STRUCTURE OF TUBULAR NERVE-FIBRES. 
75 
as the tubular and the gelatinous. The “ tubular ” fibres are so 
named because each possesses 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 Schwannand 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 unfrequentlv 
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 
