534 COMPARATIVE ELECTRO-PHYSIOLOGY 



condition is raised, and the normal excitability consequently 

 enhanced. From this point onwards the responses are con- 

 tractile or normal negative. At this stage the response of 

 the nerve exhibits the staircase increase, the nerve itself 

 showing a certain amount of tonic contraction. The 

 excitability of the nerve then attains a maximum, and the 

 successive responses become uniform. Long and intense 

 stimulation will, after this, bring on fatigue. This stage is 

 characterised, again, by a growing relaxation of the nerve as 

 a whole, and its responses may become, first, diminished in 

 amplitude, second, of a diphasic character, and, thirdly, 

 reversed to the abnormal positive, according to the amount 

 of fatigue which supervenes. We may thus, for the sake of 

 convenience, distinguish four stages in the response of 

 nerve. The first of these is the initial phase, SUB-TONIC 

 RELAXATION, and the characteristic response to individual 

 stimuli is here abnormal positive. The second phase 

 is that of the TRANSITION to normal response. The 

 characteristic responses to individual stimuli here show a 

 staircase increase, with more or less permanent contraction 

 as its after-effect. If at this stage the nerve is allowed to 

 remain long without stimulation, it slowly reverts to the 

 first stage of sub-tonic relaxation, with its growing relaxa- 

 tion and abnormal positive response. Stimulation, how- 

 ever, brings it back once more to the second or transition 

 stage. In the third stage of UNIFORM responsiveness, the 

 responses are normal and take place by equal contraction. 

 In the fourth, or FATIGUE stage, there is a tendency, as already 

 said, to relaxation on the part of the nerve as a whole, 

 and it thus outwardly mimics the stage of sub-tonicity. 

 The responses now, therefore, diminish in amplitude, and 

 show the diphasic or the abnormal positive character. 

 Further characteristics of these four stages, and their 

 relations to each other, will be treated in detail in 

 Chapter XLI. 



A few words may be said about the mechanical response of 

 the nerve, when it is in a favourable condition of excitability. 



