CHAPTER III.

INTRA-CEREBRAL PROPAGATION OF THE PROCESSES

OF SENSIBILITY.

SENSITIVE impressions pursuing their course are, as we have already seen, condensed in the masses of grey matter in the optic thalami.

These masses of grey matter represent, then, in the general economy of the nervous system, a species of point of convergence, or cross-roads, and the penultimate halting-place where impressions from the external world are united before being radiated towards the peripheral cortical regions.

But as regards these different kinds of sensitive elements which come flowing towards the grey ganglions of the optic thalami, these latter, which receive them into their mass, give them each an isolated territory—so that that division of labour of which we have already seen an example in the progressive evolution of the nervous system, here seems to receive a new confirmation, since we see the phenomena of sensibility divided, like white light, into isolated fascicles, each fascicle having a special receptive apparatus reserved for itself exclusively.

Thus purely sensitive impressions have a central ganglion where they are isolatedly condensed (Fig. 6-9.); it is the same for the optic, olfactory, and acoustic im

pressions and finally the excitations of vegetative life also find a cell-territory specially appropriated to their reception, so that as the processes of sensibility become perfected, as they penetrate more deeply into the interior of the nervous system we find them splitting up, dividing into elementary fascicles, each gifted with dissimilar specific properties, and yet united among themselves by the common bonds of their origin and evolution.

After radiating through the cerebral white fibres, into the different departments of the cortical substance, the same phenomena of division of labour again occur, and we may directly observe that the regions in which the dissemination of auditory impressions takes place are different from those where that of the olfactory, visual, etc., takes place. So that each isolated region of the brain has also to work and develop its specific energies in isolation. (See Fig. 6-4. 9. 14. and Fig. 5-7. 8. 10.)

When the sensitive excitations, whatever they may be, have been launched into the midst of the plexuses of the cortical layer, they find there also sensitive nervous apparatuses prepared to receive and absorb them, and thus co-operate in the various processes in evolution.

We have indeed already studied the remarkable disposition of the cells of the cortex (see Fig. 1.), which are arranged in isolated zones, stratified like the layers of the crust of the earth, and thus constitute a continuous network of which all the organically connected molecules are arranged so as to vibrate in unison, and to propagate the nervous undulations, either vertically or laterally

On the other hand, those myriads of nerve-cells, agglomerated into a continuous whole in the sub-meningeal regions of the cortical substance, are themselves essen

tially sensitive. They are living, impressionable, and gifted in the highest degree with that vitality which characterizes the nervous elements: and accordingly, when the perturbation from the external world, transformed by the metabolic action of the optic thalami, comes to reverberate within them, they are perturbed in their turn, and are in a manner thrown into a condition of erethism, just as the peripheral plexuses were when first agitated by the external excitation.*

Thus it is that the sensitive excitations awaken the activity proper to the elements of the cortical substance; that these are brought into play; and that the sensitive process, like a force which is incessantly transformed, loses by degrees its primordial character as it advances and enters a new territory.

We see then how gradually the processes of sensibility are transformed by incorporating themselves more and more with the organism; how, starting as simple physical elements, they end by becoming, in the last term of their long course, a living excitation, more and more. animalized and intellectualized by the special activities of the different media which they have successively called into action. In this respect they are quite comparable to those physical phenomena by virtue of which we see the luminous rays which pass through our optical instruments become subject to the modifying influence of the media they traverse-become concentrated, refracted, unequally diffused in secondary elements, to present themselves finally to our visual sensibility, perfected, purified, separated, and with their maximum of effect. Genesis of the Notion of Personality and of Moral Account of the experiments of Schiff, p. 77.

Sensibility. The processes of sensibility have not for their sole object the transformation of external excitations; they contribute in a much more effectual manner to operations of great delicacy, which are designed to co-operate in the genesis of the notion of our individual personality.

It is, indeed, through the awakening of the activity of the sensibility diffused throughout the different regions of the organism—vegetative as well as excitomotor sensibility—that this notion is engendered, developed, and maintained constantly active and alive in us.

It follows, indeed, as a natural consequence of what we have already indicated, that everything in us which is sensitive-every fibre which vibrates, every sensorial plexus which becomes erethised-produces a vibration which is concentrated in the plexuses of the cortical substance, and finds in their essential structure a vast common reservoir, the veritable sensorium commune into which all the excitations collected in the periphery separately flow, and in which they remain latent.

The result, as regards the secondary reactions of this sensorium, of the general concentration in these plexuses of all the diffuse sensibilities of the organism, is naturally that all the sensibilities of the peripheral regions of the nervous system, drained from the essential structure of our tissues, of our flesh, mucous membranes, viscera-in a word, of our whole organism-and conducted along the converging nervous filaments, as the electric fluid is along the wires which transport it to a distance, inevitably travel towards the central regions of the system, towards the sensorium commune, where they are simul

taneously distributed; and that these conceptive regions of the sensorium represent, as it were, at the other pole, the sensitive foci correlated to the peripheral regions in agitation.

All these modes of sensibility, whatever be their origin, are, then, physiologically transported into the sensorium, and there find a symmetrical region which vibrates in unison with their peripheral tonality; so that from fibre to fibre, from sensitive element to sensitive element, our whole organism is sensitive, our whole sentient personality, in fact, is conducted, transported just as it exists, as a series of isolated currents, into the plexuses of the sensorium commune.*

There we are represented in detail, there all our sensitive elements are condensed, fused, and anastomosed into an inextricable unity-a unity which is itself only an expression of the organic connection of the underlying nervous plexuses. There, in a word, the synthesis of all our dispersed sensibilities, which are united in a limited space and yet faithfully reproduced, takes place. There our personality lives and feels.

Here, by means of the conductility of sensitive excitations and the intervention of the nervous system, which represents in the truest sense an organ of per

The conductility and dispersion of sensibility in the sensorium, by means of the nerve-fibres, is so real, that in persons who have suffered amputation, when any irritation attacks the stump and engages the sensitive nerves, it immediately awakes and develops in the sensorium the old impressions in a posthumous form. It is not simply the painful state of the sensitive nerves that the patient feels, it is also the resurrection in the sensorium of a portion of himself, in consequence of the persistence of the conductors which formerly supported it and in which this sensitive portion of his personality was incar(See Muller," Physiologie," vol. i. p. 598; Sensations experienced by persons after amputation.)

nate.