306 THE HUMAN BODY 



the auricles. As the ventricles fill, back currents are set up along 

 their walls and these carry up the flaps of the valves so that by 

 the end of the pause they are nearly closed. At this moment the 

 auricles contract, and since this contraction commences at and 

 narrows the mouths of the veins opening into them, and at the 

 same time the blood in those vessels opposes some resistance to 

 a back-flow into them, while the still flabby and dilating ventricles 

 oppose much less resistance, the general result is that the con- 

 tracting auricles send blood into the ventricles, and not back into 

 the veins. At the same time the increased direct current into the 

 ventricles produces a greater back current on the sides, which, 

 when the auricles cease their contraction and the filled ventricles 

 become tense and press on the blood inside them, completely closes 

 the auriculoventricular valves. That this increased filling of the 

 ventricles, due to auricular contractions will close the valves may 

 be seen easily in a sheep's heart. If the auricles be carefully cut 

 away from this so as to expose the mitral and tricuspid valves, 

 and water be then poured from a little height into the ventricles, 

 it will be seen that as these cavities are filled the valve-flaps are 

 floated up and close the orifices. 



The auricular contraction now ceases and the ventricular com- 

 mences. The blood^n^each ventricle is imprisoned between the 

 aiiriculoyentriculsflf valves behind and the semilunar jvalves in 

 front. The former cannot yield on account of the cordse tendineae 

 fixed to their edges: the semilunar valves, on the other hand, can 

 open outwards from the ventricle and let the blood pass on, but 

 they are kept tightly shut by the pressure of the blood on their 

 other sides, just as the lock-gates of a canal are by the pressure 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 of course they might be forced open without this 

 by applying sufficient power to overcome the higher water pres- 

 sure on one side. It is in this latter way that the semilunar y .! VPS 

 are^openexl* The contracting ventncjfij^ightens its grip on the 

 blood inside it and becomes rigid to the touch. As it squeezes 

 harder and harder, at last the pres^ureorjthe_blopjl-within it be- 

 comes greater than the^pressure jexerted ^on_ the other_side of the 

 valves by the blood in the arterievihe flaps are forced oj3ejnjind_ 

 the blood begins to pass out: the ventricle continues its contrac- 



