The geniculate bodies, although intimately connected with the optic thalami, appear to be distinct from them, but very similar in struc ture. A section made into the thalamus through either of them shows a distinct line of demar cation between them. The optic tracts adhere to the lower surface of each thalamus by their inner margins, and vvhen followed backwards are found to form a very evident connection with both the geniculate bodies.
Corpora mamillaria.—These bodies may be conveniently noticed here, as formiug part of the series of gangliform masses in connection with the brain. They are of a spherical shape, covered on the exterior by very pure white matter, which is apparently derived from the anterior pillars of the fornix. When cut into, they are found to consist of a mixture of vesi cular and fibrous matter, surrounded by a thin cortex of the latter. Microscopic examination proves this structure of the interior substance to be of the same nature as that of ganglia, and to resemble the optic thalamus.
The fibrous matter is connected, at the upper part of each body with the anterior pillar of the fornix, and on the outside with a fascicu lus of fibres from the optic thalamus. It has been supposed that these two bundles are con tinuous, and that the mamillary body results from a twisting of the anterior pillar of the fornix as it changes its direction to pass into the substance of the thalamus. But the gan glionic structure of the mamillary bodies is unfavourable to this view, and renders it more probable that they are independent structures, exercising proper functions as nervous centres ; and the constancy of these bodies in, at least, the mammiferous series, increases this proba bility.
Of the commissures of the brain.—A large number of fibres connected with the hemi spheres or the two gangliform bodies just de scribed, seem to serve the purpose of connect ing different parts, either on the same side of the mesial plane, or on opposite sides of it. Those on the same side constitute the longitu dinal comrnissures ; those on opposite sides the transverse.
Of the longitudinal cormnissures.—These are four in number. They all evidently belong to the same system of fibres, separated from each other by the developement of intermediate parts. That which is on the highest plane is
the superior longitudinal commissure, which is the fibrous matter of the internal convolution. Internal and a little inferior to this is a se cond, very small, band of fibres, longitudinal tract, (Vicq. d'Azyr,) which passes from be fore backwards along the middle of the corpus callosum, parallel to the superior longitudinal comrnissure, from which it is separated only by the grey matter of the convolution. Both these commissures are separated by the corpus callosum from a third, which takes a parallel course to them, namely, the fornix, which occupies a plane considerably inferior to both. External to this and separated from it by the ventficular projection of the optic thalamus, we find a fourth band, which passes parallel to the fornix : this is the tenia semicircularis.
As these parts have been already described, it will be unnecessary to do much more at pre sent than indicate the connections which they serve to maintain.
1. The superior longitudinal commissure con nects the convolutions ,of the inferior surface of the anterior lobe with the hippocampus major; and as its fibres pass above the corpus callosum they form connections with some of the other convolutions on the internal surface of the hemisphere 395).
2. The longitudinal tracts of the corpus cal losum may be traced from about the same region of the inferior surface of the anterior lobe as the preceding commissure, near the perforated space, and they pass backwards, winding over the posterior reflection of the corpus callosum to its inferior surface.
3. The fornix* is, next to the corpus cello sum, the most extensive of the cerebral commis sures. That it consists of longitudinal fibres cannot be doubted. Although commonly de scribed as a single structure united at the body of the fornix, and spreading backwards and forwards by its crura, it nevertheless isdistinctly divisible along the middle line into two per fectly symmetrical portions. The adhesion of the transverse fibres of the corpus callosurn on its upper surface, and of the terminal fibres of its posterior reflection on its inferior surface which form the lgra, is the principal bond of union of these two lateral halves of the fornix.