iv GENEEAL PHYSIOLOGY OF NERVOUS SYSTEM 211 



cerebral cortex directly, or reflexly, by stimulation of the central 

 end of the sciatic of the opposite side. 



The discovery of these electrical phenomena, which are the 

 constant corollary of nerve stimulation, signalled a considerable 

 advance in the general physiology of nerve, since they are the 

 only external manifestation known of the transition from the 

 state of rest to that of activity. 



The negative variation of the current of rest depends on the 

 fact that the excited point of the nerve becomes for the moment 

 the seat of a negative electrical potential which is transmitted 



FIG. 136. Photograph of electrical variations produced by rhythmical tetanisation of an excised 

 nerve. (A. D. Waller.) The stimulations were sent in at intervals of a minute. After applying 

 ether (black line) the electrical responses were suspended for about 5 min., after which they 

 recommenced and became more vigorous than before. 



along the nerve as a diphasic wave, in complete analogy with what 

 we have seen for muscle. 



The strength of the negative variation is up to a certain 

 limit proportional to the intensity of stimulation (Waller). It 

 is a more reliable measure of the impulse in a motor nerve than 

 the height of the muscular contraction which the impulse induces. 

 In fact the maximum degree of muscular excitation is evoked 

 with a strength of stimulus less than that required for the 

 maximum degree of nervous excitation. When the maximal 

 muscular contraction is already obtained, it is still possible to 

 increase the value of the negative electrical variation by increasing 

 the strength of stimulation. 



The negative variation alters with the excitability and 

 conductivity of the nerve ; it is abolished or decreased by any 



