146 COMPARATIVE PHYSIOLOGY 



claimed to demonstrate increased respiratory activity of nerve 

 during the propagation of the impulse. It remains to be seen 

 w^hether these claims will be sustained by further research. 



In its most recent form the theory of excitation propounded 

 by Nernst and extended by Hill and Keith Lucas is amplified 

 by Lillie as follows. The surface-membrane of a resting 

 neurone is supposed to be impermeable to certain anions 

 which are present in greater concentration inside the fibre 

 than outside it. The kations are free to pass through the 

 membrane, but the electro attraction of the anions forces 

 the kations to remain close to the membrane, forming an 

 electrical double layer. The change in ionic concentration at 

 the membrane during excitation implies the removal of anions 

 _j- 4- ^^^ therefore the re- 



4-"^ JT "^"^ _L moval of the double 



(L 



layer (Fig. 36). This 

 depolarisation is accom- 

 panied by an increase 

 of permeability at the 

 _j__l_-l_-|__|_-^-f--l--j--}- kathode, and escape of 



anions at the permeable 



area initiates an action 

 Yic^ 36. current. The local bio- 



electric current leads to 

 a depolarisation of adjacent parts of the membrane which 

 in their turn become permeable and the seat of a bio- 

 electric current. Thus the change is propagated along the 

 nerve-fibre, the action current in one section becoming the 

 stimulus for the setting up of a similar condition in the next. 

 According to this view all stimulation is electrical. It is 

 of interest therefore to note that incident light produces an 

 electrical variation (Piper) in the denervated eye both of 

 Vertebrates and Cephalopods. 



The balance of evidence is at present in favour of the view 

 that in the phenomenon of nervous conduction we have to 

 deal with a process which is essentially of the same nature 

 as the excitation process itself. It might therefore be expected 

 that the capacity of the nerve to conduct a second impulse 



