MECHANICS OF THE HEART-BEAT 97 



by the pulse-wave in travelling from the aorta to the carotid. The 

 S.C. point, the A.O. point, or moment of opening of the auriculo- 

 ventricular valves, and the leginning of the ventricular systole, are 

 three important points of reference in the measurement and inter- 

 pretation of pulse-tracings in clinical work. The A.O. point in man 

 may be taken as a point ' 0-03 second in advance of the summit of 

 the dicrotic wave ' on the carotid pulse-tracing (Lewis). But this is 

 the most difficult of the three standard points to determine clinically 

 with anything like accuracy. 



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, or the pause. 



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

 pressure is increasing 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 of 

 ' rising pressure,' during which the ventricles are, so to say, ' getting up 

 steam.' The interval between the beginning of the ventricular systole 

 and the opening of the semilunar valves is termed the ' presphygmic ' 

 interval. 



(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 ' discharge ' or 

 ' sphygmic ' period. 



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

 than the arterial, while it still exceeds the auricular pressure, and both 

 sets of valves are closed. This is the period of rapid relaxation. The 

 interval between the closure of the semilunar and the opening of the 

 auriculo-ventricular valves is sometimes called the ' post-sphygmic ' 

 interval. 



Of the four periods, the second and fourth are exceedingly brief. 

 The third is relatively long and constant, being but slightly depen- 

 dent on either the pulse-rate or the pressure in the arteries. The 

 duration of the first period varies inversely as the frequency of the 

 heart ; with the ordinary pulse-rate it is the longest of all. 



From records taken in a person with a defect in the chest-wall which 

 rendered the heart accessible the following results were obtained as 

 to the duration of the various events of the cardiac cycle : First and 

 fourth periods together, 0-445; third period, 0-254; second period (pre- 

 sphygmic interval), 0-051 second, the pulse-rate being 80 a minute 

 (Tigerstedt). In another case with a similar defect the first period 

 lasted 0-32, the fourth period (post-sphygmic interval) 0-06, the second 

 and third periods together 0-4, and the auricular systole 0*1 second, 

 the pulse-rate being 66. 



