It is the blood that carries everywhere with its un interrupted currents the vivifying stimulation which causes the cells to feel, to become erect, and to associate for co-ordinated actions. In the purely sensitive regions, where the phenomena of conscious personality are inces santly in process of evolution, it keeps them constantly awake, and thus sustains in us the conscious idea which we possess of the external world. In the motor regions it enables the nervous elements to accumulate, as in con densers, a store of nervous influence destined to pass into the dynamic condition as soon as a call is made upon them. It is everywhere present, flowing every where, and evoking the specific innervation of each of the cell-territories which it animates and bedews, thus enabling them to renew their latent energies.
When once provided with the necessary elements of nutrition, the cerebral cell becomes capable of entering into action, and performing the dynamic function for which it is designed. This new phase under which it reveals itself is characterized : I. By an acceleration of the blood-currents in the functioning regions.
2. By a local development of heat in these regions.
T. —If it be incontestably demonstrated what an portant influence the regularity of circulatory pheno mena has in evoking the activity of nerve-cells, it is on the other hand very curious to note what an influence the activity of these same cells may have in return on the vascular irrigation designed to provide for their nutrition as well as their expenditure.
It is not, indeed, without a certain astonishment that we observe that if on the one hand, the nerve-cells play a passive part with regard to the circulation which feeds them—if they are in subjection to it, and are veritably its tributaries ; by an inverse phenomenon, from the moment they become active their position changes, and, ceasing to be subject as they were, they in turn become dominant. From the very fact that they are working— that there in certain isolated spaces they develop a state of nervous erethism—they at the same time deter mine hic et nunc a concomitant influx—they make an appeal to the blood, and even turn to their own profit the irrigation of certain neighbouring regions.* Thus the brain, as regards the phenomena of circu lation, is at the same time active and passive,; it is of necessity subject to their influence, and cannot, on pain of cessation from all work, refuse their aid ; and yet, at a given moment, it reacts, solicits them, makes appeals to them, and thus unconsciously directs the vaso motor actions designed to maintain the integrity of its vital energy.
Thus from this double influence of the phenomena of the circulation on those of cerebral activity, and those of cerebral activity on the acceleration of the flow of blood, a vicious physiological circle results, calculated to have an inevitable influence upon the infinite series of regular cerebral operations, as well as upon the progressive evolution of pathological pheno mena, which mostly are but exaggerations of the normal actions of the organism.
Every one knows how fatal chronic lesions of the capillary plexuses are to the delicate substance of the cerebral cells—how the plastic exudations which pro ceed from the vessels, the fibro-albuminous deposits which become infiltrated into the tissue and interstices of the cells, become like so many foreign bodies hostile to life, and injurious to the physiological medium whence they draw the elements of their normal constitution.
Every one knows, further, how moral causes—too energetic work, which exceeds the amount of the reserved nerve-force — prolonged vigils, which do not permit the recuperation of lost materials—pre occupations concerning a single subject, which induce a condition of chronic congestion within certain circum scribed limits—are, so many morbid modes of excite ment which maintain a permanent condition of local erethism, and thus indirectly become the causes of those repeated affluxes of blood which are so inevitably followed by exudations of all kinds and persistent new formations (the lesions of general paralysis).
Hence that preponderant influence which the whole series of moral affections exercises upon the genesis of mental maladies. Whether they be derived from an intellectual excitement prolonged beyond physio logical limits, or result from profound disturbances occurring in the emotional sphere of the sensorium, in consequence of trouble, disappointment, misfortunes of all kinds, the minute mechanism of their advent is always fundamentally the same. It is by the physio logical channel they introduce themselves into the organism it is under the form of regular excitations— shocks propagated along the normal processes of cere bral life—that they implant, develop, and perpetuate themselves ; and the incurable disorders they leave behind them are but the indirect effects of disturbances of nutrition in the nervous plexuses, proceeding from this single source, the afflux of blood too frequently provoked.* .S7ecp.—By an inverse phenomenon, if the cerebral cell, from the very fact that it is in its period of erethisrn, its working period, becomes the occasion of a call upon the blood destined for its activity, this curious fact occurs, that so soon as this activity begins to slacken, so soon as fatigue announces itself, and its histological sensibility is exhausted by the action of external impressions, the vascular irrigation is modified simultaneously. It fol lows step by step the decreasing phase of the dynamic activity of the cells that depend upon it, and at the same that the brain becomes weary, and that the sum of its functional energies diminishes, the mass of blood which flows to it becomes less, the capillaries are less gorged with blood, and the cerebral tissue insensibly becomes exsanguine.