90 A MANUAL OF PHYSIOLOGY 



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 auriculo-ven- 

 tricular 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 be- 

 ginning 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 

 1 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 dependent 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 (presphygmic 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. 



The fluctuations of pressure in the auricles, although confined 

 within narrower limits than in the ventricles, are of equal 

 interest. They have been studied of late years in considerable 

 detail both in animals and by indirect methods in man. No 

 fewer than three distinct elevations or ' positive waves,' separated 

 or followed by three depressions or ' negative waves,' have been 

 described on the curve of intra-auricular pressure (Fig. 24) . The 



