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AN AMERICAN TEXT- BO OK OF PHYSIOLOGY. 



to a ck-nsc How of current, will be excited and the muscle will contract. 

 Unless the primary current is very strong the electromotive force developed in 

 the secondary coil on the closing of the primary circuit may be too weak to 

 cause contraction, and only the effect of opening the circuit maybe observed ; 

 in any case, the effect of breaking the primary circuit will be the stronger. 

 More striking results will be obtained if the primary current be rapidly 

 made and broken by an automatic interrupter introduced into the primary 

 circuit; the muscle will then be excited by a series of rapidly following 

 shocks. 





Fig. 22.— Unipolar, localized excitation of nerve. By this arrangement a large part of the surface 

 of the nerve and muscle is brought into immediate connection with the secondary coil through the sheet 

 of gold-foil. The nerve is Locally excited at the point that is touched by the needle, because the current 

 going to charge the tin-foil conductor passes out of the nerve at this point as a dense stream. The 

 muscle (a) is supported by an insulating clamp of lead-glass and vulcanite (6), and is connected to the 

 ■writing lever by a dry lead-glass hook (c); the nerve (d) lies on a sheet of gold-foil (e*, which is also 

 wrapped about the muscle, and which rests on a block of vulcanite (/) supported by a glass rod (g); the 

 gold-foil is in close contact with the binding-post (A), and this is connected with one terminal of the 

 secondary coil (i) of an induction apparatus, the other terminal being connected with a gas pipe (j), and 

 so with the earth ; in the primary induction circuit there area battery (k) and a key {I); the needle (wi) is 

 connected with a large conductor (n), which is composed of a board covered with tin-foil, and is sus- 

 pended from glass hooks. 



For the sake of simplicity we have thus far only spoken of the charging 

 of the preparation from the secondary coil. It must be borne in mind, how- 

 ever, that the change in the electrical condition of the secondary coil lasts 

 only an instant, and the terminals of the coil and the tissues connected with 

 them immediately return to their original potential, this change being accom- 

 panied by a backward surge of the electrical wave from the muscle through 

 the nerve, electrodes, and wire to the coil, and this reverse current acts like 

 the charging current to cause excitation. The charging and discharging 

 processes follow each other with such rapidity, however, that they act upon 

 the tissues as a single excitation. 



To Prevent the Spread of Current. — As we have seen when the nerve of a 

 nerve-muscle preparation is connected by two electrodes with the poles of 



