25(5 A Manual of Veterinary Physiology. 



induced current (faradic), which is obtained by means of a 

 battery and an induction coil. 



If a moderate constant current be passed into the nerve 

 by connecting it with the poles of the battery, at the 

 moment the connection is made the muscle gives a twitch 

 or contraction and then remains perfectly quiet, though the 

 current is still streaming through it ; if the connection be 

 broken by removing one pole of the battery, the muscle 

 gives another contraction. This is termed ' a making and 

 breaking contraction,' viz., a contraction produced on closing 

 and opening the electric circuit. If instead of a moderate 

 constant current a weak or strong one be used, the results 

 on making and breaking may not be the same. 



During the period of apparent quiescence following the 

 closing of the circuit, though the muscle is giving no indi- 

 cation of the current, yet profound changes are occurring 

 in the nerve ; for if it be tested by passing into it an in- 

 duced current, it is found that the irritability is diminished 

 in the neighbourhood of the positive pole of the continuous 

 current, and increased in the neighbourhood of the negative 

 pole. This is known as electrotonus, the diminished irrita- 

 bility being known as anelectrotonus, that of increased 

 excitability as kathelectrotonus. .Between the increased 

 and reduced irritability is a zone of unaffected irritability, 

 known as the neutral point. 



During the condition of electrotonus there is no inter- 

 ruption to the natural nerve current; provided the constant 

 current takes the same direction as the current of rest 

 through the nerve, the natural current is simply increased ; 

 but if the constant current be passing in the opposite 

 direction to the nerve current, the latter is diminished. 



To state this matter more clearly, imagine a nerve 

 running to a muscle, Fig. 2:] ; at a certain pari of the 

 nerve a continuous current of electricity is passed through 

 it (the application and withdrawal of which gives rise to 

 the making and breaking contraction previously mentioned I, 

 but during the passage of the current the muscle is per- 

 fectly quiet, in spite of important changes occurring in the 



