30 LISTER AND TURNER, ON THE 



taincd. With this view we steeped in chromic acid portions 

 of the sciatic nerve of a cat just killed, and also parts of the 

 spinal cord of the same animal; and having allowed them to 

 remain between three and four weeks in the solution, we 

 commenced the investigation in Julv 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 diameters. 

 Viewed by transmitted light, it appeared as a confused opaque 

 mass ; but, by reflected light, it exhibited the structure de- 

 picted in PI. II, fig. 1,* each nerve-fibre presenting in its 

 section a carmine spot, surrounded by a yellowish-white, 

 somewhat granular ring, which, though doubtless correspond- 

 ing 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 tliey 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. 



1 1 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 distinguished, 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, 



* This sketcli, like the others illustrating this paper, was drawn by means 

 of the camera lucida. 



