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 regulates 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 spontaneously, 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 dissemination 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 the 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 regularlylinked physiological operations, all derived one from another, becoming complicated in their diverse phases, but always having a common basis of elementary operations.

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 experienced.

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 sensorium. (Genesis and evolution of the phenomena of voluntary motion.)

BOOK I.

PHASE OF INCIDENCE OF THE PROCESSES OF CEREBRAL ACTIVITY.

CHAPTER I.

ATTENTION.

THE period of incidence of the process of cerebral activity occurs at the moment when the sensorial excitations darted from the different centres of the optic thalami are distributed to the different regions of the sensorium, upon which they thus produce a consecutive impression (Fig. 6, p. 61). 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 different 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 vivified by them alone.

a

The regions of the sensorium, which are the living sources that feed all the activities of animal life as at 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 intracerebral processes, that sensorial impressions shall be regularly conducted during their period of incidence, that they shall be distributed according to the physiological laws we have described, and that, besides, they shall be received, propagated, and retained. At this precise moment of cerebral activity, a delicate, precise, 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 peripheral 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