OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 799 



first and second stages ; inasmuch as the chordae tendinese, on the contraction of 

 the muscnli papillares, drag the segments in a downward direction, to adapt them 

 to the altered conditions of the auriculo- ventricular orifice, and ventricular cavity. 

 That this downward movement, actually takes place, is proved as follows. If a 

 portion of the fluid be withdrawn by applying the mouth to the tube in the aorta, 

 so as to create a certain amount of suction, the segments of the bicuspid valve, are 

 found gradually to descend in a spiral direction (Plate XXIX. figs. 52 and 55 r s) ; 

 forming, as they do so, a spiral cone, whose apex becomes more and more defined 

 in proportion as the suction is increased ;' the water in the interior keeping the 

 margins of the segments, accurately in apposition, and thereby maintaining the 

 symmetry. If, again, the musculi papillares, be cut out of the ventricular walls 

 and made to act in the direction of their fibres, i. e., in a spiral direction from left 

 to right downwards ; they will be found, in virtue of being connected by the 

 chordae tendinese more or less diagonally to either segment of the bicuspid valve, 

 to act simultaneously on that side of the segment which is next to them; the 

 musculus papillaris marked « 5 in fig. 31, Plate XXVIII., acting spirally on the 

 margin and apex (x) of the larger or anterior segment, in the direction of the 

 arrow near it; that marked cd^ acting spirally on the margin and apex (v) of the 

 smaller or posterior segment, in a precisely opposite direction. (Compare direc- 

 tion of arrows marked a and d.) The efiect of these apparently incongruous 

 movements, on the segments, is very striking. 



The space which naturally exists between the segments (Plate XXYIII. fig. 

 31 D a;; fig. 28 m n), and which corresponds to the distance between the points 

 of origin of the two sets of chordae tendineae {a and J), is gradually but surely 

 diminished, and the segments twisted into or round each other. 



This arrangement, I may observe, while it facilitates the spiral movement 

 adverted to, absolutely forbids any other. 



It is rendered perfect by the pressure exercised on the delicate margins of 

 the segments by the spiral columns of blood as already explained. 



One complete closure of the mitral or bicuspid valve, may therefore be briefly 

 stated, as follows : — 



The segments, are first floated gently and gradually upwards, by the uniform 

 expansion of the blood forced into the left ventricle, during the diastole, by the 

 contraction of the left auricle. TJiis is a purely mechanical act^ and during its 

 performance the valve^ and chordce tendinew, are entirely passive. When, however, 

 the ventricle suddenly contracts, the margins of the valve, which were in 

 apposition, although not in actual contact, are rapidly approximated (the left 

 auriculo- ventricular opening being instantly closed*) ; and the two spiral columns 



* Regurgitation (as has been already stated) is prevented in the inverse of the rapidity with 

 which the closure takes place. 



