THE HEART BEAT. 555 



as the ventricles are concerned, falls, therefore, into three periods: 

 1, Systole; 2, diastole; 3, diastasis. Variations in heart rate affect 

 chiefly the last period; this becomes shorter and shorter the more 

 rapid the rate. When the heart rate is so rapid that the period 

 of diastasis drops out altogether and the systole begins as soon 

 as the diastole is complete, then we should have the maximum 

 output of blood per minute. An increase of rate beyond this 

 point would lead to a curtailment of the period of diastole and 

 eventually to a diminished output of blood per minute. Accord- 

 ing to the account just given, the filling of the ventricle is 

 practically completed before the auricles contract. Henderson 

 believes that the contraction of the auricles adds very little or 

 nothing to the change of blood in the ventricles, but other authors, 

 using the same methods, differ from him in this conclusion. 

 It is at least certain that the ventricles are for the most part 

 filled before the auricular contraction comes on this latter 

 act may add a greater or less amount to this charge, according 

 to the extent to which the inflow of venous blood during diastole 

 has filled the ventricle. The auricular contraction, besides initiat- 

 ing the ventricular systole, doubtless serves, by raising the tension 

 in the ventricular chamber, to bring the auriculoventricular valves 

 more completely into the position of closure. When these valves 

 are deficient, as in mitral stenosis, the contraction of the auricles 

 plays a larger part in completing the filling of the ventricles 

 (Hirschf elder) . For the cases in which it can be applied, the 

 volume curve enables us to estimate the ventricular discharge at 

 each beat and the outflow per minute. It was formerly assumed 

 that at each systole the ventricles emptied themselves completely, 

 but work of the kind described in this paragraph has shown, on the 

 contrary, that at the end of systole a considerable proportion of 

 the blood may be left in the cavity of the ventricle. The amount 

 thus left behind will vary with the rate and other conditions. 

 According to Henderson's figures for the dog, about one-third or 

 somewhat less of the ventricular charge is left in the heart after 

 systole, when the heart is beating at the normal rate (90), and 

 the quantity of blood discharged from the left ventricle at each 

 systole is approximately .002 of the body weight. It is evident 

 that when the aortic pressure rises to an abnormal level the 

 discharge of blood from the left ventricle will be or may be 

 diminished, with the result that the blood backs up in the left 

 auricle, thus raising the venous pressure in the lungs and retard- 

 ing the pulmonary circulation. On the other hand, as Hender- 

 son has especially emphasized, the outflow from the ventricle 

 must be influenced very directly by the inflow into the auricle 



