From this precise moment it leaves the purely psycho motor regions of the cortex, in the form of transient and rapid stimulations destined to be converted into articulate sounds, digital movements, or expressive ges tures ; and it proceeds, by help of the special white fibres (cortico-striate fibres), to different territories of the corpus-striatum, of which it thus excites the imme diate activity. (See 5, II, 16, Fig. 6, p. 6i.) Here, in this first stage of its outward course, it insensibly loses its original character of a purely psychical excitation, to incorporate itself more and more with the organism, to materialize itself, in a man ner, and increase its dynamic power by the addition of a new nervous element, the cerebellar innervation, which, in the condition of a static force in permanent tension, is incessantly distributed in the plexuses of the corpus striatuM.
Thus reinforced by this adventitious contingent of innervation which is engrafted into it, it continues its centrifugal course (see 7, 12, 19, Fig. 6, p. 6i), and by means of the antero-lateral fibres of the axis (cerebral peduncles) it descends, in the form of an interrupted current, to excite the dynamic activity of the different motor nuclei of the spinal axis, which, like a series of always ready to enter into action, only wait its arrival to develop their latent activity. From this moment, mixed up with the proper activity of the different spinal regions, it projects itself along the anterior roots and thus becomes, in its final phases of transformation, one of the multiple exciting causes of muscular contractility.
We see then, to sum up, from what precedes, that the processes which produce voluntary motion pass, in their evolution; through phases inverse to those of the processes of sensibility. While these latter, as they approach the central regions of the sensorium, are puri fied and made perfect, becoming more and more intel lectualized by the metabolic action of the different nervous media through which they are propagated; the former, on the contrary, conceived as psychical vibra tions at the moment of their genesis, amplify and are materialized more and more, as they descend from the superior regions. They become complicated by the addition of adventitious elements, which reinforce them as they progress (cerebellar and spinal innervation), and thus become, in the last term of their evolution, a true synthesis of agglomerated dynamic elements, which resume in themselves the vital forces of the system through which they have been developed—cerebral, cerebellar, and spinal activities.*
Conceived under this simple formula, the processes which produce voluntary motion begin by being a purely psychical excitation, and insensibly become, by the natu ral play of the organic machinery, a physical excitation. In thus becoming transformed in their successive evolu tion, they present the fascinating picture we constantly see presented to us in the working of steam-engines. We see, in fact, in this case, how a force, slight at its commencement, is capable of being transformed, and becoming by means of the series of apparatuses it sets at work, the occasion of a gigantic development of mechanical power.
In fact, at the moment when the engine begins to work, a very slight force, the mere intervention of the hand of the engine-driver who turns a handle and lets the steam rush against the upper surface of the piston, would suffice for this. This active force, once at liberty, immediately develops its strength, which is proportional to the surface over which it extends ; the piston falls, its rod draws down the beam ; the power is developed as the fly-wheel revolves, and the initial movement, so weak at its commencement, amplifies and increases continually, in proportion as the volume and power of the mechanical appliances placed at its disposal become more considerable and more powerful.
We see then, in conclusion, after an examination of all the details of cerebral physiology that we have suc cessively passed in review, that the different processes of cerebral activity finally resolve themselves into a circular movement of absorption and restitution of forces. The external world, with all its incitements, enters into us by the channel of the senses, in the form of sensorial excitations ; and the same external world, modified, and refracted by its intimate contact with the living tissues it has traversed, emerges from the organism, and is reflected outwards in the various manifestations of voluntary motor-power.