REPEATED STIMULATIONS OF THE HEART 111 



unable to respond to a stimulus. That time during which a stimulus 

 produces no response in a muscle fibre is termed its refractory 

 period. We see, then, that for the heart the refractory period lasts 

 through the whole of the period of contraction. In contrast with 

 this we have previously found that the refractory period for skeletal 

 muscle was practically identical with its latent period. 



Experiment 7. — Upon the same heart study the effect of a rapidly 

 repeated series of stimuli. Arrange for the recording surface to move 

 very slowly. Set up the coil for giving tetanising shocks, and then stimu- 

 late the heart, varying the strength of stimulus employed. 



In fig. 92 are shown three curves obtained in this way with 

 different strengths of stimulus. In i the stimulus was strongest, and 

 we see that during the whole period of stimulation, lasting during 

 the interval between the two vertical marks, the ventricle never com- 

 pletely relaxed. The important result, however, is that there is no 

 complete fusion. A heart cannot be sent into complete tetanus. At 

 first the amount of fusion was marked, but as the stimulus proceeded 

 fusion became gradually less, and a rhythmic contraction, which 

 was not very regular, was produced. Putting it in another way, 

 we may say that as tetanisation proceeded the refractory period 

 tended to increase. Note, moreover, that a contraction occurred 

 after stimulation had ceased, showing that the effect of the stimuli 

 did not stop immediately stimulation ceased. Tracing n was taken 

 from the same heart, but the secondary coil was removed 2 cm. 

 further from the primary. With this weaker stimulation there is no 

 summation of effect, except in the case of the first two beats. A 

 rhythmic beating was started which ceased on the stimulation being 

 stopped. In the third curve (in) the stimulus was still weaker, and 

 only two contractions were produced at the commencement and 

 none during the rest of the time of stimulation. 



A single break shock with the secondary coil at 13 cm. produced 

 no response, whereas at 12 cm., the position during tracing n, a con- 

 traction was produced. 



