560 THE OUTGO OF ENERGY 



writing point record exactly above the point of 

 the electromagnetic signal on a drum revolving so 

 slowly that the individual beats shall appear in 

 the curve very close together and yet far enough 

 apart to be readily counted. 



Lay the vagus nerve on the electrodes. Start 

 the drum. As soon as good curves are writing, 

 start the inductorium, and open the short-circuit- 

 ing key for about twenty seconds. The heart will 

 be inhibited. Note that the arrested heart is al- 

 ways relaxed, i. e. in diastole. The latent period 

 is short (one or two heart-beats). A brief after- 

 effect is present. If the stimulus is continued, 

 the heart will begin to beat even during the 

 stimulation, showing that the inhibitory mechan- 

 ism can be exhausted. The heart beats more 

 rapidly, and usually more strongly, immediately 

 after inhibition than before ; this probably is due 

 to the after-effect of the stimulation of augmentor 

 fibres in the vagus trunk, as explained below. 



Eepeat the stimulation, but weaken the stimu- 

 lating current by moving the secondary farther 

 from the primary coil. 



With a suitable strength of current, the heart 

 will be slowed but not arrested. The duration 

 of diastole will be markedly less; while the dura- 

 tion of systole will be changed but little if at 

 all. A stronger excitation would lengthen both 



