CLASS IV. I. Nature of the brain, its ramifications and substitutes. II. Principle of sensation and motion. more conscious of an excitement in this organ of sense than in any other: and, from the anomaly and rare occurrence of the sensation itself, find no terms by which to express it. In Germany it has of late been attempted to be shown that every man is possessed of a sixth sense, though of a very different kind from those just referred to; for it is a sense not only common to every one, but to the system at large; and consists in that peculiar kind of internal but corporeal feeling respecting the general state of one's health that induces us to exult in being as light as a feather, as elastic as a spring; or to sink under a sense of lassitude, fatigue, and weariness, which cannot be accounted for, and is unconnected with muscular labour or disease. To this sensation M. Hubner has given the name of canesthesis, and several of his compatriots that of selbstgefühl, and gemeingefühl," self-feeling or gèneral-feeling;” and its organ is supposed to exist in the extremities of all the nerves of the body, except those that supply the five external senses*. I scarcely know why these last should have been excepted: for the sensation itself is nothing more than a result of that general sympathy which appears to take place between different organs and parts of the body, expressive of a pleasurable or disquieting feeling according as the frame at large is in a state of general and uninterrupted health or affected by some cause of disquiet. II. As the nerves thus generally communicate with each other, and with the brain where this organ exists, it has been a question in all ages by what means they maintain this communication, and what is the nature of the communicated influence? or, in other words, what is the fabric of the nerves, and the quality of the nervous power? Comment. de Canesthesi. Dissert. Aug. Med. Auct. Chr. Fred. Hubner, 1794. Nature and Origin of Mental Derangement, by A. Crichton, M. D. 2 vols. 8vo. 1798. CLASS IV. II. Princi ple of sen motion. whether Upon these points two very different opinions have been entertained from an early period of the world, which under different modifications have descended to our own sation and times for by many physiologists, both ancient and mo- Nervous dern, the nerves have been regarded as solid capillaments, fabric, or tense and elastic strings, operating by tremors or oscil- solid chords lations, like the chords of a musical instrument; and by or hollow others as minute and hollow cylinders conveying a pecucylinders. liar fluid. The word NERVE, which among the ancients Original was applied to tense chords of every kind, and especially the term to bow-strings and musical strings, affords a clear proof nerve. meaning of crates and Galen ; an etherial how generally the former of these hypotheses prevailed Hypothesis among the Greeks. It was not, however, the hypothesis of Hippoeither of Hippocrates or Galen; for by them, while the nerves were regarded as the instruments of sensation and motion, the medium by which they acted was supposed to be a fine etherial fluid, elaborated in the organ of the brain; to which they gave the name of animal spirit, to distinguish it from the proper fluid of the arteries which was denominated vital spirit. "Not", says Galen," that supposed this animal spirit is of the substance of the soul, but its fluid; prime agent while inhabiting the brain"*. But with respect to the manner in which the animal spirit operates upon the nerves they spoke with great modesty; for though they thought they had been able to trace a tubular form in some of the nerves, and particularly those of vision, they had not been able to succeed in others. And hence, says Galen, " it is impossible for us to pronounce absolutely and without proof, whether a certain power may not be transmitted from the brain through the to the different members; or whether the material of the ed a comanimal spirit may not itself reach the sentient and moving parts; or, in some way or other, so enter into the nerves as to induce in them a change which is afterwards extended to the organs of motion" +. nerves ⚫ De Hippocratis et Platonis Decretis, Lib. vII. A. Tom. 1. p. 967. Ed. Basil. 1542. + Id. Sect. C. p. 969. but ed them selves un express certain how it maintain munication with the rest of the body. CLASS IV. II. Principle of sensation and motion. The ques an unset tled state. Hartley's In a state not much less unsettled, remains the subject at the present moment. Dr. Hartley, in the beginning of the seventeenth century, revived the hypothesis that the nerves are bundles of solid capillaments conveying tion still in motion, sensation, and even perception, by a vibratory power, and supported his opinion with great ingenuity and learning *; but the opposite hypothesis that they are hypothesis minute tubes filled with the animal spirit of the Greek of vibratory physiologists, had acquired so extensive a hold ever since the discovery of the circulation of the blood, which presupposes the existence of tubular vessels too subtle to be traced by the senses, that it never obtained more than a partial and temporary assent; and hence, from the times of Sydenham and Boerhaave almost down to our own day the last has been the popular doctrine; is to be traced in the general tenour of medical writings; and has been especially maintained by Sabatier and Boyer. strings, not able to subvert the hypothesis of Sydenham and Boerhaave that they are hollow cylinders conveying an animal spirit. Nervous fibres unadapted to vibrations, as inelastic: In effect, no fibres of the animal frame can be less adapted to a communication of motion by a series of vibrations than those of the nerves, since none exhibit a smaller degree of elasticity; and though we have little reason to confide in their tubular structure, or to believe that any kind of fluid is transmitted in this way, the close affinity which the nervous power' is now known to hold with several of the gasses that chemistry has of late years unfolded to us; and the wonderful influence which some however of of them possess over the moving fibres of the animal yet no proof of their being tubular. Little doubt a nervous and peculiar fluid; which, like gasses, does various not stand in need of hollow ves sels for its transmission. frame, seem to leave no question that the nervous power itself is a fluid, though not, perhaps, of their precise nature, yet resembling the most active of them in its subtilty, levity, and rapidity of movement. Nor is there upon this supposition any difficulty in conceiving of its transmission by solid fibres or capillaments of a particular kind, the neurilemma of Bichat, whilst we behold the etherial fluids, now referred to, transmitted in the same way by substances still more solid and unporous. But there is another question, closely connected with • Observations on Man, his frame, &c. his duty, and his expectations. 2 vols. 8vo. 1749. the present subject, that has also greatly interested physiologists both in ancient and modern times, and is not yet settled in a manner altogether satisfactory. CLASS IV. ple of sen II. Princi sation and motion. Whether and motion power, or a common from dis tinct The two It has appeared that the nerves are instruments both of sensation and motion. Are these two effects produced sensation by the same nervous fibres or by different? or by the same fluids or by different? That there must be two distinct kinds of fibres, or of fluids, is clear, because, as we from shall have more particularly to observe when we come to sources? treat of paralysis, the muscles of a limb are sometimes effects must deprived of both sensation and motivity at the same pe- proceed riod, sometimes of sensation alone while motivity conti- from disnues, and sometimes of motivity alone while sensation of fluids. continues. And hence Hippocrates and Galen, the last to the According of whom has treated of the subject with great minuteness Greeks, in many of his writings, while they speak of only one tinct sets of kind of animal spirit, speak of two kinds of nerves, those fibres opeof sense and of motion; equally issuing from the brain, rated upon and mostly accompanying each other, and forming parts same nerof the same organs. tinct fibres from dis vous fluid. distinct pro cerebellum. This distinction is supported by the concurrent obser- How far vations and experiments of physiologists, and especially supported by by the curious investigations of many of those of our own siologists. day, among whom should be particularly noticed the names of Fleurens, Rolando, Charles Bell, Magendie and Shaw. M. Rolando attempted to show by a long train Rolando's of interesting, but very painful, and hence unjustifiable vince of ceexperiments, carried on through animals of almost every rebrum and kind, that the cerebrum is the ordinary source of sensation, and the cerebellum of motion: for, according to his observations, in every instance in which the former is much broken down, or in any other way injured, drowsiness, stupor, or apoplexy, is sure to follow; the animal being still capable of exercising locomotive power, but without any guidance or knowledge of what it is about, or where it is moving to. But the moment the cerebellum is wounded, the locomotive power is instantly lost *. These ⚫ Saggio sopra la vera Struttura del Cervello, &c. e sopra le Fonzioni della Sistema Nervosa. Sassari, 1809. CLASS IV. II. Principle of sensation and motion. Such sepa since con firmed, but ascribed. investigations were valuable as leading on to others more accurately conducted and followed up by more correct conclusions. That these distinct portions of the brain are endowed with separate powers, as observed by Rolando, has rate powers been sufficiently ascertained by other pathologists; and especially by M. Fleurens*, who does not seem, at the Conversely time to have been acquainted with Rolando's experiments, and consequently gives us the weight of an unconnected testimony. But it seems to have been better established, as M. Magendie remarks+, since these experiments, that the converse of M. Rolando's constitutes the law and order of nature: for sensation seems now proved to be dependent upon the cerebellum, instead of upon the cerebrum, while motivity takes its rise from the cerebrum instead of from the cerebellum. Followed up by C. Bell into the spinal mar row. His demonstration. Double chord of the Like me chanism continued to Mr. Charles Bell has successfully followed up these distinct and established powers of the two departments of the braint, into the spinal marrow, which he has sufficiently proved to consist of a double chord; an anterior connected with the crura of the cerebrum, and productive of locomotion, and a posterior connected with the crura of spinal chain. the cerebellum, productive of sensation. And he has further shown that these two distinct powers are communicated to every part of the body by nervous fibres accordevery part of ing as they issue from the one or the other of these rethe body. spective channels: that, for the most part, every nervous fascicle distributed over the body and limbs, has a double origin, and issues equally from both the anterior and posterior trunk of the spinal medulla; and is consequently alike sensific and motific: while those which proceed from one alone, are limited in their power to the peculiar property of their source, of which the portio dura of the seventh nerve affords a striking example: being, when uncombined, simply a nerve of motion, without the Strikingly exemplified in the portio dura of the seventh nerve. Archives Générales de Médecine, 1. 11. + Experiences sur les Fonctions, &c. Journ. de Physiologie, Tom. II, III. passim, 1822, 1823. Idea of the Anatomy of the Brain. 1809. |