ELECTRICAL EXCITATION OF NERVE 207 



twelve cells, experienced a feeling of warmth in the arm con- 

 nected with the anode, but could not detect cold in the other 

 arm. 



VII. POLAR EXCITATION BY CURRENTS OF VERY SHORT DURATION 

 (INDUCTION CURRENTS) 



It has already been repeatedly pointed out that it is necessary, 

 in order that current should produce electrical activity in excitable 

 substances, that it should pass for a certain period, varying in 

 absolute value within a wide range, according to the nature 

 of the excitable tissue. This is especially true of the break 

 excitation by the constant current, which implies, besides ade- 

 quate intensity, a due period of closure, since the anelectrotonic 

 state (i.e. the anodic alterations of current with the disappearance 

 of which it is connected) can only develop fully under these 

 conditions. Here, with indirect stimulation of the muscle, there 

 can be no doubt that the development of an adequate an- 

 electrotonus in the nerve itself requires such an interval : on the 

 other hand, there are cases in which it may be asked whether the 

 inefficacy of a make stimulus with brief currents is due to some 

 property of the nerve, or of its peripheral end -organ (muscle). 

 If, e.g., a single impact of current, or induction shock, is effective 

 when applied to the nerve of a cross -striated muscle, and in- 

 effective when it acts upon the motor fibres of smooth muscle, 

 it may be conjectured that the absence of contraction in the 

 last case derives solely from the muscle, i.e. that the excitation 

 passing along the nerve may be of the same nature and magnitude 

 as in the first case, but that it is inadequate, or in some way 

 inept, to stimulate the more sluggish tissues. Internal variations 

 in the nerve must also be reckoned as factors. 



However this may be, the manifestations of the law of 

 contraction undergo considerable modifications with brief currents, 

 even where the most rapidly reacting preparations are employed. 

 Pfliiger's law of contraction would lead us to anticipate that 

 very brief currents effect no opening twitch, and this is supported 

 by experiment. We have already seen that induction currents 

 (which should theoretically produce a double excitation, since 

 they are equal at make and break) act in striated muscle, 

 at moderate intensity, from the kathode only ; and within a 



