THE LIMINAL STIMULUS FOR TOUCH, 927 



More critical than the larger areal methods of touch testing are 

 observations by a method introduced by Hensen (1893). 1 By using 

 the bending of a glass-wool fibre, one end of which rested on the 

 skin (face), Herisen found a pressure of less than -001 grm. easily per- 

 ceptible. A somewhat artificial but otherwise convenient way of 

 expressing the intensity of stimulus thus used, is to give the weight per 

 sq. mm., which will exert, on an area equal to the cross-section of the 

 fibre or bristle used, the pressure actually exerted by the fibre or bristle 

 employed in the observation. The following results obtained by v. 

 Frey, using Hensen's method, are expressed in that way. The contact 

 surfaces ranged between 5^- sq. mm. and T V sq. mm. 



Tongue and nose ..... 2 grms. per sq. mm. 



Lips ... . . . 2-5 



Finger-tip and forehead . . 3 ,, 



Back of finger ..... 5 ,, 



Palm, arm, thigh . . . . 7 



Forearm . . . . . 8 ,, 



Back of hand 12 



Calf, shoulder . . . . . 16 



Back of upper arnri 



Abdomen . . . 26 



Outside of thigh J 



Shin and sole ..... 28 



Back of forearm ..... 33 



Loins 48 



Applying weights to 1-10 sq. mm. area of skin of the frog's foot, 

 Steinach 2 found 2 grm. the liminal weight. 



To excite a naked nerve, the minimal mechanical stimulus is in 

 experiment '2 grm. moving at 140 mm. a second (Tigerstedt). For the 

 touch spot, v. Frey finds *2 grm. at '17 mm. a second minimal. The 

 limen is lowest with a contact area of about *5 mm. 3 



It is probable that tactile stimuli of subliminal intensity can by* 

 repetition become summated to liminal intensity. If a point of skin be 

 faradised with the secondary coil of the inductorium too distant from 

 the primary for any single shock to evoke sensation, a sensation can still 

 be evoked from the skin. 4 That is, electric shocks individually too feeble 

 as stimuli, repeated in series eventually evoke sensation. The stimuli 

 are in this experiment not, however, of the specific quality for touch 

 organs ; but the phenomenon of " latent summation " is clearly shown. 5 



An important determinant of the intensity of the limina stimulus, 

 is the speed of its application. When quickly applied, the pressure 

 required is less. 6 The energy of the stimulus is the criterion. 



When touch spots and pain spots are tested with mechanical stimuli, 

 e.g. pressure with a bristle, the liminal stimulus for touch lies lower 



than for pain ; the quotient - varies from I in the forearm to 



pam-limen 



i^-g- in the finger-tips. 7 Examined by faradic currents, the quotient of 



1 " Vortrag gegen den sechsten Sinn," Kiel, 1893. 2 Op. cit. 



3 v. Frey and Kiesow, Ztschr. f. Psychol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 

 1899, Bd. xx. S. 147. 



4 v. Frey, Ber. d. k. sacks. Gesellsch. d. Wissensch. z. Leipzig, Math. -Phys. Classe, loc. cit. 



5 Richet, " Recherches sur le sensibility " These inaugurale, Paris, 1877. 

 Stratton, Phil. Stud., Leipzig, Bd. xii. S. 556; Kiesow, Arch. ital. de biol., Turin, 



1897, tomo xxvi. p. 440 ; Blix, Ztschr. f. Biol., 1885, Bd. xxi. 

 7 v. Frey, loc. cit. 



