628 SPECIAL SENSES 



instances of this kind are on record. The blind have been known to 

 become proficients in conchology and botany, depending entirely on the 

 touch. It is well known that the blind learn to read with facility by 

 passing the fingers over raised letters but little larger than the letters in 

 an ordinary folio Bible. 



An easy method of determining the relative delicacy of the sense of 

 contact in different parts of the cutaneous surface was devised a num- 

 ber of years ago (1829) by E. H. Weber. This method consists in 

 the application to the skin, of two fine points, separated from each 

 other by a known distance. The individual experimented on should 

 be blindfolded, and the points applied to the skin simultaneously. By 

 carefully adjusting the distance between the points, a limit will be 

 reached where the two impressions upon the surface are appreciated 

 as one ; and by gradually approximating them, the subject will sud- 

 denly feel both points as one, when an instant before, with the points a 

 little farther removed from each other, he distinctly felt two impres- 

 sions. This gives a measure of the delicacy of the sense of contact in 

 different parts. An instrument, consisting of a pair of dividers with 

 a graduated bar giving a measure of the separation of the points, com- 

 bines simplicity, convenience of use and portability. This instrument 

 is called an esthesiometer. The experiments of Weber were made on 

 his own person. They showed some slight variations with the direction 

 of the line of the two points, but these are not important. The follow- 

 ing table is made of selections from the observations of Weber, taking 

 those that are most likely to be useful as a guide in pathological investi- 

 gations. The experiments of Valentin and others on different persons 

 do not vary much in their results from the figures given in the table on 

 opposite page. 



By comparing the distribution of the tactile corpuscles with the 

 results given in the table, it will be seen that the sense of contact is most 

 delicate in those- situations in which the tactile corpuscles are most 

 abundant. In the space of a little more than -^ of an inch (2.2 millime- 

 ters) square, on the palmar surface of the third phalanx of the index 

 finger, Meissner counted the greatest number of corpuscles ; namely, 

 one hundred and eight. In this situation the tactile sensibility is more 

 acute than in any other part of the skin, the mean distance indicated 

 by the esthesiometer being 0.603 of a line, or 1.27 millimeter (Valentin). 

 In the same space on the second phalanx, forty corpuscles were counted, 

 the esthesiometer marking 1.558 line, or 3.27 millimeters (Valentin), 

 this part ranking next in tactile sensibility after the red surface of the 

 lips. One can readily understand how the tactile corpuscles, embedded 

 in the amorphous substance of the cutaneous papillae, might increase 



