QUANTITATIVE STIMULATION OF NERVE 



463 



Between these two extremes of normal negative and 

 abnormal positive responses there lies the intermediate 

 diphasic. All these positive, diphasic, and negative may 

 be exhibited in the same specimen, in the course of a sus- 

 tained record of responses to single stimuli, without tetani- 

 sation. This fact is illustrated in fig. 277, where the first 

 series shows the unmixed abnormal positive. The response 

 then passes by a gradual 

 transition into diphasic- 

 positive followed by negative 

 and this phase, lastly, is 

 succeeded by a series of 

 purely negative responses. 



We come next to the ex- 

 planation of these phenomena. 

 We have seen that on account 

 of isolation the tonic condition 

 of a highly excitable tissue 

 will undergo a gradual decline. 

 On account of this its ex- 

 citability and conductivity 

 will fall below par. We have 

 also seen that in this de- 

 pressed condition the normal 

 response by negativity tends 

 to be reversed to positivity. 

 With regard to the con- 

 duction of excitation it may 

 be said that this condition of 

 depression will lower the power of the tissue to conduct true 

 excitation. Thus a stimulus of given intensity, capable under 

 normal conditions of transmission to a certain distance, will, 

 when the tissue is thus depressed, fail of conduction to the 

 same distance. It will now, therefore, be the hydro-positive 

 effect of stimulus which will make its appearance alone at 

 the distant responding point. And the electrical expression 

 of this will be galvanometric positivity. 



FIG. 276. Conversion of Abnormal 

 Positive into Normal Negative Re- 

 sponse after Thermal Tetanisation 



Abnormal positive response to left 

 converted into normal negative 

 on right, after intervening tetanisa- 

 tion. 



