THE DELICACY OF TOUCH. 363 



different parts of the skin. The points of these papillae (see Fig. 67, 

 b) are situated nearer to the surface of the skin than the general sur- 

 face of the cutis from which they project ; and the cuticle upon them 

 receives the pressure of external objects, and transmits it to the 

 papillae and their nerves. The tactile corpuscles and the Pacinian 

 bodies are not essential to the exercise of touch; but they exist only 

 in those situations in which this sense is most delicate or acute. The 

 tactile corpuscles, far more numerous, and situated so much more 

 superficially than the Pacinian bodies, the former lying in the papillae 

 of the cutis, and the latter being entirely subcutaneous, may act 

 mechanically by supporting the expansions of the ends of the tactile 

 nerves, so as to prevent their yielding to objects of touch. Thus, 

 although not essential to sensation, they may serve to intensify the 

 tactile sensibility of a part. The use of the Pacinian corpuscles is 

 quite unknown. Their analogy to the electrical organs of certain 

 fishes has not escaped attention. 



The delicacy of touch has been estimated numerically, by measur- 

 ing the power possessed by different parts of the surface, of distin- 

 guishing the double impression produced by the simultaneous applica- 

 tion of the two points of a pair of compasses. (Weber.) It presents 

 marked differences in different situations ; it is greatest at the tip of 

 the tongue and the end of the third finger. It was found, by Weber, 

 that if the eyes be closed, and the points of a pair of compasses, pro- 

 tected by cork, be applied to either of the parts just named, the 

 double impression is distinctly perceived when the points are approxi- 

 mated to within half a line of each other ; for the perception of the 

 double impression on the palmar surface of the last phalanges of the 

 thumb and fingers, the points of the compasses must be separated one 

 line; on the red surface of the lip, two lines; on the middle of the 

 dorsum of the tongue, four lines : on the lower part of the forehead, 

 ten lines ; on the sternum, twenty lines ; lastly, on the middle of the 

 forearm, on the middle of the thigh, and over the middle of the cer- 

 vical and dorsal vertebrae, the two impressions are not perceived, un- 

 less the points of the compasses are at a distance of thirty lines from 

 each other; these last are the portions of the cutaneous surface in 

 which the sense of touch is least perfect. The sensibility of the trunk 

 is said to be greater in the middle line in front and behind, than at 

 the sides. 



It will thus be seen that the delicacy of touch in the most sensitive 

 parts, is about sixty times greater than it is in the least sensitive 

 parts ; it presents, however, considerable differences in different indi- 

 viduals. It has been shown by Valentin, that some persons can dis- 

 tinguish the double impression at one-third of the distance at which it 

 can be felt by others. The smallest distance at which the two points 

 - of the compasses can be distinguished, is called the limit of confusion. 

 (Graves.) The two impressions of the points of the compasses are 

 more plainly perceived when these are placed in a direction trans- 

 versely to the trunk or limbs, than when they are applied in a longi- 

 tudinal, direction ; but it is said that at the point of the tongue and 

 the tips of the fingers, the two impressions are more easily felt when 



