STRUCTURE OF TUBULAR NERVE-FIBRES. 75 



as the tubular and the gelatinous. The " tubular " fibres are so 

 named because each poss.' ;*es a distinct tubular sheath of a 

 delicate structureless membrane (fig. 22, A), which encloses the 

 proper nerve-substance, and isolates it completely from the 



Fig. 22. STRUCTURE OF NERVE-TUBES. 

 Tubular Nerve-fibres ; A, from a nerve-trunk; B, from the substance of the brain. 



blood-vessels and other surrounding structures ; this tube 

 does not either branch or unite with others, and there is 

 reason to believe it to be continuous from the origin to the 

 termination of the nerve-trunk. Within the tube is a hollow 

 cylinder of a material known (after its discoverer) as the 

 " white substance of Schwann ;" and this encloses a sort of 

 central pith, which is transparent and semi-fluid in the living 

 state, but undergoes a kind of coagulation into a granular sub- 

 stance after death, and under the influence of chemical 

 re-agents. There is reason to believe that this central pith or 

 " axis-cylinder " is the essential component of the nervous 

 fibre, and that the hollow cylinder which surrounds it serves 

 only to isolate it more completely; for we not unfrequently 

 see the former to be alone continued, both the tubular sheath 

 and the white substance stopping short ; and this at either 

 extremity of the fibre, where it separates itself from those 

 with which it is bound up in the nerve-trunk. The proper 

 form of the fibre seems always to be truly cylindrical ; though 



