FUNCTION OF THE VALVES. 99 



orifice, and prevent any of the blood flowing back into the 

 ventricles (p. 104). 



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 contraction 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 con- 

 traction 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 Auricles, . . . 1 -f Repose of Auricles, . . 10 = 11 

 " Ventricles, . . 4 -j- " Ventricles, . 7 = 11 



Repose (no contraction of either 



auricles or ventricles), . . (' -f Contraction of either 



auricles or ventricles, 5=11 

 11 



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. Each half or side of 

 the heart, through the action of its valves, may be compared 

 with a kind of forcing-pump, like the common enema-syringe 

 with two valves, of which one admits the fluid on raising the 

 piston, but is closed again when the piston is forced down ; 

 while the other opens for the escape of the fluid, but closes 

 when the piston is raised, so as to prevent the regurgitation of 

 the fluid already forced through it. The ventricular dilata- 

 tion is here represented by the raising up of the piston ; the 

 valve thus admitting fluid represents the auriculo-veutricular 

 valve, which is closed a^aft) whenUae" piston Js;' foi;cd down, 

 i. e., when the ventricle contracts^ and ,tlye" ;opier, i. e., the 

 arterial, valye^opecs: , Tfy<^ diagrams, ,pn^ the following page 

 illustrate this <vef y wpl J. ; . "- % " ; ' ; , ; - ^ \ \ 



