274 



Experiments upon the Heart of the Dog. [May 31, 



the experiments, it was found that the outflow from the left ventricle, 

 and consequently the work done by it, increases with the venous 

 pressure, but not proportionally, up to the point of maximum work. 

 It is certain" that the most direct factor influencing the quantity of 

 blood sent out from the ventricle is the intra-ventricular pressure by 

 which it is distended during diastole. Leaving out the aspirating 

 action of the thorax, the intra-ventricular pressure during life must 

 be mainly owing to the action of the auricles, since the pressure in the 

 great vein emptying into the auricles probably never rises to any 

 important positive value, indeed, according to the experiments of 

 most observers, has a mean negative value. The contraction of the 

 auricles then must have the most important and direct effect upon the 

 work done by the ventricles. 



IV. Influence of Bate of Beat on the Work done hy the Heart. 



The rate of beat of the heart was varied in these experiments by 

 heating or cooling the blood supplied to it. In this way the pulse- 

 rate was changed in one case from 228 to 77 beats in a minute, and 

 back again to 140 in a minute, in another case from 204 to 65'5, and 

 back again to 157, and so on. The general result may be stated as 

 follows : — A diminution of pulse-rate, brought about by lowering the 

 temperature of the blood flowing into the heart, causes an increase 

 in the quantity of blood thrown out from the ventricle at each systole, 

 and consequently an increase in the work done at each systole, and 

 vice versa. The changes in the outflow from the ventricle at each 

 systole are not, however, inversely proportional to the changes in the 

 pulse-rate. The total outflow and the total work done during any 

 given period of time decreases with a diminished pulse-rate, and 

 increases with an increased pulse-rate. 



