656 



SPECIAL SENSES. 



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 upon 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 im- 

 pressions upon the surface are appreciated as one ; i. e., by gradually approx- 

 imating them, the subject will suddenly feel both points as one, when an 

 instant before, with the points a little farther removed from each other, 

 he distinctly felt two impressions. This gives a measure of the delicacy of 

 the tactile sensibility of different parts. Of course the instrument used 

 may be very simple a pair of ordinary dividers will answer but it is con- 

 venient to have some ready means of ascertaining the distances between the 

 points. An instrument, consisting simply of a pair of dividers with a grad- 

 uated bar giving a measure of the separation of the points, is the best, as it 

 combines simplicity, convenience of use and portability. This instrument is 

 called an aethesiometer. The experiments of Weber were made upon his own 

 person. They showed some slight variations with the direction of the line 

 of the two points, but these are not very important. The table which follows 

 is made of selections from the observations of Weber, taking those that are 

 most likely to be useful as a guide in pathological investigations. The ex- 

 periments of Valentin and others on different persons do not vary much in 

 their results from the figures given in the table. 



TABLE OF VARIATIONS IN THE TACTILE SENSIBILITY OF DIFFERENT POR- 

 TIONS OF THE SKIN (\VEBER). 



The tactile sensibility is measured by the greatest distance between two points at which they convey a 

 single impression when applied simultaneously. The measurements are given in lines (^j of an 

 inch, or a little more than 2 mm.). 



