156 DURATION OF THE CARDIAC PHASES. [BOOK i. 



the thorax actually negative (as we shall see in speaking of 

 respirations), remains higher than the pressure obtaining in the 

 interior of the auricle; the blood in consequence flows into the 

 empty auricle, its progress in the case of the superior vena cava 

 being assisted by gravity. At each inspiration, this flow is favoured 

 by the increased negative pressure in the heart and great vessels 

 caused by the respiratory movements. 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 much blood 

 has had time to enter the ventricle, the auricle is full, and forth- 

 with its sharp sadden systole takes place. Partly by reason of the 

 onward pressure in the veins, which increases rapidly from the 

 heart towards the capillaries, partly from the presence of valves in 

 the venous trunks and at the mouth of the inferior vena cava, 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 ex- 

 ceedingly 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 entirely on the conus arteriosus 

 and the pulmonary semilunar 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, a 

 transient negative pressure makes its appearance in the rear of the 

 ejected column of blood. This in return leads to a reflux of blood 

 towards the ventricle. The first act of this reflux however is, as 

 we have seen, to close the semilunar valves, and even if it be 

 urged that the exit of the ventricular contents does not always end 

 with sufficient abruptness to cause a negative pressure adequate 

 to produce this result, the elastic rebound of the arteries, upon 

 their receiving no fresh blood, has the same effect of closing the 

 semilunar valves, and thus of shutting off the blood in the over- 



