PHASE OF INCIDENCE OF THE PROCESSES OF CEREBRAL ACTIVITY.
THE period of incidence of the process of cerebral activity occurs at the moment when the sensorial ex citations darted from the different centres of the optic thalami are distributed to the different regions of the sensoriunz, upon which they thus produce a consecutive impression (Fig. 6, p. 6r). We have already several times insisted upon the different phases of evolution of the phenomena 'of sensibility, and shown that this simple physical impression produced by the external world is transformed, as it becomes incorporated with the organic tissues, into nervous vibrations, and that these nervous vibrations, passing through successive agglomerations of cells, undergo the action of the dif ferent media through which they pass, until they arrive transformed and purified in the plexuses of the cortical substance, which are set in motion, impressed, and vivi fied by them alone.
The regions of the sensori um, which are the living sources that feed all the activities of animal life as at a common reservoir, are then, before they react by radiating outwards the forces that they create on the spot, themselves the tributaries of excitations from the external world, which, like an electric spark dispersed throughout their tissues, suddenly excite and develop their latent energies. It is necessary, therefore, as a fundamental condition of the evolution of the intra cerebral processes, that sensorial impressions shall be regularly conducted during their period of incidence, that they shall be distributed according to the physio logical laws we have described, and that, besides, they shall be received, propagated, and retained. At this precise moment of cerebral activity, a delicate, 4precise, and rapid phenomenon takes place. This is called the phenomenon of attention. It is quite comparable to that which we have already described at the other pole of the nervous system, at the moment when sensitive impressions come into contact with the peri pheral plexuses, and when the external excitation, becoming incorporated with the nervous tissue, loses in an instant the qualities of a purely physical, to assume those of a purely nervous excitation.
At the periphery, at the precise moment when the external excitation, represented either by a luminous or a sonorous vibration, or by a material impression, impinges upon the sensorial plexuses, an inward phenomenon of impregnation or transformation of force occurs. The natural sensibility of the nervous element
is affected : it becomes erect, is arrested, is attentive; and from this intimate contact with the external vibration it enters into a new state ; a specific impression is made upon it, which passes from the external world from which it is derived, to explode in the sensorium itself.
The plexuses of the sensoriunz, which themselves represent a vast sensitive surface open to external exci tations, are the theatre of phenomena of the same kind. For there each excitation from the external world arrives in a quintessential form, intellectualized by the metabolic action of the centres of the optic thalamus. Henceforth it represents only the distant and trans formed echo of an impression, which was purely physical when it made its first appearance in the organism. Here also, in order that this incident im pression shall penetrate into the plexuses of the sensorium and become incorporated with them, it is necessary that it shall find in them proper conditions of receptivity, that their natural sensibility shall be ex cited, that it shall be seized upon, and that a species of similar erethism shall be developed. This is what, in fact, takes place at the moment when the exci tation arrives in the sensorium. Its impregnation does not take place coldly, nor without a local reaction and an active participation of the nervous element thrown into agitation. There is a period of physiologi cal erethism which this element then manifests at a given and variable point in the cerebral cortex. It is, in fact, actually recognized that, at the moment when this subtle phenomenon takes place, there is a local development of heat, which is disengaged in the cerebral region that becomes active (experiments of Schiff, see p. 77), and that this reaction expresses the active participation, the attentive state of the elements of the sensorium which receive the excitation, at the moment when they are impregnated by it, and transform the purely sensorial excitation into a psychical impression.