Chap. XL] NEC A TI l''E VA RIA TION. I I 9 



deflected by the muscle current ; the key of the 

 primary circuit is then closed, and that of the short 

 circuit opened, so that the muscle is tetanised, when 

 the needle will be found to swing back, sometimes 

 almost to zero. On closing the short circuit the 

 muscle ceases to be stimulated, tetanus disappears, 

 and the needle is again deflected, but not so much as 

 before. Care must be taken that the induction coil 

 is so far away from the galvanometer that the 

 opening and closing of its circuit have no effect on 

 the needle, and also that the position of the 

 muscle is not shifted during contraction. A good 

 way to obtain the latter with certainty is to use 

 the electrodes (Fig. 59), and to make the points 

 press accurately on the centre of cross-section and 

 longitudinal surface. To prove that the tetanising 

 current does not gain access to the galvanometer 

 circuit and cause an error, tie a piece of wet silk 

 thread round the nerve below the exciting electrodes, 

 and, everything else being unmoved, send on the 

 tetanising stream as before. The continuity of the 

 nerve for nervous stimulation has been 

 destroyed ; no tetanus occurs in the muscle, 

 and no negative variation arises. The 

 continuity of the nerve for electrical cur- 

 rents is, however, still unimpaired, so 

 that the negative variation is not due to 

 any diffusion of electrical currents from 

 the exciting electrodes. Fio . 61 



The electric currents of nerves neWe ar- 



may be demonstrated in a similar way. the ge 

 Here also it will be found convenient to poiarisabie 

 use the tube electrodes. The nerve (a long trodes. 

 piece of the sciatic nerve) may be laid 

 over one clay point turned up into a hook, and the 

 two depending ends made to touch, by their transverse 

 section, the clay point of the second electrode, placed 



