504 NERVE, 



from the state of increased stability. Of these the first is the most 

 effectual ; it is therefore produced by weaker currents than those neces- 

 sary to evoke the opening effect (Pfliiger). 



The effects in the third stage are due to such excitation, modified by 

 the polar interferences with conductivity described in the preceding 

 chapter. The nerve is locally excited, both at the cathode on closure and 

 at the anode on cessation, the stimulation being more intense than in 

 the preceding stages ; but in the case of a motor nerve the excitatory 

 state started by the cathodic excitation of an ascending current has to 

 traverse the anodic region, and is greatly diminished in the passage, so 

 that it evokes either a minimal or no muscular response. 



Similarly, the descending current excites at both poles, but that 

 started at the anode on its cessation has to traverse a region in which 

 catelectrotonus is disappearing, and its propagation is interfered with in 

 the same way, but complete block is more difficult to attain. 



In afferent nerves the reflex response is used as the index, but the 

 same laws hold good, although modified in the third stage. Since the 

 effective propagation is now towards the central nervous system, it 

 follows that in the third stage it is the descending current which fails 

 to evoke the closing, and the ascending one which fails to evoke the 

 opening reflex effect. 1 



That the excitation on closure does actually occur at the cathode, 

 and that on opening at the anode, may be directly demonstrated in nerve 

 by experiments of the following kind. If a portion of a motor nerve be 

 locally raised in excitability by such an agent as fall of temperature, 

 then the closure of a very weak galvanic current causes a local excita- 

 tion adequate to evoke a maximal muscular effect, provided that the 

 cathode coincides with the altered region ; on the other hand, when the 

 anode coincides with this region, then, with an intensity of current 

 too weak to adequately excite at closure, an opening response may 

 be obtained. By locally raising the temperature, and thus diminishing 

 the susceptibility to stimulation, the reverse effects can be produced. 

 Finally, by locally raising the temperature at one pole and lowering 

 it at the other, the passage of a current of appropriate intensity in 

 either direction will excite only at the pole which is situated upon the 

 cooled region ; if this is the cathode the excitation occurs on closure, if 

 the anode it occurs on opening. 2 



Any agent which causes a local alteration in excitability may be utilised for 

 demonstrating the polar site of the excitation ; temperature is convenient as it 

 produces no permanent effect, but the local application of NaCl may be used 

 for increasing local excitability, and gives the same results ; the closure being 

 effective when the salt is on the cathodic region, the opening when on the 

 anodic. Ammonia may be used for abolishing local excitability, and the failure 

 of cathodic closing or anodic opening excitation is then observed when either 

 of these poles corresponds with the impaired region. By similar experiments 

 it can be shown that the intrapolar region is not the seat of this sudden 

 stimulation on closure and opening ; it is limited to the neighbourhood of the 

 poles. As the nerve loses its excitability by any of the previously described 

 agencies, arrest of circulation, etc., the exciting effects of the stronger currents 

 alter from the third to the second and finally to the first of Pfliiger's three stages. 3 



1 Pfliiger, loc. cit. 



2 Gotch and Macdonald, Journ. PhysioL, Cambridge and London, 1896, vol. xx. p. 257. 



3 Rosenthal and v. Bezold, Arch. f. Anat., PhysioL u. wissensch. Med., 1859. 



