fibres, it is not our business in this work to give a detailed anatomic description of each of them. We shall only mention that, whether we consider them in the posterior, middle, or anterior regions of the brain, we find them everywhere disposed in a similar manner, and directed towards their proper centre of attraction.

Thus the converging fibres of the posterior convolutions follow a common postero-anterior direction; those of the anterior the reverse; while those of the superior convolution run from above downwards, and those of the inferior from below upwards.

Such are the special characters of the two great systems of fibres which constitute the white substance of the brain. These fibres run in a fixed direction, obey definite laws, and thus become the fundamental framework which on the one hand binds together the homologous regions of the two hemispheres, and on the other establishes the organic union between the peripheral regions and central ganglions of the brain.

This concentration of the converging fibres around the optic thalamus once effected, what becomes of these nervous elements, and how do they become lost in its. mass?

From the moment in which they are implanted in the circumference of the optic thalamus, they become dispersed by degrees, insensibly taper away, and we then see them, in the form of whitish rectilinear fibrils, continue the converging direction of the primitive fascicles, and finally lose themselves in the midst of the different agglomerations of grey matter that they meet in their passage (centres of the optic thalamus and corpus striatum).

Thus it is that each region of the cortical periphery is united, by means of these white fibres, to a symmetrical region in this common ganglion of grey matter (the optic thalamus), and that these two foci of nervous activity, the cortical periphery and the central ganglion, like two electric piles united by a common wire, are intimately united into a single instrument.

D

CHAPTER IV.

THE OPTIC THALAMUS.

HAVING thus passed in review the structure of the cortical 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 anatomical structure and general relations were scarcely known until the present day, is an ovoid body of reddish 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: (1). 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 internal surface of the third ventricle, and continuous with the grey matter of the spinal cord, which thus ascends. into the interior of the brain.

I. 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 tuberosities on the surface of the optic thalamus, which give it the multilobular appearance of a conglomerate ganglion. (See 7, 8, 9, 10, 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 animal 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.†

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

* See "Tabula Anatomicæ."-Arnold, Icones cerebri et medulla spinalis.' Turici, 1858.

+ See "Iconographie photographique des centres nerveux." Plates, 2, 4, 6, 7, 8, 26, 28.

lated masses of grey matter, composed of plexuses of anastomosing cells; and that they in reality form small independent centres in regular juxtaposition, and isolatedly communicating with special groups of afferent nerve-fibres.

Now what is their true signification from a physiological 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 unknown land, of which anatomy had barely ascertained the situation. Thus, à fortiori, it may be comprehended that it was far from possible to point out the significance of each of these isolated ganglions.

It was by applying myself to the study of the connections 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 concentration 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 tania 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,

* See Luys, "Recherches anatomiques, physiologiques et pathologiques sur les centres nerveux," 1865.