Physiologists even at an early date, misled by the apparent likeness in the conduction of excitation, especially in the nerve, to that of electricity in a metal wire, regarded both processes as identical. When, however, Helmholtz first demonstrated experimentally the rapidity of the conduction in the nerve, the thought that electrical conduction was concerned, such as takes place in a metal wire, had to be abandoned, as the velocity shows too great a difference in the two cases.
Fig. 25.
Model of a “Kernleiter.” A, B—Glass tube, with a number of side tubes filled with saline solution, through which a wire is passed. c and d—Side tubes with electrodes for stimulation. e and f—Tubes for connection with a galvanometer. (After Hermann.)
The observations, on the other hand, on the conductivity in the so-called “core model,” seemed to offer another possibility of attributing the conduction of excitation in the nerve to electric processes. Matteucci, later Hermann and finally Boruttau[114] have endeavored to apply the results obtained when electricity is introduced in a wire covered with a moist envelope (saline solution), to the explanation of conductivity in the nerve. (Figure [25].) The fact has been shown, that in such a model the application of electricity to a point, as a result of polarization between the moist envelope and the metal, produces a weak local current, which in turn disturbs the electrical potential in the next cross section and consequently a new local current is produced and so on through the whole length of the wire. (Figure [26].) This fact, in connection with the apparent similarity in the differentiation of the axial fibers and peripheral envelope in the nerve, has led Boruttau to apply the principles of conductivity in the “core model” to that of the nerve. Then, however, Nernst and Zeyneck brought forward their theory, according to which the galvanic current is operative as a stimulus in that it brings about an alteration in the concentration of the ions at the junction of two different electrolites which, in turn, produce local currents. Boruttau then dropped the assumption of the existence of a simple physical polarization between the wire and the envelope and replaced it by the assumption of an alteration in the concentration of the ions at this position. Thereby the “core model explanation” was already altered in principle, in that only the differentiation of a central fibrilla and a peripheral enveloping substance was appropriated. It seems to me that this factor can likewise be considered as completely dispensable and may, therefore, be omitted; thus nothing remains of the “core model explanation” of the conduction of excitation in the nerve.
Fig. 26.
Scheme of the conduction by local electric currents in a “Kernleiter.” (After Hermann.)
The results of continually increasing numbers of investigation in recent times make it appear almost as a certainty that the elementary fibrillæ in the axis cylinder are nothing else but skeletal substances. Wolff,[115] Verworn[116] and others have first expressed the view that the neurofibrillæ must be looked upon as skeletal fibers for the soft neuroplasm, and more recently Lenhossek[117] and especially Goldschmidt[118] have confirmed this assumption in detail. Goldschmidt has shown by extensive comparative studies of cell mechanism the rôle played by the neurofibrillæ in a physical connection as internal skeletal formations, and has proved at the same time, in complete unanimity with other investigators, their continuity with other undoubted skeletal fibrillæ. By this the numerous combinations and speculations of Apathy and Bethe concerning the part taken by the neurofibrillæ have been rendered untenable. In no case is there the slightest justification to regard the apparent “Kernleiterstructur” of the nerve as the principal condition for the process of conductivity, for should we dispense completely with this point for the theory of the conduction of the nerve, we can obtain, solely by the aid of the facts known today in physical chemistry, the foundations for a theory of the conductions of excitation which not merely renders the specific case of the conduction of the nerve intelligible, but contains at the same time the principles of the process of the conduction of excitation for all living substance.
