On the Mechanism of the Cardiac Valves. 



539 



up of the segments through the action of eddies, and that eddies can actually 

 bring about an approximation of the valve flaps is indicated by an experiment 

 of Baumgarten,* quoted by Sherrington. The quotation is as follows : — 



" If the arterial openings of the excised heart be blocked, and through the 

 auricles a momentary rush of water under about twelve inches pressure be 

 allowed to play into the auriculo-ventricular orifices, the valve flaps rise into 

 the orifice, and come together sufficiently firmly to allow of the inversion of 

 the heart without the escape of a drop of its contents." 



Thus eddies or currents may undoubtedly bring about the preliminary 

 approximation of the valve flaps under certain circumstances. 



The course of events in the normal heart is probably somewhat as follows: — 



Immediately before the moment at which the valve is timed to close, its 

 flaps are being acted upon by certain forces tending in different directions. 

 One of these forces is the stream of blood driven from the auricle, which 

 presses the flaps outwards. Another is the eddy behind them, between them 

 and the ventricular wall, which presses them inwards. 



To this latter must now be added the effect of the muscular slips described 

 as existing in the bases of the valves, which tend to raise the flaps away from 

 the ventricular wall, and towards the position of closure. The actual position 

 of the flaps at any given moment will be determined by the combined effect 

 of these forces. 



As the stream of blood driven from the auricle weakens towards the close 

 of auricular systole, the retrovalvular eddy, though possibly also somewhat 

 weakened on account of the lessened stream of blood, will have less to 

 antagonise it. It will therefore become relatively more effective. With 

 regard to the muscular action, the contraction of the auricle will have 

 commenced to die away in the upper parts. It will still be present in full 

 force in the lower parts of the auricular wall, and also in the slips of muscle 

 entering the valves. That is to say, the muscular contraction will die away 

 and be replaced by a condition of relaxation latest in this situation. 



As a result, the flaps will continue to be drawn up by the muscular slips 

 quite to the end of auricular systole, and this will not only have a direct 

 effect in closing the valve at exactly the appropriate instant, but will also 

 ensure free play to the retrovalvular eddy up to the time when the valve 

 closure begins to be finally accomplished. 



At the end of auricular systole, therefore, the forces tending to keep the 

 flaps apart will have become weakened, the forces tending to approximate 



* Baumgarten, 'Archiv f. Anat. und Physiol.,' 1843, p. 464. Quoted in Clifford 

 Allbutt's 'System of Medicine,' vol. 6 (1909). By C. S. Sherrington. Bevised by 

 James Mackenzie. 



