MECHANICAL RESPONSE OF NERVE 519 



tion of stimulus, as may be seen from the record, there is a 

 sudden" expansive movement downwards, followed by an 

 equally rapid reversed movement of contraction upwards, 

 and this followed again by a slow recovery. Each of the 

 successive stimuli evokes the same diphasic responsive 

 sequence. It must be noted that the downward twitches are 

 the preliminary, and not the after-affect. It is also interesting 

 to note, as the tissue approaches death, under the continued 

 action of chloroform, how regularly in both negative and 

 positive directions the responses decrease in amplitude. 



We shall next undertake an independent investigation 

 into the causes which bring about the three types of response 

 abnormal positive, diphasic, and normal negative known 

 to be exhibited in the electrical response of the animal, and 

 already demonstrated as occurring also in that of the vege- 

 tal nerve. While discussing these three types of electrical 

 response and their variations in Chapter XXXI. it was 

 stated that the differences of effect involved were due to 

 changes in conductivity and excitability, brought about by 

 varying tonic conditions. It was also explained, in the same 

 place, that the continued isolation of so highly excitable a 

 tissue as nerve, from its accustomed supply of energy, would 

 be sufficient of itself to depress its tonic condition below 

 par, with concomitant depression of its conductivity and 

 excitability. The result of this depression of excitability 

 will be to render inefficient a stimulus which was formerly 

 efficient, to evoke the true excitatory reaction of galvano- 

 metric negativity. The absorbed stimulus will now induce 

 only a responsive positivity. The depression of conductivity 

 also would cause the transmission of the hydro-positive, 

 instead of the excitatory negative, wave. Owing, then, to the 

 joint action of these two factors, stimulus induces a positive 

 response the so-called ' abnormal ' at a distant responding 

 point, when the tonic condition of the tissue has become 

 depressed. Absorption of stimulus, however, by supplying 

 the requisite energy, raises the tonic condition, with con- 

 sequent restoration of conductivity and excitability. As 



