CUTANEOUS SENSIBILITY 



45 



The lines pp represent the cutaneous surface, CC the central 

 surface, in which are the central stations of the nerve-fibres nn. 

 When point 1 on the skin is touched, the excitation is conducted 

 to the corresponding point 1 in the cortex. The intensity of 

 the excitation is expressed by the ordinate ab, and the curve led 

 represents the spread of the excitation from the point of arrival 

 (which corresponds with the apex) to surrounding points. The 

 same phenomena occur when point 2 is touched separately ; the 

 curve fgh represents the spread of the excitation when it reaches 

 the centre. Since points 1 and 2 lie in the same cutaneous tactile 

 circle, there is a single sensation of contact when they are excited 

 simultaneously. On Bernstein's theory this is because the two 

 excitations represented by the two curves led, fgh summate 



FIG. 23. Diagram to show how excitatipns from two points on the skin summate at the centre 

 into a single sensation. Explanation in text. 



geometrically, and form a single resulting curve ikl, the apex i of 

 which is at a point equidistant from the two component apices If. 

 When the distance between the two cutaneous points touched is 

 increased, the two curves intersect at a point w, which becomes 

 increasingly lower till they no longer cross ; the two stimuli are 

 then perceived separately. 



While undoubtedly ingenious, this theory has weak points. 

 Why do exercise and attention restrict the tactile circles ? On 

 what does the enormous difference between the area of the tactile 

 circles in the more sensitive and the less sensitive regions depend ? 

 And even admitting that the theory explains why the two points 

 of the aesthesiometer produce a single sensation within the limits 

 of any circle, while beyond this circle two separate sensations are 

 perceived, on what does our power of localising these sensations 

 at the periphery depend ? 



