CUTANEOUS SENSATIONS 549 



Distance 

 Skin region of touch spots 



Volar side of finger tips . . . . 0-1 



Palm of hand 0-1 



Fore-arm (flexor side) . . . . . - 0-5 



Upper arm ....... 0-6 



Back 0-4 



The compass points are perceived to lie apart with a special dis- 

 tinctness when they are applied to touch spots lying on different lines 

 which radiate from the hair follicles. The figures given in the first 

 table have no relation to touch spots, but show the average distance 

 over which an excitation can be perceived as double. 



The delicacy of discrimination of any part is largely associated 

 with its mobility. Thus in the arm the delicacy increases continuously 

 from the shoulder to the finger-tip. If the localising power for 

 touch on the shoulder be taken as 100, that of the finger-tips will be 

 represented by 2582. In the same way there is a continuous decrease 

 of the distances of discrimination as we pass along the cheek from the 

 ear to the lip, i.e. from the non-mobile to the mobile part. The power 

 of discrimination is increased to a certain extent by practice and 

 largely diminished by fatigue. Any factor which diminishes the tactile 

 sensibility of the part, such as cold, will also diminish the power of 

 discrimination. 



The fact that we can localise the point of stimulation shows that 

 every tactile sensation derived from the surface of the body, besides 

 the qualities of intensity and extensity, has also associated with it a 

 characteristic quality dependent on its position. This localised 

 quality of a tactile sensation was called by Lotze ' local sign.' 

 Among psychologists there has been much discussion as to how far 

 this * local sign ' is an inborn attribute of the sensation of every 

 point on the body surface, or how far it is acquired by experience 

 and based on memory of movements and muscular impressions. 

 In the retina we have a sense-organ which, like the skin, possesses 

 local sign, but in far higher degree, the power of discrimination 

 of the retina being three thousand times as great as that of the most 

 sensitive part of the skin. Cases of congenital cataract occur in which 

 the subjects have been blind from birth. By extraction of the cataract 

 we can give such persons the power of sight. It is found that at 

 first there is no power of localising visual impressions. The ' local 

 sign ' is only developed in response to experience, by comparing simul- 

 taneous visual, tactile, and motor sensations. By analogy we might 

 ascribe the local sign of cutaneous sensations to a similar causation. 

 Our study of the spinal animal has indeed given us a physical or 

 histological conception of local sign. We know that stimulation of 

 any part of the body evokes an appropriate reaction, the nature of 



