234 



NERVE. 



The potential at the electrodes on the nerve is varied by 

 short-circuiting through the derivation circuit formed by the 

 rheocord. 



Three strengths of potential will be sought for in order to 

 obtain the following result. 



The following is given as an approximate guide : 



1. Weak : One cell with the block of the rheocord close 

 to the return terminal. 



M 



FIG. 46. Pfliiger's Law. L Cells. K Key. B Movable block off R, short circuits 

 when placed on rheocord wire R. C Commutator. N Non-polarisable electrodes on 

 the nerve. M Muscle. 



2. Medium : One or two cells with the block on the 

 second or third stretch of the wire from the return terminal. 



3. Strong : Three or four cells with the block off the 

 rheocord. 



Modify the current so as to obtain the following results : 



Weak 



Medium 



Strong 



Explanation : Excitation is either due to production of 

 katelectrotonus or to disappearance of anelectrotonus. 

 The former is the more efficient stimulus. 



