310 



BIOLOGY 



it will readily be seen how the valves work to prevent the 

 backward passage of the blood, and to force it onward when the 

 walls of the heart contract. The blood forced from the heart 

 is received in elastic blood vessels, the arteries, which branch 

 and grow smaller as they pass from the heart, and finally break 

 up into extremely minute and even microscopic vessels. After 



FIG. 136. DIAGRAM OF ONE SIDE OF THE HEART, SHOWING 

 THE MECHANISM OF THE VALVES 



A, in the state of relaxation; B, at the time of contraction. In A the open valves admit 

 the flow of blood from the veins into the ventricles. In B the valve connecting with the 

 auricle is closed and the contraction of the heart forces the blood up through the semiluuar 

 valve, as is shown by the arrows. Upon relaxation of the ventricle, the semilunar valve 

 closes, and prevents the flow of the blood back into the ventricle, while the auriculo- 

 ventricular valve opens and allows blood to enter from the vein. 



a, auricle; 



avv, auriculo-ventricular valve; 



sh, semilunar valve; 

 v, ventricle. 



passing these capillaries, the vessels are again united into 

 larger tubes which, by combining with each other, form the 

 large veins that flow back to the heart. The whole action of 

 this system is mechanical; and we can arrange a series of 

 elastic rubber tubes with a central beating force-pump, in a 

 manner to imitate the chief functions of the circulation. Into 

 the details of this matter we need not go; for our purpose it 

 is sufficient to understand that the circulation of the blood is 

 a mechanical phenomenon which can easily be imitated by 



