WORKING OF THE HEART AND BLOOD-VESSELS. 229 



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

 the same time the blood in those vessels opposes some resist- 

 ance to a back-flow into them, while the still flabby and 

 dilating ventricles oppose much less resistance, the general 

 result is that/the contracting 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 auriculo- 

 ventricular valves. That this increased filling of the ventri- 

 cles, 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 

 commences. 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 cordae tendinese 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 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 

 pressure on one side. It is in this latter way that the semi- 

 lunar valves are opened. The contracting ventricle tightens 

 its grip on the blood inside it and becomes rigid to the touch. 

 As it squeezes harder and harder, at last the pressure on the 

 blood within it becomes greater than the pressure exerted on 

 the other side of the valves by the blood in the arteries, the flaps 

 are forced open and the blood begins to pass out : the ventri- 

 cle continues its contraction until it has obliterated its cavity 

 and completely emptied itself; this total emptying' appears, 

 at least, to occur in the normally beating heart, but in some 

 pathological conditions and under the influence of certain 

 drugs the emptying of the ventricles is incomplete. After 



