Experimental physiology has proved on its side, that in living animals, as the beautiful experiments of Flou rens long ago showed, it is possible, by methodically removing successive slices of the cerebral substance, to cause these animals reciprocally to lose either the faculty of perceiving visual, or that of perceiving auditory impressions.* More than this, Schiff, in his recent experiments, as ingeniously contrived as delicately executed, succeeded in demonstrating in a precise manner, that in the animal under experiment, the cerebral substance was subject to local increase in temperature, according as it was suc cessively excited by such or such kinds of sensorial impressions ; and that thus, in the brain of a dog, which was made to hear unexpected sounds, such or such a region of the cortical substance was heated, and that in another, in which tactile, olfactory, or gustative sensation was excited, other regions of the brain were reciprocally erethised and heated in an isolated manner.1 Following up the process of the migration of sensorial excitement from the peripheral to the central regions of the system, we see that all sensorial impressions arrive, in the last stage of their transit, at the plexuses of the cortical substance ; that they arrive transformed by the action of the intermediate media through which they have passed in trnsitu ; and finally that they there die away and are extinguished, to revive under a new form, by bringing into play the regions of psychic activity where they are at last received.
As soon as the sensorial excitation is dispersed in the midst of the plexuses of the cerebral cortex, new phenomena unfold themselves.
Here mere analogy leads us to think that the sen sitive cells of the brain may behave like those of the spinal cord, and that in the presence of the physiological excitations proper to them, they will react in a similar fashion. We may, therefore, suppose that at the moment when the cerebral cell receives the impregnation of the external impression, it becomes erect, as it were, develops its peculiar sensibility, and disengages the specific ener gies which it contains.
Thus it is that the impression which is communicated and which manifests itself by a development of heat in certain regions of the cortex (as in the experiments of Schiff), is propagated through the circumjacent plexuses, and, according to the laws of undulatory movement, develops by degrees the latent activities of new groups of satellite cells, which in their turn become new foci of activity for the neighbouring cells. with which they are intimately anastomosed.
In this manner we can conceive how, in consequence of a simple sensorial impression, all the agglomerations of nervous elements of which the cerebral cortex is com posed, may isolatedly become successively engaged ; how the movement is communicated from point to point (Fig. 6-5. la 15.) how the individual sensibility of the nervous elements begins to take part in the phenomenon ; how life is awakened in regions at first silent ; and how, in a similar manner, the incident excitation, after having thrown into agitation different zones of the cortical substance, is finally transformed into a centrifugal excitation, and externally discharged in the form of a motor act. (Fig. 6-6.
I. 16.) Having followed step by step, the phenomena of cerebral activity just explained, and interpreted them in ordinary language, we may conclude that sensorial excitations radiated from the periphery reach the regions of psychic activity, and that there, coming under the influence of the elements of which it is composed, they become transformed into persistent impressions—ideas corresponding to their origin ; that they bring into play the sensibility and emotivity proper to these regions ; that they become associated, anastomose one with another in a thousand ways, by means of the organic tissue through which they are evolved ; that they are amplified and transformed by the different zones of cells through which they are sifted ; and that finally, they are exported and reflected outwards in the form of voluntary motor manifestations, expressions more or less indirect of a primordial phenomenon of sensibility.
Now, from the premisses of the structure of the cortical substance, comprehended as already indicated, it may be possible to deduce data which will enable us to appreciate the dynamic functions of the different zones of cells contained in it.
We have already established, that the elements which compose it have very distinct morphological characters ; that the zones of small cells occupy the sub-menin geal regions, and that the zones of large cells occupy the deep regions. In the minute constitution of the spinal cord we find similar appearances as regards the distribution of the nervous elements ; and we further know that the regions of small cells are the seat of sensitive, those of large cells the point of departure of motor, phenomena. The laws of analogy therefore lead us to suppose that morphological imply physio logical analogies, and that in the succession of the multiple activities of the cortical substance, we may pro bably suppose that the sub-meningeal regions, occupied by small cells, are more particularly the regions fitted for the reception of sensitive impressions, while the deeper layers, occupied by the large cells, appear to be more particularly centres of emission appropriated to motor phenomena.
This granted, we arrived at the following conclusion : That in the plexuses of the cortical substance, there is in those formed by the small cells a special sphere for the dissemination and reception of sensitive impressions, which all impinge here and bring into play the peculiar sensibility of the cells ; and that these zones, which are anatomically demonstrable, and which represent the posterior sensitive regions of the spinal cord, receive in their essential structure all the particular sensibilities of the organism, and produce a union between them.