NERVOUS FIBRES. I 17 



surroundecl 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 docs not contain any white substance, 

 presents no appearance of a fibrous structure j 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 

 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 explana- 

 tion, the accuracy of which must be decided by further 

 researches, for much more extensive investigations and a sepa- 

 rate and distinct consideration arc 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, lstly. An external, pale, 

 thin cell-membrane, having a granulated but not a fibrous 



