122 THE CIRCULATION. 



separates these valves from contact with each other, 

 and presses them back against the sides of the artery, 

 making a free passage for the stream of blood. Then, as 

 soon as the ventricular contraction ceases, the elastic walls 

 of the distended artery recoil, and by pressing the blood 

 behind the valves, force them down towards the centre of 

 the vessel, and spread them out so as to close the orifice 

 and prevent any of the blood flowing back into the ven- 

 tricles (p. 127). 



As soon as the auricles have completed their contraction 

 they begin again to dilate, and to be refilled with blood, 

 which flows into them in a steady stream through the 

 great venous trunks. They are thus filling during all the 

 time in which the ventricles are contracting ; and the con- 

 traction of the ventricles being ended, these also again 

 dilate, and receive again the blood that flows into them 

 from the auricles. By the time that the ventricles are 

 thus from one-third to two-thirds full, the auricles are 

 distended ; these, then suddenly contracting, fill up the 

 ventricles, as already described. 



If we suppose a cardiac revolution, which includes the 

 contraction of the auricles, the contraction of the ventricles, 

 and their repose, to occupy rather more than a second, the 

 following table will represent, in tenths of a second, the 

 time occupied by the various events we have considered. 



Contraction of Auricle . . . I Eepose of Auricles . . 10 

 Ventricles . . 4 Ventricles . 7 

 6 Contraction 5 



n 



Action of the Valves of the Heart. 



The periods in which the several valves of the heart are 

 in action may be connected with the foregoing table ; for 

 the auriculo-ventricular valves are closed, and the arterial 

 valves are open during the whole time of the ventricular 

 contraction ; while, during the dilatation and distension of 

 the ventricles the latter valves are shut, the former open. 



