TOUCH. 



1175 





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. 



Sense of Weight. This is usually derived 

 from a double source ; namely, the impression 

 made upon the cutaneous surface by the sim- 

 ple pressure of the body: and the conscious- 

 ness of the muscular effort employed to re- 

 sist that pressure. The latter enables us to 

 compare the weights of different bodies much 

 more accurately than the former, which is li- 

 able to excite fallacious ideas. The extent of 

 surface, for example, which is in contact with 

 the skin, greatly modifies the estimate of the 

 pressure of a heavy body ; the body feeling 

 lightest when its pressure is distributed over a 

 larger surface, and vice versa. Thus, a trun- 

 cated cone seems heavier when it rests (with- 

 out any effort being made to raise or support 

 it) upon its small extremity, than when it rests 

 upon its large extremity on the same part of 

 the surface. At first sight this fact appears 

 altogether antagonistic to the one just stated 

 with regard to the sense of temperature, the 

 impressions on which are more powerful when 

 they are made over a large than over a small 

 surface. But it is to be borne in mind, that 

 in the latter case an absolutely larger amount 

 of calorific influence is exerted, when the large 

 surface is exposed to the action of heat ; 

 whilst in the former, the amount of pressure 

 is really the same, whether it be distributed 

 over a larger or a smaller area. The experi- 

 ments of Weber on the relative information 

 derived from the mere sense of pressure, and 

 from the sense of muscular effort, in the ap- 

 preciation of weights, are very instructive. 

 He found that if the two hands of the same 

 individual be placed upon cushions, and un- 

 equal weights be placed upon the right and 

 left hands respectively, the eyes being kept 

 shut, it will not be possible to say on which 

 hand the heavier weight lies, unless the dif- 

 ference be very considerable ; but a compara- 

 tively small amount of difference is at once dis- 

 criminated, when a muscular effort is made to 

 lift the hands from the cushions. This power 

 of comparison is capable of being rendered 

 more exact by practice ; so that men accus- 

 tomed to estimate weights by poising them in 

 their hands, will readily distinguish between 

 two which differ only by one-thirtieth part. 

 It is found that the power of comparison is 

 much greater (as in the estimation of tempe- 

 rature) when the impressions are successively, 

 than when they are simultaneously made, pro- 

 vided that the interval be not too long. Thus 

 in the comparison of two weights, the greatest 



nicety is attained by poising the one, and 

 immediately afterwards the other, in the same 

 hand ; but the intervention of a few seconds 

 between the poising of the first and that of the 

 second does not prevent their accurate com- 

 parison. The interval may amount to twenty 

 seconds, and yet a just estimate may still be 

 made ; but when it amounts to forty seconds 

 all accuracy is lost. Professor Weber has 

 further ascertained, that, in the estimation of 

 weights by their simple pressure on the sur- 

 face, the left side and extremities have usually 

 a more acute perception than the right ; for 

 out of fourteen individuals, he found this to 

 be the case in eleven; in two, the contrary 

 was observed ; whilst in one, no difference 

 was perceptible. 



Sense of Direction. The combination of 

 the muscular with the simply tactile sense 

 enables us also to judge in some degree of the 

 direction of the pressure. Of this we gain no 

 information whatever from the tactile sense 

 alone, which always suggests the idea that the 

 pressure is made vertically to the surface, 

 when it is not corrected by the sense of mus- 

 cular effort called forth to antagonise it. The 

 following example, given by Weber, shows 

 how completely involuntary may be this effort, 

 yet how large a share it has in communicating 

 to us the information we derive from an im- 

 pression, of whose direction we are rendered 

 cognisant. When a hair of the head is pulled, 

 he remarks, we can judge perfectly well of 

 the direction of the traction ; this power of 

 discrimination is not, however, derived (as 

 might at first be supposed) through the sen- 

 sation originating in the bulb of the hair, but 

 from the sense of the muscular effort which is 

 called forth to antagonise the traction, and to 

 keep the head steady during its continuance. 

 If we prevent these muscles from being called 

 into play, by steadily holding between the 

 hands the head of the person operated on, 

 and if we also prevent the traction from call- 

 ing forth the muscular action of the scalp, by 

 surrounding the point from which the hair is 

 pulled with a firm pressure by the fingers, 

 we find that the discriminating power is 

 completely lost ; the subject of the experi- 

 ment being totally unable to distinguish the 

 direction from which the hair is pulled. 



It is by this combination, too, that we judge 

 of the rate and direction of the passive motion 

 of our bodies, when we have no other means 

 of guidance. If, for example, a person be 

 seated in a carriage, with his eyes closed, and 

 the carriage be suddenly put in motion, the 

 inertia of his body causes it to be thrown in 

 the contrary direction ; and, in order to re- 

 cover and sustain its equilibrium, a muscular 

 effort is required, which is greater in propor- 

 tion to the rate of motion. If the motion 

 continue uniform, however, this effort becomes 

 so habitual that he ceases to be conscious of 

 it ; and he only becomes cognisant of the mo- 

 tion by its cessation, the equilibrium of the 

 body being then again disturbed by its inertia, 

 which tends to impel it in the direction in 

 which it was previously moving, so as to re- 

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