176 ELECTRO-PHYSIOLOGY CHAP. 



If different points of the nerve are excited from time to 

 time with constant distance of electrodes (about 1-2 cm.) by weak 

 currents, it is usually found that the opening stimulus acts sooner 

 in the part of the nerve corresponding with the plexus, than in 

 deeper parts. It is evident that the saline will take effect more 

 quickly at points where the bulk of fibres in the nerve are still 

 distributed into single bundles, than at a lower part where they 

 are joined into a single trunk, and this along with the greater 

 sensibility of central tracts of the nerve to injuries (Efron, Clara 

 Halperson) explains the above reaction. If only the part of 

 the nerve below the plexus is treated with KN0 3 near the lower 

 point of bifurcation, by placing it between two pads soaked in 



FIG. 194. Frog's nerve-muscle preparation. A weak ascending or descending current only dis- 

 charges a closure twitch (fe) with any position of electrodes. After 5 min. bath of 1 % KNO^, 

 the excitability of the nerve is lowered. The same current now discharges opening twitches of 

 equal height with the closure twitches (c, d) from all points of the altered tractjof nerve. After 

 washing with physiological saline (15 min.) the opening twitch disappears again (g, h). 



saline, the same strength of current will produce uniform break 

 twitches at all points of that part of the nerve. But if in such 

 a preparation the electrodes are laid closely together upon the 

 plexus, there will either be a closure twitch alone, with both 

 directions of current, if the connection with the spinal cord is 

 still intact, or else, from the proximity of the cross-section, there 

 will be closing and opening twitches, with descending direction 

 of current. 



If the action of the potash saltpetre does not affect the 

 immediate proximity of the muscle, but is arrested at about 2 cm. 

 off it, it may be demonstrated that by gradually shifting the 

 electrodes at uniform distance (1 cm.) from the centre to the 

 periphery, the break twitch becomes less and less as the normal 

 portion of the nerve is approached, and an increasing part of it 

 included under the anode and finally disappears altogether 



