CLASS IV. I. Nature of the brain, its ramifica nerve seems sixth nerves to voluntary muscles, which receive filaments from no other source, prove clearly that these nerves are voluntary nerves as well as conducive to muscular sensation. "Perhaps", says Mr. Herbert Mayo, "it is not tions and unfair to argue analogically from the preceding instances substitutes. that the same surface of the brain or spinal chord furnishes to each voluntary muscle of the body its voluntary and sentient nerves, if the two are not identical". There is in like manner reason for believing that the fifth nerve which, at its origin, consists of two portions, is not only a nerve of voluntary motion, but furnishes branches to the special senses, and even communicates general sensation to the muscular fibres; and that its gustatory twig is a nerve of both touch and taste at the same time. Several of these phænomena may indeed be resolved, though not the whole, into that close interunion which some parts of the brain maintain with other parts by means of ganglions, commissures and decussations of nerves; whence injuries on one side are often accompanied with loss of motion or feeling in the organs of the other side. So the curious and ingenious, but, I fear, scarcely justifiable experiments, lately instituted by Dr. Philip †, and to which we shall have occasion to return presently, sufficiently prove that stimuli of a certain kind, as spirit of wine, applied to the posterior part of the naked brain of an animal, produce the same effect on the heart, and equally increase its action, as if applied to the anterior part. To affect the heart, however, it seems necessary that the stimulus should spread over a pretty large extent of the brain; so as to take in, by the range of its excitement, some of the ganglions of the brain, whose office, as Dr. Philip conceives, is "to combine the influence of the various parts of the nervous system, from which they receive nerves, and to send off nerves endowed with the combined influence of those parts". He hence accounts for some • Anatomical and Physiological Commentaries, No. II. p. 1. 8vo. Lond. 1822. + Phil. Trans. 1815. P. 5.-90. Phil. Trans, 1815. p. 436. As the same at times subservient to purposes different or different nerves to the same pur pose, I. Nature of the brain, its ramifica CLASS IV. organs of the frame being affected by every part of the nervous system, and others by only certain small parts of it: and the wide influence possessed by the great sympathetic nerve, which is less a single nerve than a string of substitutes. ganglions. We are also hereby shown why the intestines, like the heart, sympathize with every portion of the nervous system. tions and From all this, however, it is clear that there is much yet to be learnt concerning the actual arrangement of the brain, or of its partition into three divisions, and of the respective share which the different parts take in producing a common effect: and consequently it seems to be altogether a wild and idle attempt to subdivide these perceptible inscrutable regions of the brain into still smaller and merely imaginary sections, and to allot to each of them a determinate function and faculty. fanciful to pretend subdivisions to the senses and to endow them with hypo thetical powers. The brain maintains a That a sensorial communication, however, is maintained between some part or other of the brain and every part of the body, and that this communication is conducted by the nerves, is unquestionable from the following facts: If we divide, or tie, or merely compress a nerve of any kind, the muscle with which it communicates becomes almost instantly paralytic; but upon untying or removing the compression the muscle recovers its appropriate feeling and irritability. If the compression be made on any particular part of the brain, that part of the body becomes motionless which derives nerves from the part compressed. And if the cerebrum, cerebellum, or medulla oblongata be irritated, excruciating pain or convulsions, or both, take place all over the body: though chiefly when the irritation is applied to the last of these three parts. For, according to the laws of the nervous action as collected from a variety of experiments by Dr. Philip*, and stated in a subsequent paper to that just referred to, "Neither mechanical nor chemical stimuli (irritating the brain by a knife, or pouring spirits of wine upon it) applied to the * Phil. Trans. 1815. p. 444. CLASS IV. I. Nature nervous system, excite the muscles of voluntary motion, brain, its ramifications and Number and general character of the nerves. The nerves issue in pairs, one of each pair being allotted to either side of the body. The whole number of pairs is substitutes. thirty-nine; of which nine rise immediately from the great divisions of the brain under which we have just contemplated it, and are chiefly, though not wholly appropriated to the four local senses; and thirty from the spinal marrow through the foramina of the bone that encases it, and are altogether distributed over the body to produce the fifth or general sense of touch and feeling, which powers, however, are by some physiologists regarded as distinct from each other, and to communicate, in an especial degree, irritability to the muscles. the spinal marrow is sues from the brain, the latter the opinion We have thus far represented the spinal marrow as Whether issuing from the brain, in conformity with the general doctrine that has hitherto been held upon the subject. It has of late years, however, been contended by various physiologists, and particularly by Drs. Gall and Spurz- from the heim, that the spinal marrow itself is the origin or trunk spinal marof the nervous system, and that instead of issuing from the brain, it gives birth to it. The argument is derived of Gall and from the existence of a spinal marrow alone in acephalous Spurzheim. monsters, and of a nervous chord without a brain, answering the purpose of a spinal marrow, in most invertebral animals. Whence it is inferred that the nervous column is the radical part of the system, and that the brain is an increment from it in the more perfect classes +. Ground of their opi nion. The question is not of much importance, though there is something ingenious in thus tracing animal life from its simpler forms. Yet the opinion seems to be in direct Opposed by opposition to a well-ascertained fact we shall have to analogy. • Anatomie du Cerveau, contenant l'Histoire de son developpement dans le fœtus avec une exposition comparative de sa structure dans les animaux, par A. J. Jourdan, &c. Paris, 1823. + Anatomie et Physiologie du systéme nerveux, &c. par F. J. Gall et G. Spurzheim, 4to. Paris, 1810. facts and I. Nature of the brain, its ramifications and CLASS IV. advert to presently, namely, that the magnitude of the brain and the extent of its intellectual powers hold an inverse proportion to the size of the spinal marrow, and, consequently, upon this hypothesis, to their apparent substitutes. means of supply. Nor is it the mode of induction usually adopted by physiologists on like occasions; since they generally describe the arteries as issuing from the heart, instead of giving rise to it, notwithstanding that the heart, like the brain, has been found totally wanting in some monsters, and the circulation carried on by an artery and a vein alone, of which Mr. Hewson gives a very singular instance*; and that most of the worm genera are equally without a heart though they are in possession of circulatory vessels. We only see in these arrangements that neither a brain nor a heart are essentially necessary to animal life: and that the great Author of nature is the lord, and not the slave, of his own laws; and is capable of effecting the same general principle by a ruder as well as by a more elaborate design. System and ganglions of the intercostal nerve. There is one part, however, of the system of nervous power in the more perfect classes of animals that is particularly worthy of our attention, as furnishing a rule peculiar to itself, and being without a parallel in any other part: and that is the origin, structure, and extensive influence of the great sympathetic or intercostal nerve, which forms a kind of system in itself, an epicycle within the two cycles of cerebral and vertebral influence. It is connected both with the brain and spinal marrow, and may be said to arise from either. Admitting the brain to be its source, it is an offset from the sixth pair of nerves, on either side, and in its course receives a small tributary twig from the fifth, and branches from all the vertebral, from whose union and decussation it is studded with numerous ganglions or medullary enlargements, of which there are not less than three in the neck alone tinted by an addition of cineritious substance, a larger number in its line through the chest, and others as it descends. On the Lymph. Syst. Part I. p. 15. I. Nature of the ramifications and substitutes. From its structure and exten course, an sympathy. still deeper, independently of various confluences of CLASS IV. smaller branches that unite and form extensive net-works. Having reached the hollow of the os coccygis, it meets its brain, its twin from the opposite side which has pursued a similar course, and been augmented by similar contributions. Thus equally enriched with the nervous stores of the brain and the spinal marrow, it sends off radiations as it takes the course of the aorta, to all the organs of the tho- sive interracic, abdominal, and hypogastric regions, to the lungs, instrument the heart, the stomach, and intestines, the bladder, uterus, of general and testes; and thus becomes an emporium of nervous commerce, and an instrument of general sympathy: and what is of infinite importance in so complicated a frame as that of man, furnishes to the vital organs streams of nervous supply from so many anastomosing currents, that if one, or more than one, should fail or be cut off, the function may still be continued. To this it is owing, in a very considerable degree, that the organs of the upper and lower belly, exhibit that nice fellowship of feeling which often surprises us, and that most of them are apt to sympathize in the actual state of the brain. There is no animal whose brain is an exact counterpart to that of man: and it has, hence, been conceived that by attending to the distinctions between the human brain and that of other animals, we might be able to unfold a still more mysterious part of the animal economy than that of sensation or motion, and account for the superior intellect with which man is endowed. But the varieties are so numerous, and the parts which are deficient in one animal are found connected with such new combinations, modifications, and deficiencies in others, that it is impossible for us to avail ourselves of any such diversities. Aristotle endeavoured to establish a distinction by laying it down as a maxim, that man has the largest brain of all animals in proportion to the size of his body; a maxim which has been almost universally received from his own time to the present period. But it has of late years, and upon a more extensive cultivation of compara The human brain has no exact coun terpart in other ani mals. But no reasoning of the hu- lect. pared with the brain by How com Aristotle. |