102 SOME OBSERVATIONS ON THE 
At the margins of longitudinal sections of the cord, the contrast, both in 
structure and in tint, between the axial cylinder and the medullary sheath 
showed itself very beautifully. It often happened that a projecting isolated 
fibre was, near its extremity, more or less divested of the white substance of 
Schwann, so that the delicate, carmine-tinted axial cylinder was exposed, though 
presenting here and there colourless flakes of the medullary sheath adhering to 
its surface, while in parts where the nerve was still entire, the pink colour 
of the central fibre could be distinctly discerned through the intervening white 
substance. Fig. 11 represents a large fibre under such circumstances, and 
Fig. 12 one of considerably smaller size ; and these sketches also display the 
remarkable fibroid arrangement which we find the white substance of Schwann 
invariably assumes under the influence of chromic acid. 
In conclusion, we may remark that the successive employment of chromic 
acid and carmine seems likely to afford valuable aid in discriminating nerve- 
fibres among other structures, there being, so far as we are aware, no other 
form of tissue which, after the use of these means, exhibits fibres having a central 
carmine axis and peripheral uncoloured sheath. 
Supplementary Observations by Mr. LISTER 
The fibroid arrangement of the white substance of Schwann in nerves 
hardened by chromic acid has been minutely described by Stilling, in his 
elaborate treatise on the ‘ Nerve-fibre and Nerve-cell’,! a work which we had 
not seen when the foregoing communication was written, but a copy of which was 
kindly lent me by Professor Goodsir, soon after Mr. Turner had left Edinburgh 
for the vacation. According to Stilling, the medullary sheath is, even in per- 
fectly fresh nerves, composed of a network of fibres, which are continuous 
with others in the axial cylinder and in the proper investing membrane; so 
that, in his opinion, these three constituents of the nerve-fibre differ from each 
other only in the manner in which their elements are disposed.2 This view 
is not only quite novel anatomically, but is opposed to the generally received 
physiological opinion, that the axial cylinder is the essential part of the nerve- 
fibre, and the medullary sheath an insulating investment. Considering the 
high estimation in which the writings of Stilling on the anatomy of the nervous 
centres are deservedly held, and the influence which therefore attaches to his 
opinions, it seems fortunate that we have been able to present so clear a demon- 
stration that the axial cylinder is chemically as well as morphologically totally 
distinct from the medullary sheath. 
* Ueber den Bau dev Nerven-Primitivfaser und der Nervenzelle. Won Dr. B. Stilling. 1856. 
ERO. /Cits, (P. °6 
