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for the same thing. They ought, however, to have been aware, that it is metaphysically erroneous to confound an external sense with an internal intellectual faculty. All metaphysicians agree, that a sense receives passively an impression, communicated from an external material object to the brain by a specific nerve, and that there its function ends. An internal intellectual faculty is an active power, to which the sense ministers; and although it has a
; marked prominence of brain, and corresponding development of cranium, it has of itself no direct organic communication with the external material world. Hence it should have been recollected and noticed, that Dr Brown and Dr Spurzheim, if speakingof the same thing, could not both be right, when the one called this power only a sense, and the other only an internal faculty.
In the harmonious relation of the Creator's works, which renders true science, however branched for human convenience, really one and indivisible, a new and brilliant light was soon to be shed on this difficult question, by talent engaged in a different pursuit, and expecting a different result and a different reward. On the 16th February 1826, Sir Charles Bell, in a paper read before the Royal Society of London,* announced the discovery
of the fact that each muscle of the body is supplied with two nerves,-one, the motor nerve as formerly known, to contract and move the muscle; and the other to convey a sensation to the brain of the state of the muscle, that the necessary power may be transmitted, through the motor nerve, for adequate contraction or relaxation, as may be required. It is physiologically true that the same nerve cannot act in both directions. By satisfactory experiments on these two nerves, separately and alternately, as mentioned in his paper, Sir Charles Bell demonstrated the existence and the distinct and different function of each; and although he found this new nerve of muscular sensation likewise associated with the nerve which supplies sensibility to the skin,—viz. of pain, and heat, and cold (for the bared muscle does not feel pain acutely, and heat and cold not at all,)-he concluded it probable, that these two nerves are as distinct from each other, as is the motor nerve from the nerve of muscular sensation. † Be this, however, as it may, he has demonstrated that there is a muscular sensation which informs the brain of the state of the muscle, in other words its demand for motive power; and that that power is conveyed by the motor nerve, the other part of what he calls a nervous circle connecting every voluntary muscle with the brain. In the note referred to below, I adduced an instructive instance communicated by a medical gentleman of Edinburgh, of the alternate failure of the energy of the motor nerve and nerve of muscular sensation. It is at once so illustrative and so decisive, that I
Transactions, vol. cxvi, page 163.
am tempted to give it again. " I was consulted by the son of a gentleman in the country, who has had a singular paralytic affection. He lost the power of motion in his arms, but retained sensation acutely, and felt another person's hand cold or warm, as the case might be. [This concerns the sensitive nerves of the skin.] Now, at the distance of three weeks, he has regained the power of motion, but has lost the sense of the state of the muscles so completely, that he cannot adapt his muscular contractions to the purposes he has in view. (This is the proper nerve of muscular sensation.] In seizing a small object, he bears down upon it with his extended hand, gathers it in,
and grasps it like a vice, not aware of the disproportion of his effort. He has at the same time the complete command of his muscles as to contractions and relaxations [another word for the energy of the motor nerve), but wants only the sense of their state.
The discovery of Sir Charles Bell, * is broad enough to reconcile Dr Brown and Dr Spurzheim, not as having given different and incompatible names to the same thing, but as having each furnished a name for which there is a distinct corresponding thing: in other words, the constituent part of man in question is compounded of both a passive sense and an active internal faculty; and this I humbly think is demonstrable. The nerve of muscular sensation conveys to the brain information of the state of the muscle, and does no more; it neither has nor can have an 'ulterior function. · The state of the muscles is another phrase for the degrees of the impression of resistance upon the muscular frame, a sensation produced by an external material cause, and therefore beyond all question as much a sense as smelling or tasting. The will is under no moral necessity to act upon the sensation or message from the muscle conveyed by the nerve of muscular sensibility, and to command the motor nerve to do its duty, and change the state of the muscle. That it invariably does so, and with the speed of electricity t, is nothing to the purpose : not only are we able to conceive the sensation without the responsive action, but we can suspend that action, and endure the sensation, provided it be not intolerably painful or dangerous, quite long enough to
• This discovery is claimed by a physician of Turin, named, by singular coincidence, Carlo Bellingeri ; and the medical journals have lately maintain. ed a warm controversy on the question of priority. Bellingeri claims to have distinguished the motor nerves from what he vaguely calls the sensitive nerves. From him therefore, I never should have got the idea of a nerve conveying to the brain a sense of the state of the muscle as to contraction ; of this important function, which is every thing for my purpose, I owe my knowledge to Sir Charles Bell. Bellingeri may have discovered two nerves formerly believed to be one; but Sir Charles Bell has thrown greatly more light on their respective functions.
+ Mr Wheatstone, Professor of Experimental Philosophy in King's Col. lege, London, has discovered a means of measuring the velocity of electricity, which he declares to be equal to that of light, 190,000 miles in a second.
demonstrate that it is a sensation independent of any action whatever-in short, that the newly discovered nerve of muscular sensation is as much the nerve of a sense, of which the whole muscular frame is the external organ, as the optic, auditory, or olfactory, are the nerves of their respective senses. That this sense was so long unobserved may be accounted for by the fact of the universality of its external organ: a limited organ, like the eye or ear, points out a sense by the sense's failure when its organ is destroyed; but there was niuch less obvious reason to refer a sense to the entire muscular frame, in other words to know that it is a sense at all.
At the point where the passive sensation of the muscular state ends, and something to be done at the command of the will begins, there is a necessary and most evident distinction of agents as well as operations. Here commences a function for which an external sense is altogether inadequate, an act of the will for which the higher power of an active internal faculty is necessary; and for this faculty there is a specific servant, namely the motor nerve. The conclusion appears to me unavoidable, that in every change, produced by an act of the will through the instrumentality of the motor nerves, in the state of the minutest of above four hundred muscles with which the human body is furnished, two distinct functions are exercised, two separate operations performed: the muscular sense does its specific duty, and reports to the brain the state of the muscle, whether in repose or tension, and in what degree of tension ; and subsequently, although instantaneously, the faculty of muscular adaptation performs its part, and, with the most perfect calculation of the counter resistance required, changes the degree of contraction, in other words the state of the muscle.
Important consequences follow from this distinction between the sense and the internal faculty. While the precise function and extent of function of both are philosophically discriminated, the discovery of their almost invariably joint operation will render yet clearer than hitherto, the manifold phenomena of the rela. tion of animals to the mechanical laws of the material world. Take, for example, those two most constant of all resistances, gravitation and the impenetrability of the earth's surface, for, to the sense, they are both resistances. Gravitation, by attracting our bodies in the line of the earth's centre, prevents them from flying off as portions of matter into infinite space; while the resistance of the solid ground antagonises that attraction, and saves our bodies from being actually carried to the centre; the result being that they are retained on the surface. These joint though counter resistances are felt by us in consequence of their producing a certain state of our muscular frame. Suppose us at rest, sitting upon a chair, the whole frame in equilibrio—the muscular sense informs the
brain, whether we are attending to the information or not, that the muscles are in a certain satisfactory state; a state the disturbance of which is painful to us, by failure of support, or the sense, however brief, of falling; for falling is nothing else but the operation of gravitation, not antagonised by the counter resistance of the earth's surface, or something solid resting thereon, as a chair, a floor, a horse, or a carriage. The fall or prostration of our body itself, from the vertical to the horizontal position, depends upon the failure of another kind of antagonising power, to be presently adverted to. We wish to change our position and stand erect: the sensation of the state of the muscular frame, at the moment of forming this resolution, in other words the mere sense, would never enable us to perform the act of standing erect ; under its province we should simply continue to feel passively the state of the muscular frame which gravitation and the earth's resistance produce, and this without the slightest possible variation. But a distinct power comes now into action, and let us see what it does for us. The instant the will resolves that we shall rise up from the sitting posture, not only is a message sped by the faculty under consideration to those muscles which, by their contraction, draw the legs close to our chair, but, to the end of aiding and antagonising, the muscles of the trunk, neck, and arms, are subjected to a new contraction ; perhaps few muscles in the body are left totally unsummoned and unchanged for the important and complicated feat of standing erect. That this is an exertion, and a great exertion, of muscular power, is familiar to every one who has experienced the fatigue of long standing, when the muscles are successively appealed to in vain for a posture of ease, and the sufferer longs to sit or lie down. Suppose us erect, the duty of the faculty is by no means done: it has mounted guard, and it never deserts its post or relaxes its watchfulness. All unconscious as we may be, our instinct of calculated force is active, and, in a manner which would excite our wonder and admiration could we but see and comprehend it, is keeping to their duty some hundred muscles, each performing, by nicest calculation of its directing power, the precise part allotted it, and exercising the exact measure of force which is necessary to antagonise the power with which gravitation is unceasingly endeavouring to bring our body, so much longer than it is broad, and placed on so narrow a base as the feet, down to the safer base of its back, breast, or side, on the ground. The mus. cular frame antagonises the resistance of gravitation ; but this latter resistance itself assists in keeping us erect, and we could not stand without it: it is the basis of the whole operation,—the perpendicular support, antagonised by the resistance of the ground in the vertical direction; while it is the duty of the muscles to keep the body laterally in that position in which the resistance of
gravitation and of the earth's surface are vertically applied. If the muscles lack strength to preserve that position in which a perpendicular from the centre of gravity of the whole body shall fall within the space occupied by the feet, the body will fall; and it falls instantly, in consequence either of weak muscular power, or of the suspension by some cause of unconsciousness, as syncope, intoxication, or death, of the faculty for applying the necessary force to preserve the balance. We have an appetite for the perpendicular, and any defect of it is painful, and instantly felt by the muscular frame; this will be apparent to any one who is pushed or pressed out of his perpendicular, and staggers to save himself from falling; and this it is which renders the mere sensation of falling, independent of and before the blow to be received, so alarming and painful.
Again, it may be that having succeeded in standing erect, we desire to walk. To produce motion it is evident that equilibrium must be disturbed, for equilibrium is essentially rest. In standing erect, so long as the muscles have force to keep the body in that position in which the two resistances of gravitation and the ground are vertically applied to it, there is, theoretically at least, a state of rest. · What, then, is our first movement to attain the end of walking ? Before we move, the brain is cognisant, by the nerves of muscular sensation-and there are no such faithfully minute and prompt informers -of a certain state of tension of the muscles, necessary, and no more than necessary, to preserve at rest the upright position; but their state must be changed, else there would be no change from the vertical position, and therefore no advance. The faculty under consideration changes the state of almost every muscle in the body, by the required contraction, and no more, and the body is slighily thrown forward. This inclination in walking is so slight that it is scarcely recognised; but its necessity can be put to the test, by any one attempting to advance by merely planting the foot forward,
without an accompanying forward movement of the body. By the inclination of the body, the equilibrium of the standing position is destroyed; but the faculty, in obedience to the necessity of equilibrium, instantly restores the disturbed balance, by that specific muscular contraction which advances the foot. This, which is the first step of walking, is a new position of equilibrium or rest, and, because of an enlarged base, a stronger position than that which the body before occupied. A second step is to be taken, and the faculty, informed by the sense of the state requiring to be changed, repeats the first operation, and does so again and again till the walk is finished, and we change the state of the muscles to prepare for standing still and erect, and again for sitting or lying down. Nothing can be imagined more exquisitely precise than the measure of force allotted to the muscles by this wonderful instinct, for each and all