In the production of vocal sounds, again, that nice adjustment of the muscles of the larynx, which is requisite to give forth deter minate tones, is ordinarily directed by the auditory sense; being learned in the first instance under the guidance of the sounds actually produced ; but being subsequently effected voluntarily, in accordance with the mental conception (a sort of inward sensa tion) of the tone to be uttered, which concep tion cannot be formed, unless the sense of hearing has previously brought similar tones to the mind. Hence it is that persons who are born deaf; are also dumb. They may have no malformation of the organs of speech ; but they are incapable of uttering distinct vocal sounds or musical tones, because they have not the guiding conception, or recalled sensation, of the nature of these. By long training, however, and by imitative efforts directed by the muscular sense of the larynx itself, some persons thus circumstanced have acquired the power of speech ; but the want of a sufficiently definite control over the vocal muscles is always very evident in their use of the organ. It is very rarely that a person who has once enjoyed the sense of hearing, afterwards becomes so completely deaf, as to lose all auditory control over his vocal organs. An example of this kind, however, has been made known to the public by a well known author, as having occurred in himself; and the record of his experiences * contains many points of much interest. The deafness was the result of an accident occurring in child hood, which left him for some time in a state of extreme debility ; and when he made the attempt to speak, it was with considerable pain in the vocal organs. This pain probably resulted from the unaccustomed effort which it was necessary to make, when the usual guidance was wanting ; being analogous to the uneasiness which we experience when we attempt to move our eyes with the lids closed. His voice at that time is described as being very similar to that of a person born deaf and dumb, but who has been taught to speak. With the uneasiness in the use of the vocal organs was associated an extreme mental in disposition to their employment; and thus, for some years, the voice was very little ex ercised. Circumstances afterwards forced it, however, into constant employment; and great improvement has subsequently taken place in the power of vocalisation, evidently by atten tion to the indications of the muscular sense. It is a curious circumstance, fully confirming this view, that the words which had been in use previously to the supervention of the deafness, are still pronounced (such of them, at least, as are kept in employment) as they were in childhood ; the muscular movements concerned in their articulation being still guided by the original auditory conception, in spite of the knowledge derived from the in formation of others, that their pronunciation is erroneous. On the other hand, all the words subsequently learned are pronounced according to their spelling; the acquired as sociations between the muscular sensations and the written signs being in this case the obvious guide.
The perception of "effort" which we derive through the impressions made on the muscular sense, is one which, as we shall presently see, is of immense value, in com bination with simple tactile sensation, in informing us of the sensible properties of external objects. In its simplest exercise, however, it enables us to appreciate the degree of muscular force which is being exerted ; and excites in our minds our most definite idea of power. It is true that we might, by the exercise of our other senses, have arrived at the conception of a tendency in bodies to attract one another, or to com municate motion one to another ; but the notion of the force with which they do so is entirely founded, directly or indirectly, upon the conception of the muscular exertion which would be required to produce or to antagonise the movement. Thus it is, too, that when we are about to make a muscular effort, the amount of force which we put forth is governed by the mental conception of that which will be required, as indicated by the experience of former sensations ; just as the contractions of the muscles of vocalisa tion are regulated by the conception of the sound to be produced. Hence if the weight be unknown to us, and it prove either much heavier or much lighter than was expected, we find that we have put forth too little or too great a muscular effort.
It is through the " muscular sense," in combination with the visual and tactile, that those movements are regulated, which are concerned alike in ordinary progression, and in the maintenance of the equilibrium of the body. That the visual sense has, in most persons, a large share in this regulation, is evident from the simple fact that no one who has not been accustomed (to the deprivation of it can continue to walk straightforwards, when blind-folded, or in absolute darkness, towards any point in the direction of which he may have been at first guided. But the blind man, who has been accustomed to rely exclusively upon his muscular sense, has no difficulty in keeping to a straight path ; and moves onwards with a confidence which is in remarkable contrast with the gait of a man who has been deprived of sight for the oc casion only. In fact, as Mr. Mayo has well
remarked *, in our ordinary movements, " we lean upon our eyesight as upon crutches." And when our vision, instead of aiding and guiding us, brings to the mind sensations of an antagonistic character, our movements become uncertain, from the loss of that power of guidance and control over them which the harmony of the two sensations usually gives. Thus a person unaccustomed to look down heights feels insecure at the top of a tower or a precipice, although he knows that his body is properly supported ; for the void which he sees below him contradicts (so to speak) the tactile sensations by which he is made conscious of the due equilibrium of his body. So, again, any one can walk along a narrow plank which forms part of the floor of a room, or which is elevated but a little above it, without the least difficulty, and even without any consciousness of effort. But let that plank extend across a chasm, the bottom of which is so far removed from the eye that the visual sense gives no assistance ; and even those who have braced their nerves against all emotional distraction feel that an effort is requisite to maintain the equilibrium during the passage over it ; — that effort being aided by the withdrawal of the eyes from the abyss below, and the fixation of them on a point beyond, which at the same time helps to give steadiness to the move ments, and distracts the mind from the sense of its danger. The degree in which the muscular sense is alone sufficient for the guidance of such movements, when the mind has no consciousness of the danger, and when the visual sense neither affords aid nor contributes to distract the attention, is re markably illustrated by the phenomena of Somnambulism; for the sleep-walker traverses, without the least hesitation, the narrow para pet of a house, and crosses narrow and insecure planks, chambers, roofs, &c., under circum stances that clearly indicate the nature of the guidance by which they are directed (see SLEEP, p. 694 The dependence of our ordinary power of maintaining our equili brium upon the combination of the guiding sensations derived through the sight and the touch, is further well illustrated, as Mr. Mayo has pointed out *, by what happens to a landsman on first going to sea. " It is long before the passenger acquires his sea legs.' At first, as the ship moves, he can hardly keep his feet ; the shifting lines of the vessel and surface of the water unsettle his visual stability ; the different inclinations of the planks he stands on, his muscular sense. In a short time, he learns to disregard the shift ing images and changing motions, or acquires facility in adapting himself (like one on horseback) to the different alterations in the line of direction in his frame." Before this power, however, has been gained, the pas senger has usually to experience most dis tressingly that peculiar feeling of want of support, which is consequent upon the pitch ing and rolling of the ship, but more par ticularly upon the former. As the part of the vessel on which he is standing, sitting, or lying, rises beneath him, there is a comfortable sense of support ; but as it sinks, the want of support is most disagreeably felt ; and the continual repetition of this sensation gives rise to nausea and vomiting. The tendency is increased by the sight of continually shifting lines and surfaces, which of itself, with many individuals, disposes to the same state; and hence it is that the sickness may often be kept at bay by simply closing the eyes, so as to exclude these objects ; whilst, on the other hand, the effort to stand or walk only serves to augment the distress, by increasing the sense of instability. — The giddiness and nausea produced by rapidly turning round, are the results of the same sensations. They are usually excited more through the visual than through the tactile sense ; but that the latter is of itself quite sufficient to produce them, is obvious from the fact that they are experienced when the eyes are closed, as well as by blind persons. The feeling of disturbed equilibrium is more persistent than most other sensations ; thus when a person has turned round quickly several times in suc cession, and then suddenly stops, he feels a whirling sensation, which excites a disposition to continued motion in his limbs, and the surrounding objects appear to move before his eyes. But if, as Mr. Wheatstone has pointed out, the person who is turning round holds a large sheet of paper before and near his face, so as to exclude all sight of the room, and fixes his eyes upon a point— a letter, for instance, in the middle of the paper,— when he stops, he finds his head perfectly steady, and the surrounding objects have no apparent motion ; but his legs feel unsteady, as if they continued actually turn ing round. And it is thus clearly proved that the cause of the giddiness lies in the affection of the senses, and not (as is usually imagined) in disturbed cerebral action.