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 Remains 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 

 cell-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 comdction of its distinct and sepa- 

 rate existence, did not opportunities of observing it in 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, 



