NERVE 595 



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 peri- 

 phery. As time goes on, the excitability 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 excita- 

 tion 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 neighbour- 

 hood of the kathode the excitability is increased (condition of 

 katelectrotonus), while around the anode it is diminished 

 (anelectrotonous). 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 increased. In the intrapolar area there is one point the 



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