93 6 



CUTANEOUS SENSATIONS. 



" average liminal distance," certain precautions must for accuracy be 

 taken. When the stimuli are successive instead of simultaneous, the 

 distance is much reduced, e.g. from 20 mm. to 5 mm. on dorsum of hand. 1 

 The sesthesiometer points must both be at skin temperature. The 

 average liminal distance varies greatly in different skin regions. When 

 children and adults are compared, it is found to be less in the former. 



Table of Average Liminal Distances in Millimetres.' 2 ' 



In the limbs, the " average liminal distance " for touch is greater 

 parallel with the long axis of the limb than across it. The distance can 

 be reduced by practice. The blind, who partially supply by other senses 

 their deficiency of visual space-perception, recognise small shapes, e.g. 

 letters, coins, with enhanced nicety by practised touch. 3 Type-setters 

 similarly discriminate between letters, correctly and rapidly by touch. 

 Under repeated examination by Weber's method, an area of skin will, in 

 even a few hours, reduce its average liminal distance by a half. 4 The 

 " education " is at first slow, then quicker, and finally slower, dwindling 

 to cessation when a certain decrement of the distance has been effected. 

 The decrement obtained by " education " is completely lost by a few 

 months of want of practice. In those inaccessible and relatively im- 

 mobile regions of the body surface, where the liminal distance is very 



1 When two pressure stimuli are applied simultaneously near together, the areas of 

 deformation of the skin surface are likely to meet and overlap. This will not occur when 

 the same two stimuli are applied to the same two points successively. This, however, is an 

 explanation insufficient to account entirely for the reduction of the average liminal distance 

 obtained by applying the stimuli successively instead of simultaneously. Cf. Judd, Phil. 

 Stud., Leipzig, Bd. xii. S. 409; v. Frey, Sitzungsb. d. phya.-med. Gesellsch. zu Wurzburg, 

 1899, Bd. xii. 



2 The observations on the adult are condensed from Weber's list in the "Handworter- 

 buch," with a few additions from the " Programmata, " 1834, fasc. 1. The observations on the 

 boy are from Landois, "Lehrbuch," 1881, 2te Aufl. S. 929. The distances given are the 

 minimal from which double sensation is obtained, when stigmatic excitation of selected 

 touch spots is not undertaken. 



3 Czermak, loc. cit. ; Stanley Hall on Laura Bridgeman, Mind, London, April, 1879. 

 The question has been recently examined by F. Galton, " Inquiries into Human Faculty," 

 London, 1883. 



4 Volkmann. Ber. d. k. sticks. Gessellsch. d. JFissensch., Leipzig, 1858. 



