800 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



of blood set suddenly in motion by the ventricular systole, force the segments of 

 the valve^ in an upward spiral direction^ rendering them tnore and more tense 

 until they reach the level of the ventricular orifice ; at which pointy they are twisted 

 and wedged into each other ; the chordae tendineae limiting the amount of upward 

 motion, to prevent retroversion and regurgitation. As, however, the blood 

 finds its way through the aorta, which it does the instant the segments of the 

 valve are screwed home,* the segments gradually hut rapidly descend in an opposite 

 direction to that hy which they ascended, their descent being occasioned, regulated, and 

 minutely graduated hy the contraction of the musculi papillares aided hy the chordce 

 tendinece and hy the ascending spiral columns of hlood ; an arrangement which insures 

 that the delicate margins of the segments, are always closely and accurately 

 applied to each other ; for the chordae tendineae and the blood in the auricles act- 

 ing from above, while the spiral columns of blood in the ventricles act from beneath, 

 the delicate margins in question, are effectually prevented from falling towards 

 the ventricular walls.f This downward action of the valve, musculi papillares, 

 and chordae tendineae, which is of essential importance in adapting the former to 

 the diminishing condition of the left auriculo- ventricular orifice and ventricular 

 cavity, continues until the blood is completely ejected, and the segments of the 

 valve are twisted or plaited into each other to form a dependent spiral cone, 

 whose apex, is directed towards the apex, of the ventricle. By the time this 

 happens, i. e., by the time the blood is ejected from the ventricle, and the cone in 

 question fairly formed, the left auricle is distended ; and due advantage being 

 taken of the extra space aftorded by the descent of the valve, the blood assumes 

 a spiral and conical or wedge-shaped form, which is the best possible for pushing 

 aside the segments, already in the most favourable position for falling away from 

 the ventricular axis, towards the ventricular walls. The same phenomena are 

 repeated with unerring regularity, with each succeeding action of the heart. 

 What has been said of the manner of closure of the mitral or bicuspid valve, 

 applies, I need scarcely add, with slight modifications to the tricuspid. 



Eesume. 



The points which have been more particularly dwelt upon in the present 

 investigation, and on which the writer has endeavoured to throw additional light, 

 are these : — 



* When the segments of the mitral valve are screwed home, the whole force of the ventricular 

 contraction is expended in raising the aortic semilunar valve, and until the screwing home has taken 

 place, the latter action is impossible, as the ventricle up till this point is compressible. 



\ The serious results which might arise from the segments of the valve falling towards the 

 ventricular walls, or away from the axis of the cavity, is especially prevented by the attachments of 

 the chordae tendineae ; the principal and more internal chordse tendineas (Plate XXVIII. fig. 33 r s) 

 being, as I have shown, attached to the backs or more external surfaces of the segments, an arrange- 

 ment which makes their rapid approximation towards the ventricular axis inevitable. 



