HAVING thus passed in review the structure of the cor tical substance, and the direction of the white fibres which emerge from it, it is now necessary to begin the study of the optic thalamus and corpus striatum, in the substance of which these white fibres are lost ; these being, as it were, the natural pivots around which all the elements of the system gravitate.
The central mass of grey matter which is usually designated the optic thalamus, and of which the anato mical structure and general relations were scarcely known until the present day, is an ovoid body of red dish colour, situated in the very middle of the brain, a fact easily verifiable with a pair of compasses. It is in a manner the centre of attraction of all the fibres, the grouping and direction of which it thus governs.
It is composed : (r). Of a series of small isolated ganglions of grey matter, situated one behind another in a line which runs in an antero-posterior direction ; (2). Of two slender bands of greyish material, lining the inter nal surface of the third ventricle, and continuous with the grey matter of the spinal cord, which thus ascends into the interior of the brain.
1. The isolated ganglions are four in number. These have already been described by anatomists, Arnold in particular,* with the exception of the median ganglion, the existence of which has been revealed by my own researches. They are arranged, as has been said, in an antero-posterior direction, and form successive tuberosi ties on the surface of the optic thalamus, which give it the multilobular appearance of a conglomerate gang lion. (See 7, 8, 9, Do, Figs. 5 and 6.) The anterior ganglion is the most prominent. It is very much developed in the animal species in which the development of the olfactory nerve is very well-marked (corpus album subrotundum of anatomists).
Immediately behind comes a second, the middle ganglion, which in man is comparatively the most apparent and the most fully developed. In those ani mal species in which the optic nerves are rudimentary, the mole in particular, this ganglion is on the contrary scarcely visible.
Behind the preceding, and in the very centre of the optic thalamus, we meet with a third ganglion, of the size of a large pea, and whitish in appearance, which from its situation I propose to call the median centre.
Finally, behind, in the neighbourhood of the superior tubercula quadrigemina, we find another ganglion, of which the contours are in general vaguely defined, and which constitutes the posterior centre.t By means of a series of sections, either vertical or horizontal, we may convince ourselves that these small ganglions form circumscribed and very distinctly iso lated masses of grey matter, composed of plexuses of anastomosing cells; and that they in reality form small independent centres in regular juxtaposition, and iso latedly communicating with special groups of afferent nerve-fibres.
Now what is their true signification from a physio logical point of view ? Up to the last few years the function of this mass of grey matter which forms the optic thalamus was an insoluble problem for anatomists. It was like an un known land, of which anatomy had barely ascertained the situation. Thus, a fortiori, it may be comprehended that it was far from possible to point out the signifi cance of each of these isolated ganglions.
It was by applying myself to the study of the con nections of each of these little isolated centres with the peripheral nervous expansions which are distributed to them, and by confronting these new data with the facts which comparative and pathological anatomy had revealed to me, that I was led to consider them as so many small isolated and independent foci of con centration for the different kinds of sensorial impressions which are conveyed to their substance.* Thus, if we take the anterior centre, for instance, (Fig. 6), we see that it is directly united, by means of a series of curvilinear fibrils, described by anatomists under the name of tonia semicircularis, with a particular mass of grey matter situated at the base of the brain, and itself directly receiving the external root of the olfactory nerve. Direct anatomical examination shows, then, that there are intimate connections between the anterior centre and the peripheral olfactory apparatus. (20, Fig. 6.) On the other hand, in confirmation of this, in the animal species in which the olfactory apparatus is very much developed, this ganglion itself is proportionally very well marked.