STRUCTURE OF NERVE-FIBRES. 31 



that the pellucid rings in preparations treated with turpen- 

 tine consisted of the white substance rendered transparent by 

 that reagent. 



The point at issue was thus satisfactorily decided ; but for 

 the sake of confirmation we made some further observations, 

 the results of which seem deserving of mention. 



On examining the hardened sciatic nerve, without tinting 

 the preparations 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 spe- 

 cimens 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 remark- 

 able, 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 essen- 

 tial 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 sub- 

 jected to the action of chromic acid, imbibes the carmine 

 colour with peculiar facility, although the medullary sheath 

 is entirely untinged by it.f 



We next applied the high magnifying power to extremely 

 thin slices of the spinal cord prepared in the same way. 

 In transverse sections of the columnar regions the white 



* It must be mentioned that a similar brown colour is seen in the super- 

 ficial 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. 



f In a boiled fresh nerve also the medullary sheath remains unaffected by 

 ammoniacal solution of carmine, while the axial cylinder assumes a distinct 

 though very faint pink tint. — J. L. 



