-STRUCTURE OF TUBULAR NERVE-FIBRES. 75 
as the tubular and the gelatinous. The “ tubular ” fibres are so 
named because each poss: <ses a distinct tubular sheath of a 
delicate structureless membrane (fig. 22, a), which encloses the 
isolates it completely from the 
Fig. 22.—Structure or Nerve-Tuses. 
‘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- 
ce after death, and under the influence of chemical 
agents. There is reason to believe that this central pith or 
“axis-cylinder” is the essential component of the nervous 
re, and that the hollow cylinder which surrounds it serves 
nly to isolate it more completely; for we not unfrequently 
se the former to be alone continued, both the tubular sheath 
the white substance stopping short ; and this at either 
ity of the fibre, where it separates itself from those 
ith which it is bound up in the nerve-trunk. The proper 
rm of the fibre seems always to be truly cylindrical ; though 
