34 



PRACTICAL PHYSIOLOGY 



electrodes, study the stimulation of nerve and muscle, using the 

 protected limb of the frog from the last experiment. Isolate the 

 nerve to the top of the thigh and cut it across as high up as possible. 

 Stimulate by closing and opening the mercury key, allowing a 

 second or two to elapse between each stimulation, and increasing 

 the strength of the current by moving the handle on the switch-board 

 from W to S, or by moving the bridge of the rheocord. Note any 

 difference at making and at breaking the current, taking the contrac- 

 tion of the muscles as an index of the stimulation of the nerve. 



2. Do the Two Poles act in the same way? 



Dip the end of the nerve furthest from the muscle momentarily 

 into boiling water to kill it. This part will not now respond to the 

 electric current although it will conduct it. 



Place the dead bit over one wire and the nerve near the muscle 

 over the other. Now make and then break the current (1) with the 

 anode on the living bit of nerve, and (2) with the cathode on the 

 living bit of nerve, and record your results on the subjoined 

 table : 



FIG. 24. To show the stimulation, at the cathode on closing and at the 

 ariode on opening in the exposed nerve. 



(From Xoel Paton's Essentials.) 



II. Faradic or Induced Current. 



That the current instantaneously induced in the secondary coil 

 by closing or opening the primary circuit stimulates nerve has 

 already been learned in the curare experiment above. With the 

 two electrodes from the secondary coil of an inductorium applied 

 to the living bit of nerve in the preparation just used gradually 

 weaken the induced current by moving the secondary coil away 

 from the primary, and note that the stimulation on breaking con- 

 tinues after that on making has disappeared. Explain this from 

 your knowledge of the physics of the induction coil (Fig. 25). 



