528 NERVE. 



in their resting difference ; 1 but in medullated nerves a succession of 

 stimuli must be employed. The proofs that the negative variation pro- 

 duced by such successive stimuli is not a uniform continuous electro- 

 motive change, but a series of the same rhythm as the excitation, are 

 similar in character to those which exist in the case of muscle, and 

 differ merely in detail. 



Observations with rheotome. — If the differential rheotome referred 

 to in connection with muscle is employed, it is found that when the 

 rate of revolution is 20 per second, a large negative variation occurs 

 if the galvanometric contacts of the instrument are so arranged as to 

 comprise a time of from one to ten thousandths of a second after each 

 excitation ; whilst with contacts arranged in any part of the remaining 

 twentieth of a second, which intervenes between the successive excita- 

 tions, there is no such effect. 2 



Observations with the telephone — If a delicate Siemen's telephone 

 is arranged so as to be in metallic contact with the surface and cross 

 section of a large excised nerve or group of nerves, and the nerve be 

 excited near its further end by a rapid series of induced currents (100 

 per second), a note similar to that of the vibrating exciter is heard. That 

 this is due to the production and transmission of a series of true electrical 

 responses, and not to the physical spread of the exciting induced 

 currents, is proved by observing that it may be augmented or diminished 

 by placing the excited region in the condition of catelectrotonus and 

 anelectrotonus respectively, that it may be blocked by ether, and other 

 agents which impair nerve conductivity, and that it is abolished by 

 destroying the functional continuity between the excited and observed 

 regions. 3 



Observations with the capillary electrometer. — An instrument com- 

 bining great sensitiveness and rapidity of movement is highly magnified, 

 and the image of the mercurial meniscus thrown upon a photographic 

 plate which travels at about 50 cms. in 1 sec. When the instrument is 

 connected with the excised sciatic nerve of the frog, a movement of the 

 level of the meniscus is recorded on the plate in response to each single 

 stimulus of the nerve trunk, whether electrical or mechanical. When 

 the nerve is excited by a series of stimuli of 100 per second or less, a 

 movement is shown on the record corresponding to each member of the 

 series of ex citations. 4 



Observations with the physiological rheoscope. — The phenomena of 

 secondary tetanus can be obtained when two excitable nerves are in 

 juxtaposition. 5 Such excitation of one nerve by the excitatory electrical 

 response in an adjoining one was first demonstrated by Hering as follows. 

 In the cooled frog the sciatic branches are prepared near the knee, 

 ligatured and divided below this, and then dissected up a short way. 

 On now exposing and dividing the upper sciatic plexus, the excitation of 

 the lower sciatic branches evokes a tetanic contraction in the adductor 

 muscles. This contraction is not due to an electrotonic escape, such 

 as that which causes the paradoxical contraction of du Bois-Keymond. 

 This is proved by the circumstance that it is evoked with a pro- 



1 Biedermann, Sitzungsb. d. k. Akad. d. Wlssensch., Wien, loc. cit. 



2 Bernstein, " Untersuch. ueber Nerv. u. Musk.," Heidelberg, 1891. 



3 Wedenski, Comjrt. rend. Acad. d. sc, Paris, 1893, tome cxvii. p. 4. 



4 Goteh and Burch, Proc. Roy. Soc. London, 1898, vol. lxiii. p. 300. 



5 Hering, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1882, Bd. lxxxv. Abtli. 3, S. 237. 

 See also "Biological Memoirs," Oxford, 1889. 



