546 ADDENDUM. 



cases the electrotonus ceased abruptly about ^^" after the polarising 

 current was broken. 



Prof. Bernstein carefully avoids drawing any theoretical conclu- 

 sions from his experiments, which must be regarded as a continuation 

 of those embodied in his well-known * Researches on the excitatory 

 process in Muscle and Nerve/ published in 1871. He regards 

 extrapolar electrotonic currents as indications of a ' state of polar- 

 isation peculiar to living nerve, which is propagated in nerve fibre 

 from section to section/ but declines to express any opinion as to its 

 relation to ordinary physical polarisation. 



2. TIME-RELATIONS OF THE EXCITATORY PROCESS IN MUSCLE AND 



NERVE. 



An investigation recently conducted in Prof. Hering's laboratory 

 by Mr. Head of Cambridge 1 is of sufficient novelty in relation to 

 the excitatory process in nerve to make it desirable that the reader 

 should be informed on the subject. 



Our knowledge of the time-relations of the process have been 

 hitherto exclusively founded on the experiments of Bernstein with 

 the repeating rheotome, recorded in his well-known monograph ' On 

 the excitatory process in Muscle and Nerve.' Mr. Head's experi- 

 ments on the subject have been made with an improved form of 

 instrument, the construction of which renders it possible to subject 

 a nerve to excitations of a frequency much greater than was attain- 

 able with the original rheotome of Bernstein, and thereby, as well 

 as by employing a galvanometer better adapted to the purpose, to 

 obtain indications of much greater delicacy than those afforded by 

 Bernstein's method. As regards the duration of the negative 

 variation, due to a single excitation, the new determinations show 

 that Bernstein's estimate, which has up to the present moment 

 been universally accepted, is very much too short, that the period 

 of negativity lasts, even in the nerves of summer frogs, for more 

 than a hundredth of a second, and in winter frogs twice as long. 

 But this point, however interesting and important, was not the 

 main object of the enquiry. Mr. Head's efforts were chiefly directed 

 to obtaining determinations of the conditions which lead to the 

 positive after effect which, as Hering showed in his paper ' On 

 tetanic excitation of Nerve/ succeeds the concomitant diminution 



1 Henry Head, Ueber die negativen u. positiven Schwankungen des Nerven- 

 aystems. Pfltiger's ' Archiv,' vol. 40. p. 207, 1887. 



