INNERVATION OF THE HUMAN SKIN 229 



5. Table V. also gives results of the mode of distribution of 

 cold and warmth spots, examined with punctiform stimuli. The 

 " local sign " for cold is higher than that for warmth spots, and 

 two of these are distinguished as double when only 0.8 millimetres 

 apart on the palm, cheek, chin and forehead, whereas on the 

 upper arm, back and thigh, they are only distinguished as double 

 when separated by two millimetres, and this distance is the minimum 

 at which warmth spots are distinguished as two, that is 2 mm. on 

 the palm, and five on cheek, chin, forehead and back. This tells 

 the same story as Table IV., of past stimuli of cold and warmth. 



6. Table VI. deals more elaborately than the others with double 

 sensation in different areas of the skin, the tip of the tongue being 

 the most accurate in this respect of all examined, and the tip of the 

 index finger next, which is to the great toe as 2.3 to 11 .3, the palmar 

 surface of a finger half as accurate again as the dorsal surface, 

 the palm of the hand twice as accurate as the surfaces of the fore- 

 head and back of ankle, nearly four times as much so as the dorsum 

 of the foot and six times as the skin of the middle of the back. 



There is here a very close relation between the amount of 

 exposure of these various regions to tactile stimuli and their present 

 equipment of ability to discriminate between two small objects. 



7. Table VII. deals with the sensation of pressure in certain 

 groups of areas, and shows that change of pressure is perceived 

 about three or four times as accurately on the forehead, lips and 

 tongue, as on the finger nail, back of forearm, hand, or fingers, 

 and more than three or four times on the back of the foot, and sole, 

 and surface of leg and thigh. In this group of observations also 

 the rule is followed that the greater and more frequent in man's 

 ancestral past have been the exposure of his skin to variations of 

 pressure, the greater is his present power of accurate discrimination 

 of them. 



There are some scattered facts mentioned by Professor 

 Sherrington which are in keeping with the line here taken, 

 that the formation of receptors in the skin have their origin in 

 accumulated stimuli. He refers to the vain endeavours of Goltz to 

 evoke the reflex croak of the female frog by applying electrical 

 stimuli to the skin, whereas non-nocuous mechanical stimuli were 

 the only stimuli that proved effective. 



He never was able to elicit the " extensor thrust " in the 

 " spinal dog " by any form of electrical stimulation, but only by a 

 particular kind of mechanical stimulus. This peculiarity was 

 also found in the pinna reflex of the cat. 



