BY DR. MICHAEL FOSTER. 381 



CHAPTER XXVI. 

 ELECTRIC CURRENTS OF NERVES. 



I. Natural Currents. Obs. I. Bring the galvanometer 

 into as sensitive a condition as possible. The shunt will be 

 unnecessary in this case except to be used as a key. Prepare 

 as long a piece of nerve as possible with the least possible 

 injury. Hang the middle of the nerve over a bent non-polari- 

 zable electrode, and bring both ends to rest on the plug of 

 another electrode, as represented in fig. 289. In this way, 

 one electrode will be in contact with the equator, and the other 

 with both transverse sections. The current from the equator 

 to each transverse section being the same in direction, the re- 

 sult of this arrangement will be to double the effect on the 

 needle. 



The current in the nerve, far feebler than thai in muscle, is 

 as in the muscle from the equator (or mid-longitudinal point) 

 to the transverse section. 



Obs. II. By doubling a long piece of nerve and laying 

 different points on the electrodes, it may be determined that 

 the arrangements of the currents are the same in one case 

 as in the other. Naturally, the various points in the minute 

 transverse section cannot be examined. 



II. Negative Variation in Nerve. Obs. III. Prepare 

 as long a piece of nerve as possible ; lay the transverse section 

 of the central end and a point in the longitudinal suri'ace at 

 some little distance from that end on the pair of galvanometer 

 electrodes. Lay any two points at the other end (peripheral) 

 of the nerve on a pair of exciting electrodes connected with an 

 interrupted current. 



Determine the amount of deflection given by the natural 

 current. Send an interrupted current through the exciting 

 pair. There should be a slight but distinct diminution, a slight 

 negative variation, of the current. When a nerve is excited, 

 the natural current suffers a negative variation. 



Obs. IV. Repeat the observation, placing the peripheral end 

 of the nerve on the galvanometer electrodes, and the central on 

 the exciting electrodes. 



There will be, as before, a diminution, a negative variation 

 of the current. 



