538 NERVE. 



nounced negativity, which is the evidence of the electrical response, 

 it blends with the main effect, and may be regarded as the normal wake 

 of the single response. With uninjured contacts, this wake presumably 

 occurs as the successor of both the change under the proximal and that 

 under the distal contact, and the algebraic sum of the two practically 

 reduces it to nothing. If, however, the distal tissue is rendered less 

 capable of responding (injury, local heat, etc.), the unbalanced effect under 

 the proximal electrode becomes evident. The negative after effect seen 

 under these conditions is thus the tail or wake of the explosion, and 

 occurs under normal conditions as a constant concomitant of this. 

 On the other hand, diminished capacity of the tissue under the 

 proximal contact must allow the wake of the change under an un- 

 injured distal contact to appear. It must show itself as positivity 

 of the proximal contact, and thus give rise to a positive after effect. 

 There are, however, other conditions capable of developing after 

 effects of this type. Thus in the nerves of winter or cooled frogs, 

 the negative variation, as evidenced by the galvanometer, is often 

 succeeded by augmentation of the resting difference. Head, who 

 first drew attention to the phenomenon, considered that the essential 

 condition which favoured its appearance was vital power or vigour. 

 In summer frogs it gives place to the customary demarcation 

 decrement. 



Another set of conditions which favours the production of positive 

 after effects has been observed by Waller. He found that the excised 

 sciatic nerve of the frog, if kept for some time in physiological salt 

 solution, responds to electrical stimulation in quite a different manner 

 to that of the freshly excised nerve. 1 



In such " stale " nerves, the following electromotive phenomena are 

 observed. After immersion for a few hours, the variation evoked by 

 a short period of rapid stimulation is often of a biphasic character, 

 the deflections indicating surface negativity, followed by positivity. 

 These effects are succeeded by the usual demarcation decrement, i.e. 

 decline in the resting difference. 



If the nerve has been kept for a still longer time in the bath, 

 the positive phase of the variation becomes still more prominent. The 

 excitatory variation now becomes one in which a positive phase may 

 precede the negative, and may finally be such as to indicate positive 

 phases only. 



Photographic reproductions of such positive deflections are given 

 in Fig. 277. 



The frequent repetition of the excitation causes a gradual diminu- 

 tion in these positive effects, and thus the most modified condition, 

 in which the variation is purely positive, may become transformed into 

 one showing a positive succeeded by a negative, or into one showing a 

 negative followed by a positive. 



There are thus four distinct stages as regards the character of 

 the excitatory galvanometric effects 



First stage. Each set, with effect - (unmodified nerve). 

 Second stage. Each set, first - then + (slightly modified nerve). 

 Third stage. Each set, first + then - (more modified nerve). 

 Fourth stage. Each set, + only (profoundly modified nerve). 

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



