ix ELECTRICAL EXCITATION OF NERVE 193 



As regards the break excitation, it should be observed that 

 it requires stronger currents (here as everywhere) than the make 

 effect, and may, like this last, produce opposite changes of form 

 in the muscle under certain conditions. In consequence of the 

 inferior strength of the opening stimu- 

 lus, however, it only excites the muscle 

 in the majority of cases, and seldom 

 reaches sufficient proportions to inhibit 

 a preexisting tonus. But if in such 

 a case the exciting action of the 

 current fails to find expression, the r ~ 

 effects of stimulation both at make ' 

 and at break of the circuit may 



consist in a transitory relaxation of the tonically contracted 

 muscle : the curve then presents two depressions, one beginning 

 at closure, and disappearing only during the passage of the 

 current, the other, less considerable corresponding with the 

 break of the exciting circuit (Fig. 199). 



In view of the double, partly inhibitory, partly exciting 

 effect of stimulating the two muscles of the crayfish claw with 

 the constant current, the important question arises whether 

 under the presumption of pure polar action of current the two 

 effects, at make on the one hand, at break on the other, 

 proceed from the same electrode, or whether there is an 

 antagonism between the respective discharges of excitation and, 

 inhibition. 



It has already been stated of the atonic adductor muscle that 

 the order of excitatory effects corresponds throughout with 

 Pfltiger's law ; i.e. both on applying very strong currents, and after 

 excluding the influence of the central electrode by partially 

 killing the nerve, the descending current takes effect at make, 

 the ascending current at break only of the circuit. 



Since in the first case owing to the enormous resistance in 

 the exciting circuit, and low density within the part traversed 

 the current must be of very considerable intensity in order to 

 obtain the third stage of Pfltiger's law (with unpolarisable elec- 

 trodes), the second of the methods given above seems the most 

 appropriate. 



By this it is easy to ascertain, with given intensity of 

 current for both adductor and abductor muscle, that inhibition 



VOL. II 



