146 NERVOUS FIBRES. 
a secondary deposit upon the inner surface of the cell-mem- 
brane, being chemically distinct from the latter, and the 
remainder of the cell-cavity may then, and not until then, be- 
come filled up by Remak’s band. 
It will be seen that the above question is analogous to 
that raised when we were treating of muscle, viz., whether 
the proper muscular substance be a thickening of the original 
eell-membrane itself, or a secondary deposit upon it. The 
reply is not, in either instance, essential to the proof of the 
origination of nerves or muscle from cells, but it is of so much 
the more importance for the explanation of the structure of a 
perfectly-developed nerve. If any conclusion may be drawn 
from the few observations which I have made on this point, 
the latter view appears to me the most probable, viz., that 
the white substance is a secondary deposit upon the inner 
surface of the cell-membrane. The white substance of each 
nerve is surrounded externally with a structureless and peculiar 
membrane, which appears to be minutely granulated. This 
membrane presents itself as a narrow, clear border, which 
is readily distinguished from the dark contours of the white 
substance. This membrane seems hitherto to have been in- 
cluded with the neurilema or with the cellular tissue, which 
surrounds the nervous fibre, and although its external outline 
is generally very sharply defined in the nerves of the frog, it 
would be difficult, on examination of the entire nerve of a 
mammal, to arrive at any conviction of its distinct and sepa- 
rate existence, did not opportunities of observing it im an 
isolated state present themselves. Pl. IV, fig. 9 a, represents 
such a preparation, taken from the cranial portion of the 
nervus vagus of a calf. The continuity of the white substance 
has here been broken by the process of preparation; but 
where it still exists, the double contours, (and thus the thick- 
ness of the white matter), may be clearly distinguished. But 
the nerve still exists at the part where the white substance is 
separated, its sharply-defined external margins may be seen, 
although their contours are but pale, and it may be observed 
that this pale outline does not pass into the external dark 
one of the white substance, but is continued on the outside of 
it as a narrow border, parallel to the two outlines of the 
white substance. The white substance of nerve is, therefore, 
f 
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Ps 
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