26 EXPERIMENTAL PHYSIOLOGY 



of the variation in amount of current ; but if quick, the muscle-nerve 

 responds to each movement of the bridge. 



The rheonome may also be made with a copper bridge, the ends 

 of which dip into solution of sulphate of copper. 



Excitation by induced current ; Determination of excitability of 

 a nerve. An induction shock is a stimulus, and the break induction 

 shock a far stronger stimulus than the make. Get the minimal 

 effect of each by sliding the secondary coil to the necessary distance 

 from the primary, and make a note as to the respective positions 

 of the secondary coil. This gives a measure of the excitability of the 

 nerve. Its excitability . to tetanisation (use the Neef s hammer 

 with and without the Helmholtz wire) is determined in the same way. 



FIG. 31. Stimulation of nerve by condenser discharge. 



Reaction of nerve and muscle to galvanic and faradic excitation. 

 If the above experiment be tried (a) with a nerve-muscle preparation 

 and (b) with nerveless muscle (the nerves within the muscle are 

 paralysed by curari), it will be found that the nerve will react to a 

 weaker stimulus than the muscle when induction shocks are employed ; 

 but if the make and break of a galvanic circuit be used as the stimulus 

 the muscle will respond to the weaker stimulus. Plain muscle shows 

 this even more strikingly. 



G-otch's experiment. Cooling a nerve causes it to react to galvanic 

 and faradic stimulation in the same way as muscle. The upper end 

 of the nerve of a nerve-muscle preparation is cooled by laying it on a 

 tube through which ice-cold water is passing ; the muscle itself must 

 not be allowed to come anywhere near the cold tube. If, now, the 

 cooled part of the nerve be stimulated, it will be found to be less 

 sensitive to faradic and more sensitive to galvanic stimulation than a 

 part nearer the muscle which has not been cooled. Cooling the nerve 



