1865.] Fine Fibres with Dark-bordered Fibres. 241 



Of the fine fibres running with the dark-bordered fibres. 

 We find in the same nerve-trunk fine and coarse dark-bordered fibres, 

 and we often observe exceedingly fine pale fibres running with dark- 

 bordered fibres, the essential difference between these two classes of fibres 

 in the same trunk being that the former fibre is nearer to its ultimate 

 distribution than the latter ; but in some instances it is probable that the 

 fine fibre is a branch of the sympathetic. The fine fibre runs in the same 

 transparent matrix (sheath) with the dark-bordered fibre. In fact the idea 

 of tubular membrane or sheath being an essential and separate anatomical 

 constituent of every individual dark-bordered fibre must be given up. For, 

 as I showed in 1860, several dark-bordered fibres and fine fibres might run 

 together in the same sheath or matrix. The opinion that the fine fibres 

 which I hold to be nerve-fibres running in the same sheath with the dark- 

 bordered fibres, are not nerve-fibres at all, but modified connective tissue, is, 

 however, still entertained by many observers. As I have before stated, 

 their continuity with true dark-bordered fibres may often be seen, and the 

 same fibre may in some instances be followed to its ultimate distribution. 



The different and incompatible views existing between continental observers 

 and myself are in some measure due to this sheath question. The so-called 

 sheath is not a "tube" or "membrane," or "tubular membrane," which con- 

 tains the other constituents of the nerve-fibre ; nor is it a sheath which 

 invests them, but it is simply a transparent matrix, in which nerve-fibres, 

 coarse and fine, are imbedded. The so-called sheath is not formed as a 

 special structure to invest the nerve-fibres, but it results from changes oc- 

 curring in the nerve-fibres themselves. This " sheath " or " tubular mem- 

 brane " of the so-called dark-bordered fibre precisely corresponds to the 

 transparent connective tissue, in which the fine nerve-fibres are imbedded. 

 It is a form of connective tissue, and in many situations where nerves 

 existed at an earlier period, nothing but this so-called sheath remains. All 

 the soluble fatty matters have disappeared, and this material, which is not 

 readily absorbed, is left behind. Vessels may waste, and ducts and glands 

 may waste, and leave behind them the same sort of transparent connective 

 tissue. Moreover, as I have before stated, it is altogether a fallacy to 

 suppose that near the peripheral distribution, every single branch of nerve- 

 fibre is surrounded by its own separate sheath. The drawings of the so- 

 called axis-cylinder near the terminal distribution of the nerves also seem to 

 me to be diagrammatic, founded rather upon a theoretical idea of the consti- 

 tution of the nerve-fibre than upon the results of actual observation. 



Many of the pale fibres accompanying the dark-bordered fibres are 

 doubtless sympathetic fibres, for it has been shown that there are fine fibres 

 springing from ganglion- cells which retain the same character from their 

 origin to their distribution (see p. 236). Not only has the nervous nature 

 of the fine fibres above described been proved by tracing them from 

 their connexion with ganglion-cells, but a dark-bordered fibre has often 

 been observed to be drawn out so as to form a line as fine as these fine 



