THE NERVOUS SYSTEM. 355 



tions of the whole contents of the nerve-tube, and have regarded their 

 formation as a proof against the pre-existence of the axis-fibre, but 

 incorrectly. They belong to the medullary sheath only, which, in the 

 interior of all nerve-fibres with only a double contour, is still for some 

 space perfectly clear and bright. An axis-fibre and a clear space, in 

 fibres having a double contour, are therefore by no means identical, and 

 it is not at all surprising, nor opposed to the existence of an axis-cylin- 

 der, that a multitude of drops with a double contour and clear contents 

 should be obtained from such fibres. The medullary sheath may also 

 coagulate entirely, and then the axis-fibre remains evident, sometimes as 

 a transparent streak of uniform breadth throughout ; sometimes, when 

 the grumous particles are more numerous, it may be concealed by them, 

 so that the entire contents of the nerve appear to be coagulated. They 

 are so, however, only in appearance, the clear fibre always lying in the 

 interior ; and I have never yet seen it coagulated or grumous. Non- 

 medullated nerve-fibres occur in many situations. I enumerate among 

 them : 1, the pale fibres in the Pacinian bodies ; 2, the nucleated 

 pale fibres in the terminations of the olfactory nerves ; 3, the per- 

 fectly transparent, non-nucleated nerve-fibres in the cornea; 4, the 

 pale, branched, and partially anastomosing terminations of the nerves 

 in the electrical organ of the Torpedo and Ray (R. Wagner, Ecker) ; 

 5, the similarly constituted terminations of the nerves in the skin of 

 the Mouse (vid. " Micr. Anat." 11); 6, the pale processes of the 

 nerve-cells in the central organs and ganglia, even though they may 

 not all pass into dark-bordered fibres. Of these fibres, those which 

 occur at the extremities of nerves were, even by the earliest observers 

 of them, unconditionally regarded as nerve-fibres ; and as respects the 

 processes of the nerve-cells, I described this to be their nature as early 

 as the year 1846 ; but these views could not be considered as fully es 

 tablished, until the relation of the fibres with the elements presenting 

 the dark borders was completely elucidated. But since it has been 

 ascertained by Schwann, Ecker, and myself, that the nerve-fibres of 

 the embryo are in precisely the same condition as the pale fibres now 

 in question, and since, I, Wagner, Robin, and Bidder and Reichert, 

 have shown that the pale processes of the nerve-cells pass into dark- 

 bordered fibres, it has become more possible to arrive at positive 

 conclusions on the subject. R. Wagner was the first to broach the 

 supposition, that the pale fibres in the Pacinian bodies, and in the elec- 

 tric organs, were nerve-sheaths, with axis-cylinders, and that the pro- 

 cesses which pass into nerve-fibres, were themselves bare axis-cylinders, 

 and, moreover, that the entire granular contents of a nerve-cell are 

 nothing but an axis-cylinder enlarged into a globular form ; and after 

 I had demonstrated the constant existence of the axis-cylinder in the 

 living nerve, and that it was a structure distinct from the medullary 



