LOCALIZATION OF TACTILE SENSATIONS. 561 



must have definite unchangeable limits, which experiment 

 .shows that they do not possess. Suppose the small areas in 

 Fig. 153 to each represent a peripheral area of nerve distribu- 

 tion. If any two points in c were touched we would accord- 

 ing to the theory get but a single sensation; but if, while 

 the compass points remained the same distance apart, or were 

 even approximated, one were placed in c and the other on a 

 contiguous area, two fibres would be stimulated and we ought 

 to get two sensations; 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. 



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 distinct. If we suppose twelve unexcited nerve 

 areas must intervene, then, in Fig, 153, 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 distri- 

 bution 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; if we could discriminate the excitations of each of 

 these from that of its immediate neighbors we would get 

 the sensation of a series of points touching us, one for each 

 nerve region excited; but in the absence of intervening 

 unexcited nerve areas the sensations are fused together. 



The ultimate differentiation of tactile areas takes place in 

 the central organs, as will be more fully pointed out in the 

 next chapter. Afferent nerve impulses reaching the spinal 

 cord from a finger-tip enter the gray matter and tend 

 to radiate some way in it; from the gray region through 

 which they spread, impulses are sent on to perceptive 

 tactile centres in the brain; if two skin-points are so close 

 that their regions of irradiation in the cord overlap, then 

 the two points touched cannot be discriminated in con- 



