520 
Proceedings of Royal Society of Edinburgh. [jan. 31 , 
The former method is suited solely for naked eye observation. 
The main objection to Weigert’s stain is, as mentioned by its 
author, that it cannot be adapted to cutting the tissue in the freezing 
microtome. The modification above referred to has been introduced 
with the view of overcoming this. The reaction of the nerve- 
medulla is quite as intense if not more so than that obtainable 
by the original procedure. 
Could one unite the aniline-black stain of Sankey and Lewis for 
nerve cells with this modification of Weigert’s stain for nerve fibres, 
little would remain to be desired for microscopic demonstration. 
The difficulty, however, is that the aniline-black dye will give its 
proper reaction only when the brain is perfectly fresh, whereas the 
hsematoxylene will act upon the nerve-medulla only when it has 
been hardened in a chrome salt. I have, however, already managed 
to partially combine the two, and see no insuperable barrier to 
complete success. 
The Callosal Fibres . — It is, I think, almost universally believed 
at the present day that the corpus callosum is a commissure ; that 
anatomically it unites equivalent areas in the two cerebral hemi- 
spheres, and that physiologically it serves to bring these into 
functional harmony. Some years ago, when working at the pathol- 
ogy of the brain, I came upon certain appearances which tended to 
shake my belief in the commissural theory, and which led to an 
inquiry, part of the results of which is embodied in this paper. 
The appearances alluded to are to be seen in the brain of any 
mammal when it has been hardened in Muller’s fluid, but best 
in those in which the organ is of large size, as in Man. It 
was in Man that I first noticed the appearance, but I have since 
then found that it exists in all the mammalian brains I have 
examined. The Muller’s fluid, in the case of a large brain such 
as that of Man, must be injected from the main vessels at the 
base in order to insure that it will penetrate deeply and in sufficient 
quantity. 
If such a brain, when completely hardened, be simply cut into a 
series of perpendicular transverse segments, each of about half an 
inch in thickness, the following can be readily seen with the naked 
eye or with a simple lens : — 
Coming out of the corpus callosum at each side is a large arched 
