106 THE CONTRACTILE TISSUES. 



the kathode where the nerve passes suddenly into katelectrotonus ; when 

 the current on the other hand is " broken " the impulse is generated only at 

 the anode where the nerve passes suddenly back from anelectrotonus into 

 a normal condition. We have an indirect proof of this in the facts to 

 which we drew attention a little while back, viz., that a contraction some- 

 times occurs at the " breaking" only, sometimes at the " making" only of 

 the constant current, sometimes at both. For it is found that this depends 

 partly on the strength of the current in relation to the irritability of the 

 nerve, partly on the direction of the current, whether ascending or descend- 

 ing ; and the results obtained with strong, medium and weak descending 

 and ascending currents have been stated in the form of a " law of contrac- 

 tion." We need not enter into the details of this " law," but will merely 

 say that the results which it formulates are best explained by the hypothe- 

 sis just stated. We may add that when the constant current is applied to 

 certain structures composed of plain muscular fibres, whose rate of contrac- 

 tion we have seen to be slow, the making contraction may be actually seen 

 to begin at the kathode and travel toward the anode, and the breaking con- 

 traction to begin at the anode and travel thence toward the kathode. 



Since in katelectrotonus the irritability is increased, and in anelectrotonus 

 decreased, both the entrance from the normal condition into katelectrotonus 

 and the return from anelectrotonus to the normal condition are instances of 

 a passage from a lower stage of irritability to a higher stage of irritability. 

 Hence, the phenomena of electrotonus would lead us to the conception that 

 a stimulus in provoking a nervous impulse produces its effect by, in some 

 way or other, suddenly raising the irritability to a higher pitch. But what 

 we are exactly to understand by raising the irritability, what molecular 

 change is the cause of the rise, and how either electric or other stimuli can 

 produce this change, are matters we cannot discuss here. 



Besides their theoretical importance, the phenomena of electrotonus have 

 also a practical interest. When an ascending current is passed along a nerve 

 going to a muscle or group of muscles, the region between the electrodes and 

 the muscle is thrown into anelectrotonus and its irritability is diminished. 

 If the current be of adequate strength, the irritability may be so much 

 lessened that nervous impulses cannot be generated in that part of the nerve 

 or cannot pass along it. Hence, by this means the irregular contractions of 

 muscles known as "cramp" may be abolished. Similarly, by bringing into 

 a condition of anelectrotonus a portion 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 irrita- 

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

 stant 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. 



76. In a general way muscular fibres behave toward an electric cur- 

 rent very much as do nerve fibres ; but there are certain important dif- 

 ferences. 



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 



