x ELECTROMOTIVE ACTION IN NERVE 233 



The circuit was closed by applying a third pad quickly, where- 

 upon the muscle contracted (Fig. 201). The point here being 

 adequate rapidity of closure and opening, the two pads on which 

 the nerve is resting may hang freely over the edge of a glass 

 plate, a vessel of salt solution being rapidly raised or lowered 

 below them (Hering, I.e.), or two blocks of salt clay can be employed, 

 which are readily moulded into any form (Kuhne, Lc,). The 

 twitches resulting from this last method are, as Kuhne showed, 

 most energetic. In excitable preparations Hering obtained 

 vigorous make and break twitches, when the tract of nerve 

 between the clay blocks was lengthened to 1 cm. This gave 

 reason to anticipate that a nerve may be tetanised by its 

 own current as well as by the interruptions of a battery 

 current. With this object, Kuhne employed a vibrating mercury 

 key; Hering, on the other hand, constructed a special apparatus, 

 by which he obtained a "tetanus without metals." "The rapid 

 raising and dropping of the closure pad (supra) was effected by 

 the teeth of a rotating cog-wheel, which lifted the one-armed 

 lever, and the closure-pad attached to its free end, while a spring 

 fastened to the lever drew it down again after each rise." " The 

 simplest means of exciting a nerve by its own current is, as 

 remarked by Hering (Lc. p. 241), to let its end fall upon a second 

 isoelectric moist conductor." Metals (platinum, amalgamated 

 zinc) are less suitable for this purpose, because they very shortly 

 set up polarising currents. " If the end of the nerve falls upon a 

 drop of lymph, blood-serum, or weak salt solution, the effect usually 

 occurs once only, because the fluid that clings to the nerve when 

 it is lifted out again short-circuits it permanently and effectively. 

 But if the nerve falls upon a coagulated drop of blood, or a block 

 of clay saturated with 0*6 per cent NaCl, the experiment may 

 be freely repeated." An isoelectric, muscle will obviously be a 

 convenient sub-stage for the same purpose. " If a nerve still 

 connected with the leg is allowed to fall upon the gastrocnemius 

 muscle, the resulting twitch is no proof that the nerve is excited 

 by a muscle current, although this may generally be the case." 

 Czermak pointed out that frogs' legs of great excitability will 

 contract when their nerves fall upon portions of the intestine, 

 kidneys, or liver of a rabbit, which no more proves the pre- 

 existence of a P.D. in those parts than the observation of Bonders 

 that frogs' legs may twitch under certain conditions, when the 



