* CHAP, iv.] THE VASCULAR MECHANISM. 233 



auricle. The valves in the coronary vein are however probably 

 of some use in preventing a reflux into that vessel. 



As the blood is being driven by the auricular systole into the 

 ventricle, a reflex current is probably set up, by which the blood, 

 passing along the sides of the ventricle, gets between them and 

 the flaps of the tricuspid valve and so tends to float these up. 

 It is further probable that the same reflux current, continuing 

 somewhat later than the flow into the ventricle, is sufficient 

 to bring the flaps into apposition, without any regurgitation into 

 the auricle, at the close of the auricular systole, before the ventri- 

 cular systole has begun. 



The auricular systole is as we have said immediately followed 

 by that of the ventricle. Whether the contraction of the ven- 

 tricular walls (which as we shall see is a simple though prolonged 

 contraction and not a tetanus) begins at one point and swiftly 

 travels over the rest of the fibres, or begins all over the ventricle 

 at once, is a question not at present definitely settled ; but in any 

 case the walls exert on the contents a pressure which is soon 

 brought to bear on the whole contents and very rapidly rises to a 

 maximum. The effect of this increasing intra- ventricular pressure 

 upon the valve is undoubtedly to render the valve more firmly 

 and securely closed; but the exact behaviour of the valve in 

 thus firmly closing is a matter on which observers are not agreed. 

 From the disposition of the flaps of the valve, and their relations 

 to the papillary muscles, the chords tendinese of a papillary 

 muscle being attached to the edges of and spreading over the 

 surfaces of two adjacent flaps, we may infer that when the 

 papillary muscles contract, taking their share in the whole ventri- 

 cular systole, they on the one hand bring at least the edges if not 

 part of the surfaces of adjacent flaps into opposition, and on the 

 other hand tend to pull down the whole of the valve, more or less 

 in the form of a narrow funnel into the cavity of the ventricle. If 

 we assume, as some observers do, that the papillary muscles begin 

 their contraction at the same time as the rest of the ventricular 

 wall, we may conclude that the valve is in this manner firmly 

 closed by their action at the very beginning of the systole. Other 

 observers find that a tracing, obtained by attaching a hook to the 

 apex of one of the flaps of the valve and connecting it with a 

 thread passing through the auriculo-ventricular orifice and the 

 auricle to a lever, indicates that the apex of the flap does not 

 begin to move downwards until some appreciable time after the 

 beginning of the systole. This they interpret as meaning that the 

 papillary muscles do not begin to contract until some time after 

 the ventricular wall has begun its contraction ; (and the tracing 

 in question similarly indicates that the papillary muscle ceases its 

 contraction before the ventricular wall does). If we assume this 

 interpretation of the tracing to be correct we must conclude that 

 at the first the pressure exerted by the commencing systole would 



