184 THE CARDIAC CYCLE. [BOOK i. 



tend, while bringing the edges of the flaps together, to bulge the 

 whole valve upwards towards the auricle, but that, later, when the 

 papillary muscles contract, these pull the valve in a funnel shape 

 down into the ventricle with the edges of the flaps in complete 

 apposition. On the one view, the papillary muscles serve merely to 

 secure the adequate closure of the valve ; on the other view, they 

 add to the pressure exerted by the ventricular wall, by pulling 

 the already closed valve down on the ventricular contents, or, 

 according to an old opinion, obviate, by their shortening, the 

 slackening of the chordae which might result from the shortening 

 of ventricle during the systole. Whichever view be taken, it may 

 be worth while to remark that the borders of the valves 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. 



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 

 ventricle continues to press with greater and greater force on its 

 contents, making the ventricle hard and tense to the touch, the 

 pressure within the ventricle becomes at length greater than that 

 in the pulmonary artery, and this greater pressure forces open the 

 semilunar valves, and allows the escape of the contents into the 

 artery. The ventricular systole may be seen and felt in the 

 exposed heart to be of some duration ; it is strong enough and long 

 enough to empty the ventricle more or less completely, indeed, in 

 some cases, it may last longer than the discharge of blood, so that 

 there is then a brief period during which the ventricle is empty 

 but yet contracted. 



During the ventricular systole the semilunar valves are pressed 

 outwards towards but not close to the arterial walls, reflux currents 

 probably keeping them in an intermediate position, so that their 

 orifice forms an equilateral triangle with curved sides ; they 

 offer little obstacle to the escape of blood from the cavity of the 

 ventricle. The exact mode and time of closure of the semilunar 

 valves is a matter which has been and, indeed, is still disputed, 

 and which we shall have to discuss in some detail later on. 

 Meanwhile it will be sufficient to say, after the blood has ceased 

 to flow from the ventricle into the aorta, whether this be due to 

 the cessation of the ventricular systole, or to the whole of the 

 ventricular contents having been already discharged, a reflux of 

 blood in the aorta towards the ventricle at once completely fills 

 and renders tense the pockets, causing their free margins to conn' 

 into close and firm contact, and thus entirely blocks the way. 

 The corpora Arantii meet in the centre, and the thin, membranous 

 festoons or lunulae are brought into exact apposition. As in the 



