216 On the Construction of the 



or denuded from them (Figs. 16, 17, and 20), are always met with, 

 on wliicti their granular-fibrillous structure can be distinctly recog- 

 nized. In man especially, they can always be demonstrated ; the 

 same is the case with the sheath surrounding the bundle of fibrils, 

 which in many instances manifests itself by a very sharply-defined 

 double contour. 



In examining, now, one of these axis cylinders, previously pre- 

 pared with a weak chromic acid solution, with an amplification of 

 about 275 to 300 diameters, and with central illumination, nothing 

 more is recognized on it than an exceedingly fine, longitudinal 

 striation ; but in increasing the magnifying power to about 500 

 diameters, the aspect is changed, and we observe in the course of the 

 striae a number of dark points, separated from each other by minute 

 interspaces, and which, with this illumination, may be easily looked 

 upon as minute granules (Fig. 14). The view thus obtained cor- 

 responds also with Max Schultzes description of the structure of the 

 axis cyhnder. If now, however, the same object is illuminated with 

 oblique light by means of an achromatic prism, it will be found 

 that the dark points are in reality no granules, but are only pro- 

 duced by minute depressions. At the same time, fine, pale shadows 

 will be observed, proceeding from the latter and crossing the fibrils 

 of the axis cylinder, while, between these lines of shadow, minute 

 elevations appear in the form of clear, shining points, giving to the 

 whole the appearance of rows of minute granules partially fused 

 with each other (Figs. 15, 16, c — 17, and 20). In referring to the 

 diagram (Fig. 19), which represents, on a larger scale, an axis 

 cylinder obliquely illuminated, we shall observe its three fibrils, 

 composed of granules, running in such a manner parallel to each 

 other that, transversely, their granules come to lay also in a straight 

 hne. As the intermediate substance connects the latter prin- 

 cipally at those points where they come in direct contact with each 

 other, the greatest depression will naturally be found at such points 

 where they are most distant from each other, that is, in the centre 

 between every four granules placed in immediate apposition to each 

 other. And it is just at this place where the above-mentioned 

 dark, granule-like points are seen. If we now imagine the whole 

 illuminated with obhque light, it follows that one side of the 

 granules will be more strongly illuminated than the other, by which 

 their roundness is brought out; wliile, at the same time, those 

 depressions between every four granules must appear darkest. The 

 whole arrangement and appearance of these granules resembles 

 somewhat the hexagonal elevations seen on some of the Diatomaceae, 

 as, for instance, on the Pleurosigma angidatum. 



The granular-fibrillous structure of the axis cylinders is very 

 distinctly recognized on those of the finer nerve fibres of the spinal 

 marrow or the brain, consisting of two or even only one fibril, and 



