NERVOUS FIBRES. 147 
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 6.) 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 ap- 
pears 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 
m the pale border, which surrounds the white substance. 
Fig. 9, ¢ and d, exhibits them from the nervus vagus of a 
ealf. At ¢ 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 explana- 
tion, the accuracy of which must be decided by further 
researches, for much more extensive investigations and a sepa- 
rate and distinct consideration are absolutely necessary for 
aceurate 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, Istly. An external, pale, 
‘thin cell-membrane, having a granulated but not a fibrous 
