476 NERVE. 



had to be augmented, and that if such augmentation was not made, the 

 muscular response was an initial twitch. 1 Kronecker used magneto- 

 induced currents, produced by the vibration of a steel core, within the 

 primary coil of a specially constructed induction apparatus. The 

 production of a note indicating 20,000 vibrations per second was 

 effectual for the excitation of the nerve, the muscular response being at 

 first a tetanus, but on repetition it became an initial twitch, and 

 subsequently failed. 2 In this connection attention may be drawn to the 

 fact that the Hertz oscillations, which are of a different order, being 

 far in excess of any of these rates, only excite when very intense, and 

 then generally evoke initial twitch. 3 



It thus appears that the nerve at 12 C is capable of being excited 

 by stimuli recurring at very rapid intervals, but that the susceptibility 

 to excitation is greatest at the commencement of such stimulation, and 

 diminishes with the repetition. In explanation of this, Bernstein and 

 others have laid stress on the possibility of functional interference due 

 to the overlapping of the successive nerve responses, which thus become 

 fused ; it is, however, probable that no such fusion occurs. The dura- 

 tion of the excitatory disturbance in nerve is suggested by that of the 

 electrical nerve response. This has been variously estimated in frog's 

 nerve as lasting from y-Vir sec - to T ^ sec., and undoubtedly the dura- 

 tion is greatly increased at low temperatures. An overlapping is thus 

 possible, particularly at low temperature, but the real question is how 

 far the response to a second stimulus is prohibited under these con- 

 ditions, by the fact that the response to a previous stimulus is still in 

 progress. In other words, is there an inexcitable or " refractory " period 

 during activity ? Wedenskii determined the electrical excitatory changes 

 by connecting a telephone with four sciatic nerves, placed side by side, 

 and exciting with rapid stimulation at two points. The telephone note 

 caused by the excitatory changes was always the same pitch, whether 

 the excitation was double or single, the only perceptible difference 

 being that in the former case the character and intensity of the note 

 varied at rhythmical intervals, due to alterations in nerve excitability. 4 



The graphic records of the electrical responses, evoked in nerve by 

 two successive stimuli, and obtained by the capillary electrometer 

 method, appear to show that a second response cannot be evoked if 

 the change evoked by its predecessor is still in progress. The exact 

 interval necessary for the second stimulus to be effective thus obviously 

 depends upon the duration of the first change, and this is largely 

 determined by temperature. 5 



Finally, it must be borne in mind that the experiments just referred 

 to deal almost exclusively with the medullated motor nerves of the frog 

 and mammal. At present there is no reliable information as to the 

 possession of the same attributes by the non-medullated nerves or the 

 afferent medullated nerves, as the determination by means of either 

 the muscular index or the electrical nerve response presents great 

 technical difficulties. On general grounds, it may be assumed as 



1 Roth, Arch.f. d. ges. PhysioL, Bonn, 1888, Bd. xlii. S. 91. 



2 Kronecker. "Die Genesis des Tetanus," Arch. f. AnaL, PhysioL u. wissensch. Med., 

 1868. 



3 Lodge and Gotch, llep. Brit. Ass. Adv. Sc., London, 1894, p. 818. 



4 Wedenskii, Compt. rend. Soc. de biol., Paris, 1893, tome cxvii. p. 4. 



5 Gotch and Burch, " Electrical Response of Nerve to Two Successive Stimuli," Journ. 

 PhysioL, Cambridge and London, 1899, vol. xxiv. p. 410. 



