ACTION OF THE VALYES OP THE MAMMALIAN HEART. 193 



position. Hence, when even a slight engorgement of one side of the heart occurs, we may 

 have an incomplete occlusion and regurgitation. This state of things is well seen in 

 the heart of a rabbit which is much engorged. Marked regurgitation into the right 

 auricle occurred when the ventricles were dipped in the boiling solution. In all my 

 experiments with engorged hearts, this regurgitation occurred very much more readily 

 on the right side. The safety-valve action of the tricuspid is to be explained in this 

 way. 



Again, it is often difficult to explain on the usual theory of occlusion how organic 

 lesions so modify the action of the auriculo- ventricular valves as to allow of regurgita- 

 tion. In these cases, if water be injected from the aorta, the mitral is floated up and 

 seems to act satisfactorily. And yet during life, regurgitation occurred. A roughening 

 or crumpling which would not be sufficient to prevent the adaptation of the segments 

 to one another in the horizontal position, might be sufficient to prevent their close 

 adaptation, face to face, and might thus allow of a back flow through the valve. 



Aortic and Pulmonary Valves. 



Support of Valves. 



In connection with these valves, an extremely interesting mechanism is to be observed, 

 whereby the cusps are protected from and supported against the great strain of the 

 arterial pressure. 



Aortic Valve. — An examination of antero-posterior sections (figs. 8 and 9), and 

 of preparations of the base of the heart (fig. 4), shows that the anterior cusp of the 

 aortic valve is placed upon the top of a muscular cushion formed by the upper part 

 of the septum ventriculi. Upon this cushion the blood filling the Sinus of Valsalva 

 will rest. Now Pettigrew (Transactions of the Royal Society, 1864) has shown by a 

 series of casts in plaster of Paris that this cusp closes before the other two, which, to use 

 his expression, are twisted down upon it. Thus the muscular cushion supporting 

 the pressure in the anterior sinus will also support the pressure in the sinuses of 

 the two posterior cusps, and will thus diminish the strain put upon the cusp of the 

 valve. 



Pulmonary Valve. — Though not so well marked, a similar cushion arrangement is to 

 be found in connection with the pulmonary valve when the postero-sinistral cusp of the 

 valve is set upon the upper part of the septum ventriculi, which forms a cushion 

 underneath it. The two other cusps are at a somewhat higher level and will rest upon 

 the first, thus participating in the support of the septum. 



I was for long unable to find any reference to this mechanism, but Sir William 

 Turner referred me to a paper by Savory, Lancet, vol. ii., 1854, in which this 

 muscular cushion is clearly described and figured. 



