CHAP, iv.] THE VASCULAR MECHANISM. 



251 



the foregoing remarks as to the approximate or even uncertain 

 character of some of the data must be borne in mind. 



Systole of ventricle before the open- 

 ing of the semilunar valves, while 

 pressure is still getting up 



(probably rather less than) '1 

 Escape of blood into aorta (about) "1 

 Continued contraction of the emptied 

 ventricle 



(possibly rather more than) *1 ^ 

 Total systole of the ventricle 



(probably rather more than) 

 Diastole of both auricle and ventricle, 

 neither contracting, or " passive in- 

 terval " (probably rather less than) 

 Systole of auricle (about or less than) 

 Diastole of ventricle, including relaxa- 

 tion and filling, up to the beginning 

 of the ventricular systole 



(probably rather less than) 

 Total Cardiac Cycle 



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 

 removed by the closing of the tricuspid valves, offers but little re- 

 sistance 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 gra- 

 vity. 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 unbroken stream from the venae 

 cavas 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 pressure in the veins, which increases 



