142 THE MECHANISM OF THE VALVES. [BOOK i. 



that it lasts for a comparatively long time; in the figure this 

 part of the curve embraces more than four-tenths of a second. These 



[features, the sudden rise, the long duration, and the rapid fall 

 of the pressure exerted by the ventricle are seen in all tracings of 

 the ventricles engaged in a cardiac beat whatever be the method 

 employed. They mean of course that the muscular contractions 

 which constitute the ventricular systole come on suddenly, that 

 they last altogether a considerable time, and that relaxation is 

 also rapid. With the end of the ventricular systole the cycle 

 represented in figure ends, and a new cycle begins, repeating 

 the same changes. The meaning of the features on the curves 

 marked e and d, &c., as well as a more complete discussion of 

 the changes thus briefly described, we must defer till we have 

 spoken of 



The Mechanism of the Valves. 



The auriculo-ventricular valves present no difficulty. 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 

 valve (whether tricuspid or mitral). As the pressure of the 

 auricular systole diminishes, the same reflux current floats the 

 flaps up, until at or immediately after the close of the systole they 

 meet, and thus the orifice is at once and firmly closed, at the very 

 beginning of the ventricular beat. The increasing intraventricular 

 pressure serves only to render the valve more and more tense, and 

 in consequence more secure, the chordae tendineae (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 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. Since the same papillary muscle is in 

 many cases connected by chordae with the adjacent edges of two 

 flaps, its contraction also serves to keep these flaps in more 

 complete apposition. Moreover the extreme borders of the valves, 

 outside the attachments of the chordae, are excessively 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. 



