158 



THE VASCULAK MECHANISM. 



two auricles are similarly synchronous in action. It has been maintained, 

 however, that the synchronism may at times not be perfect. 



Before we attempt to study in detail the several parts of this compli- 

 cated series of events, it will be convenient to take a rapid survey of what 

 is taking place within the heart during such a cycle. 



116. The cardiac cycle. We may take as the end of the cycle the 

 moment at which the ventricles having emptied their contents have relaxed 

 and returned to the diastolic or resting position and form. At this moment 

 the blood is flowing freely with a fair rapidity, but as we have seen at a very 

 low pressure, through the venae cavse into the right auricle (we may confine 

 ourselves at first to the right side), and since there is now nothing to keep 

 the tricuspid valve shut, some of this blood probably finds its way into the 

 ventricle also. This goes on for some little time, and then comes the sharp, 

 short systole of the auricle, which, since it begins, as we have seen, as a wave 

 of contraction running forward along the ends of the venae cavse, drives the 

 blood not backward into the veins but forward into the ventricle ; this end 

 is further secured by the fact that the systole has behind it on the venous 

 side the pressure of the blood in the veins, increasing, as we have seen, back- 

 ward toward the capillaries, and before it the relatively empty cavity of the 

 ventricle, in which the pressure is at first very low. By the complete con- 

 traction of the auricular walls the complete or nearly complete emptying of 

 the cavity is insured. No valves are present in the mouth of the superior 

 vena cava, for they are not needed ; and the imperfect Eustachian valve at 

 the mouth of the inferior vena cava cannot be of any great use in the adult, 

 though in its more developed state in the foetus it had an important function 

 in directing the blood of the inferior vena cava through the foramen ovale 

 into the left auricle. The valves in the coronary vein are, however, probably 

 of some use in preventing a reflux into that vessel. 



As the blood is being driven by the auricular systole into the ventricle, a 

 reflux current is probably set up, by which the blood, passing along the 

 sides of the ventricle, gets between them and the flaps of the tricuspid valve 

 and so tends to float these up. [Figs. 50, 51.] It is further probable that 



[Fio. 50. 



FIG. 51. 



Diagrams of Valves of the Heart. After Dalton.] 



the same reflux current, continuing somewhat later than the flow into the 

 ventricle, is sufficient to bring the flaps into apposition, without any regurgi- 

 tation into the auricle, at the close of the auricular systole, before the ven- 



