THE CAUSATION OF THE HEART BEAT 



1003 



fibres composing its wall, so we arrive at a statement similar to that made 

 previously for voluntary muscles, namely, that the energy of contraction is 

 a function of the length of the muscle fibres, i. e. to the extent of active surface 

 involved. This reaction of the heart to increasing distension has long been 

 known but was ascribed to the excitatory influence of tension on the muscle 

 fibres. It is evident that in a resting heart increasing distension of its 

 cavities will tend to stretch its muscle fibres and therefore to exert a 



FIG. 448. 



The 



Effect of alterations in venous supply on volume of heart. Heart, 67 gms. 



Arterial Venous Output of heart 



pressure pressure in 10 sees. 



A 124 == 95 = 86 



130 

 124 



145 

 55 



140 

 33 



e curved line at the side represents the value of the cardiometer excursions 

 in capacity of ventricles in, c.c. 



tension on them. By an accurate record of the pressure changes within 

 the contracting ventricle under varying conditions, it is possible to exclude 

 the tension on the fibres as the determining factor. In a heart beating 

 regularly the inflow of blood is proceeding during diastole, during the relaxa- 

 tion of the ventricles, i. e. the muscles are giving before the inflowing blood. 

 The latter is therefore able to distend the heart without exercising more than 

 a minimum pressure on its walls, and it is found that the pressure in the ven- 

 tricles may be approximately zero at the end of diastole whether the heart 

 is contracting against a resistance of 80 mm. Hg. or a resistance of 120 mm. 

 Hg., or whether it is receiving 5 c.c. or 10 c.c. during the period of diastole. 



