GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 45 



the secondary coil of an induction apparatus, if a large electromotive force is 

 developed in the secondary coil, a current not only passes through the part 

 of the nerve bridging the electrodes, but through the part of the nerve 

 between the electrodes and the muscle. This spread of current may be in 

 part prevented by connecting the electrode nearest to the muscle with a gas- 

 pipe and leading the charge through this to the earth (Hermann). Another 

 method which has been suggested is to connect the two sides of the nerve 

 beyond the electrodes by a loop of good conducting metal, so that the spread- 

 ing currents shall be short-circuited (Hering). 



Application of the Unipolar-excitation Method to the Localization of Excita- 

 tion. — The principle that a flow of current will excite at the point where the 

 current is dense can be employed to obtain definitely localized excitations by 

 the unipolar method of irritation. For example, Kiihne employs the follow- 

 ing arrangement to show isolated contraction of muscle-fibres by localized 

 excitation. A thin parallel-fibred sartorius muscle of a frog is curarized to 

 shut out the effect of excitation of the nerve so that only the muscle-fibres 

 which are directly excited will contract. The preparation is then placed 

 on an insulated copper plate, which is connected with one pole of the sec- 

 ondary coil of an induction apparatus, the other pole being connected with 

 the earth (see Fig. 23). The muscle makes no contractions when the key in 



Fig. 23.— Unipolar localized excitation of the sartorius muscle. The muscle (a) rests on a sheet of 

 copper (b), which is on a plate (c), resting on a sheet of vulcanite (d) ; the binding-post (e) on the copper 

 plate is connected with one terminal of the secondary coil of the apparatus (/), the other terminal being 

 connected with a gas pipe (g), and so with the earth ; the primary coil (h) is connected with a battery (i) 

 and a key (j); the muscle is locally excited by the current, which passes in a dense stream through it 

 to the needle (k) held in the hand. 



the primary circuit is closed or opened. Its potential is undoubtedly changed, 

 but its capacity is small, and it is charged from many points; the charging 

 current is at no place sufficient in quantity or density to excite. If now the 

 experimenter touch the top of the muscle near one side with the point of a 

 needle held in the hand, the muscle twitches on the side touched each time 

 the current is opened, and if the current is strong each time it is closed. 

 The contraction is limited to the fibres just below the needle, because this is 

 the point where the current charging the body of the observer passes through 

 the muscle in a dense stream. The effect is more striking if, instead of 

 using single shocks, the primary current be frequently interrupted. The 

 fibres on the side stimulated will then be continuously contracted and the 

 muscle will curl toward the stimulated side. 



