58 EXPERIMENTAL PHYSIOLOGY 



sciatic become stimulated ; in an excitable preparation this will occur 

 even with very weak currents. The experiment can also be performed 

 with ordinary metallic electrodes connected with the secondary coil of 

 an inductoriuin (Fig. 49). On stimulating the peroneus nerve by 

 closing the mercury key the gastrocnemius is made to contract. If 

 the peroneus branch is tied or crushed near its junction with the 

 sciatic the effect can only be got with currents strong enough to spread 

 in the tissues generally. The experiment should be repeated by 

 placing a thread, wetted with Ringer, along the sciatic nerve, and 

 laying its free end on the electrodes. In this case the excitation, 

 which is, of course, caused by spread of current, will only show itself 



FIG. 50. To test the polar effects of a constant current on nerve excitability. 



with strong currents. These experiments show that the spread of 

 current is assisted by the structure of the nerve ; even with a weak 

 current it takes place for a considerable distance along an intact 

 nerve, but it is easily blocked if the nerve be tied or crushed. 



Electrotonic effects of constant current on excitability. Take a 

 pair of non-polarisable electrodes and connect with a battery of at 

 least two cells, inserting a rheochord, a commutator, and a mercury 

 key into the circuit (polarising circuit). l Another circuit is also 

 prepared (exciting circuit), including cell, induction coil, and mercury 



1 A variation of the experiment is to replace the exciting circuit by a few 

 crystals of salt and wait until the penetration of this begins to excite the nerve 

 fibres. The rheochord may be dispensed with in the polarising circuit. 



