}<>80 SIXTH REPORT — 1836. 



incapable of drawing this flap from the adjoining surface ; and 

 were tlie s5Stole to commence while this portion of the valve 

 was applied to the side of the ventricle, the impulse of the 

 blood would be expended upon its auricular, instead of its ven- 

 tricular surface. In many quadrupeds, for instance, in the calf, 

 the papillary muscles are almost altogether wanting in both 

 sides of the heart : the tendons of the valves are inserted direct- 

 ly into the surfaces of the ventricles, and are so short that it is 

 manifestly impossible that the flaps can be laid against the 

 sides of those cavities when they are distended with blood. 



In the dead heart, placed in water, the valves do not hang 

 down in the fluid, but assume a cup-like form, and their free 

 edges are puckered together ; thus manifesting a disposition to 

 acquire, without the aid of the muscles, that figure and position 

 which are most favourable for receiving the impulse of the 

 blood. 



When the ventricular systole begins, the valves are closed by 

 the nmscular power of the ventricles transmitted to them 

 through the blood, and the papillarj^ muscles, commencing their 

 contraction at the same moment, are enabled to resist the im- 

 petus by which, but for their aid, the valves would be driven 

 unduly towards the auricles. The valvular flaps are now in 

 contact with each other by a portion of their auricular surfaces 

 adjacent to their free edges, and their form is curvilinear like 

 that of a sail distended by the wind ; a form of surface which, it 

 may be observed, enables the papillary muscles to resist the 

 impetus of the blood by a much less expenditure of their power 

 than had the valves been rendered tense in the first instance, 

 and drawn to a point by the action of these muscles alone. As 

 the systole proceeds, all the parts of the ventricles approach, 

 more or less, to the base ; and thus the distance which at the 

 beginning of the systole intervened between the auriculo-ven- 

 tricular openings and the more remote extremities of the papil- 

 lary muscles, is gradually abridged. The gradual contraction 

 of these muscles serves to compensate for the diminution of 

 this distance, and thus the valves are retained in an unaltered 

 position from the beginning till the end of the systole. 



This view of the purpose which the contractile power of the pa- 

 pillary .muscles is intended to fulfil, is strengthened by observing 

 that those papillary muscles are the longest which have their 

 origin from the substance of the ventricles most remote from 

 the base of the heart, and that they are found to be shorter in 

 proportion as their origins are nearer to that part ; and the ten- 

 dinous cords of the smaller flap of the tricuspid valve, which 



