THE CIKCULATION 227 



tough membrane, but are comparatively thin. The following 

 direction for deciding at an autopsy whether or not they were 

 healthy at the time of death was given many years ago by a sur- 

 geon of repute : " You ought to be able to see the dirt under your 

 thumbnail when you place it beneath one of the flaps." Surgery 

 has improved in cleanliness as well as in other ways ; indeed, 

 the possibility of advance has been due to the recognition of 

 the need for transcendental cleanliness. But this is a digres- 

 sion. The margins of the flaps are crenulated. Threads 

 chordae tendineae are attached to them like the stay-ropes of 

 a tent. At their other end these tendons are attached to the 

 musculi papillares already mentioned. The bunch of tendons 

 from each papillary muscle spreads, to be inserted into the 

 contiguous margins of two flaps. We have mentioned some of 

 the difficulties which have been overcome in the construction 

 of the pump. (1) The flaps do not flatten back against the wall 

 of the ventricle during systole of the auricle. It must be re- 

 membered that during diastole of both chambers blood is 

 flowing through the auricle into the ventricle. The latter being 

 partly filled before systole of the auricle commences, the flaps 

 are floated up. This is greatly favoured by the form of the 

 inner wall of the ventricle. It is not flat, but raised in pillars 

 columnae carneae. The spaces between these pillars cause 

 backwash currents, which lift the flaps and help to bring them 

 into apposition as soon as systole of the ventricle commences. 

 (2) No blood which has entered the ventricle is thrown back 

 into the auricle. The valve " balloons " over the blood in 

 the ventricle before the contraction of the auricle has ceased. 

 The thin margins of its flaps come together with great rapidity. 

 The tendinous cords holding their edges on the ventricular 

 side, they meet, not edge to edge, but folded flap to folded 

 flap. (3) The valve does not bulge into the auricle. On the 

 contrary, at the height of systole it is pulled into the ventricle 

 by the contracting musculi papillares. As the ring to which 

 the valve is attached is diminished in size, by the contraction 

 of the base of the heart, which continues, it will be remembered, 

 until after the apex has begun to relax, the edges of the flaps 

 are folded farther and still farther over by the pull of the 

 musculi papillares, and the blood is squeezed out from between 

 the wall of the ventricle and the indrawn valve. 



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