BY DR. MICHAEL FOSTER. 393 



Send a single weak induction shock through each pair of 

 electrodes, and record the contraction ; or determine the 

 minimum stimulus for each pair of electrodes. 



Repeat the observation at intervals during the day. 



It will be found that after the temporary increase due to 

 section, the irritability gradually diminishes from the central 

 cut end towards the peripheiy, the extreme muscular branches 

 being the last to die. 



Be careful that no part of the nerve is more exposed than 

 others. 



Obs. XVIII. Repeat the observation in a frog whose brain 

 and spinal cord have been destroyed, but the blood current 

 not interfered with. 



The irritability will disappear much more slowly, but in the 

 same centrifugal manner. 



CHAPTER XXIX. 

 PHENOMENA ACCOMPANYING A NERVOUS IMPULSE. 



THE only phenomenon definitely and certainly known to 

 accompany the passage of a nervous impulse is the negative 

 variation of the nerve current (see Chap. XXVI., sec. II.). 



In the case of muscle, the negative variation shown in teta- 

 nus by the galvanometer was proved by the rheoscopic frog to 

 consist of a series of successive negative variations (see Chap. 

 XXIV., sec. III.). 



At first sight a similar proof seems to be afforded by the be- 

 havior of nerves. 



Obs. I. Prepare a nerve-muscle, and also a separate piece of 

 nerve as long as possible. Place the nerve-muscle B (fig. 291) 

 on a glass plate ; place the nerve A over the nerve of B, in either 

 of the positions shown in fig. 291, 1. II. ; connect the end of A 

 with an induction coil. 



A single shock sent through A will produce a contraction in 

 B; an interrupted current will throw B into tetanus. 



Obs. II. Ligature A between the electrodes and the end touch- 

 ing B. No contractions will appear in B on sending shocks 

 through the electrodes. This proves that the results of Obs. 

 I. were not due to any simple electrical conduction through A 

 or to any escape of the current to B by other means. 



The same thing is shown in the so-called "paradoxical con- 

 traction." 



06s. III. In the leg of a frog, the sciatic nerve divides at the 



