CHAP, iv.] THE VASCULAR MECHANISM. 265 



Summary. 



137. We may now briefly recapitulate the main facts con- 

 nected with the passage of blood through the heart. The right 

 auricle during its diastole, by the relaxation of its muscular fibres, 

 and by the fact that all backward pressure from the ventricle is 

 prevented by the closing of the tricuspid valves, offers but little 

 resistance to the ingress of blood from the veins. On the other 

 hand, the blood in the trunks of both the superior and inferior 

 vena cava is under a pressure, which though diminishing towards 

 the heart 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 (as we 

 shall see in speaking of respiration) is favoured by the diminution 

 of 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 un- 

 broken stream from the venae cavae into the ventricle. How far 

 the entrance of blood from the auricle into the ventricle is, under 

 ordinary circumstances, aided by the negative pressure in the 

 ventricle following the close of the systole, must, as we have said, 

 be left for the present uncertain. In a short time, 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 pressure in the veins, which increases 

 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. 



