218 TEE HUMAN BODY. 



that the contracting auricles send blood mainly into the 

 ventricles and hardly any back into the veins. The in- 

 creased current into the ventricles produces a greater back 

 current on the sides, which, as the auricles cease their con- 

 traction, and the filled ventricles become tense and press on 

 the blood inside them, completely closes the auriculo-ven- 

 tricular valves. 



The auricular contraction now ceases, and the ventricu- 

 lar begins. The blood in each ventricle is imprisoned be- 

 tween the auriculo-ventricular valves behind and the semi- 

 lunar valves in front. The former cannot yield on account 

 of the chordaB tendineae fixed to their edges; the semilunar 

 valves, on the other hand, can open outwards from the ven- 

 tricle and let the blood pass on; but they are kept tightly 

 shut by the pressure of the blood in the aorta and pulmo- 

 nary artery, just as the lock-gates of a canal are by the pres- 

 sure of the water on them. In order to open the canal-gates 

 water is let in or out of the lock until it stands at the same 

 level on each side of them; but they might be forced open 

 without this by applying sufficient power to overcome the 

 higher water pressure on one side. It is in this latter way 

 that the semilunar valves are opened. 



The contracting ventricle tightens its grip on the blood 

 inside it. As it squeezes harder and harder, at last the 

 pressure on the blood in it becomes greater than the pres- 

 sure exerted on the other side of the valves by the blood in 



What is the consequence of the increased flow into the ventricles 

 due to the auricular contraction? 



What happens when the ventricle begins to contract? Why can- 

 not the imprisoned blood escape back into the auricle? How are the 

 semilunar valves kept closed? Illustrate. How might we force open 

 the gates of a canal lock without bringing the water to the same level 

 on each side? 



"'How are the semilunar valves opened? 



