Any farther detail in addition to what has now been said would contribute little to our essential knowledge of the nervous system. We have traced it from its commencement in a series of independent centres, and we have seen these becoming successively conjoined in a greater and greater degree, as if in obedience to a law of attraction, whose tendency was to collect these various nuclei from every part of tbe body towards a common centre. This dis position to centralization has, in its turn, given a satisfactory explanation of the most remark able differences observed in the disposition of the ganglions and of the nervous cords among the different types of the class, however dissimilar tbese may be one from another. We may, therefore, here conclude, as has been done already in my work especially devoted to this subject, that the nervous system of the Crustacea consists uniformly qf medullary nuclei (ganglions), the normal number of which is the same as that of the members or rings of the body, and that all the modifica tions encountered, tvhether at different periods of the incubation, or in dffferent species of the series, depend especially on the approximation, more or less complete, of these nuclei, (an ap proximation tvhich takes place from the sides towards the median line as well as in the longi tudinal direction,) and to an arrest of develop ment occurring in a variable number (g. the nuclei.
In a paper upon the nervous system of the Lobster recently published,'" Mr. Newport mentions an interesting fact hitherto overlooked by anatomists. Ile found that the double omnolionic chain of this Crustacean is composed 0 0 of two orders of fibres, forming distinct and superposed fasciculi or columns, which the author designates colunins y. sensation and of motion, following the analogy which he be lieved he had traced between these fasciculi and the anterior and posterior columns of the spinal cord of the higher animals. The fas ciculi here indicated are but indistinct in the interganglionic cords, but become extremely apparent in the ganglions themselves, for these enlargements belong exclusively to the inferior or sensitive fasciculi, and the superior or motor fasciculi pass over their dorsal surface without penetrating their substance at all.
Before going on to the study of those organs the object of which is the application, if we may be allowed the expression, of the nervous system to the perception of the existence of outward objects, and of those in which the reaction designated volition is immediately effected, that is to say, the organs of the senses and the muscles, it may be as well to say a word upon the general functions of the nervous system itself in its different parts. The experiments made by M. Audouin and me, with a view to solve the principal problems which may be proposed on this subject, have confirmed the inductions to which we had been led by views arrived at a priori wholly from anatomical researches, of which the preceding may be regarded as the sumniary. Thus:—
istly, The nervous is the system which en tirely presides over the sensations and motions.
2dly, The nervous cords are merely the organs of transmission of the sensations and of volition, and it is in the ganglions that the power of perceiving the former and of pro ducing the latter resides. Every organ sepa rated from its nervous centre speedily loses all motion and sensation.
3dly, The whole of the ganglions have analogous properties : ,the faculty of determin ing motions and of receiving sensations exists in each of these organs; and the action of each is by so much the more independent as its development is more isolated. When the ganglionic chain is nearly uniform through its whole length, it may be divided without the action of the apparatus being destroyed in either portion thus isolated,—always under stood, that both are of considerable size ; because when a very small portion only is isolated from the rest of the system, this appears too weak, as it were, to continue its functions, so that sensibility and contractility are alike speedily lost. But when one portion of the ganglionic chain has attained a develop ment very superior to that of the rest, its action becomes essential to the integrity of' the functions of the whole.
It must not be imagined, however, from this that sensibility and the faculty of exciting muscular contractions are ever completely con centrated in the cephalic ganglions, and it seems to us calculated to convey a very inaccurate idea of the nature and functions of these ganglions to speak of them under the name of brain, as the generality of writers have been led to do, seduced by certain inconclu sive analogies in point of form and position.
It is nevertheless to be remarked that in these animals an obscure tendency to the centra lization of the nervous functions is observable in the anterior portion of the ganglionic chain; because if in the Lobster, for instance, it be divided into two portions, as nearly equal as possible, by severing the cords of communica tion between the ganglions belonging to the first and second thoracic rings, sensibility, and especially mobility, are much more quickly lost in the posterior than in the anterior half; and this disproportion is by so much the more manifest as the division is performed more posteriorly ; still there is a great interval between this first indication and the concen tration of the faculties of perception and. of will in a single organ—the brain, of which every other portion of the nervous system then beco:nes a mere dependency.