394 PHENOMENA ACCOMPANYING A NERVOUS IMPULSE. 



lower end of the thigh into the peroneal nncl tibial branches. 

 Dissect out one, say the peroneal, and divide it at its periphery. 

 Divide the sciatic trunk high up, and place the peroneal branch 

 on the electrodes of an induction coil. This will virtually con- 

 vert the leg into a preparation similar to tig. 291, III.; the 

 peroneal and tibial branches running, so to speak, side by side 

 in the sciatic trunk. 



Irritating the peroneal nerve A, with an interrupted current, 

 will produce contractions in the muscles to which the tibial B 

 is distributed. 



All these " secondaiy contractions" cease when the nerve A 

 is ligatured between the electrodes and the nerve B. 



With each making (competent to give rise to a nervous im- 

 pulse) of the exciting current through A, two events take place 

 which must be kept distinct in the mind of the student. 



First, there is the electrotonic increase (in the anelectrotonic 

 region) or decrease (in the katelectrotonic region) of the 

 natural nerve current. This increase or decrease remains 

 during the whole time of the passage of the exciting current, 

 and disappears with the breaking. 



Secondly, there is the negative variation of the natural cur- 

 rent which travels with the nervous impulse indifferently in 

 either direction, and which, in any given point of the nerve, is 

 over and gone in an exceedingly short time after the act of 

 making the exciting current. 



During the time of the passage of the (uniformly constant) 

 current, there is no negative variation, as there is no nervous 

 impulse. 



On breaking the exciting current, a fresh negative variation 

 sweeps along the nerve, if the current is of such a character 

 that the breaking of it gives rise to a nervous impulse. 



With a single induction shock there is also the double event 

 of a negative variation, and, as well, of a momentary electro- 

 ton us ; with an interrupted current there is a succession of 

 sueh double events. 



In both these cases the secondary contraction, as in Obs. I., 

 II., III., may be due to either half of the double event: to the 

 negative variation, or to the electrotonic change; or to both. 

 To which of them it is really due cannot be decided by the use 

 of such currents only. 



If, however, the electrotonic increase is itself competent to 

 cMuse a secondary contraction, the contraction ought to be ob- 

 tainable at an}- period during the passage of an exciting con- 

 stant ei.rrent, at a time when" the negative variation is absent. 



(Ms. IV. Connect A (placed on a glass plate) with a constant 

 cunvnt of two cells, the positive pole towards the long free end ; 

 suspend the nerve of B in such a manner over A that, when de- 

 sired, it can be let fall so as to lie upon A in the position I. or 

 II. (fig. 291). 



