CUTANEOUS SENSATIONS 491 



The sensitiveness of the sense-organs in the skin is probably much 

 greater than that of the nerve-trunks themselves. Thus Tigerstedt found 

 that the minimal mechanical stimulus necessary to excite the exposed 

 nerve amounted to O2 grm. moving at 140 mm. per second. For the touch 

 spots von Frey found that 0*2 grm. moving at O17 mm. a second is an 

 adequate stimulus. 



In testing the sensibility of any surface it is important to remember 

 that the hairs themselves form very effective tactile organs. The touch 

 spots are distributed in greatest profusion around hair follicles, and there 

 is a rich plexus of nerve fibres round the root of each hair. A slight touch 

 applied to the hair acts on these as on the long end of a lever, the hair being 

 pivoted at the surface of the skin, so that pressure on the hair is transmitted, 

 increased five or more times in force, to the hair follicle and the surrounding 

 nerve-endings. The actual sensibility of any part is therefore much dimin- 

 ished by removal of the hairs. On 9 sq. mm. of the skin, from which the 

 hairs had been shaved, the minimal stimulus necessary to evoke a tactile 

 sensation was found to be 36 mg., whereas on the same surface before it was 

 shaved 2 mg. was effective. 



WEBER'S LAW. The smallest increment or decrement of stimulus 

 which determines a perceptible difference of sensation must, according 

 to Weber's law, always bear the same ratio to the whole stimulus. In 

 measuring such differences it is best to apply the stimulus successively to 

 the same surface of the skin rather than simultaneously to adjoining areas. 

 The time interval between two successive stimuli should not be more than 

 five seconds and the duration of the stimuli should be equal. Weber found 

 that in the terminal phalanx of the finger the minimal perceptible difference 

 was about one-thirtieth, but the ratio was not the same for all regions of 

 the skin nor for all individuals. The following represents the liminal 

 difference in various skin regions : 



Forehead, lips, and cheeks . . . l/30th to l/40th 

 Back of fore-arm, of leg, and of thigh ; A 



back of hand, and first and second I l/10th to l/20th 



phalanx of finger, &c. . . J 



All parts of the foot, surface of leg, and 



thigh . . . . more than 1 /10th. 



THE SPATIAL QUALITY OF TOUCH. DISCRIMINATION. If any 

 part of the skin be stimulated the subject of the experiment can tell at once 

 the exact situation of the excited spot. If two points be stimulated simul- 

 taneously excitation is perceived as double, i.e. as proceeding from two 

 points, provided the distance between the points exceeds a certain amount, 

 varying in different parts of the body. The power of discrimination, i.e. 

 of judging whether a stimulus- isAgb-^ double, can be- tested by armin 

 the points of a pair of compasses with small pieces of cork and then seeing 

 how far apart the points must be when pressed on the skin in order that the 



