EVOLUTION OF THE PROCESSES OF CEREBRAL ACTIVITY.
thus far considered the elements of cerebral activity as individual simple forces in the statical con dition, we shall, in this third part of the work, consider them from a dynamic point of view, as living forces in movement, in combination one with another, effect ing reciprocal reactions, and co-operating in the different modes of mental activity.
One general fact governs the essential organization of the cerebral cortex (see p. 15). This fact is the admir able order, the regular subordination which is established in the grouping and methodical distribution of all the elements of this cortical substance. In all its regions the zones of cells are arranged one below another in thicker or thinner strata ; they are strictly united one with another, both transversely and horizontally as regards this substance ; the regions of small cells, moreover, everywhere occupy the superficial sub-meningeal zones, while the regions of large cells are localized in the deep regions, and communicate with the preceding by a series of intermediate links—strata of cells which serve as a transition between these two isolated regions. If we compare this simple disposition, which is the anatomic formula in which the economy of the constitution of the cerebral cortex is epitomized, with that which regu lates the reciprocal relations of the nerve-cells of the spinal cord, we immediately perceive that it presents certain analogical characters which in a manner explain themselves ; and we cannot avoid recognizing the fact that if in both instances there are analogies from an anatomic point of view, that is to say regions of large cells indirectly anastomosing one with another, there should similarly be physiological analogies as regards the mechanism of the activity of these similar ele ments.
Now, as experience proves that the nervous Currents pass across the spinal cord from the smaller to the larger cells, and that these latter never enter into activity spon taneously, but merely in consequence of an incidental excito-motor excitation, which they simply reflect, we cannot help admitting, from the most legitimate analogy, that the nervous actions must be evolved in a similar manner throughout the stratified elements of the zones of the cerebral cortex. We may therefore conclude that the regions of small cells in the cortex represent in the brain the posterior grey regions of the spinal cord, and that, like them, they are the territory of dissemi nation of sensitive impressions, designed to retain them, store them up, and afterwards propagate them to the subjacent zones.
From the clear analogies which exist between these two spheres of nervous activity, the spinal cord and the brain, we are therefore led to the conclusion that the different zones of the cortical substance, taken as a whole, represent, as it were, a series of sensori-motor organs conceived on the same plan as that of the similar organs of the spinal axis ; that the nervous activities are developed throughout its tissue as throughout that of the spinal grey matter ; and that in both instances the processes which take place are always —except for differences of medium, the different qualities of the elements called into play, the amplitude and complexity of the different phases of which they are composed—similar processes, reducible to the same primordial phenomena. It is always a phenomenon
of sensibility that produces movement, and excites the activity of the motor cell ; and the motor act itself, whether we have to do with the spinal cord or the brain, is always, as regards its dynamic signification, merely a secondary and subordinate phenomenon, the return effect of a sensitive impression transformed.
This being the case, the phenomena of cerebral activity, as regards their successive development, may be briefly reduced to a series of processes—of regularly linked physiological operations, all derived one from another, becoming complicated in their diverse phases, but always having a common basis of elementary opera tions.
It is always a phenomenon of sensibility, an anterior sensorial impression, present or past, that marks the point of departure, and becomes, in a more or less sensible form, the primary stimulation that induces the movement. In a word, it is always an agitation of the sensorium, an emotion of the personality, that expresses, through the infinite series of cerebral opera tions, the condition of erethism which it has ex perienced.
Hence there are three natural phases under which we shall successively consider the mode of evolution of the different processes of cerebral activity.
I. A phase of incidence. which corresponds to the moment when the external impressions arrive in the plexuses of the sensorium and are perceived there (phenomenon of attention—genesis of the notion of personality—conscious perception).
2. An intermediate phase, during which the affected elements of the cortical substance enter into active participation with the external impression, transformed into a psycho-intellectual excitation. (Dissemination of sensorial impressions in the psycho-intellectual sphere —evolution and transformation of these impressions— operations of the judgment, etc.) 3. A phase of reflexion, which corresponds to the moment in which the primordial excitation, being propagated through the plexuses of the cortex, passes outwards, and expresses, by voluntary motor reactions, the different states of the previously impressed senso rium. (Genesis and evolution of the phenomena of voluntary motion.)