34 ELECTRO-PHYSIOLOGY CHAP. 



the key to nervous activity was always looked for in electrical 

 manifestations. But as in muscle, so in. nerve, these hopes were not 

 to be realised in their original sense, and although Albrecht (1) has 

 recently attempted to revive the old doctrine of the identity of 

 the " nervous principle " with current electricity, there can be no 

 serious discussion on these lines. In nerve, as in muscle, electro- 

 motive action must be viewed as the concomitant of chemical 

 processes, while again as in muscle its proper significance is 

 not yet adequately established. The nervous system invariably 

 consists of cellular elements (ganglion- or nerve-cells) and of 

 fibres, which must be reckoned as processes of the former, so 

 that the cell with its fibre forms an anatomical and physiological 

 unit (" Neuron',' Waldeyer ; " Neurodendron" Kolliker). At 

 their first appearance, both phylogenetically and ontogenetically, 

 the nerve-fibres are pale fibrous structures, greater or less in 

 length, which always spring from special cell-bodies (ganglion- 

 cells), and either pass branched or unbranched into the peripheral 

 end-organs, or establish mutual communication between different 

 ganglion-cells. While in the lower forms of animals this condition 

 is permanent, it is only a temporary phase of development in the 

 higher species, since various sheaths are added later (in parts at 

 least) to the originally naked fibres, so that the structure of the 

 individual nerve-fibre may become very complicated. The fibres 

 may be classified into different groups according to the very 

 different properties of these sheaths or investments, the most 

 characteristic being those termed medullated and non-medullated 

 fibres. The nervous system of vertebrates is composed almost 

 exclusively of the former, while the latter predominate among 

 invertebrates and the lowest vertebrata. 



Hence we may conclude that the one essential constituent of a 

 nerve-fibre is, functionally speaking, the substance of the cell process 

 or " axis-cylinder" as it is termed from the sheath which usually 

 envelops it, and serves mainly as a protective covering. We 

 shall, therefore, understand by " nerve-fibre " the single axis- 

 cylinder, whether as the process of a central or peripheral 

 ganglion- cell, or as the branch of such a process ; irrespective of 

 its being per se naked, or enclosed in a sheath, or if the 

 same sheath encloses few or many axis-cylinders. The last case 

 occurs most frequently in invertebrates. The nerves of the pro- 

 boscis of many Nemertines, e.g., contain within a tolerably thick 



