960 



PHYSIOLOGY 



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 tension 



VP 



FIG. 439. Effect of alterations in venous supply on volume of heart. Heart, 67 gms. 



Arterial 

 pressure 



124 



130 



124 



Venous 

 pressure 



95 

 145 



55 



Output of heart 

 in 10 sees. 

 86 

 140 

 33 



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

 in capacity of ventricles in c.c. 



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

 tracting ventricle under varying conditions it is possible however 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 minimal 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. 



