TOUCH, TEMPERATURE, MUSCULAR SENSE. 483 



one millimeter may still be perceived as distinct points; 

 at the tip of the finger we perceive as two objects, points 

 as close as two millimeters ; on the wrist and arm the power 

 to localize becomes much less and a distance of four or five 

 millimeters apart hardly produces a double sensation. The 

 lips require 4 mm. ; the tip of the nose 6 mm. ; the eyeball 

 10 mm. ; the forehead 20 mm. ; the back of the hand 28 

 mm., and the middle of the back and neck require as much 

 as 60 mm. between the two points to be perceived as double. 

 That the absolute sensibility and the power to localize 

 are different is evident in the case of the forehead, where 

 absolute sensibility is possibly greater than in any other 

 portion of the skin, but where the power of localization is 

 only possible when the points applied are as much as 20 

 mm. apart. This relative power of localization has been 

 generally described in terms of touch circles. The diam- 

 eters of such circles being the least distance between two 

 points to be perceived as double. The touch areas, there- 

 fore, on the point of the tongue are exceedingly small, only 

 1 mm. in diameter, while on the other extreme, the touch 

 circles on the middle of the back are as much as 60 mm. in 

 diameter. Two points, then, placed within such a circle 

 are perceived as one, or in other words, to perceive two 

 points placed on the skin as two, requires that the two 

 points shall fall in different touch circles. It is very neces- 

 sary, however, to remember that these touch areas are not 

 definite anatomical structures, and that, for instance, it is 

 quite impossible with a pencil to divide the skin of the back 

 into such circles so that in every case two points coming 

 within a circle would produce a single sensation, or placed 

 in adjacent ones, a double sensation. For, if one point 

 should be placed close to the edge of one such area and the 

 other point right on the boundary line of the adjacent area 

 so that the actual distance between the two points should 

 be less than 60 mm., they are perceived as one. It is, 

 therefore, not at all possible that the existence of these cir- 

 cles are brought about by the fact that each circle has 



