CONDUCTION IN NERVE 51 



Blocking by a galvanic current Bernstein's experiment (Fig. 44). 

 To exhibit the blocking effect of a galvanic current upon nerve, take 

 a muscle-nerve preparation with long nerve and attach the muscle to 

 the myograph lever in the usual manner, so that its contractions may 

 be recorded upon a slowly moving drum. Apply stimulating electrodes 



FIG. 44. Diagram of Bernstein's experiment to illustrate the blocking effect 

 of a constant current upon conduction of nerve impulses. 



from the secondary coil to the part of the nerve near the vertebral 

 column, using the Neef's hammer (with Helmholtz wire) for tetanisa- 

 tion. Apply a pair of non-polarisable electrodes, connected through 

 a mercury key with a battery of three Daniell cells, arranged so that 

 the current can be passed up the nerve 1 (polarising circuit). Take 

 a tracing of the muscle during tetanisation, and whilst this is progressing 

 close the polarising circuit. The tetanus at once ceases, to be renewed 

 on again opening that circuit, and so on repeatedly. 



1 If passed downwards it is itself liable to produce stimulation should the 

 preparation be very excitable (see p. 56). 



