82 ELECTROTONIC CURRENTS. [BOOK i. 



We may speak of the conditions which give rise to this electrotonic 

 current as a physical electrotonus analogous to that physiological electro- 

 tonus which is made known by variations in irritability. The physical 

 electrotonic current is probably due to the escape of the polarizing 

 current along the nerve under the peculiar conditions of the living 

 nerve; but we must not attempt to enter here into this disputed 

 and difficult subject or into the allied question as to the exact con- 

 nection between the physical and the physiological electrotonus, though 

 there can be little doubt that the latter is dependent on the former. 



An induction-shock is a current of very short duration developed 

 very suddenly and disappearing more gradually. Hence, when it 

 falls into a nerve, the nerve undergoes a sudden transition from its 

 normal condition to the katelectrotonic phase, and consequently a 

 nervous impulse giving rise to a contraction is the result. The 

 return from the anelectrotonic phase to the normal condition 

 appears from a number of considerations to be less effective as 

 a generator of nervous impulses than the change from the normal 

 condition to the katelectrotonic phase. Hence in the induced 

 current we have to deal with a ' making ' contraction only, the 

 breaking contraction being absent. This is true whether the induced 

 current be generated by the making or by the breaking of a con- 

 stant current. 



The constant current applied directly to a muscle from which 

 the purely nervous element has been eliminated by urari poisoning, 

 has effects similar to and yet somewhat different from those which 

 it has upon a nerve. The efficacy of the rise of katelectrotonus 

 and the fall of anelectrotonus respectively in producing contraction 

 is the same as in a nerve. In one respect the muscle is more 

 striking, and offers a support of the hypothesis mentioned above. 

 The making contraction may under favourable circumstances be 

 seen to start from the kathode and the breaking contraction from 

 the anode. Besides the make and break spasm a partial tetanus 

 during the whole time of the passage of the current through a 

 muscle is very often seen. Another marked difference between 

 muscle and nerve is that in muscle the current must act for a 

 much longer time upon the tissue before it can call forth a con- 

 traction. This is what we might expect from the more sluggish 

 nature of the muscular impulse-wave. Hence muscular tissue 

 which has lost its nervous elements or does not possess them, is 

 far less readily affected by the almost momentary induction-shocks 

 than are nerves. - 



