INNERVATION OF HEART AND BLOOD-VESSELS 557 



magnetic signal. Prepare the sympathetic as 

 directed above. Expose the heart (page 75). 

 Place it in the heart-holder. Should the heart 

 beat rapidly, slow it with ice. Let the writing 

 point record above the point of the electromag- 

 netic signal on a drum revolving so slowly that 

 the individual beats shall appear in the curve 

 very close together, yet far enough apart to be 

 readily counted. Divide the observation into 

 nine periods of twenty seconds each. Place the 

 electrodes beneath the sympathetic, with the 

 short-circuiting key closed. Adjust the heart 

 lever to write its curve. Let the assistant call 

 the, beginning of each period as he marks it on 

 the drum. At the beginning of the second pe- 

 riod, open the short-circuiting key ; at the begin- 

 ning of the third period, close the short-circuiting 

 key. Lower the drum when one circuit is 

 completed. 



Count the number of beats in each period. The 

 frequency will be increased. The force of con- 

 traction will also be increased. 1 The latent period 

 of excitation is long and there is a prolonged 

 after-effect. The former frequency is regained 

 more rapidly after short than after long stimula- 

 tions. The speed of the cardiac excitation wave 



1 The stimulation of the augmentor fibres is difficult and 

 often fails in winter frogs. 



