252 SUMMARY OF CARDIAC EVENTS. [BOOK i. 



rapidly from the heart towards the capillaries, and which, at some 

 distance from the heart is assisted by the presence of valves in the 

 venous trunks, but still more from the fact that the systole begins 

 at the great veins themselves and spreads thence over the auricle, 

 the force of the auricular contraction is spent in driving the blood, 

 not back into the veins, but into the ventricle, where the pressure 

 is still exceedingly low. Whether there is any backward flow at 

 all into the great veins or whether by the progressive character of 

 the systole the flow of blood continues, so to speak, to follow up 

 the systole without break so that the stream from the veins into 

 the auricle is really continuous, is at present doubtful ; though a 

 slight positive wave of pressure synchronous with the auricular 

 systole, travelling backward along the great veins, has been 

 observed at least in cases where the heart is beating vigorously. 



The ventricle thus being filled by the auricular systole, the 

 play of the tricuspid valves described above comes into action, 

 the auricular systole is followed by that of the ventricle, and the 

 pressure within the ventricle, cut off from the auricle by the 

 tricuspid valves, is brought to bear on the pulmonary semilunar 

 valves and the column of blood on the other side of those valves. 

 As soon as by the rapidly increasing shortening of the ventricular 

 fibres the pressure within the ventricle becomes greater than 

 that in the pulmonary artery, the semilunar valves open and the 

 still continuing systole discharges the contents of the ventricle 

 into that vessel. 



As the ventricle thus rapidly and forcibly empties itself, 

 .either the transient negative pressure which makes its appear- 

 ance in the rear of the ejected column of blood or the elastic 

 action of the aortic walls leads to a reflux of blood towards 

 the ventricle, the effect of which however is to close the semi- 

 lunar valves, and thus to shut off the blood in the distended 

 arteries from the emptied ventricle. Either immediately at or 

 more probably some little time after this closing of the valves, 

 the ventricular systole ends and relaxation begins ; then once 

 more the cavity of the ventricle becomes unfolded and finally 

 distended by the influx of blood, a negative pressure developed 

 by the relaxation probably aiding the flow from the auricle and 

 great veins. 



During the whole of this time the left side has with still 

 greater energy been executing the same manoeuvre. At the same 

 time that the venae cavse are filling the right auricle, the pulmonary 

 veins are filling the left auricle. At the same time that the right 

 auricle is contracting, the left auricle is contracting too. The 

 systole of the left ventricle is synchronous with that of the right 

 ventricle, but executed with greater force ; and the flow of blood 

 is guided on the left side by the mitral and aortic valves in the 

 same way that it is on the right by the tricuspid valves and the 

 valves of the pulmonary artery. 



