The separation of its anterior pillars in front affords strong indication of its double form. These pillars pass downwards in a curved course, through the grey matter of the tuber cinereum, to the mamillary bodies, which are connected to the optic thalami by a bundle of fibres which may be easily traced into them. This bundle is described by Red as the root of the anterior pillar of the fornix. From the gangliform structure of the corpus mamillare, I prefer to regard this band as a medium of connection with the optic thalamus, and to trace the anterior pillars to the ma millary body.
The parts with which the fornix is connected in front are the optic thalami, the mamillary bo dies, and the septum lucidurn, which consists of fibres having the same physiological import as those of the corpus callosum, altered however in d irection by the backward position of the anterior pillars, which adhere to the body of the fornix. The tuber cinereurn and the grey matter which adheres to the lower half of the inner surface of each optic thalamus, are connected- with it ; and as its anterior pillars pass upwards in this situation, they receive fibres from the neigh bouring convolutions. These pillars remain separate from the mamillary bodies to the fo ramen of Monro, where they adhere to each other and form the apex of the body of the fornix. Traced backwards, the fibres of the fomix pass into the posterior and inferior horns of the lateral ventnele. In the former they connect themselves with the hippocampus minor by expanding over it, and in the latter they spread over the hippocampus major, forming the posterior pillar of the fornix, or tmnia hippocampi.
The relation of the anterior pillars of the fornix to the foramen commune anterius has been already sufficiently described. Superiorly the fornix adheres to the corpus callosum or to the septum lueadum, and the anterior commis sure crosses in front of its anterior pillars, and almost touches them.
4. The fourth longitudinal commissure is the fiertia semicircularis. It may be traced from the corpus mainillare outwards and back wards in the groove between the optic thala mus and corpus striatum into the inferior horn of the lateral ventricle, when its fibres mingle with those of the middle lobe. It is evidently part of the same system of fibres with the fornix.
Take away the corpus callosum, the grey matter of the internal convolution, the.ventri cular prominence of the optic thalamus, and all these commissures fall together and become united as one and the same series of longitu dinal fibres.
It is very remarkable how few fibres pass between the great mass of the cerebrum and the cerebellum. The proccssus cerebelli ad testes are the only fibres which can be regarded as forming commissures between these two seg ments of the encephalon, and they are of the nature of longitudinal commissures.
The transverse commissures are the corpus callosum, the anterior commissure, the posterior commissure, the soil conanissure.
1. The corpus callosum, so called according to some from the density of its tissue, is the great commissure of the lateral halves of the brain proper—rommissura cerebri maxima of Soemmering. The fasciculated character of this structure is as obvious as that of any nerve in the body, and the direction of the fibres is clearly from one hemisphere to the other.
From the description already given of the corpus callosum, it is evident that its fibres sink into the white substance of each hemisphere above the level of the corpora striata, as well as into that of the anterior and of the posterior lobes. By its principal or horizontal portion it connects the white matter of the lesser cen trum ovale of each side, and by the fibres which form the anterior and posterior reflexions it connects the anterior and posterior lobes. It needs only a very superficial dissection to ascer tain thus much.
To determine the precise fibres with which those of the corpus callosum are continuous, and the relation Which they bear to the lateral ventricles, demands a much more minme dis section. This mnst be done, according to the directions of Foville, wbo has' given a most elaborate description of this cominissure, by carefully separating it in the transverse direc tion from the internal convolution, on a har dened brain. Pursuing the dissection in this direction, it may be detached from the sub stance of the hemispheres as far outwards as the external border of the corpus callosum and optic thalamus. Along this edge the fibres curve downwards and inwards, and appear to become continuous with some of those whidt radiate from those bodies. The anterior and posterior fibres enclose the anterior and pos terior horns of the lateral ventricles in radi ating forwards and backwards from the corpora striata and optic thalami to those parts of these cavities.