OF THE TUBULAR MEMBRANE OR NERVE- SHEATH. 247 



The dark-bordered fibres distributed to the muscles 

 of the frog often divide into two finer fibres, as 

 represented in several of my figures, Plate YII, 

 fig. 2, and these at last divide into two very fine fibres, 

 which may be followed for a long distance. See 

 PL IX, fig. 4, and PL XI, p. 253. The fine fibres 

 appear pale and granular, and connected with them 

 at varjdng intervals are bioplasts (nuclei). These 

 pale nucleated fibres in the frog are often less than 

 ^^^ Tod oo <^f '■^^ "ich in diameter. They are never- 

 theless compound, consisting of bundles of still finer 

 fibres. These in fact, although much narrower, corre- 

 spond to the pale, granular, but nucleated intermus- 

 cular nerves fu'St described by me in the muscles of the 

 mouse, Plate IX, fig. 1, copied from the Phil. Trans., 

 1860. The very fine compound fibres still continue 

 to divide and subdivide, and assist to form plexuses 

 and networks in precisely the same manner as the 

 dark-bordered fibres, of which they are the continua- 

 tion. It is quite certain that these pale fibres are true 

 nerve-fibres, for they are directly continuous with the 

 dark-bordered fibres. Instead of breaking up into 

 one or more bundles of fine fibres, a dark-bordered 

 fibre not unfrequently divides into a finer dark-bor- 

 dered fibre, and a bundle of fine fibres, as represented 

 in one of my drawings from the frog's mesentery. 



274. Of the tubular membrane or nerve-sheath. — 

 As already stated, 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, § 235, the essential difference 

 between these two sets of fibres in the same trunk 

 being that the former set 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 trans- 

 parent matrix (sheath) with the dark-bordered fibre. 

 The idea of tubular membrane or sheath being" an 



