CUTANEOUS SENSATIONS 547 



the skin, we know exactly the force which we are applying to the 

 skin. Von Frey employed hairs of different thickness for the same 

 purpose (Fig. 241). The following represents the minimal excita- 

 bility of the surface of different parts of the body when tested in 

 this wav. 



(/ 



(inn. per sq. mm. 



Tongue and nose . . 2 



Lips ... 2-5 



Finger-tip and forehead .... 3 



Back of finger ... 5 



Palm, arm, thigh ... . 7 



Fore-arm ... . . . 8 ' 



Back of hand . 12 



Calf, shoulder . ... 16 



Abdomen . . . . . . .26 



Outside of thigh . . 26 



Shin and sole ..... 28 



Back of fore-arm ...... 33 



Loins 48 



FIG. 241. Hair mounted on a wooden handle, and used 

 by von Frey for testing tactile sensibility. 



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 0-2 grm. moving at 140 mm. per second. 

 For the touch spots von Frey found that 0-2 grm. moving at 0-17 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 -end ings. The 

 actual sensibility of any part is therefore much diminished 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 nag., whereas on the same surface before it was 

 shaved 2 mg. was effective. 



