464 COMPARATIVE ELECTRO-PHYSIOLOGY 



We have also seen that a tissue which is not in the 

 highest tonic condition may have its tonicity increased by 

 the action of impinging stimulus, with consequent enhance- 

 ment of its excitability. I shall also demonstrate, in 

 Chapter XXXIV, that the effect of an impinging stimulus 

 on a sub-tonic tissue is a similar enhancement of con- 

 ductivity. The result of this will be either (i) that a tissue 

 which has already conducted a moderate intensity of 



FIG. 277. Gradual Transition from Abnormal Positive, through 

 Diphasic, to Normal Negative Responses in Frog's Nerve 



Cf. similar effect in response of skin of gecko, fig. 191. 



stimulus to a distant point will show, after continuous stimu- 

 lation, an enhanced power of conduction ; or (2) that in a 

 very sub-tonic tissue, in which true excitation has at first 

 failed to reach the responding point, the true excitatory 

 negative is subsequently transmitted instead of the hydro- 

 positive alone. 



Under actual experimental conditions, where the stimulus 

 is applied at a distant point, the twofold effects of exaltation of 

 excitability and conductivity under tetanisation both come 

 into play. In normally responding nerve, the increased con- 



