230 THE CARDIAC CYCLE. [BOOK i. 



the foetus it had an important function in directing the blood of 

 the inferior vena cava through the foramen ovale into the left 

 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 reflux 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. According to some authors however the 

 closure of the valve is effected, at the very beginning of the ven- 

 tricular systole, by the contraction of the papillary muscles ; the 

 chordae tendineae of a papillary muscle are attached to the adjacent 

 edges of two flaps, so that the shortening of the muscle tends to 

 bring these edges into apposition. 



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 only effect of this increasing intra-ventricular 

 pressure upon the valve is to render the valve more and more 

 tense, and in consequence more secure, the chordae tendiiiese (the 

 slackening of which through the change of form of the ventricle is 

 probably obviated by a regulative contraction of the papillary 

 muscles) at the same time preventing the valve from being inverted 

 or even bulging largely into the auricle, and indeed, according to 

 some observers, keeping the valvular sheet actually convex to the 

 ventricular cavity, by which means the complete emptying of the 

 ventricle is more fully effected. The connection, to which we 

 have just referred, of the chordae of the same papillary muscle 

 with the adjacent edges of two flaps, also assists in keeping the 

 flaps in more complete apposition. Moreover the extreme borders 

 of the valves, outside the attachments of the chordae, are exces- 

 sively thin, so that when the valve is closed, these thin portions 

 are pressed flat together back to back ; hence while the tougher 

 central parts of the valves bear the force of the ventricular systole, 

 the opposed thin membranous edges, pressed together by the blood, 

 more completely secure the closure of the orifice. 



At the commencement of the ventricular systole the semilunar 

 valves of the pulmonary artery are closed, and are kept closed by 

 the high pressure of the blood in the artery. As however the 



