THE CIRCULATION 



57 



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 with- 

 out fresh blood, fainting is accompanied by unconsciousness. Recov- 

 ery 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 flat- 

 tens down. So with the action of 

 the blood upon the valves in the 

 veins. (Repeat Exp. 6 in class.) 



How Muscular Exercise aids the Heart. 



,, , . , , , FIG. 58. VALVES IN VEINS. 



When a muscle contracts, it hardens and 



' ., (Jegi.) 



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 con- 

 tracts 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. 



