180 THE HUMAN BODY 



Tongue-tip 1.1 mm. (0.04 inch) 



Palm side of last phalanx of finger 2.2 mm. (0.08 inch) 



Red part of lips 4.4 mm. (0.16 inch) 



Tip of nose 6.6 mm. (0.24 inch) 



Back of second phalanx of finger 11.0 mm. (0.44 inch) 



Heel 22.0 mm. (0.88 inch) 



Back of hand 30.8 mm. (1.23 inches) 



Forearm 39.6 mm. (1.58 inches) 



Sternum 44.0 mm. (1.76 inches) 



Back of neck 52.8 mm. (2.11 inches) 



Middle of back 66.0 mm. (2.64 inches) 



The localizing power is a little more acute across the long axis 

 of a limb, and is better when the pressure is only strong enough 

 just to cause a distinct tactile sensation, than when it is more 

 powerful; it is also very readily and rapidly improvable by practice. 

 It might be thought that this localizing power depended di- 

 rectly on nerve distribution; that each touch nerve had connec- 

 tion with a special brain-center at one end (the excitation of 

 which caused a sensation with a characteristic local sign), and 

 at the other end was distributed over a certain skin area, and 

 that the larger this area the farther apart 

 ^^QlUIIfev m ight two points be and still give rise to 

 only one sensation. If this were so, how- 

 ever, the peripheral tactile areas (each be- 

 ing determined by the anatomical distribu- 

 tion of a nerve-fiber) must have definite 

 unchangeable limits, which experiment 

 shows that they do not possess. Suppose 

 each of the small areas in Fig. 68 to repre- 

 sent a peripheral area of nerve distribu- 

 tion. If any two points in c were touched 

 we would according to the theory get but 

 a single sensation; but if, while the compass 



points remained the same distance apart, or were even approxi- 

 mated, one were placed in c and the other on a contiguous area, 

 two fibers would be stimulated and we ought to get two sensa- 

 tions; but such is not the case; on the same skin region the 

 points must be always the same distance apart, no matter how 

 they be shifted, in order to give rise to two just distinguishable 

 sensations. 



