Dr Graves on the Sense of Touch, 'ffc 



and in what manner are they so arranged, that, in consequence 

 of the shape of that surface in the convolutions of the internal 

 ear, each line of direction resulting from the vibration communi- 

 cated to any point, may be parallel to the various other lines of 

 direction which result from vibrations, simultaneously communi- 

 cated to all other points of the nervous surface? 



These are extremely difficult questions, but it is l)y no means 

 improbable that they may be hereafter satisfactorily resolved. 

 But, to return to the sense of touch ; in some parts of the surface, 

 an exception seems to occur to the general rule of perpendicu- 

 larity ; thus, when a hair of the head is pulled, we can judge 

 perfectly well of the direction in which it is pulled. The most 

 obvious explanation of this fact, which refers to the discrimina- 

 tion of the line of traction to the bulb of the hair, Weber proves 

 to be erroneous, and he shews that we judge of the direction in 

 which the hair is pulled, by means of the muscles called into ex- 

 ertion to counteract the pull, and keep the head steady during 

 its continuance. If these muscles be not called into play, which 

 is the case when the head is held steadily by the hands of one 

 person, while another, by surrounding the point in which the 

 hair is pulled, with a firm pressure made by the fingers, thus 

 prevents the least motion in the enclosed portion of the skin, 

 then no matter in what direction the hair is pulled, the person 

 cannot judge of it. 



Weber's experiments on the faculty the skin possesses of esti- 

 mating and comparing different pressures made on its surface, 

 ought not to be altogether passed by in this report. One chap- 

 ter he entitles, *' De Suhtilitafe Tactus in cognoscendo corpo- 

 rum pandere.'^'' If both the right and the left hand of the same 

 individual are supported on cushions, and that he keeps his eyes 

 shut, while unequal weights are placed one on each hand, he 

 will, if the difference between the weights is considerable, be 

 able to tell on which hand the heavier lies ; slight differences of 

 weight cannot be thus estimated, but they at once become percep- 

 tible if the hands be raised from the cushions ; the muscles that 

 now support the weight give great assistance in estimating its force. 

 Thus we judge of the weight of any heavy body, partly by 

 the pressure it produces against our surface, but chiefly by the 

 quantity of muscular force it requires us to use in lifting or sus- 



