CUTANEOUS AND INTERNAL SENSATIONS. 265 



eter is that devised by von Frey. The two points in this case are 

 made by long, rather stiff hairs whose pressure can be made quite 

 uniform. According to the older measurements, the localizing 

 sense of different parts of the skin varies greatly, as is shown by the 

 accompanying table: 



Tip of the tongue 1.1 mms. 



Tip of finger, palmar surface 2.3 



Second phalanx finger, palmar surface 4.5 



First phalanx finger, palmar surface 5.5 



Third phalanx finger, dorsal surface 6.8 



Middle of palm 8to9 



Second phalanx finger, dorsal surface 11.3 



Forehead 22.6 



Back of the hand 31.6 



Forearm 40.6 



Sternum 45 



Along the spine 54 



Middle of neck or back 67.7 



The tips of the tongue and the fingers are, therefore, the most 

 delicate surfaces, and that the tongue surpasses the fingers in this 

 respect is easily within the experience of everyone who will recall 

 the ease with which small objects between the teeth are detected by 

 the tongue as compared with the fingers. From the above data it 

 is evident also that the whole skin may be imagined as composed 

 of a mosaic of areas of different sizes, the sensory circles of Weber, 

 in each of which two or more simultaneous stimulations of the pres- 

 sure nerves give only one pressure sensation. The size of these 

 areas, particularly where they are large, may be reduced by practice, 

 as is shown by the increased tactile sensibility of the blind. The 

 fact that we can recognize two simultaneous pressure stimuli of the 

 skin as two distinct sensations implies that the two sensations have 

 some recognizable difference in consciousness. This difference is 

 spoken of as the local sign. We may believe that every sensitive 

 point upon the skin has its own distinctive local sign or quality, and 

 that by experience we have learned to project each local sign more 

 or less accurately to its proper place on the skin surface. Two points 

 on this surface that are a great distance apart are easily recognized 

 as different ; but as we bring the points closer together the difference 

 becomes less marked and finally disappears when the distance 

 corresponds to the area of the sensory circle for the part of the skin 

 investigated, for instance, 1 mm. for the tongue, 22 mms. for the 

 forehead, etc. The ultimate limit of the power of discrimination 

 was assumed by Weber to depend upon the area of distribution of 

 a single nerve fiber. Assuming that each nerve fiber at its termi- 

 nation spreads over a certain skin area, it was suggested that the 

 size of this area forms a limit to the power of discrimination, 

 since two stimuli within it would affect a single fiber and therefore 

 would give a single sensation. 



