682 



A MANUAL OF PHYSIOLOGY 



been shown both for muscle and for nerve that the cooler tissue 

 requires a smaller current strength for its excitation when the 

 current is of long duration. With brief currents this effect is 

 masked, either partially or completely, by the greater increase of 

 current strength needed in the case of the cooler tissue to com- 

 pensate for a given 

 decrease in duration 

 (p. 685) (Lucas and 

 Mines). This is the 

 reason that for induction 

 shocks or voltaic cur- 

 rents of short duration, 

 the excitability of the 

 nerve seems to be in- 

 creased by a rise of tem- 

 perature (up to about 

 30 C. in the case of frog's 

 nerve), and diminished 

 by cooling. 



Drying of a nerve at 

 first increases its ex- 

 citability ; and the same 

 is true of separation of 

 a nerve from its centre. 

 In the latter case the 

 increase of irritability 

 begins at the proximal 

 end of the nerve, and 



E, changes of excitability during the flow of travels towards the peri- 



the current, according to Pfliiger. The ordinates -L \ j.- p-oes 



drawn from the abscissa axis to cut the curve P J' . ., 



represent the amount of the change. C(i), on, the excitability 



changes of conductivity during the flow of a diminishes and ulti- 



moderately strong current. Conductivity ^^pU, Hkarmpar* in the 



greatly reduced around kathode ; little affected matel Y disappears 



at anode. C(2), changes of conductivity during same order (Ritter-Valll 



flow of a very strong current. Conductivity Law) . At a certain stage 



reduced both in anodic and kathodic regions, , i r -j ,-i n 



but less in the former. C', changes of con- jt ma Y be found that a 



ductivity just after opening a moderately given Stimulus causes a 



strong current. Conductivity greatly reduced smaller and Smaller COn- 



in region which was formerly anodic ; little , ,-1 c ,-, -t 



affected in region formerly kathodic. tractionthefartherdown 



the nerve that is, the 



nearer to the muscle it is applied. On this was based the now 

 abandoned ' avalanche theory,' according to which the impulse 

 continually unlocked new energy as it passed along the nerve, and 

 so gathered strength in its course like an avalanche. It is now 

 known that no material change takes place in the intensity of 

 the excitation while it is being propagated along a normal un- 



FIG. 252. DIAGRAM OF CHANGES OF EXCIT- 

 ABILITY AND CONDUCTIVITY PRODUCED IN 

 A NERVE BY A VOLTAIC CURRENT. 



