1174 



TOUCH. 



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 clanger. 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 



Outlines of Physiology, p. 355. 



* Loc. cit. 



