216 THE HUMAN BODY 



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 

 ^ a t the larger this area the farther apart 

 might 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 

 xbe shifted, in order to give rise to two just distinguishable sen- 

 sations. 



^ It is probable that the nerve areas are much smaller than the 

 tactile; and that several unstimulated must intervene between 

 the excited, in order to produce sensations which shall be di 

 tinct. If we suppose twelve unexcited nerve areas must inter- 

 vene, then, in Fig. 68, a and b will be just on the limits of a single 

 tactile area; and no matter how the points are moved, so long as 

 eleven, or fewer, unexcited areas come between, we would get a 

 single tactile sensation; in this way we can explain the fact that 

 tactile areas have no fixed boundaries in the skin, although the 

 nerve distribution in any part must be constant. We also see 

 why the back of a knife laid on the surface causes a continuous 

 linear sensation, although it touches many distinct nerve areas; 



