n THE ACTION OF THE VALVES 53 



will depend upon the relative resistance offered to it in 

 these two directions. Towards the great veins it is 

 resisted by the mass of the blood contained in the veins. 

 Towards the ventricles, on the contrary, there is no resist- 

 ance worth mentioning, inasmuch as the valves are open, 

 the walls of the ventricles, in their uncontracted state, 

 are flaccid and easily distended, and the entire pressure 

 of the arterial blood is taken off bj' the semilunar valves, 

 which are necessarily closed. The return of blood into 

 the veins is further checked by a contraction of the great 

 veijis at their point of junction with the heart which im- 

 mediately precedes the ^stole of the auricles, and is 

 practically continuous with it. 



Therefore, when the auricles contract, little or none of 

 the fluid which they contain will flow back into the veins ; 

 all the contents or nearly so will pass into and distend the 

 ventricles. As the ventricles fill and begin to re.sist 

 further distension, the blood, getting behind the auriculo- 

 ventricular valves, will j)ush them towards one another, 

 and indeed almost shut them. The auricles now cease to 

 contract, and immediately that their walls relax, fresh 

 blood flows from the great veins and slowly distends them 

 again. 



But the moment the auricular systole is over, the 

 ventricular systole begins. The walls of each ventricle 

 contract vigorously', and the first effect of that contraction 

 is to complete the closure of the auriculo-ventricular 

 valves and so to stop all egress towards the auricle. The 

 pressure upon the valves becomes very considerable, and 

 they might even be driven upwards, if it were not for the 

 chordce tendineft: which hold down their edges. 



As the contraction continues and the capacities of the 

 ventricles become diminished, the points of the wall of 

 the heart to which the chorda tendinece are attached ap- 

 proach the edges of the valves ; and thus there is a ten- 

 dency to allow of a slackening of these cords, which, if 

 it really took place, might permit the edges of the ^■alves 



