THE HEART BEAT. 551 



opening of the auriculoventricular valves, the ventricles are 

 dilated rapidly by the inflow of venous blood. Henderson has 

 emphasized the fact that the filling takes place nearly as rapidly 

 as the emptying, owing doubtless to the fact that at the end of 

 ventricular systole the auricles are dilated under some pressure, 

 so that their contents escape at once into the ventricles as soon 

 as the intervening valves are opened. The diastolic curve comes 

 back nearly to the base line and then forms a shoulder (4) from 

 which it approaches gradually to the base line up to the moment of 

 auricular contraction (5). The period of gradual dilation of the 

 nearly filled ventricles, which on the curve is shown from 4 to 5, is 

 called the period of diastasis by Henderson. The heart cycle, so far 

 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 



