OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 793 



the chordae tendineiie into the ventricular cavities to form two dependent cones ; 

 this downward movement of the segments permitting the blood in the auricles to 

 descend into the ventricles, so as to relieve the congestion of the former. 



Granting that the foregoing hypothesis is correct, there is yet another point 

 as to the manner of the closure, to which I am particularly anxious to direct at- 

 tention, as it is of primary importance, and appears to me, by some unaccountable 

 means, to have hitherto escaped observation. I refer to the spiral form assumed 

 by the blood in the ventricular cavities, which, 'as has been already partially ex- 

 plained, causes it, towards the end of the diastole and the beginning of the systole, 

 to act in spiral waves mechanically (Plate XXIX. fig. 49 / q) on the segments, with 

 the effect of twisting and wedging them into each other in a sjnral upward direction 

 (Plate XXIX. fig 51 m n, and figs. 53 and 54 m i n, rs). I allude, also, to the spiral 

 course pursued by the musculi papillares (Plate XXIX. figs. 49 and 50 X2/) ; these 

 structures, as the systole advances, contracting in such a manner as occasions the 

 spiral descent of the segments into the ventricular cavities (Plate XXIX. figs. 52 

 and 55 mn, rs), to form two s^nral dependent co7ies, the apices of which are directed 

 towards the apices of the ventricles. As the decrease of the blood in the ventricles 

 is followed, as has been stated, by a corresponding increase in the auricles ; the 

 blood in the latter assists in keeping the free margins and apices of the segments 

 from being everted by the uniform pressure exercised on them by the blood in 

 the former, during the systole. From this account of the closure of the auriculo- 

 ventricular valves, it will be perceived that the valvular segments form two 

 moveable partitions or septa, which rise and fall during the action of the heart, 

 in the same way that the diaphragm rises and falls during the respiratory efforts. 

 The advantages arising from such an arrangement are very great. When the ven- 

 tricles are full of blood, and the auricles empty or comparatively so, the valvular septa 

 are convex towards the base of the heart, and protrude into the auricular cavities. 

 When, however, the aiiincles are fidl of Mood, and the ventricles all but drained of 

 it, the valvular septa descend so as to protrude in a downward or opposite direc- 

 tion. Certain portions, therefore, of the auriculo-ventricular cavities are common 

 alike to the auricles and to the ventricles ; and it is important to note this fact, as 

 the valvular septa by their rising and falling, at one time increase the size of the 

 ventricular cavities while they diminish the auricular ones, and vice versa. The 

 principal object gained by the descent of the segments into the ventricles is 

 the diminution of the ventricular cavities towards the base ; the dependent cones 

 formed by the valves fitting accurately into the conical-shaped interspaces situated 

 l)etween the slanting heads of the musculi papillares and the auriculo-ven- 

 tricular tendinous rings. As the musculi papillares, on the contraction of the 

 ventricles, mutually embrace and twine round each other, the obliteration of 

 the ventricular cavities is by this arrangement rendered very complete. " That 

 the ventricles empty themselves during the systole, is rendered probable 



VOL. XXIII. PART III. 10 c 



