488 APPLIED BIOLOGY 



To demonstrate the action of the auriculo-ventricular valves, 

 cut away the auricles from a heart with the apex removed, and then 

 plunge it with the ventricle held downward into water so as to float 

 the valves into the closed position. 



Slit open the aorta to expose its valves (semilunar) ; and also cut 

 the side of a ventricle to show attachment of the auriculo-ventricular 

 valves. 



411. The cause of circulation is the constant, rhythmic 

 beating of the heart, which is a muscular force-pump. Its 

 general plan of structure is illustrated by an ordinary rubber 

 bulb such as is used for atomizers and syringes. In such a 

 bulb there are two valves arranged so that when the bulb 

 is filled with water and then compressed, one valve (inlet) 

 closes and prevents the water from flowing outward, while 

 the other valve (outlet) remains open and allows the water 

 to escape into the outlet tube. Then if the bulb be allowed 

 to expand, the outlet valve is closed by the back pressure of 

 the water in the outlet tube, and water rushing in from the 

 supply, tube opens the inlet valve. The next compression 

 will again arrange the valves as first described above. (Draw 

 diagrams to illustrate positions of valves and with arrows 

 show direction of flow through a two-valved bulb.) 



In a similar manner, there are valves arranged in the heart 

 in two places, one valve to prevent a flow of the blood back 

 into the veins (inlet) when contraction occurs, the other to 

 prevent the return flow from the arteries (outlet tube) when 

 relaxation or dilation takes place. And, as can be easily 

 demonstrated with a rubber bulb having one inlet and one 

 outlet valve, repeated contraction and expansion will cause 

 fluids to flow through always in the same direction, which is 

 determined by the arrangement of the valves. Hence, the 

 blood circulates because the heart is a pump with valves so 

 arranged as to force the blood to flow in only one direction, 

 out through one valve, and thence around through tubes 

 which lead back to the inlet valve. 



