THE TIME RELATIONS OF THE EFFECTS. 497 



is thrown upon the accuracy of Asher's conclusions by recent work done in 

 Hermann's laboratory. 1 (See also footnote.) 



Although the extrapolar development of the excitability alteration 

 and of the electrotonic effects may not at present be considered as con- 

 temporaneous, the time relations of the development of these two states 

 at the actual poles are closely similar. This parallelism between the 

 formation of the electrotonic currents and the production of the excita- 

 bility changes suggests a very close connection between the two classes 

 of effects. It is probable that the electrotonic effects are due to what may 

 be termed two different reactions in the nerve fibre, one the formation of 

 ions, an electrolytic process rendered possible by the physical conditions 

 presented by the living nerve structure, the other • the establishment of 

 prolonged excitatory changes with their accompanying electromotive 

 phenomena. The further consideration of this may with advantage be 

 postponed to the section dealing with the electromotive phenomena of 

 nerve, but it may be pointed out that this second reaction is to a great 

 extent the electrical indication of the alteration in excitability, which 

 forms the proper subject of this section. It is not, therefore, surprising 

 to find that the time relations of the development, in close proximity to 

 the poles of excitability and of electrotonic changes, are to a large 

 extent identical. 



The two changes, excitability and electrotonic, both commence at the 

 moment of closure, and they are both developed in increasing amount 

 during the first period of such closure. Investigation with the rheotome 

 shows that the anodic fall of excitability, and the anelectrotonic change, 

 gradually increase at the anode during the continued flow of the current, 

 and then slightly decrease; on the other hand, the cathodic rise and 

 catelectrotonic change are both developed quickly, the development 

 culminates much sooner than that of the anodic states, and may subside 

 whilst the latter is still going on, if the closure is sufficiently long. 



It is for this reason among others that duration of closure, apart from 

 the intensity of the current, has such an appreciable effect upon the 

 production and spread of excitability changes ; the longer the duration, 

 the more will the anodic fall overpower the subsiding cathodic rise, the 

 indifference point moving at first nearer the cathode, until finally the 

 anodic state may spread into the cathodic extrapolar region. For similar 

 reasons a very short closure produces a cathodic rise of excitability, 

 which is more obvious than the anodic fall, and thus a series of alter- 

 nating induced currents, recurring at rapid intervals, will excite 

 sufficiently to evoke a flickering muscle response, when one induced 

 current of similar intensity and character is inadequate. 



The excitability changes on the cessation of the current flow are to 

 be regarded as due to a rebound of the tissue equilibrium, when released 



1 Hermann and Weiss, Arch. f. d. ges. Physiol., Bonn, 1898, Bd. lxx. S. 237-296. 

 The writer of this article agrees with the position taken up by Hermann, namely, that both 

 the excitability and the electrotonic modifications commence almost instantaneously in 

 all parts of the extrapolar regions. The fact that such modifications develop the more 

 slowly and less extensively in the extrapolar region, as the region investigated is removed 

 from the pole, accounts for many of the contradictory results hitherto obtained. A certain 

 degree of change must occur before evidence, whether of decreased excitability or electro- 

 tonic effect, can be obtained ; this degree can be attained instantaneously at the pole, but 

 it may not be detected in the extrapolar region, although a feeble modification may have 

 begun instantaneously even in this part. Such would be pre-eminently the case with the 

 anodic effects, which have been those chiefly investigated. 



VOL. II. — 32 



