548 



CIRCULATION OF BLOOD AND LYMPH. 



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

 1, Systole; 2, diastole; 3, diastasis or pause. 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 prac- 

 tically completed before the auricles contract. The auricular con- 

 traction, besides initiating the ventricular systole, doubtless serves, 

 by raising the tension in the ventricular chamber, to bring the 

 auriculoventricular valves more completely into the position of 



Fig. 234. — Diagram of tlie normal volume curve (plethysmogram) of the dog's heart 

 when beatmg at a slow rate (after Hirschf elder). The up-stroke represents the systole, 

 the down-stroke the diastole; 4 to 5 the period of diastasis {Henderson). At 5 the auricular 

 contraction causes a slight additional dilatation of the ventricle. 1, 2, and 3 represent the 

 time of occurrence of the first, second, and third heart-sounds respectively. 



closure. 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 sys- 

 tole 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, the arterial resistance, the state of the 

 musculature, and other conditions. 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 l^e diminished, with the 

 result that the blood backs up in the left auricle, thus raising the 

 venous pressure in the lungs and retarding the puhnonary circula- 

 tion. On the other hand, the outflow from the ventricle is influenced 

 and, indeed, mainly controlled by the inflow into the auricle from 

 the veins. Variations in the size of the blood-vessels, such as 



