724 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



nary 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 depriva- 

 tion of it can continue to walk straight-forwards, when blindfolded, or in abso- 

 lute 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 exclu- 

 sively 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 occasion only. In fact, 

 as Mr. Mayo has well remarked, 1 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 feds, 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 sensa- 

 tions 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 equili- 

 brium during their 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 movements, 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 remarkably illustrated by the phe- 

 nomena of Somnambulism; for the sleep-walker traverses, without the least hesi- 

 tation, the narrow parapet of a house, and crosses narrow and insecure planks, 

 clambers roofs, &c., under circumstances that clearly indicate the nature of the 

 guidance by which he is directed (see Sect. 7). The dependence of our ordi- 

 nary power of maintaining our equilibrium upon the combination of the 

 guiding sensations derived through the sight and the touch, is further well 

 illustrated, as Mr. Mayo has pointed out (loc. cit.), by what happens to a lands- 

 man on first going to sea. " It is long before the passenger acquires his l 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 shifting 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." And when a person who has 

 thus learned by habit to maintain his equilibrium on a shifting surface, first 

 treads upon firm ground, he feels himself almost as much at fault as he did 

 when he first went to sea; and it is only after being some time on shore, that 

 he is able to resume his original manner of walking. Indeed, most of those 

 who spend the greater part of their time at sea acquire a peculiar gait, which 

 becomes so habitual to them that they are never able to throw it off. 



753. But, further, there is very strong physiological evidence that the Sen- 

 sory Ganglia are not merely the instruments whereby our voluntary movements 



1 "Outlines of Physiology," p. 355. 



