THE HEART. 175 



of pressure in the heart and great vessels caused by the respiratory move- 

 ments. Before this flow has gone on very long, the diastole of the ventricle 

 begins, its cavity dilates, the flaps of the tricuspid valve fall back, and blood 

 for some little time flows in an unbroken stream from the venae cavse into 

 the ventricle. In a short time, however, probably before very much blood 

 has had time to enter the ventricle, the auricle is full ; and forthwith its 

 sharp sudden systole takes place. Partly by reason of the backward pres- 

 sure in the veins, which increases rapidly from the heart toward the capil- 

 laries, 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 con- 

 tinues, 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 pul- 

 monary 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 pul- 

 monary 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 tran- 

 sient negative pressure which makes its appearance in the rear of the ejected 

 column of blood, or the elastic action of the aortic walls, leads to a reflux 

 of blood toward the ventricle, the effect of which, however, is to close the 

 semilunar 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 relax- 

 ation begins ; then once more the cavity of the ventricle becomes unfolded 

 and finally distended by the influx of blood, a negative pressure devel- 

 oped 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 con- 

 tracting, 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 pul- 

 monary artery. 



The Work Done. 



127. The most important factor of all in the determination of the work 

 of the vascular mechanism is the quantity of blood ejected from the ventricle 

 into the aorta at each systole. The general result of some of these calcula- 

 tions gives about 180 grammes (6 ozs.) as the quantity of blood which is 



