CHAP, iv.] THE VASCULAR MECHANISM. 231 



ventricle continues to press with greater and greater force on its 

 contents, making the ventricle hard and tense to the touch, the 

 pressure within the ventricle becomes greater than that in the 

 pulmonary artery and this greater pressure forces open the semi- 

 lunar valves and allows the escape of the contents into the artery. 

 The ventricular systole may be seen and felt in the exposed In-art 

 to be of some duration, it is strong enough and long enough to 

 empty the ventricle completely ; indeed, as we shall see, it probably 

 lasts longer than the discharge of blood, so that there is a brief 

 period during which the ventricle is empty but yet contracted. 



During the ventricular systole the semilunar valves are pressed 

 outwards towards but not close to the arterial walls, reflux currents 

 probably keeping them in an intermediate position, so that their 

 orifice forms an equilateral triangle with curved sides ; they thus 

 offer little obstacle to the escape of blood from the cavity of the 

 ventricle. The ventricle as we have seen propels the blood with 

 great force and rapidity into the pulmonary artery and the whole 

 contents are speedily ejected. Now, when a force which is driving 

 a fluid with great rapidity along a closed channel suddenly ceases 

 to act, the fluid, by its momentum, continues to move onward after 

 the force has ceased; in consequence of this a negative pressure 

 makes its appearance in the rear of the fluid, and, sucking the 

 fluid back again, sets up a reflux current. So when the last 

 portions of blood leave the ventricle a negative pressure makes 

 its appearance behind them, and leads to a reflux current from 

 the artery towards the ventricle. This alone would be sufficient 

 to bring the valves together; and, in the opinion of some, is 

 the real cause of the closure of the valves; others however, as 

 we shall see later on, maintain that subsequent to this reflux 

 due to mere negative pressure a somewhat later reflux, in 

 which the elastic reaction of the arterial walls is concerned, 

 more completely fills and renders tense the pockets, causing 

 their free margins to come into close and firm contact, and 

 thus entirely blocks the way. The corpora Arantii meet in the 

 centre, and the thin membranous festoons or lunulae are brought 

 into exact apposition. As in the tricuspid valves, so here, 

 while the pressure of the blood is borne by the tougher bodies 

 of the several valves, each two thin adjacent lunulse, pressed 

 together by the blood acting on both sides of them, are kept in 

 complete contact, without any strain being put upon them ; in 

 this way the orifice is closed in a most efficient manner. 



There is no adequate foundation for the view put forward by 

 Briicke that during the ventricular systole the flaps are pressed 

 back flat against the arterial walls, and in the case of the aorta 

 completely cover up the orifices of the coronary arteries, so that the 

 flow of blood from the aorta into the coronary arteries can take place 

 only during the ventricular diastole or at the very beginning of the 

 systole, and not at all during the systole itself. 



