CHAPTER XXXI 



ON A NEW METHOD FOR THE QUANTITATIVE 

 STIMULATION OF NERVE 



Drawbacks to use of electrical stimulus in recording electrical response Response 

 to equi-alternating electrical shocks Modification of response by decline of 

 injury Positive after-effect Stimulation of nerve by thermal shocks 

 Enhancement of normal response after tetanisation Untenability of theory of 

 evolution of carbonic acid Abnormal positive response converted into normal 

 negative after tetanisation Gradual transition from positive to negative, 

 through intermediate diphasic Effect of depression of tonicity on excitability 

 and conductivity Conversion of abnormal into normal response by increase 

 of stimulus-intensity Cyclic variation of response under molecular modifica- 

 tion. 



IN the study of the electrical effects of excitation on the 

 nerve, the chief experimental difficulty lies in the selection 

 of a form of stimulus which can be made quantitative. In 

 such investigations it is usual to employ the electrical form 

 of stimulus, because of the great facilities which it offers. 

 A marked drawback to its use, however, lies in the fact that 

 unless extraordinary precautions are taken it is liable to lead 

 to serious error. It must be remembered that for the detec- 

 tion of responsive variations in the nerve an extremely 

 sensitive galvanometer has to be employed. The excitatory 

 effect which is to be detected being indicated by the relatively 

 feeble electrical response, and the form of stimulus being also 

 electrical and being of high intensity, the results are liable to 

 be disturbed in an unknown manner by leakage of the stimu- 

 lating current. 



In some cases it is possible to take the bold step of 

 including the experimental nerve itself in a circuit in which 

 the exciting coil and the galvanometer are in series. Under 

 these circumstances, and employing strictly equr-alternating 



