270 THE MECHANICS OF THE HEART 



convincingly that they are instrumental in approximating the different 

 valve flaps. Thus, by permitting the different flaps to be moved into 

 a position transversely across the orifices and no farther, they serve 

 a purpose very similar to that of the guy-ropes of a sail. Secondly, as 

 a number of chordae are always inserted upon the central area or body 

 of the flaps, they prevent the bulging or bellying of the entire valve 

 into the auricular cavity. Thirdly, as the papillary projections from 

 which the chordae arise are usually placed vertically below the space 

 between two adjoining flaps, and as the individual strings are inserted 

 upon the margins of both, they must necessarily exert a traction toward 

 a common center which is situated midway below the planes of the two 

 flaps. In this way the margins of the different flaps are pulled together 

 transversely and are then held firmly in place. It may be assumed 

 that the papillary muscles take part in the general contraction of the 

 ventricles, thereby furnishing a more solid basis for the chordae to act 

 upon; in fact, it may be said that the contraction of these projections 

 exerts a certain traction upon them which facilitates their unfolding 

 and the approximation of the valve-flaps. 



The auriculoventricular valves are opened very soon after the ces- 

 sation of the contraction of the ventricles. Gradually, as the blood 

 flows into the auricles from the central veins, the intra-auricular pres- 

 sure is raised above that prevailing in the now passive ventricles. 

 In consequence of the higher pressure exerted upon their upper sur- 

 faces, the flaps are forced slightly apart with the result that the blood 

 now rushes into the ventricular cavity. It should be remembered, 

 however, that the flaps are not moved as a door would be on opening 

 it, because their basal portions are attached to a rather rigid cushion 

 of tissue and remain, therefore, relatively fixed. Their tips, on the 

 other hand, are bent sharply downward so that each flap assumes the 

 shape of a crescent, the concavity of which faces the ventricle. 



The auricular contraction following very shortly after the initial 

 opening of the auriculoventricular valve, renders the orifice between 

 these chambers more funnel-shaped. The blood being driven directly 

 into the narrowest part of this passage opposite the tilted tips of the 

 flaps, is thus directed into the central expanse of the ventricles without 

 being able to form secondary currents or whorls which might seriously 

 impair its flow. Quite naturally, when this column of blood traverses 

 the ostium, the flaps are pushed far apart, but are not brought into 

 actual contact with the ventricular wall. The space between them 

 and the surface of the latter is filled with blood. This is of great dynam- 

 ical importance, because if thefiaps were forced against the wall, it 

 would be difficult to dislodge them and to move them into the position 

 of closure. Obviously, the latter movement can only be effected if 

 their under surfaces remain exposed to the ventricular pressure. 



The contraction of the auricles fills the ventricles to their utmost 

 capacity so that their walls become fully distended and remain so 

 until the end of the auricular contraction. Directly thereafter, 



