a secondary deposit upon the inner surface of the cell-mem- brane, being chemically distimct from the latter, and the remainder of the cell-cavity may then, and not until then, become 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 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 included 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 separate 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 thickness 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,
