THE CIRCULATION OF THE BLOOD AND LYMPH 87 



the curves belong to the type of the peak or of the plateau, 

 no sudden change of curvature, no nick, or crease, or undu- 

 lation reveals the moment of opening or closure of any 

 valve. But by experimentally graduating a pair of elastic 

 manometers, and obtaining with them simultaneous records 

 of the pressure in auricle and ventricle, we can calculate at 

 what points of the ventricular curve the pressure is just 

 greater than and just less than the pressure in the auricle. 

 The first point, it is evident, will correspond to the instant at 

 ,vhich the mitral or tricuspid valve, as the case may be, is 

 closed, and the second to the instant at which it is opened. 

 And in like manner, by comparing the pressure-curve of the 

 aorta with that of the left ventricle, the moment of opening 

 and closure of the semilunar valves may be determined 

 (Figs. 23 and 24). According to the best observations, the 

 closure of the semilunar valves takes place at a time corre- 

 sponding to a point on the upper portion of the descending 

 limb of the intraventricular curve. 



The study of the curves of endocardiac pressure enables 

 us to add precision in certain points to the description of 

 the events of the cardiac cycle which we have already given, 

 and, as regards the ventricles, to divide the cycle into four 

 periods : 



(1) A period, during which the pressure is lower in the ventricles 

 than either in the auricles or the arteries, and the auricula- 

 ventricular valves are consequently open, and the semilunar valves 

 closed. This is the period of ( filling ' of the heart. 



(2) A period, beginning with the ventricular systole, during 

 which the pressure is rising abruptly in the ventricles, while they 

 are as yet completely cut off from the auricles on the one hand 

 and the arteries on the other by the closure of both sets of valves. 

 This is the period during which the ventricles are, to use a homely 

 but expressive phrase, 'getting up steam.' 



(3) A period during which the pressure in the ventricles overtops 

 that in the arteries, and the semilunar valves are open, while the 

 auriculo -ventricular valves remain shut. This is the period of 

 1 discharge. 1 



(4) A period during which the pressure in the ventricles is again 

 less than the arterial, while it still exceeds the auricular pressure. 



