AFTER EFFECTS OF EXCITATION. 



537 



tory process in nerve fibres ; it is evidently an equally striking feature of 

 the electrical response. 



The initial stage of augmentation is undoubtedly related with the 

 conditions produced in the first few responses, and is thus increased if 

 such changes occur on a more extensive scale. 



It will be remembered that nerve excitability, as gauged by the 

 indirect muscular re- 

 sponse, is raised by 

 previous excitation ; 

 the negative variations 

 show a similar aug- 

 mentation. 



If the nerve is sub- 

 jected to a prolonged 

 series of rapid alternat- 

 ing induced currents, 

 during the regular and 

 periodic excitation by 

 the shorter series of 

 such currents, then, on 

 the cessation of the 

 prolonged stimulation, 

 the negative variations 

 evoked by the periodic nerve during period x xl> the additional electrical re _ 



sponses thus evoked are fused, and the members of the 

 series are thus masked ; on the cessation of the fara- 

 disation, the subsequent members of the series are aug- 

 mented. The series of events is to be read from right 

 to left. After Waller. 



Fro. 276. Effect of faradisation upon the electrical response 

 evoked by each of a series of stimulations, repeated at 

 considerable intervals. The faradic currents stimulated 



series become aug- 

 mented in amount 

 (Fig. 276). This aug- 

 mentation closely re- 

 sembles that produced 



in nerve through subjection to a CO., environment. It has been, 

 therefore, supposed by Waller, that the augmentation following pro- 

 longed activity is due to the C0. 2 set free in the nerve in consequence 

 of its activity. There is, undoubtedly, a close correspondence between 

 the two classes of effect, those of external C0 2 and of internal 

 prolonged activity, which will be made more evident by the facts to 

 be next detailed. The absence of any thermal effects during nerve 

 activity renders it, however, somewhat difficult to accept this explanation 

 of the effect. 1 



Negative and positive after effects. It has been already pointed 

 out that the excitatory negative variation observed in a nerve, when 

 connected by surface and cross sectional contacts with a galvano- 

 metric circuit, may exhibit itself as a prolonged change in the resting 

 demarcation current. This is generally of such a character as to cause 

 its diminution, but the presence of effects in the other direction may be 

 detected under certain conditions. The diminution of the difference is 

 termed a negative after effect, the augmentation a positive one, these 

 terms being here employed to indicate a prolonged decrement or incre- 

 ment of the demarcation difference. 



It appears from the capillary records previously referred to, that any 

 circumstance causing an impairment in the physiological condition of 

 the tissue under the distal contact (i.e. that farthest from the seat of the 

 excitation) produces a negative after effect; as this follows the pro- 



1 Waller, Croonian Lecture, Phil. Trans., London, 1896. 



