70 THE HEART 



and systolic capacities of the ventricle, which, instead of 

 being 8 c.c. and c.c. respectively, are now 14 c.c, and 

 6 c.c. The increased work of the heart is associated with 

 increased distension at diastole and incoinjdete emptying at 

 systole. 



5. Changes in the Venous Inflow. — The result of changing 

 the venous inflow is seen in Fig. 16. It will be seen that 

 rise of venous pressure, like rise of arterial pressure, causes 

 a gradual distension and again no change in the rate of the 

 beat. The difference is that there is now an increase in 

 the output per beat, shown in the increased excursion of the 

 cardiometer. 



The factors which these two experiments have in common 

 are the increased work performed by the heart, and the 

 increased distension at diastole. How arc these factors 

 related ? The greater energy of contraction cannot be 

 due to the stretching of the fibres owing to increased 

 tension through abnormal filling, for no such increase in 

 tension exists. As the blood flows in, the ventricle wall 

 simply gives, the pressure at the end of diastole being 

 practically nil whatever the capacity of the chamber. 



In discussing skeletal muscle we have seen that the 

 energy of contraction varies directly with the initial length 

 of the fibre (p. 46). The same rule applies to the heart, 

 and is the cause of the phenomena we have been discussing. 

 This is called by Starling the Law of the Heart. " Within 

 physiological limits the larger the volume of the heart, the 

 greater are the energy of its contractions and the amount of 

 chemical change at each contraction.''' It must be remem- 

 bered, however, that with the heart in situ, the amount of 

 dilatation which it can undergo is limited by the inextensible 

 pericardium. 



The Influence of the Nervous System upon the Heart 



Having shown the power of adaptation possessed by the 

 isolated heart, we pass on to consider what further modifica- 



