1887 .] Prof. D. J. Hamilton on the Cortex of the Brain. 527 
is contiguous to the caudate nucleus below, but the tissue of the 
one is separated from that of the other by a sharp line of demar- 
cation. It passes for a short way underneath the corpus callosum, 
and at its lower extremity posteriorly, seems to he united with 
the taenia semicircularis. Its fibres are directly continuous with 
the fibres of the plexus in the white matter just referred to. It is 
most developed anteriorly in the region of the head of the caudate 
nucleus. 
In PL XIY. fig. 4 I have given a drawing of the plexus constituting 
this body as it appears when magnified about 350 diameters. The 
part from which the drawing was taken was immediately adjacent to 
the inner capsule at the point x. in fig. 2. The plexiform nucleus 
(j).n., p.n.,p.n.,) is seen to the right ; a few of the fibres of the inner 
capsule (i.c.) to the left. It will be noticed that the main bulk 
of the body is made up of an intertwining felt-work of nerve fibres. 
They stain deeply with Weigert’s copper-hsematoxylene dye, and 
between them, as in other regions of the brain, a quantity of 
granular neuroglia is interposed. It is only lately that I have made 
out the true nature of this body, and on account of its structure I 
propose to name it the plexiform nucleus. 
Points of Origin of the Callosal Fibres . — The most of the callosal 
fibres which come down from the vertex appear to run directly 
into the corpus callosum. Their usual appearance and direction 
are represented in fig. 2 ( v.c.f . , v.e.f). In passing downwards 
they interlace with those leaving the corpus callosum, and which 
are turning downwards to the two capsules. 
Those, however, which are derived from the lower third or half 
of the cortex between the Sylvian fossa and the great longitudinal 
fissure (fig. 2, p.nf p.n.f.) do not appear to run directly into the 
corpus callosum, but pass first of all into the plexiform nucleus just 
described. Shortly after issuing from the grey matter they become 
united into loose bundles which penetrate through the fibres of the 
crossed callosal tract, and which seem to lose themselves within the 
plexiform nucleus by breaking up into its reticular network. From 
this reticular network fresh fibres appear to arise, and to enter 
the corpus callosum. In all probability, these turn downwards 
on the opposite side into the two capsules as fibres of the crossed 
callosal tract. This plexiform nucleus would thus possibly represent 
