Attention, which marks the first phase of all the pro cesses of cerebral activity, is, then, a phenomenon similar to all those developed in the peripheral plexuses of the system when they are impressed by excitations from the external world. It is the sensorium itself, the sensitive plexuses of our organism, the conceptive regions wherein the notion of our personality dwells, that are immediately engaged and become conscious of the inward phenomenon which occurs. It is from this very fact that the operations of the attention are always, par excellence, conscious operations, which imily the necessary participation of the entire human person ality.
Thus, then, in order that the processes of cerebral activity, by virtue of which attention is exercised, shall be evolved in a regular manner, it is necessary that two indispensable conditions shall concur : on the one hand the registration of the first sensorial impression, regu larly effected in the peripheral plexuses at the moment of its genesis ; and on the other hand the active, spontaneous, and original participation of the elements of the sensorium, which must vibrate in a concordant manner and enter into unison with the impressions radiated from the peripheral regions. It is necessary, then, that between these two poles of the system, there shall be a simultaneous co-operation.
On the other hand, it is also necessary that at the moment when the excitation from the external world arrives in the sensorium. it shall be introduced methodi-. tally, and in a gradual manner ; that it shall work its passage independently ; and that, at the moment at which it is there deposited, it shall vibrate alone, and alone imprint the records of its presence upon the plexuses of the sensorium. In a word, it is neces sary even in optical experiments, when we wish to study the elementary properties of a luminous ray, that we shall carefully eliminate the rays of diffused light, and cover the head with a black veil to eliminate from the eye the incident rays—and just so, for the perfect ac complishment of the phenomena of conscious attention, in order that these shall produce their maximum of effect it is necessary that simultaneous and approximate im pressions shall not come to join the principal impression, and eclipse by their presence its intra-cerebral radiation. To be attentive it is, then, necessary simultaneously to receive impressions from without, and to admit them only in a gradual and successive manner. Without these fundamental conditions the process is abortive, and con fusion of impressions and want of precision in the notions acquired, are the sole and ultimate result of this abortive operation.
It is necessary,'then, that one single impression at a time shall be imprinted upon the sensorium, and that, moreover, the elements of the sensorium shall them selves be in a kind of silence and relative calm.
In fact, where lively preoccupations, or a prolonged intellectual effort maintain in certain zones of the cere bral substance a period of erethism more or less per sistent, the result is that this local over-activity, simply from its being in possession of the field where it has originated, will stifle by its intensity impressions from the external world. Subjectivity predominates, and
itself alone absorbs the cerebral activity ; so that exter nal impressions grow dull on arriving, only penetrating into the regions of conscious personality, when fore stalled by excitations originating on the spot. They are consequently as though they had never arrived.. It is necessary, therefore, that at the moment when the external impression arrives in the brain, it shall find the sensitive regions available, in a healthy con dition, and free from every local cause of internal excitation.
In order that the process shall be completely effected, another special condition is finally necessary—a condi tion of receptivity similar to that which, as we have seen, must exist in the peripheral regions of the system. It is necessary that the impressed cerebral cell shall, like the cell of the sensorial plexuses, be endowed with a certain special retentive power, and with a certain energy for supporting fatigue ; for it is at the expense of its substance that it produces movement, vibrates, enters into erethism, and becomes attentive.* We all know that it is impossible to prolong efforts of attention beyond certain limits,—that we cannot, for instance, fix our attention in an undetermined manner for a prolonged period upon some petty fact, which only involves a single sensorial impression. It is only by mnemonic artifices that we succeed in reviving the fleeting impression by a series of suc cessive instigations producing continuance of the act of attention.* This distraction takes place by reason of the vital forces of cerebral activity them selves ; for as certain regions of the brain, fatigued by sustained attention become inactive, other cell terri tories, reposed and fresh, automatically come into action, by virtue of their native energies, and monopolize the vital forces of these regions of the conscious per sonality for their own profit. Thus, we may say that the failure of attention, and easy distraction, imply the rapid fatigue or need of repose of the cerebral cells ; so that we are led to the conclusion that the vigour of attention is, to some extent, a measure of the degree of vigour of the mental faculties ; that it is the external expression of the energy and vitality of the cerebral elements ; as, in the appreciation of motor phenomena, the continuity of effort is proportional to the disposable motor force.t Functional Perturbations.—The processes of attention represent, then, as we have just explained, a synthesis of the active operations of the brain, in which the phe nomena which occur in the periphery of the nervous system in the sensorial regions on the one hand, and the phenomena which are developed in the central regions of the sensorium where they come directly in contact with external excitation, on the other hand, are fused together. We can therefore comprehend how, when these two fundamental conditions happen to be disturbed in their constituent elements, whether in the peripheral or central regions, the processes of attention are, at the same time, disturbed and arrested in their regular evolu tion.