258 PHYSIOLOGY CHAP. 



action current of the nerve. To demonstrate this fact it is only 

 necessary to send a polarising current of brief duration through 

 the model, and to lead off from more or less distant points by the 

 galvanometer electrodes. Electrotonic currents make their appear- 

 ance at a time when the polarising circuit has already been broken. 

 How these waves, which are analogous to the action currents of 

 nerve, can be generated in the artificial conductor is still obscure. 

 The apparent similarity of the two phenomena is interesting, and 

 justifies the conjecture that conduction of the impulse in nerve 

 consists in the spreading in wave form of a physico-chemical 

 molecular process, comparable with that observed on the core- 

 model. This hypothesis agrees better with the known facts of the 

 velocity of transmission of the nerve impulse (which we have seen 

 to be about 40 m. per second) than any other theory, on which 

 the active state of nerve is assumed to be a chemical modification 

 accompanied by metabolic phenomena. 



Biedermann objected to the hypotheses of Hermann and 

 Boruttau that conduction of excitation is a general property, 

 common to many tissues very different from nerve. In some of 

 these tissues conduction takes place from cell to cell, e.g., in ciliated 

 epithelial cells, in hydroid colonies, in the cells of cardiac muscle, 

 etc. Yet, as Boruttau remarks, in these cases cited by Biedermann 

 the transmission of the impulse can be measured by millimetres 

 or centimetres per second. These phenomena are in a different 

 category from those manifested in the homogeneous elementary 

 fibrils of the axis-cylinder, in which the velocity may reach 60 m. 

 per second. 



On the other hand it must be pointed out that nerves have 

 recently been found in invertebrate animals with a comparatively 

 sluggish rate of conduction, while many gradual transitions exist 

 between the most rapid conduction of vertebrate nerves and that 

 of other tissues, so that there is no justification for assuming a 

 fundamental difference in the process of nerve conduction. As 

 we have seen, the latest investigations on asphyxia and fatigue in 

 nerve have proved that its metabolism, however small, is by no 

 means a negligible factor, and must be taken into account in 

 any comprehensive theory of nervous activity. The chemical 

 theory, which refers the conduction of the excitatory impulse in 

 nerve, like that in all other tissues of the body, to the propaga- 

 tion of a process of chemical change, and regards the electrical 

 phenomena solely as accessory, is, therefore, at least in .theory, as 

 acceptable as the purely physical theory. 



Of late the theory of axial conduction seems to be yielding 

 more and more to modern concepts of physico- chemistry, by 

 which the bio-electric phenomena are referred to the principle of 

 concentration cells (Ostwald, Tschagowetz, Macdonald, Oker-Blom, 

 Bernstein, and others). Nernst and Zeynek (1899), on the strength 



