VII 



STRUCTURE AND ORGANISATION OF NERVE 



35 



nucleated sheath of connective tissue a whole bundle of the finest 



axis-cylinders, each springing from one nerve-cell (2). On 



hastily examining preparations stained intra vitam 'with methylene 



blue, such a bundle of fibres might easily be taken for a single 



nerve-fibre with one axis-cylinder and a thick sheath ; closer 



inspection, however, shows the central filament to be composed of 



excessively fine fibres, deeply 



stained, and all doubt as to 



their nature is removed by the 



connection in every case with 



a separate nerve-cell (Fig. 149). 



The term " nerve-fibre " must, 



therefore, only be applied to 



the minute fibrils which branch 



off from the central bundle of 



the nerve-trunk to supply the 



peripheral end-organs. 



While in this case the rela- 

 tions of calibre between the 



single fibres which constitute 



FIG. 149. Section of a nerve from the proboscis 

 of Amphiporus marmoratus with paired cells. 

 (Methylene-blue preparation from O. Burger.) 



the bundle are tolerably uni- 

 form, we elsewhere find marked differences. In insects and crus- 

 tacea, e.g., broad band-shaped axis-cylinders frequently run alongside 

 very small and fibril-like filaments in the same sheath, and this 

 not merely in the coarser nerve-trunks, but in the finest terminal 

 branches also, where the calibre would justify us in reckoning them 

 individually as nerve-fibres. But it is in these muscular nerves 

 of Arthropoda with their copious ramifications (e.g. in the familiar 

 instance of the crayfish), that it is most easy to show that no 

 essential difference in structure exists between the coarser and finer 

 branches of the nerve, apart from the number of axis-cylinders 

 contained within one sheath of connective tissue. Neither the one 

 nor the other must be designated as nerve-fibres, but must be 

 treated simply as bundles. Notwithstanding therefore that, 

 even in the minutest ramifications, and actual terminal branches, 

 there may be several axis-cylinders in a common sheath, the 

 morphological definition of a nerve-fibre must be strictly con- 

 fined to one such axis-cylinder. It follows that the richly 

 developed connective-tissue sheath of the finest nerve-branches 

 in invertebrates must not be regarded as analogous to the 



