We see then, to sum up, by means of this simple physiological sketch, what an all-important part these two central ganglions play in the phenomena of cere bral activity, and how completely different is the mode of action of each.
The elements of the optic thalami purify and trans form by their peculiar metabolic action impressions radiating from without, which they launch in an intel lectualized form towards the different regions of the cortical substance. The elements of the corpus striatum, on the contrary, have an inverse influence upon the stimuli starting from these same regions of the cortical substance. They absorb, condense, materialize them by their intervention ; and, having amplified and incorporated them more and more with the organism, they project them in a new form in the direction of the different motor ganglions of the spinal axis, where they thus become one of the multiple stimulations destined to bring the muscular fibre into play.
if we group synthetically the anatomical pro positions we have tried to establish in the course of this work, we see that the brain is a geminate organ, formed of two hemispheres, of which the elements are strictly associated with one another, by means of a series of commissural fibres which unite them in timately, and produce a certain tendency in their mole cules to vibrate in unison. (Figs. 3. and 4.) Each of these two lobes, or hemispheres, is funda mentally formed of masses of grey matter irregularly distributed—the grey matter of the central ganglions (the optic thalami and corpora striata) and that of the cerebral cortex.
These two regions of cerebral activity are united to one another by a series of white fibres, which serve as a bond of union between them, and as a channel of pro pagation for nervous currents passing from one to the other, either centrifugally or centripetally.
The opto-striate central ganglions of each lobe may be ideally conceived as occupying the centre of a hollow sphere, of which the circumference is represented by the undulations of the cerebral cortex ; and the white fibres would thus represent an infinite number of radii uniting the central with the peripheral regions of the sphere. (Fig. 5.) The anatomical study which we have just made, of the grey matter of the optic thalamus and that of the corpus striatum, has enabled us to observe distinct differences between them, and consequently to formu late the unlike dynamic aptitudes with which each of these two ganglions is gifted.
We have thus seen that the function of the optic thalamus in particular seems to be that of receiving, con densing, and transforming, like a true nervous ganglion, impressions radiating from the sensorial periphery, before launching them into the different regions of the cortical substance ; and that, inversely (Fig. 6.-14, 9. 4.), the
corpus striatum, in connection with exclusively motor regions, appears to be a place of passage and rein forcement for stimuli radiating from the different psycho-motor zones of the cortical periphery.
These anatomical connections being admitted as fun damental data, as regards the structure and mode of agency of the nervous elements ; let us now see what use we may make of this, from the standpoint of the particular interpretation of certain phenomena of cerebral activity.
Let us take things as they normally occur, following the natural channels by which excitations from the external world penetrate into the organism. Let us take, for example, the impression upon a sensitive nerve —a vibratory phenomenon which calls into activity the cells of the retina or those of the acoustic nerves ; what then takes place in the secret recesses of the nervous conductors ? Immediately following the direction of their natural channels, these vibrations applied to each par ticular sensorial nerve, bring into play the specific activities of the different cells of the centres of the optic thalami. (Fig. 6.-3, 13.) These immediately take up the vibration, and by means of the radiating fibres which unite them to the different regions of the cortical periphery, transmit to their sensitive partner-cells, the new dynamic conditions in which they have just been placed by the fact of the ex ternal excitation.—(See Fig. 6.-5 and i5.) External sensorial impressions do not therefore propagate them selves through and through from the plexuses of the sensorial to those of the cortical periphery, until they have awakened various intermediate cell-territories which give them a new form, cause them to undergo a peculiar metabolic action, and only launch them into the different plexuses of the cortical zones, after they have animalized them, and rendered them somehow more assimilable. (Fig. 6.-4. 9. 14.) Each special kind of sensorial excitation is thus dispersed, and quartered upon a special area of the periphery of the brain. (Fig. 6-15 and 5.) Anatomy shows, then, that there are definite localiza tions of limited regions, organically designed to receive, to condense, and to transform such or such particular kinds of sensorial impressions.