100 SOME OBSERVATIONS ON THE 
same animal; and having allowed them to remain between three and four 
weeks in the solution, we commenced the investigation in July of the present 
year, 1859. 
A transverse section of the hardened sciatic nerve having been placed for 
a time in the carmine solution and then dried, we submitted it, without the 
application of turpentine, to microscopic examination with a power of 130 dia- 
meters. Viewed by transmitted light, it appeared as a confused opaque mass ; 
but, by reflected light, it exhibited the structure depicted in Plate IV, Fig. 1, 
each nerve-fibre presenting in its section a carmine spot, surrounded by a yellow- 
ish-white, somewhat granular ring, which, though doubtless corresponding to 
the pellucid rings in the preparations of the cord before alluded to, was clearly 
composed of some solid material, in short, of the white substance of Schwann 
altered by the action of the chromic acid. 
We next examined sections of the cord treated in the same way, but found 
that these dry specimens were so incrusted with carmine that they gave no 
definite results. It happened, however, that one of the sections treated with 
carmine still remained moist, and, after washing away all superfluous colouring 
matter, we examined it by transmitted light. A very beautiful appearance 
now presented itself, carmine points being seen in the columnar regions, as 
in Mr. Clarke’s preparations, surrounded by rings; but the latter, instead of 
being transparent like mere spaces, were dead white; the carmine points, on 
the other hand, appearing in the thinnest parts of the section as illuminated 
spots amid the general opacity. This is represented in Fig. 5. 
It will be seen from this sketch, which is drawn on the same scale as Fig. 1, 
that the nerve-fibres varied very much in their diameter, the largest being of 
about the same size as those of the sciatic nerve, while others were of extreme 
minuteness ; but in all cases in which they were sufficiently large to be dis- 
tinguished, they had the same character of a white circle with a central carmine 
spot from one-fourth to one-third the diameter of the whole fibre. It was 
obvious that, in the cord, as in the sciatic nerve, the carmine central part of 
each fibre was the axial cylinder, and the opaque circumferential portion the 
medullary sheath ; and, therefore, that the pellucid rings in preparations treated 
with turpentine 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 
* This sketch, like the others illustrating this paper, was drawn by means of the camera lucida. 
