Abbildungen der Seite
PDF
EPUB

In the animal kingdom sensibility reveals itself in its origin by phenomena exactly comparable with those which we have just sketched.

There, in the form of amoeboid movements of the white corpuscles and ciliated cells, and contractility of the protoplasmic cells, it shows itself to us under the appearance of purely histological sensibility, and not as yet in the shape of sensibility belonging to a living autonomous individuality.

In the protozoa, rhizopods, and certain polyps, it becomes more and more distinct, and by the very complex operations through which it manifests itself, we perceive how well these protorganisms of the animal kingdom are provided with active and vital energy, and how distinctly general sensibility is inherent in them and combined with their substance.

In these elementary forms of animal life, the phenomena of sensibility are first united with an organized tissue. They are divided among as many cells as the individual contains; and they exist in a vague and diffuse manner, without there being as yet a special system of anatomical elements, designed to serve them as an appropriate receptacle.

Soon, as we ascend in the series of beings, new factors are added to the preceding; the phenomena become complicated as they grow more perfect, and we then see that in proportion as animal organisms develop themselves, and their agglomerations of cells become more numerous, there takes place among them, as it were, a natural selection of the physiological work to be performed. Some are gifted with such or such specific * Wund, "Physiologie," p. 83.

aptitudes, and appropriate such or such a function, while others, gifted with such or such a different aptitude, reserve themselves for such or such another. For its better performance there is a division of labour.

This natural division of the living forces of the living individual, which are thus distributed among the different departments of its substance, constitutes the first outline of the nervous system.

It soon appears, like an organ of perfectionment implanted in the organism. It is henceforward the grand dispenser of sensibility in general, and is designed to collect, to drain all the scattered forms of sensibility, to fegulate their course, to condense them in its own reservoirs, to purify them by the participation of its substance, to make them leap forth in the form of motor excitations, or to transform them, like the perfected products of its own industry, into subtle and quintessential materials, destined to co-operate in the most subtle phenomena of psycho-intellectual life.

Humble in its origin, the nervous system, as F. Leydig has pointed out, makes its first appearance in the midst of the living tissues in the form of three or four cells, independent one of another.* One step further, and the cells are united within a common envelope, a first nervous ganglion being thus constituted. Little by little the work of evolution completes itself; ganglion is united to ganglion; these soon dispose themselves in the form of two lateral rows, which emit, right and left, radicles which plunge into the surrounding tissues, and soon these two lateral chains, approaching, become fused together, and thus constitute a central unity,

* Claude Bernard, “Système nerveux,” vol. i. p. 506.

or axis, around which all the nervous radii emerging from the peripheral regions converge. At the same time, a superior ganglion, destined to be the brain, is developed, and uniting itself to the axis, becomes in a manner the crowning of the edifice thus successfully perfected.

From this moment the nervous system is constituted as a central force destined to condense in its plexuses sensitive excitations, in order to transform them by its own metabolic action into co-ordinated motor reactions. From this moment the living forces of the organism are duly subordinated and distributed in a methodic fashion; the physiological task is regularly divided; one group of elements is connected with sensibility, one centre with motor-power, and another with the functions. of organic life.

Sensation is henceforward neatly isolated in special regions of the system, neatly collected in particular organs, and from the very fact that it is attracted, like an electric fluid, by means of nervous conductors, from the peripheral regions towards the central, it becomes a disposable mobile force, transmissible to a distance like dynamic electricity.

Once concentrated in the central regions of the system, it thus represents, with all the diverse elements of which it is composed, a true synthesis of all the partial sensibilities of the living being, and the true generating element of its living and feeling unity.

The phenomena of sensation in the superior animals are not, then, simple phenomena, constituted by the mere reaction of a tissue in the presence of external

excitations; they are the complex subordinated operations of the nervous activity which require the participation of a great many organs successively brought into play, in order to arrive at their complete evolution. We shall now study these different conditions in succession.

CHAPTER II.

EVOLUTION OF THE PROCESS OF SENSIBILITY, THROUGH THE MECHANISM OF THE NERVOUS SYSTEM--UNCONSCIOUS SENSIBILITY CONSCIOUS SENSIBILITY

(SENSATION).

THE nervous system being constituted, as we have just explained, by a central axis, plunging by its lateral roots into the surrounding tissues, and crowned at its superior extremity by a central ganglion, the brain, gifted with its special activity, we shall now see how the phenomena of sensibility, existing per se as fundamental histological properties, behave in presence of the machinery which the nervous system places at their disposal; how they become incorporated with it; how, arriving in the form of centripetal excitation, they become refracted in the plexuses, reappearing as a centrifugal reaction, through the peculiar influence of the new media they have put in requisition; and how at last, in the most elevated regions of their journey, they come to play a primary part in the evolution of the essential phenomena of psycho-intellectual activity.

In taking their departure from the peripheral regions of the nervous system, which physiologically represent the frontiers of the organism, sensitive impressions, wherever they may have originated, once implanted in

« ZurückWeiter »