LABORATORY MANUAL OF PHYSIOLOGY. 



indicated in the figure. The auricles will again begin to beat, but 

 at a different rate from that of the ventricle. 



Tie a fourth ligature about the base of the ventricle as indicated 

 by line 4, Fig. 30. The ventricle will again cease to beat. 



XII. MAXIMAL RESPONSE OF HEART MUSCLE TO MINIMAL 



STIMULUS. 



i . Stop the rhythmical contraction of a frog's heart by applying 

 the first Stannius ligature. 



Set up inductorium for single induction shocks. Connect tip of 

 ventricle with the cardiograph lever. Arrange the drum for move- 

 ment by hand. With the 

 secondary coil removed as 

 far as possible from the pri- 

 mary, apply the electrodes 

 to the ventricle and break 

 the primary circuit. No con- 

 traction will probably occur. 

 Move the secondary nearer 

 the primary and repeat the 

 breaking of the circuit, at 

 intervals of ten seconds, un- 

 til a stimulus is found which 

 will cause the ventricle to 

 contract. 



Move the drum slightly, 

 increase the strength of stim- 

 ulus, and record again. 

 Repeat with stronger and 

 stronger stimuli. 



The contraction in re- 

 sponse to the strongest stimulus is no greater than the one in 

 response to the weakest stimulus that will cause a contraction. 

 The heart muscle, therefore, responds to a minimal stimulus by 

 a maximal contraction. 



[96] 



FIG. 30. Schematic Frog's Heart, to Show 

 Application of the Stannius Ligatures, i, 

 Between sinus (S) and auricles G4); 2, 

 middle of auricles; 3, between auricle (A) 

 and ventricle (V), at auriculo-ventricular 

 groove; 4, about base of ventricle, below 

 groove. 



