IX 



ELECTRICAL EXCITATION OF NERVE 



187 



tensity, are such as are in a state of moderate tonic contraction, 

 and therefore react by corresponding changes of form to both 

 effects of excitation. 



If with increasing intensity of current the preparation is 

 excited with alternately ascending or descending currents, or 

 with uniform direction of current, there are, as a rule, certain 

 obvious characteristics of the mode of reaction which given the 



FIG. 195. Adductor of crayfish claw. Excitation of nerve with constant currents. The existing 

 tonus is little if at all increased by closure of weaker currents (a, b), which essentially inhibit 

 it. Closure of a strong current on the other hand = c. 



previous data re tetanisation of the nerve distinguish the 

 adductor muscle sharply from the abductor. 



In the first place, it is evident that in preparations of the 

 former muscle weak currents and medium currents have a 

 predominantly inhibitory action, while with stronger currents 

 the effects of excitation preponderate, or alone appear (Fig. 

 195, a, ft). This is expressed, on the one hand, in the fact 

 that the augmentation of tonus that invariably corresponds with 

 the moment of closure, i.e. shortening of the muscle, increases 

 with increasing intensity of current to a certain upper limit, 

 which (from the mechanical conditions of the experiment) is 



