STIMULATION OF MUSCLES AND OF NERVES 



421 



producing any contraction; whereas a much weaker current closed suddenly pro- 

 duces a maximal effect. And conversely, a stronger current if reduced very 

 gradually, may be brought down to nil without causing an excitation; whereas 

 the sudden opening of a much weaker current is accompanied by a strong 

 contraction. 



But under certain circumstances a constant current flowing through a motor 

 nerve may stimulate not only at the moment of closing, but during the entire 

 period of closure. This happens for example with frog's nerves when the latter 

 are taken from frogs which have been kept for a long time at a temperature 

 below 10 C. (v. Frey) ; also with the nerves of warm-blooded animals when the 

 current is not too weak. Again, if a constant current has been flowing through 

 a nerve for a sufficient time, on opening the current there often appears a pro- 

 longed contraction instead of a simple short contraction. This continued state 

 of contraction is often spoken of as " Hitter's tetanus" Often also after the 

 summit of the closing contraction has been passed, a cross-striated muscle does 

 not recover its natural length immediately, but remains more or less shortened 

 (" Wundt's tetanus"}, and only returns to its resting condition when the cur- 

 rent is broken i. e., in case no opening contraction occurs. If the stimulus is 

 very weak, the constant excitation is only a local one, spreading over a limited 



FIG. 159. Details of the Wagner hammer or interrupter of the induction coil. c, primary 

 coil; i, secondary coil. The primary current is generated in the battery K and the secondary 

 or induced currents are led off by electrodes attached to the ends of the secondary coil. 



portion of the muscle. Finally, when a constant current is applied to an afferent 

 nerve, a distinct sensation is felt during the entire period of closure, even when 

 the peripheral end organs are excluded (Griitzner, Langendorff, Biedermann). 



We find therefore so many exceptions to Du Bois-Beymond's law, that in 

 its original form it can no longer be regarded as of general application, 

 although, so far as muscle alone is concerned, the excitation of large masses 

 appears to depend upon sudden changes at the place of direct stimulation. 

 Moreover, the law must take into account the nature of the irritable tissue : 

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