EXCITATION AND INHIBITION 385 



period when a stimulus at I. is effective as regards the muscle and a stimulus at 

 II. is not. Ihus the full line shows the disturbance starting at II., undergoing 

 in d a decrement which does not lead to complete extinction, emerging into 

 normal nerve between d and d', where it persists at reduced magnitude, and 

 then undergoing in d' a further decrement which does lead to extinction. The 

 broken line shows the disturbance starting at I. undergoing in d' a decrement 

 which does not lead to extinction and then passing on to the muscle in this 

 reduced condition. 



"Fig. 105, C shows what will happen if the disturbance recovers after leaving 

 the region of decrement. In this case, the disturbance from II., when it travels 

 through the normal tissue between d and d', is fully equal to the disturbance 

 which starts at I. (broken line). Thus stimuli at I. and II. will become ineffective 

 at the same moment when the narcotic has acted for such a time that a full-sized 

 disturbance is extinguished in the length d or d'. 



"Consequently, the only data required for the solution of our problem are 

 the times from the beginning of narcosis to the moments when stimuli from 

 electrodes I., II., and III. cease to evoke a muscular contraction. If the stimulus 

 fails first at III. and afterwards at I. and II. simultaneously, the disturbance 

 must recover to its original size after leaving the area of decrement ; if the stimulus 

 fails first at II. and III. together and afterwards at I., the disturbance does not 

 recover." 



The actual experimental method used will be found in the original paper. 

 Suffice it to say here that the results prove conclusively that a disturbance, after 

 having been decreased by passing through a region of decrement, recovers its 

 original magnitude when it re-enters a normal area. We may look upon the 

 various magnitudes of the disturbance, as it emerges from regions of various 

 degrees of narcosis and enters on the normal region, as being different degrees 

 of intensity of a stimulus applied to the normal nerve. Experiment shows 

 that the impulse then present in the normal legion is the same in all cases and 

 maximal, whatever its strength was after subjection to decrement. 



Attention may be called to the method of measuring the strength of an impulse by the 

 extent of decrement it can suffer without extinction. An important point in Adrian's woik is 

 that the strength was not measured by the magnitude of the electrical change alone, since the 

 actual relationship of this change to the propagated disturbance itself is not, as yet, eom- 

 pletety known. 



Another way, in which the result is confirmed, is by applying stimuli of various 

 strengths and allowing the impulses produced to pass through a narcotised region. 

 It was found that the same degree of narcosis abolished all, so that they must 

 have been of equal intensity. 



By similar methods it was shown that, if the impulse is altered in magnitude 

 by passing through a cooled area, it regains its original size on emerging into 

 normal tissue. 



Space does not permit discussion here of the results obtained by previous observers, which 

 appeared to show a gradation of impulses in nerve fibres. Adrian has shown that they do not 

 warrant the interpretation put upon them. There is one point, however, which should be 

 referred to. It was thought at one time that a nerve might still be able to conduct a pro- 

 pagated disturbance through a narcotised area, when unable to respond to a stimulus applied 

 directly to this area. The results of Adrian show that the phenomenon can be explained 

 without this assumption, which, therefore, introduces an unnecessary complication. The 

 phenomenon known as " Wedensky's inhibition" depends on the stage of diminished 

 excitability immediately following the passage of an impulse which is known as the " refractory 

 period " and will be discussed presently. With regard to the supposed distinction between 

 conductivity and excitability, referred to above, the fact of the local excitatory change, which 

 is antecedent to the setting up of a propagated disturbance and will be discussed below, should 

 be kept in mind. This local state is not propagated and it does not appear improbable that 

 the possibility of its occurrence might be prevented by the action of certain agents, although 

 the nerve might still be able to conduct an impulse started elsewhere. 



It appears to me that the results obtained by Adrian show quite clearly that 

 there is no gradation of excitatory state in the normal condition, so that the fact 

 must be accepted whatever consequences may follow from it. Its application to the 

 phenomena in nerve centres and to heart muscle will be referred to later, but its 



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