1849.] Systems of Nerves. 549 line or rather chord of gray matter, and passing out through the surrounding layer of white substance, the efferent nerves go to all the muscles indeed, but more especially to those which are employed in respiration, deglutition, and other actions or motions equally necessary to the maintenance and perpetuation of life. The afferent nerves arising mainly, either from the external surface or from the internal membranes, and gathering themselves successively into twigs, branches, and trunks, make their way to different points of the spinal marrow, and having penetrated the outer layer of fibrous matter, terminate in the same line of ganglia from which the efferent take their rise. As in the preceding system, the nervous circles are thus completed without anywhere coming into relation with the spirit. Irritation of the afferent fibres immediately calls into action the efferent, neither sensation nor volition intervening. In this manner a large and important class of actions are provided for independently of any effort or knowledge on the part of the individual, so that they continue to be performed not only during the unconscious state of sleep but even after the entire paralysis of the nerves of both sense and will. Besides the mechanical acts of respiration and deglutition already alluded to, the moistening and lubrication of the eye by the frequent passage of the lid over it, the dilation or contraction of the pupil according to the degree of light, the spontaneous and spasmodic closure of the glottis against whatever, if admitted, would do harm to the lungs, the defence and control of all the entrances to the body, together with the habitual tone or tension of the muscles generally, depend upon this system. In many of the lower animals, it is much more largely developed and performs a greater number of offices than in man. These two systems, the sympathetic and the excito-motary, are the first which are called into action and the last which cease to act. Their functions commence with the organic life of the individual, and continue without interruption or cessation till that life is terminated. It is under the influence of the latter of these systems that the last dying struggles take place, after, it is believed, in many instances, all sensibility and consciousness have ceased-after, it may be, the spirit has already been loosed from its connection with the body. 3. Of the senso-motary system. This has its origin in a chain of ganglia situated at the base of the brain, and in intimate connection, on the one hand with the cerebrum, and on the other with the spinal marrow. The nerves of sensation, including those of the special senses, constitute the afferent branches and a corresponding set going to the different muscles-the efferent. This system is in immediate relation with the spirit. All our sensations are experienced through it, and a large class of our actions are dependent upon it. It is not however alone capable of awakening perceptions, nor are its efferent branches directly acted upon by the will. Sensation and not volition is their proper stimulus. Hence the motions to which they give rise are spoken of as consensual. Of this character are the violent muscular contractions occasioned by the sudden application of heat and cold to the surface, the involuntary closing of the eye from excess of light, coughing, sneezing, vomiting, laughing, weeping, and numerous other similar acts which are performed not only without assistance from the will, but in direct opposition to it. Of the same character also are the various instinctive actions, which though comparatively few in the case of man, make up by far the greater part of the motions executed by the lower tribes of the animal kingdom. In all of these instances, the nervous circle is completed without the intervention of either perception or volition. There is no knowledge, no plan, no purpose. Some form of sensation or of emotion is the immediate and sole cause of the action. 4. Of the cerebral system. This is situated entirely within the cranial cavity, of which it occupies the larger portion. Its structure is far more compact than that of the preceding systems and the relative disposition of the two kinds of matter entering into it, very different. The gray, instead of occupying the centre, is situated at the circumference of the cerebrum, surrounding and enclosing the white on all sides, except where the latter, as already mentioned, forms connections with the ganglia of the senso-motary system. It consists in man of a layer, about the eighth of an inch in thickness, which follows the surface of the brain through all its irregularities, now passing over its eminences, and now descending into its depressions, so that if it were detached from the included mass, and all its numerous bendings unfolded, it would be of sufficient dimensions to contain a body eight or ten times as large. Indeed, it is not improbable that the chief object of these remarkable inequalities of surface is the extension of this peripheral layer of gray substance, and the consequent increase of its power as a secreting organ. More points are also thus presented for the reception of the innumerable fibres of the white or conducting matter, which radiating from the senso-motary ganglia, go to bury themselves in this layer. These fibres are closely packed, and together with the insulating material by which they are surrounded, and the veins and arteries which serve to nourish them, make up the whole interior portion of the brain. Their sole use, as it would seem, is to connect the ganglia at its base with the periphery of the organ. Such is a general account of the structure of the cerebrum. Its 1849.] The Cerebellum. 551 office as gathered from the teachings of comparative anatomy, from experiments made upon the lower vertebrated animals, and from various pathological phenomena observed in the case of man, is on the one hand, the awakening of ideas in the mind, and on the other, the transmission of volitions to the different members of the body. Impressions made upon the outward senses, are capable of exciting through the preceding system only sensations. Through this, they give rise to perceptions. The will has no power over the preceding system. By this its mandates are received, and conveyed to the muscles whose duty it is to execute them. Whether the fibres of this system, afferent and efferent, pass out through the senso-motary ganglia, and go to the several organs of sense and motion, or whether they terminate in those ganglia-the impressions made and received by them being transmitted through the preceding system-is not quite certain. Most physiologists incline to the latter opinion, although the analogy of the other parts of the nervous structure would seem to favor the former. Further anatomical investigations conducted by aid of the microscope are necessary for settling the question. 5. Of the cerebellum. This like the cerebrum is situated within the cranium, occupying the lower and, back part of that cavity. Although of far inferior dimensions, the relative disposition of the two kinds of matter composing it, is the same,-the gray forming the peripheral, and the white the central portion of the organ. Its connections are with the cerebrum, the ganglia at the base of the brain and the spinal marrow,-more especially the two latter. From the study of its comparative development in the different orders of the lower vertebrated animals, as well as experiments made upon those animals, it is believed to be immediately concerned in the regulation and subordination of the different muscular contractions necessary to the execution of the more delicate and complex movements. Indeed there are scarcely any motions so simple as not to require for their production the simultaneous contraction of several muscles; and it not unfrequently happens, that the same muscle forms one, of two, three, or four different sets of muscles producing by their combined action as many different motions. Now the several muscles constituting each of these sets, are associated through the cerebellum, it is believed, in such a manner that their simultaneous and due contraction is determined by a single act of the will, directed by the idea of the object to be accomplished. When the cerebellum is removed from the brain of a bird, which it may be without materially affecting the vital functions, the bird, although retaining its powers of sensation, perception, and voluntary motion, is no longer able to execute with any precision movements requiring the combined and harmonious action of several muscles. If on the other hand, the cerebrum be removed, it retains the power of sensation and continues to perform a great variety of instinctive and consensual motions, but it no longer shows signs of either perception or will. If food be placed in the mouth, it is swallowed, and life may in this way be sustained for weeks or even months. But no intelligent, voluntary effort is made by it for procuring the means of sustenance. From the brief view which we have thus presented of the different nervous centres and of the functions respectively connected with them, it will be seen that our present inquiry has to do more especially with the cerebrum. This is the seat of perception and of volition. Through the instrumentality of this, ideas are awakened, which supply material to the intellect, and furnish, as it were the basis of all its operations. When quickened into action in any part by impressions conveyed to it from without, through the medium of the sensory ganglia, the ideas to which it gives rise are recognized as perceptions. When, on the contrary, the action originates within, whether spontaneously through the power of habit, or from the influence of the will, the ideas evolved are recognized by us as conceptions. In both cases, however, the same parts of the cerebral structure are concerned in their production. Each of the senses has its own separate ganglion, in which the nerves ministering to it all terminate. There is one for seeing, another for hearing, a third for smelling, a fourth for feeling, and perhaps a fifth for tasting, although it is not quite certain whether this sense is anything more than a modified form of touch. These ganglia, as already stated, are intimately connected with the cerebrum, by dense bundles of fibres which radiate from them to its circumference, and which are supposed to be the immediate instrument in awakening our varied perceptions. Now what we would have especially observed is, that only one of these ganglia-we mean on the same side, for the brain like the external parts of the organization is throughout double-is found to each of the senses. There is not one ganglion for awakening the perceptions of sight and another similar and associated ganglion for awakening the corresponding conceptions. The same is true of hearing and smell and touch. In the case of no one of them, is there but a single interior organ, which can be supposed to have any part in developing the ideas, whether perceptions or conceptions belonging to it. Remove from the brain the sensory ganglia, together with the entire mass of fibres which go from them to the gray matter of the cerebrum, and there would remain only the parts subsidiary to motion. From the structure therefore of the great nervous centre with which the mani 1849.] Mental operations independent of the Brain. 553 festation of the mental powers is immediately connected, as well as from the close resemblance of our conceptions to our perceptions and the readiness with which associations are established among the former, through the latter, we conclude that both classes of phenomena are dependent upon the same parts of the brain; and that as we have already said, these parts are excited to action in the one case by outward causes, over which we have no voluntary power, and in the other by influences within, to a greater or less extent under the control of the will. In both cases, the organs are the same, and the kind of action is the same. The only difference is in its origin and degree. In pursuance of our purpose of tracing the organic dependence of the several classes of mental phenomena, we shall next consider recollection. This is more complex than any of the acts or states of the spirit, to which we have thus far directed our inquiries. It involves conception and something in addition to it. In its simplest form, it consists of the reproduction and recognition of a former idea. The first of these is a mere act of conception, which is accomplished as in any other case, through the instrumentality of certain parts of the brain. In its ordinary, spontaneous form, it is the result of some association which has been established between that, and the act or state immediately preceding it in the order of the mental train. The second is a purely spiritual cognition, wholly independent of the material organization, except so far as that is necessary for evolving the ideas to which it relates. It is an intuitive judgment which the mind forms on comparing what it is at present thinking or perceiving, with what it has previously thought or perceived, including, it may be, along with the main idea, more or less of the attendant circumstances-a judgment by which the two things are affirmed to be identical. As this judg ment which the mind thus passes, has exclusive reference to its own consciousnesses awakened indeed by the action of the cerebrum, but not themselves organic, it is obvious, that it must be a simple act of the spirit, and would therefore continue the same, were the brain entirely dissolved, provided the same mental states should be produced through the medium of any other agent or organ. Besides the cognitions of memory, there are numerous other intuitions of the spirit, which are equally independent of the cerebral organism, and which would in like manner continue to arise, in whatever way the ideas forming their subject should be presented. In truth the entire class of interior perceptions, which by transcendental philosophers are referred to the pure reason, would seem to be of this character; and if we mistake not, it is by this character, and this alone, that they are distinguished from the so-called phenomena of the VOL. VI. No. 23. 47 |
