118 EFFECTS OF CONSTANT CURRENT. [Book i. 



of a sensory nerve in which violent impulses are being generated, 

 giving rise in the central nervous system to sensations of pain, the 

 impulses are toned down or wholly abolished, and the pain ceases. 

 So on the other hand we may at pleasure heighten the irritability 

 of a part by throwing it into katelectrotonus. In this way the 

 constant current, properly applied, becomes a powerful remedial 

 means. 



Lastly, though we are dealing now with nerves going to muscles, 

 that is to say, with motor nerves only, we may add that what we 

 have said about electrotonus and the development of nervous 

 impulses by it appears to apply equally well to sensory nerves. 



^ 73. In a general way muscular fibies behave towards an 

 electric current very much as do nerve fibies ; but there are 

 certain important differences. 



In the first place, muscular fibres, devoid of nerve fibres, are 

 much more readily thrown into contractions by the breaking and 

 making of a constant current than by the more transient 

 induction-shock ; the muscular substance seems to be more 

 sluggish than the nervous substance and requires to be acted upon 

 for a longer time. This fact may be made use of, and indeed is in 

 medical practice made use of, to determine the condition of the 

 nerves supplying a muscle. If the intramuscular nerves be still in 

 good condition, the muscle as a whole responds readily to single 

 induction-shocks because these can act upon the intramuscular 

 nerves. If these nerves on the other hand have lost their irrita- 

 bility, the muscle does not respond readily to single induction- 

 shocks, or to the interrupted current, but can still easily be thrown 

 into contractions by the constant current. 



In the second place, while in a nerve no impulses are as a rule 

 generated during the passage of a constant current, between the 

 break and the make, provided that it is not too strong, and that it 

 remains uniform in strength, in an urarized muscle on the other 

 hand, even with' moderate and perfectly uniform currents, a kind of 

 tetanus or apparently a series of rhythmically repeated contractions 

 is very frequently witnessed during the passage of the current. 

 The exact nature and cause of these phenomena in muscle, we 

 must not however discuss here. 



