EXCITABILITY AND COND UCTIVITY IN NER VES. 48 1 



minimal intensity, evokes a contraction of the muscle opening the 

 claw ; with stronger stimuli the normal tonicity of this is actually 

 diminished, whilst the muscle closing the claw is thrown into action. 

 The results appear to show that there are two sets of nerve fibres, 

 augmentor and inhibitory, for each muscle ; hence the degree of 

 excitability of these four sets of neuromuscular mechanisms is different 

 for each set. 



It is well known that the vago-sympathetic of the frog contains 

 two sets of nerve fibres, the excitation of which causes in the one case 

 inhibition and diminution, in the other augmentation and acceleration 

 of the cardiac contraction. The former are stimulated by electrical 

 currents which are too weak to excite the latter ; on the other hand, 

 the inhibitory fibres lose their excitability more rapidly than the 

 augmentor. 1 



The difference in the excitability of different nerves of the same 

 animal, of which the above are illustrations, might be exemplified by 

 many other instances ; probably no two nerves are equally excitable. It 

 is not, therefore, surprising that great differences exist in the nerves of 

 widely different animals. One of the most striking instances of this is 

 furnished by the electrical nerves supplying the electrical organs of 

 fishes (Torpedo, Malapterurus, Gymnotus, etc.). These all possess a 

 comparatively low degree of excitability. E. du Bois-Reymond claims for 

 the whole nervous system of these fish, a low excitability, which confers 

 upon them relative immunity to excitation by their own electrical 

 shocks. Although the existence of such lessened excitability is ques- 

 tioned by Schonlein, 2 there is, in the opinion of the writer, little doubt 

 as to its existence in Malapterurus. In this fish the nerve can only 

 be adequately excited by an intensity of current out of all proportion 

 to that requisite for the excitation of the frog's sciatic, the ratio being 

 at least 100 to 1 ; whilst in neither Torpedo nor Malapterurus is the 

 nerve at all readily excited by chemical agencies. 3 



A strict comparison of the excitability of different nerves is un- 

 doubtedly more or less vitiated by inequalities in the electrical resist- 

 ance, by variations in the position and character of the sheathing 

 investments, and still more by the unknown extent of alterations 

 produced by exposure and dissection. Hence variations in excitability 

 exist even with different regions of any single nerve, such as the frog's 

 sciatic. Any dissection must inflict local injuries, and thus cause local 

 heightened excitability. For this reason the upper two-thirds of the 

 frog's sciatic, for some little time after dissection, is more excitable 

 to electrical stimulation than the lower third, since more branches 

 must be divided in exposing the upper part. 4 The character of 

 the electrical stimulus appears also to be a factor which has to be 

 taken into account. Thus Fleischl found the excitability different 

 for ascending and descending exciting currents, the former evoking a 

 muscle response more easily in the distal than in the central portions, 

 the latter the reverse. 5 It has been already mentioned, that the 



1 See the article on the " Heart," by Gaskell, in this volume. 



2 Ztschr.f. JBioL, 1898, Bd. xxxi. 



3 Schonlein, ibid., Bd. xxi. S. 461 ; Gotch and Burch, Phil. Trans., London, 1896, 

 vol. clxxxvii. JB, p. 370. 



4 Fleischl ; Heidenhain, Stud. d. physiol. Inst. zu Breslau, Leipzig, 1861. 



5 Fleischl, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1880, Bd. Ixxiv. Abth. 3. See 

 also Weiss, Arch. f. d. ges. Physiol. , Bonn, 1898, Bd. Ixxii. S. 15. 



VOL. II. 31 



