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EXPERIMENTAL PHYSIOLOGY 



Arrange the heart to record as before, and place the nerve upon a pair of elec- 

 trodes. Carry out the experiment upon a similar plan to that used for the 

 vagus, and so obtain a series of tracings showing the effect upon the heart 

 with different strengths of stimulation. If the heart be beating rapidly the 



Fig. 101. — Tracing i Shows the Effect of Tetanisation of the E. Vagus ; 

 ii of the L. Vagus ; and ni of the Crescent. In all Cases the Same 

 Strength of Stimulus. 



influence of the stimulation is not very marked, in which case the heart 

 should be cooled by allowing a little normal saline, which has been cooled by 

 ice, to drop on to it until its rate is slowed. 



Fig. 102 shows a typical result of such an experiment : the upper 

 two tracings are from the same heart with different strengths of cur- 

 rent, the third from a different heart. The first shows that during 

 stimulation the following changes occur : — 



i. Acceleration of the beat. Before stimulation this was 15 per 

 minute, during stimulation 30 per minute. After stimulation ceased 

 the heart gradually slowed again until its initial rate was once more 

 reached. 



