vii MECHANICS OF THE HEART 193 



which is simultaneously brought nearer the valves by the efflux 

 of blood from the arteries. The effect of this mechanism would be 

 a negative pressure in the auricle during systole. But the idea 

 that the valves, when the heart is functioning normally, can form a 

 sort of hollow cone during systole, which presses into the cavity of 

 the ventricles like a stamp, is contradicted by the observations of 

 Krehl, as already cited. 



In order to form a correct notion of the mechanism of the 

 auriculo-ventricular valves, it is indispensable to determine their 

 position during the entire period of perisystole, presystole, and 

 systole. 



1. During perisystole, a centrifugal eddy of regurgitation must 

 be produced at the base of the ventricles by the kinetic energy of 

 the blood which is rushing from the auricles to the ventricles. 

 This vortex, directed from apex to base of the ventricle, is capable 

 of maintaining the folds of the valves (which are easily floated in 

 consequence of their light weight) in the half-open position. 



2. During presystole, ventricular pressure is slightly increased, 

 owing to the wave of blood that is driven from auricle to ventricle. 

 The walls of the ventricle dilate passively, and are thrown into 

 elastic tension, while the wave of regurgitation from the centri- 

 fugal vortex increases, and the borders of the valvular folds tend 

 to approach, and take up a position approximating to closure. 

 The auriculo-valvular fibres of Kiirschner contract simultaneously, 

 which tends to shorten the valves and maintain them in the 

 upright position. 



3. At the presystolic dead point there is a cessation of the 

 flow from auricle to ventricle, while the centrifugal vortex persists, 

 in consequence of the dynamic force acquired by the blood ; this 

 causes the valves to close with perfect apposition, not merely of 

 their borders, but also of a considerable portion of their internal or 

 superior surfaces. 



4. At the commencement of systole the valves which are 

 already closed expand and vibrate, in consequence of the sudden 

 rise of ventricular pressure, and form an irregularly curved arch at 

 the level of the auriculo-ventricular orifice. This can be detected in 

 larger animals by passing the finger through an opening made in 

 the auricular appendix, as far as the auriculo-ventricular orifice 

 (Chauveau), and results from the varying lengths of the chordae 

 tendineae affixed to the inferior surface of the valvular folds 

 (Krehl, Fig. 59). 



5. During the systolic evacuation of the ventricle, the auriculo- 

 ventricular orifice becomes more and more restricted until it almost 

 entirely closes. At the same time the valvular folds are drawn 

 forcibly onwards by the contraction of the papillary muscles, so 

 that increasingly larger portions of their inner or upper surface are 

 brought fully into contact, and deflected so as to become vertical. 



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