526 Proceedings of Boyal Society of Edinburgh. [jan. 31 , 
employed perosmic acid and ammonia, but since then the reagent 
used for the purpose of demonstrating it has almost exclusively been 
Weigert’s hsematoxylene dye previously referred to.* In the cortex 
of the cerebellum the plexus is probably densest, but it is present 
in all parts of the cerebral cortex as well. 
Continuity of Cortical Plexus with that in White Matter . — What 
I would specially wish to direct attention to at present, however, 
is that this plexus not only prevails in the cortical grey matter, 
but that it appears to intertwine itself round the nerve fibres 
throughout a great part of the white. The large medullated nerve 
fibres from the cortex run into the white matter, but almost 
immediately become surrounded by a dense padding or casing of 
this nerve network. At first it might be supposed to be simply 
connective tissue, and it has in bygone times been always regarded, 
when indistinctly seen by less favourable means of demonstration, 
simply as the branching neuroglia. The plexus I refer to, however, 
as pervading the white matter of the brain is a true nerve structure, 
and that which is found in the cortex of the cerebrum and cerebellum 
is an extension or outcrop of this. The appearance presented by 
it a short way within the cortical grey matter, of the motor region, 
towards the vertex is shown in PI. XV. fig. 3. The large medullated 
trunks (a., a.) are seen coming down from the grey cortex, but shortly 
after penetrating into the white matter of the centrum ovale they 
become encased, as it were, in a dense and complex mass of medullated 
fibres (d.). Between its fibres is the granular neuroglia (c.), which 
seems to fill all the meshes formed by it. 
The Plexiform Nucleus . — A similar medullated plexus also exists 
in certain of the ganglia, such as the thalamus and lenticular 
nucleus. In the former it is in a high state of development, but 
there is one part of the brain in which it reaches even a higher 
grade of complexity. I refer to a little comma-shaped body (fig. 2, 
j o.n.) which lies in the angle constituted by the under aspect of the 
tectorial part of the corpus callosum and the upper surface of the 
caudate nucleus. This body, whose presence I do not remember 
having seen referred to, is one mass of a dense and complicated nerve 
plexus, and, so far as I am able to discover, is without nerve cells. It 
* Since this paper was read various modifications of Exner’s method have 
been introduced by Pal of Vienna and others. 
