148 SPECIAL CARDIAC PHASES. [BOOK i. 



.period in this ease is only about 3-50ths of a sec. The period from c to d is that 

 during which the ventricle having grasped its contents is emptying its cavity and 

 remaining contracted. It can be seen that only during the first half of this period 

 is there any marked descent of the lever point ; in other words the antero-posterior 

 diameter does not continue to diminish during the whole period of the systole, 

 indicating that little or no blood was thrown out during the second half of this 

 period, the ventricle remaining simply contracted after having emptied its cavity. 

 The period from d to a is that during which the ventricular muscle is relaxing. 

 Here, as is frequently the case, there is no period of pause between the close of 

 the relaxation of the ventricle and the commencement of the succeeding distension. 

 The tracing gives no evidence as to the time of closure of the semilunar valves. 



The chief interest and the chief difficulties are attached to the* f 

 systole of the ventricles. In order to understand this, the most 

 important of the cardiac events, it must be borne in mind that, 

 as we have already seen, the pressure of the blood in the aorta 

 is always considerable. This pressure closes and keeps closed the 

 semilunar valves ; and it is not till the pressure in the ventricle 

 becomes greater than the pressure in the aorta that these valves 

 open to allow of the escape of the ventricular contents. The blood 

 therefore does not begin to pass from the left ventricle into the 

 aorta until some time, and that a variable time, after the commence- 

 ment of the systole of the ventricle ; and the same may be said of 

 the right ventricle and pulmonary artery, it being understood that 

 the arterial pressure on the right side is less than on the left. In 

 Fig. 24 the ventricular lever reaches its maximum c at once, 

 gradually declining afterwards till the more sudden fall begins, 

 and we may suppose that the escape of blood from the ventricle 

 begins at the moment when the maximum is attained; and this 

 view is confirmed by carefully comparing a tracing of the expansion 

 of an artery with the cardiac tracing. It is quite possible however 

 to conceive that owing to circumstances, such as an increasing con- 

 traction of the ventricular fibres or deficient expansion of the 

 arteries, the pressure might continue to increase even after blood 

 was escaping from the cavity of the ventricle. And indeed in some 

 curves, the ventricular lever after the first sudden leap continues 

 to rise gradually and does not reach the maximum point until 

 afterwards. In such cases the summit of the first rise must be 

 taken as marking the beginning of the flow from the ventricle. 



By the sudden systole the blood is ejected with considerable 

 force and rapidity from the ventricle, and as the ventricle becomes 

 empty a negative pressure, as we have seen, makes its appearance 

 behind the column of blood which leaves the cavity and leads to 

 the closure of the semilunar valves. Much dispute has taken place 

 as to the exact condition of the ventricle at the moment of closure 

 of the semilunar valves. The slight rise e in Chauveau and 

 Marey's curves (Fig. 21) in the ventricular curve, seen also in the 

 auricle at e and in the cardiac impulse at e", and which has been 

 taken to indicate the shutting of the semilunar valves, appears quite 

 at the close of the descent of the ventricular lever. This would 

 mean that at the moment of the closure of the valves the ventricle 



