492 PHYSIOLOGY 



stimulus may be perceived as double. The following Table represents this 

 distance for various regions of the body : 



DISTANCE IN MM. 



Skin region mm. 



Tip of tongue . . . . . 1-1 



Volar surface of finger tip . . . 2-3 



Dorsum of third phalanx .... 6-8 



Palm of hand 11-3 



Back.of hand 31-6 



Back of neck 54-0 



Middle of back, upper arm, and thigh . . 67-1 



When touch spots are sought out for stimulation with the points of a 

 compass, the distance at which the excitation is perceived as double is 

 much diminished, as is shown by the following Table of distances for the 

 touch spots in millimetres : 



Skin region Distance 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 .... 04 



The compass points are perceived to lie apart with a special distinctness 

 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 discrimina- 

 tion 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 

 factile 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 sensa- 

 tion of every point on the body surface, or how far it is acquired by ex- 

 perience 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 



