THE ELECTROMOTIVE PHENOMENA 317 



heart still resting caused a marked increase in 

 the demarcation current, in other words, a posi- 

 tive variation. No visible change in the form of 

 the heart was observed. 



Positive After Current. Compensate the de- 

 marcation current of nerve by the method de- 

 scribed on page 294. When compensation is 

 secured, note the position of the meniscus on the 

 scale, and tetanize the nerve. The meniscus will 

 be displaced by the current of action. Note the 

 direction of the current. Break the stimulating 

 current. The meniscus will return to and pass 

 the position which it held when the demarca- 

 tion current was compensated, showing thus a 

 current opposed in direction to the action 

 current. 



The positive after current is absent in weak- 

 ened or fatigued nerves. 



Contraction secured with a Weaker Stimulus 

 than Negative Variation. Place the non-polariz- 

 able electrodes on the longitudinal surface of the 

 nerve of a nerve-muscle preparation. Connect 

 them through the usual short-circuiting key 

 with the electrometer. Bring the meniscus into 

 the field. Arrange the inductorium for break 

 currents. Place the secondary coil some dis- 

 tance from the primary. Stimulate the nerve 

 in the extrapolar region. Approach the coils 



