130 ON NERVE-EXCITATION BY THE NERVE-CURRENT. 



used a little apparatus, specially constructed for the purpose, which 

 I propose to call a tetanomotor for the production of tetanus without 

 metals. In this apparatus the rapid raising- and lowering of the 

 cushion is effected as follows : The teeth of a toothed wheel raise 

 a lever, to the free end of which the cushion is attached, and a 

 spring depresses the lever after each elevation ; by means of a 

 screw the amplitude of movement can be exactly regulated, and 

 by varying* the rapidity of revolution of the toothed wheel the 

 frequency of the makes and breaks may be varied. 



Excitation of a nerve by its fall upon a moist conductor. 



The easiest way to effect an excitation of nerve by its own current 

 is to let its end fall upon some currentless moist conductor. It is less 

 advisable to let it fall on platinum or amalgamated zinc, partly on 

 account of the instantaneous polarisation, partly on account of the 

 admixture of currents due to electrical differences at the points of 

 metal with which the nerve is in contact. The possibility of 

 mechanical excitation, which can only occur when the nerve is too dry, 

 is easily excluded by control-experiments with non-conductors. 



The fall of the end of a nerve upon a drop of lymph, serum, or 

 normal salt-solution, is, in general, successful only once, because, on 

 removal of the nerve, the adherent fluid permanently short circuits 

 the current, but the experiment can be repeated several times if 

 the nerve is allowed to fall upon a coagulated drop of blood, or a 

 block of clay made with salt-solution -6 p.c. 



It need hardly be explained, after what has been said, that a 

 contraction follows also if the end of nerve is dropped upon a com- 

 pletely currentless muscle or any other currentless organ. If one 

 drops the sciatic nerve left in connection with the leg-muscles upon 

 these muscles in the ordinary way, the consequent contraction is 

 by no means a sufficient proof that the nerve has been excited by 

 a muscle-current, although indeed this will generally be the case. In 

 this favourite but not at all unequivocal experiment, the contraction 

 may be due to any one of three causes if the leg has been stripped 

 of its skin : either (i) that the leg gives an ascending current sufficient 

 for excitation, which traverses the nerve from its point of entrance 

 into the muscle to the point where it is in contact with the muscles 

 of the leg, or (2) that the nerve lies upon electrically different points 

 of muscle, or (3) that the nerve-current is suddenly closed through 

 the muscle. It is easy to understand that in most cases all three 

 factors may concur or conflict with one another. 



