786 



NERVE 



increased. In the intrapolar area there is one point the excita- 

 bility of which is not altered. This indifferent point, as it is called., 

 shifts its position when the intensity of the current is varied, moving 

 towards the kathode when the current is increased, towards the 

 anode when it is diminished. 



It is only under certain definite conditions that these phenomena, first 

 described by Pfluger, appear in their purity and uncomplicated by other 

 changes. The nerve should be quite fresh, the current a weak or at 

 most a moderately strong one, and the stretch of nerve employed 

 should be as far as possible from the cross section, and from the cross 

 sections of branches. The middle region of the frog's sciatic nerve is 

 the best. When all these conditions are fulfilled, the whole stretch 

 of nerve in katelectrotonus i.e., the part on both sides of the kathode 

 and at the kathode itself shows an increased stimulation effect, the 

 more pronounced the nearer to the kathode the point of stimulation. 

 This condition, however, only lasts an instant. Then the excitability 

 begins to sink sharply first at the kathode, then on both sides of it, till 



it ultimately becomes decidedly less than 

 the initial excitability. This secondary 

 depression of excitability, always most 

 marked at the very kathode, is just as con- 



Fig. 273 . Katelectrotonus. Weak 

 tetanus of muscle (the right-hand 

 elevation), greatly intensified in 

 katelectrotonus of the motor nerve 

 (th left-hand elevation). 



Fig. 274. Anelectrotonus. Strong 

 tetanus of muscle (left-hand ele- 

 vation), lessened in strength by 

 anelectrotonic condition of the mo- 

 tor nerve (right-hand elevation). 



stant a phenomenon as the preliminary increase. The stronger the cur- 

 rent the more profound is the depression, the more quickly it is de- 

 veloped, and the greater is the distance to which it spreads along the 

 nerve. With a certain strength of current the depression appears so 

 rapidly that the preliminary increase of excitability may be completely 

 missed. When the current is opened the excitability quickly increases 

 again, but with strong currents it may remain depressed for a while. At 

 the anode changes in the reverse direction may be observed, although 

 they are less pronounced than at the kathode. Thus at the anode 

 during the passage of the current the initial depression of the excitability 

 tends to give place to an increase (Werigo). 



These statements have been made on the strength of experiments in 

 which the height of the muscular contraction was taken as the index of 

 the excitability of the nerve at any given point. It is difficult, however, 

 to disentangle the effects of alterations in the excitability from the 

 effects of alterations of conductivity i.e., of the power of a portion of 

 the nerve to conduct an impulse set up elsewhere. Whether these two 

 properties are distinct or not is a question which will be considered a 

 little later on. But it is perfectly clear that in deducing conclusions 



