THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 759 



are taken to keep the current intensity the same at the various 

 temperatures compared, it is found that cooling of a (frog's) nerve, 

 even to 5 C., increases the excitability for currents of long duration 

 (several hundredths of a second). It has, indeed, 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 dura- 

 tion. 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 compensate fora given decrease in duration 

 (p. 763) (Lucas and Mines). This is the reason that for induction 

 shocks or voltaic currents of short duration, the excitability of the 

 nerve seems to be increased by a rise of temperature (up to about 

 30 C. in the case of frog's nerve), and diminished by cooling. 



Drying of a nerve at first increases its excitability ; 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 travels towards the periphery. As time goes on, the excita- 

 bility diminishes, and ultimately disappears in the same order 

 (Ritter-Valli Law). At a certain stage it may be found that a 

 given stimulus causes a smaller and smaller contraction the farther 

 down 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 uninjured nerve. For instance, experiments on the 

 phrenic nerve, in its natural position, and with all its connections 

 intact, have shown that with a given strength of stimulus the 

 amount of contraction of the diaphragm is the same whether the 

 nerve be excited in the upper, middle, or lower portion of its course. 

 In the above experiment on the isolated, and therefore injured, 

 nerve, the contraction varies in height with the distance of the 

 point of stimulation from the muscle, not because the excitation 

 grows as it travels, but because it is already greater at the moment 

 when it sets out from a point near the central end of the nerve 

 than at the moment when it sets out from a point near the muscle. 



Electrotonus. Although the constant current does not, unless 

 it is very strong or the nerve very irritable, cause stimulation during 

 its passage, it modifies profoundly the excitability and conductivity 

 of the nerve. In the neighbourhood of the kathode the excitability 

 is increased (condition of katelectrotonus), while around the anode 

 it is diminished (anelectrotonus) . Immediately after the opening 

 of the current these relations are for a brief time reversed, the 

 excitability of the post-kathodic area (area which was at the kathode 

 during the flow) being diminished, and that of the post-anodic 



