CH. XVI.] 



179 



the usual way, the exciting electrodes being placed on the nerve 

 near the muscle. A polarising circuit is also arranged, and in- 

 cludes a battery, key, and reverser ; the current is passed into 

 the nerve by means of non-polarisable electrodes. When the 

 polarising current is thrown into the nerve, or taken out, a con- 

 traction of the muscle occurs, but these contractions may be dis- 

 regarded for the present. 



The exciting circuit is arranged with the secondary coil so far 

 from the primary that the muscle responds to break only, and 

 the tracing may be recorded on a stationary blackened cylinder. 

 Move the cylinder on a short distance, and repeat this. The 

 height of the lines drawn may be taken as a measure of the 

 excitability of the nerve. Now throw in the polarising current 



Cell 



Key 



Coil 

 EXCITING CIRCUIT 



Muscle 



Fig. i HQ. Diagram of apparatus used in testing eloctrotonic alterations of excitability. 



in a descending direction (i.e., towards the muscle) ; the kathode 

 is then the non-polarisable electrode near to the exciting elec- 

 trodes. While the polarising current is flowing, take pome more 

 tracings by breaking the exciting current. The increase in the 

 excitability of the nerve is shown by the much larger contrac- 

 tions of the muscle ; probably a contraction will be obtained now 

 at both make and break of the exciting current. After removing 

 the polarising current, the contractions obtained by exciting the 

 nerve will be for a short time smaller than the normal, but soon 

 return to their original size. 



Exactly the reverse occurs when the polarising current is 

 ascending, i.e., from the muscle towards the spinal cord. The 

 non-polarisable elertrode near the exciting electrodes is now the 

 anode. While the polarising current is passing, the excitability 

 of the nerve is diminished so that induction shocks which pre- 

 viously produced contractions of a certain size, now produce 



N 2 



