PHENOMENA OF THE HEARTS BEAT. 221 



of the aorta and the pulmonary artery are closed, so that no 

 blood can flow back from those vessels. The whole heart, 

 however, is soft and distensible and yields readily to blood 

 flowing into it from the pulmonary veins and the venae 

 cavae; this passes on through the open mitral and tricuspid 

 valves and fills up the dilating ventricles, as well as 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 contrac- 

 tion 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 contract- 

 ing auricles send blood mainly into the ventricles, and 

 hardly any back into the veins. At the same time the in- 

 creased direct current into the ventricles produces a greater 

 back current on the sides, which, as the auricles cease their 

 contraction and the filled ventricles become tense and press 

 on the blood inside them, completely close the auriculo- 

 ventricular valves. That this increased filling of the ven- 

 tricles, due to auricular contractions, will close the valves is 

 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 

 commences. The blood in each ventricle is imprisoned 

 between the auriculo-ventricular valves behind and the 

 semilunar valves in front. The former cannot yield on 

 account of the cordae tendineas 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 



