940 CUTANEOUS SENSATIONS. 



" sensory circle." It is in most regions oval, since discrimination is most 

 delicate usually in some direction at right angles to the direction of 

 least delicacy, e.g. across the limb. It might be supposed that in each 

 "sensory circle" the two stimuli generate an indivisible sensation, 

 because they both excite the terminal field of a single sensory nerve 

 fibre, — that in fact each " sensory circle " is the area of ending of a single 

 nerve fibre. But a sensory circle where large, e.g. in the back, with a 

 diameter of more than six 6 cms., contains more than 200 touch spots ; 

 and histological examination shows that no cutaneous nerve fibre breaks 

 up into anything like that number of end-fibres. Besides, if the sensory 

 circles be represented by brackets {a + b + c) (d+e+f) (g + h + i) con- 

 taining cutaneous points a, b, and c, and if a + b + c represent the ending 

 of one sensory nerve fibre, and d + e +f the ending of another, and if the 

 explanation of the sensory circle be that two simultaneous touches can- 

 not be felt as separate, if affecting only one and the same nerve fibre, 

 the " average liminal distance " (that is, the size of the " sensory circle ") 

 will vary greatly in one and the same skin region, according as the 

 simultaneous stimuli fall on points c and d or / and g, or on a and c, or 

 d and /. The sensory circles must be larger than the anatomical units 

 of the sensory surface. Moreover, a few hours' use and practice can 

 reduce the size of the sensory circles in the finger to one quarter their 

 previous area. No morphological alteration could be effected in that 

 time. Weber urged that for two simultaneous touches to be distinguish- 

 able, a certain number of unstimulated nerve-endings must intervene 

 between the stimulated ones. The now recognised existence of " touch 

 spots," each coinciding with a tiny cluster of or a single tactile end- 

 organ, bears out Weber's view. When the compass points are felt as 

 double, a striking part of the sensation is the feeling of an interval 

 between the two points. The smaller " average liminal distance " for the 

 fingers than for the back, is not explicable entirely by greater number of 

 " touch spots " per unit surface in the former than in the latter. A sen- 

 sory circle in the back may contain many more than 200 touch spots, 

 while a digital sensory circle may contain many less than 200. " Local 

 sign " seems possessed in higher degree, therefore, by sensations generated 

 through digital " touch spots " than through " touch spots " in the back. 



The division of the skin into a mosaic of tiny sensifacient areas, each 

 of diameter less than the radius of a " sensory circle," explains perception 

 of contact along an unbroken line, even when the object in contact with 

 the skin has spatial gaps, e.g. edge of comb or saw, so long as the gaps 

 are less wide than are the " sensory circles." 



That two stimuli, identical in quality and quantity, evoke when 

 applied to two different sensory circles tactual sensations correspond- 

 ingly different, is certainly referable to occurrences situated in the higher 

 nervous " centres." A cutaneous nerve trunk stimulated in its course 

 evokes tactual sensations referred to the skin area of the peripheral distri- 

 bution of the nerve. The sensation evoked, although one towards which 

 the end-organs have not collaborated, possesses in considerable degree its 

 " local sign." It is noteworthy that at the distal ends of the limbs, 

 where local sign is better developed than in the proximal parts of the 

 limb, the nerve roots in their distribution to the skin overlap especially 

 widely. 1 This means that the area of embouchment into the central 



1 Tlirck, Denksckr. d. k. Akad. d. Wisseuseh. Wiev, 1855 ; Sherrington, Phil. Trans., 

 London, 1892. 



