146 



Scientific Proceedings (113). 



right ventricle is at once elevated. As the heart continues to 

 dilate during diastole, this elevation of pressure increases more 

 and more. After the second beat the pressure-maximum is also 

 increased in the right ventricle. Two to three beats are usually 

 required before the initial and maximum pressures in the left 

 ventricle are similarly altered. Systolic discharge and the intraven- 

 tricular pressure-maximum continue to increase only so long as 

 initial tension also continues to increase. In these cases, increased 

 diastolic distention is, therefore, never dissociated from increased 

 initial tension. Tonus changes may simultaneously operate to 

 lengthen the muscle fibers independent of initial tension but, if so, 

 their effects are entirely obscured. If this condition of increased 

 inflow persists in a stationary manner, for a matter of 15 to 20 

 minutes, however, it may happen that then the heart dilates 

 further, even while the initial pressures in the right and left 

 ventricles decline. Such a dilatation, evidently due to a decrease 

 in tonus, is always accompanied by a reduction in systolic discharge 

 and in the pressure-maximum in both ventricles. 



2. If arterial resistance is suddenly elevated during partial 

 compression of the thoracic aorta, the systolic discharge is de- 

 creased for a few beats (usually 2-3) resulting, as also shown in 

 Patterson, Piper and Starling's results, in a diastolic distention 

 and increased initial length. Systolic discharge returns to normal 

 about the fourth or fifth beat. Careful study shows that the 

 pressure-maximum is elevated at once in the left ventricle, and 

 by the third beat the initial pressure is also measurably although 

 but slightly increased. At the fourth or fifth beat where the 

 systolic discharge returns to normal there is a significant increase 

 in the initial pressure in the left ventricle also. Then, for the 

 first time, initial tension and pressure-maximum in the right 

 ventricle also increase. 



These results favor the conclusions, (1) that initial tension 

 changes are apparently always associated with changes in initial 

 length resulting from alterations in venous inflow or arterial 

 resistance; (2) that in the intact animal, changes in initial tension 

 play the predominant role in determining the response of the 

 mammalian ventricle. 



