200 CIRCULATION. 



general venous system, and particularly in the veins of the 

 extremities, do not exist in the venae cavoe. 



The continuous flow of blood from the veins into the 

 auricles, the feeble character of their contractions, the ar- 

 rangement of the fibres around the orifices of the vessels, and 

 the great size of the auriculo-ventricular openings, are condi- 

 tions which provide sufficiently well for the flow of blood into 

 the ventricles. 



Auriculo - Ventricular Valves. After the ventricles have 

 become completely distended by the auricular systole, they 

 take on their contraction ; which, it will be remembered, is 

 very many times more powerful than the contraction of the 

 auricles. They have to force open the valves which close the 

 orifices of the pulmonary artery and aorta, and empty their 

 contents into these vessels. To accomplish this, at the moment 

 of the ventricular systole, there is an instantaneous and com- 

 plete closure of the auriculo-ventricular valves, leaving but 

 one opening through which the blood can pass. That these 

 valves close at the moment of contraction of the ventricles is 

 demonstrated by the experiments of Chauveau and Faivre, 

 who introduced the finger through an opening into the auri- 

 cle, and actually felt the valves close at the instant of the ven- 

 tricular systole. 1 



This tactile demonstration, and the fact that the first 

 sound of the heart, which is produced in great part by the 

 closure of the auriculo-ventricular valves, is absolutely syn- 

 chronous with the ventricular systole, leave no doubt as to 

 the mechanism of the closure of these valves. 



It is probable that as the blood flows into the ventricles 

 the valves are slightly floated out, but they are not closed un- 

 til the ventricles contract. A German physiologist, Btiuni- 

 garten," has attempted to show that the valves are closed by 

 the contraction of the auricles, basing this opinion upon the 

 fact that when the auricles are cut away, and fluid is poured 



1 Op. cit., p. 21. * MILNE-EDWARDS, op. cit., tome iv., p. 31. 



