surrounded externally with a thin, pale membrane, which has a sharply-defined external margin. If the membrane be very thin, it cannot be recognised as the pale border round the nervous fibre; it is still, however, distinctly visible at situations where the white substance is destroyed. (See fig. 9 b.) The mere fact of the membrane possessing a defined external border, is evidence against its being composed of areolar tissue ; and even the portion which does not contain any white substance, presents no appearance of a fibrous structure ; it simply appears to be somewhat minutely granulated. If this be correct, the membrane can have no other signification than that of cell-membrane of the nervous fibre, or secondary nerve-cell. The white substance is then a secondary deposit upon its inner surface. The position of the cell-nuclei is also favorable to this view. Most of the cell-nuclei, presented by the nervous fibres in their earliest and as yet pale condition, disappear during the formation of the white substance, a circumstance which is common to most other cells. Some, however, appear to remain for a longer period; occasionally, although rarely, a cell-nucleus is here and there seen upon the side of a nerve, (the white substance of which is completely developed), lying in the pale border, which surrounds the white substance. Fig. 9, c and d, exhibits them from the nervus vagus of a calf. At c the white substance, corresponding to the nucleus, even forms a slight projection into the cavity of the fibre. This nucleus seems therefore actually to belong to the fibre, and to lie upon the inner surface of the cell-membrane, while the white substance is so deposited, that the nucleus remains situated external to it. The band discovered by Remak would then be the proper cell-contents. Meanwhile I beg that the above may be regarded simply as an attempt at an explanation, the accuracy of which must be decided by further researches, for much more extensive investigations and a separate and distinct consideration are absolutely necessary for accurate decision of so important a subject.
According to the foregoing explanation, therefore, each nervous fibre is, throughout its entire course, a secondary cell, developed by the coalescence of primary nucleated cells. With respect to these cells, we remark, 1stly. An external, pale, thin cell-membrane, having a granulated but not a fibrous
