STRUCTURE OF NERVE-FIBRES IOI 
with carmine, we found that in extremely thin slices the transverse sections 
of the nerve-fibres, viewed by transmitted light, appeared as brownish rings 
with central transparent colourless spots (see Fig. 3), whilst by reflected light 
the central parts appeared black, as shown in Fig. 2. In fact, under a low 
power the axial cylinders had, in these specimens of the sciatic nerve, as much 
the appearance of mere spaces as the medullary sheaths had in preparations 
of the cord treated with turpentine. But on applying a fine glass of high power 
a granular appearance was disclosed in the pellucid central portion, showing 
that it was in reality a solid substance, though of a transparency which was 
very remarkable, considering that it had been so long subjected to the action 
of chromic acid; and on afterwards treating similar sections with carmine we 
found that this part alone became coloured. The higher magnifying power 
also brought out an appearance of irregular concentric lines in the brown! 
medullary sheath; and this, together with the granular aspect of the axial 
cylinder, is represented in Fig. 4. 
These facts afford a very striking illustration of the essential difference 
in chemical composition between the axial cylinder and the medullary sheath, 
the former being totally unaffected by chromic acid, though the latter is rendered 
opaque and brown and concentrically striated under its influence, while, on 
the other hand, the axial cylinder, after being subjected to the action of chromic 
acid, imbibes the carmine colour with peculiar facility, although the medullary 
sheath is entirely untinged by it. 
We next applied the high magnifying power to extremely thin slices of 
the spinal cord preparedin thesame way. In transverse sections of the columnar 
regions the white substance of Schwann presented, in the larger fibres, the same 
concentrically arranged appearance as we had observed in the sciatic nerve, 
as is illustrated by Figs. 6 and 7, of which Fig. 6 is one of the largest met with, 
being 1-gooth of an inch in diameter, while Fig. 7 is as small as 1-3000th of an 
inch in transverse measurement. In the very minute fibres no appearance of 
concentric lines could be detected, yet, wherever the existence of an axial cylinder 
was indicated by a carmine point, a ring of medullary sheath was always visible, 
presenting the same proportion to the axial cylinder as in fibres of larger size. 
This may be gathered from Figs. 8, 9, and ro, of which Fig. 8 measures 1-5000th 
of an inch across, Fig. 9 1-800o0th, and Fig. ro only 1I-r4oooth. 
1 It must be mentioned that a similar brown colour is seen in the superficial parts of a cord which 
has been steeped in chromic acid, but the deeper portions of the organ are comparatively only slightly 
coloured, so that in individual nerve-fibres seen under a high magnifying power the brown tint is not 
observed. 
* In a boiled fresh nerve also the medullary sheath remains unaffected by ammoniacal solution 
cf carmine, while the axial cylinder assumes a distinct though very faint pink tint.—J. L. 
