THE CAUSATION OF THE EXCITATION. 507 



locally warmed. It can be easily demonstrated that an excitation of 

 minimal exciting intensity becomes more effectual when the cathode of 

 the induced current is on the region whose excitability to the stimulating 

 agent has been thus locally augmented, but remains unaltered when the 

 anode is on this region. 



It has, however, been already pointed out that it is possible to 

 obtain evidence of anodic alterations in both excitability and conductivity 

 with rapid induced currents of sufficient intensity, and by similar means 

 it is possible to demonstrate the existence of opening anodic excitation. 

 The experiments previously referred to, in which, through a series of 

 ascending induced currents of gradually increasing intensity, a series of 

 muscular responses are evoked, which attain a maximum, then fail, and 

 finally reappear, afford an illustration of this. The responses occurring 

 after the failure are characterised by two peculiarities : the earlier ones 

 have a prolonged period of delay, the characteristic of those evoked 

 by weak opening anodic excitation, and the later ones are often larger 

 than those of a single twitch. This is most probably due to a sum- 

 mation of effect in the muscle through two effective nerve stimulations 

 occurring in succession, on closure at cathode and on opening at anode. 

 The anodic block is insufficient to resist the intense state transmitted 

 from the cathode, whilst the response to the anodic opening excitation, 

 since it occurs later, is sufficiently retarded to produce with the former 

 a summated muscular effect. In similar experiments with descending 

 currents of short duration, there is no such failure, but the muscular 

 response with gradually increasing intensity becomes augmented. The 

 absence of failure is in strict accordance with Pfliiger's law of contrac- 

 tion, since in this case the cathodic seat of the closing excitation is 

 nearest the muscle ; the sudden development of the augmented response 

 implies a summated effect, due to the propagation of two successive nerve 

 impulses, the first evoked on closure at the cathode being followed on 

 the cessation of the brief current by one evoked at the anode. 1 



That both poles can be the seats of an effective nerve excitation by 

 intense induced currents, is confirmed by experiments in which the 

 anodic pole is placed on the nerve near the muscle, and the cathodic 

 near the central end. If the cathode only were the exciting point, then 

 the period of delay between excitation and muscular response must be 

 longer when the current is ascending than when it is descending, since 

 the cathodic seat of excitation is .in the former case distal, in the 

 latter proximal to the muscle. If, however, a double excitation can 

 occur at both poles, there should be, with adequate current intensity, 

 but little perceptible difference in the time relations dependent upon 

 change of direction. The latter appears, according to Mares", to be the 

 case. 2 



Pfliiger's law of contraction in its first and second stages, and to 

 some extent in its third stage, thus holds good for currents of short 

 duration, though the anodic results are only obtained with currents of 

 very considerable intensity. 



The causation of the closing and opening excitation. As stated 

 by Pfliiger, the closing excitation is associated with the sudden establish- 

 ment of catelectrotonus, the opening one with the sudden disappearance 

 of anelectrotonus. 



1 Fick, loc. cit. 



2 Sitzungsb. d. k. bohm. Gesellscli. d. Wissensch., Prag, 1891. 



