148 NERVOUS FIBRES. 
aspect, the inner surface of which constantly exhibits cell- 
nuclei in the very early period of the development of nerve ; 
but in the somewhat more advanced stage, when the white 
substance is developed, they are only occasionally found. 2dly. 
That the white, fat-like substance to which the peculiar appear- 
ance and distinct outline of the nerves are chiefly referable, is 
deposited upon the inner surface of this cell-membrane. When 
this deposit is thick, its double contour (to which the nerve is 
indebted for its tubular appearance), may be recognised ; this, 
however, escapes observation when only a thin stratum of 
white substance is present. Morphologically considered, it 
therefore corresponds to the peculiar substance of muscle, for 
that is likewise developed asa secondary deposit upon the mem- 
brane of the secondary muscle-cell. 3dly. That the rest of the 
cell-cavity appears to be filled up by a firm substance, namely, 
the band discovered by Remak. There seems to be no struc- 
ture analogous to this band in perfectly-developed muscles, for 
there, the secondary deposit, that is, the formation of the pro- 
per muscular substance, proceeds until the cavity of the 
secondary cell is completely filled. 
We have thus traced the development of nerve to its per- 
fect state, without those irregular globules and little cylinders 
with the dark outlines, (which were mentioned at page 143, 
as occurring at a middle stage of the development of nerve 
in addition to the pale fibres and the matured nervous fibres), 
having proved to be a transitional step in the process. I 
am inclined to regard them as an artificial product, caused 
by pressure and the action of water upon the as yet very 
delicate nerve. If, for example, water penetrate through the 
cell-membrane by imbibition, the oil-like white substance re- 
tracts into separate rounded bodies, and the facility with which 
this takes place is proportionate to its slight degree of con- 
sistence. This is often seen even in fully-developed nerves ; 
an entire nerve frequently separates from this cause into sepa- 
rate globules or little cylinders, which have sharply-defined 
outlines, so that merely the cell-membrane proceeds uninter- 
ruptedly, in the form of a pale stripe, from the external wall of 
one of the dark portions to that of the other. Valentin has 
given a delineation of such a state of the nervous fibre, (Acta 
Acad. Leopold. Nat. Curios. vol. xviii, pl. III, fig. 7). As the 
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