TOUCH. TEMPERATURE SENSATIONS. 581 



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

 enough to just cause a distinct tactile 

 sensation, than when it is more power- 

 ful ; it is also very readily and rapidly 

 improvable by practice. 



It might be thought that this local- 

 izing power depended directly on nerve c "' 

 distribution ; that each touch nerve had 

 connection with a special brain-centre 

 at one end (the excitation of which 

 caused a sensation with a chaiWri^ristic 

 local sign), and at the other end was 

 distributed over a certain skin area, and 

 that the larger this area the farther 

 apart might two points be and still give 

 rise to only one sensation. If this were so, however, the 

 peripheral tactile areas (each being determined by the ana- 

 tomical distribution of a nerve-fibre) must have definite un- 

 changeable limits, which experiment shows that they do not 

 possess. Suppose the small areas in Fig. 172 to each repre- 

 sent a peripheral area of nerve distribution. 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 approximated, 

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

 fibres 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 distinguish 

 able sensations. 



It is probable that the nerve areas are much smaller thac 

 the tactile ; and that several unstimulated must intervene be- 

 tween the excited, in order to produce sensations which shall 

 be distinct. If we suppose twelve unexcited nerve areas 

 must intervene, then, in Fig. 172, a and # 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 boun- 

 daries in the skin, although the nerve distribution hi any part 

 must be constant. We also see why the back of a knife laid 

 on the surface causes a continuous linear sensation, although 



