Dr Graves on the Sense of Touch. 71 



nervous pulp does not, of itself, constitute the sense of touch, 

 but that, as in the sense of hearing or of sight, there is an appa« 

 ratus, all the parts of which must be in unison to be perfect. 

 If any one of the five constituent parts be wanting, touch can- 

 not be exercised, and the derma, neurilema, and proper epi- 

 dermic membrane, are to the papilla, what the complicated ap- 

 paratus of sight and hearing are to the optic and acoustic nerves. 

 The analogy goes farther, for the optic and acoustic nerves, on 

 entering the structure of the eye and ear, undergo the same 

 change as the tactile nerve entering the derma, with this diffe- 

 rence, that the two former remain in their cavities, where light can 

 penetrate to the one and sound to the other ; but the nerve of 

 touch must advance, as it were, to meet impressions. The 

 following very curious phenomenon is recorded by Weber :— 

 " If the points of a compass, distant from each other one or two 

 lines, applied to the cheek, just before the ear, be then moved 

 successively to several parts of the cheek, we shall find, on ap- 

 proaching the angle of the mouth, that the points will appear 

 to recede from each other ; this is produced by the great differ- 

 ence of tactile power in these parts. It is a general law, that 

 the more sensitive portions of the skin regard any two points as 

 farther asunder from each other, than equidistant points appear 

 to be to a less sensitive portion. The same experiment may be 

 tried by holding together the extremities of the fore-finger and 

 thumb, and then passing the tips of both in a line from the ear 

 to either the upper or the under tip ; as they approach the lat- 

 ter, they will feel to the check as if they were becoming more 

 and more distant from each other." 



Another fact was observed by Weber — " If the legs of the 

 compass be applied to two contiguous surfaces, enjoying the 

 functions of voluntary motion, they will appear to be much 

 more distant from each other, than when th^y are applied to one 

 of these surfaces separately. Thus, if the points are distant 

 half a line, they are not perceived to be distant when applied 

 to one lip, but when one point is applied to the under lip, and 

 another to the upper, they are at once felt to be two." 



Another very remarkable conclusion announced by Weber 

 deserves consideration : " Apply the legs of a compass to two 

 portions of the skin, differing from each other remarkably, either 



