632 IRRITABILITY OF NERVES. 



its disappearance. In like manner any rapid increase or decrease of the current 

 passing through a nerve has a strong irritant effect. If, on the other hand, the 

 current be allowed to pass gradually into the nerve-trunk, or to disappear gradu- 

 ally, or if the current passing through the nerve be gradually increased or dimin- 

 ished, the visible signs of nerve-irritation are much less marked. In general, the 

 stimulation is most pronounced the more rapid the current-variation within the 

 nerve, that is, the more suddenly the strength of the current passing through the 

 nerve is increased or diminished. 



This law applies, however, only to the rapidly moving muscles and their 

 nerves (frog, warm-blooded animals) . For muscles that move slowly (toad) , the 

 unstriated muscles and those of some invertebrates, slowly acting and slowly in- 

 creasing electrical stimuli are the most effective. 



If linear variation in the current (v. Fleischl's orthorheonome, v. Kries' 

 spring rheonome) be employed as the stimulus, the intensity of the current must 

 be the greater in order to obtain the same irritant effect the more slowly such 

 linear variation takes place. 



The electrical current must have a definite strength before it becomes active 

 (threshold- value) . With uniform increase in the strength of the current the size of 

 the muscular contractions first increases rapidly and then more slowly. An 

 electrical current must continue at least for 0.0015 second in order to stimulate the 

 nerve; a current of shorter duration will have no effect. If the duration of the 

 current be somewhat longer the stimulation on opening is wanting. The duration 

 of closure of a constant current that continues for a time just short enough to 

 be inactive need be prolonged only from 1.3 times to twice as long in order to 

 attain the most complete effects. 



The electrical current, further, is most effective when it is passed through the 

 long axis of a nerve. It is ineffective when applied at right angles to the axis 

 of the nerve. The muscle also is less responsive to electrical currents that pass 

 transversely through its fibers than to such as pass longitudinally. The greater, 

 further, the length of nerve through which the current passes the smaller need 

 the electrical stimulus be. 



When the constant current is applied to a motor nerve it exerts its most 

 pronounced stimulating effect on closing and on opening. The stimulation, 

 however, does not completely cease during the period the current is closed, for if 

 the current be of moderate strength the muscle supplied by it remains in a con- 

 dition of tetanus galvanotonus or closing tetanus. The analogous reaction of 

 the muscle on direct application of the constant current is considered on p. 563. 

 When strong currents are employed this tetanus, it is true, subsides, but it does 

 so because as a result of the action of the current in the nerve through diminution 

 of its irritability resistances are developed that prevent the stimulation from 

 reaching the muscle. According to Hermann, a descending current excites this 

 tetanus more readily if the current is passed through the nerve at some distance 

 from the muscle ; while an ascending current excites the tetanus more readily when 

 applied in the neighborhood of the muscle. The constant current manifests its 

 stimulating influence upon sensory nerves in most marked degree at the moment 

 of closing and opening. During the period of closure feeble stimulation is per- 

 ceptible, but strong currents may under such circumstances give rise to un- 

 bearable sensations. Closing, opening, and the passage of the current stimulate 

 all centripetal fibers, and also the vasodilators of the skin. The constant current 

 has no effect upon vasoconstrictor and secretory fibers. 



The following observation of Wedenskij is noteworthy: If the sciatic nerve of 

 a frog be irritated by means of strong and frequent currents the tetanus induced 

 soon disappears on account of exhaustion of the portion of nerve concerned, but 

 begins again if the stimulus is either weakened or made less frequent. If the 

 muscle is relaxed after the strong irritation, it contracts again if stimulated directly 

 by a current of moderate intensity after the nerve-stimulation has been re- 

 moved. The fact appears noteworthy that if two tetanizing stimuli are ap- 

 plied to the nerve of a frog-preparation, the two stimulations may counteract each 

 other. If, for example, sodium chlorid be applied to the portion of nerve near 

 the leg until the muscles are thrown into a tetanic contraction, this will cease 

 if at the same time a portion of the nerve higher up is irritated. Both stimulating 

 effects may, however, act together. 



The phenomenon of deficiency is discussed on p. 664. 



If the individual short current-shock occurs in rapid succession the related 

 muscle is thrown into a state of tetanus. 



