960 CUTANEOUS SENSATIONS. 



examination of touch, employed the following method for observing the 

 differential limen or the absolute limen of cold sensation. By opening a valve, 

 a steady blast of air at the temperature of the room was allowed to pour from 

 a pressure-bag at a velocity of 14 cms. per second along a lengthy tube of 3*5 

 mm. diameter. This draught, at 120 cms. away from the end of the tube, 

 was distinctly detectable by the moistened skin of the face to be cool. It was 

 sufficient to just bend a glass-wool fibre held at 90 cms. from the opening of the 

 tube. By measuring the elasticity of the fibre by pressure on a chemical 

 balance, the force of the draught was found, at 90 cms. from the opening, to 

 be much less than *01 mgrm. A thermometer wrapped in moist linen at 

 90 cms. began to fall after 1 1 seconds, and soon had fallen '0Q° C. The sensation 

 of cold was quite clearly perceptible after 7 "5 seconds of the draught. The 

 stimulus was probably quite intangible, for it scarcely caused flickering in a 

 candle-flame 110 cms. distant from the mouth of the tube. 



Both by the methods of successive stimuli and of simultaneous 

 stimuli, the differential limen of stimulus value is very different in 

 different regions of the skin. This is clear from Weber's and from 

 Nothnagel's observations. Weber 1 determined the degree to which at 

 various regions of the body surface distinction can be made between 

 glass phials filled with oil and then warmed or cooled in water, but 

 applied dry. He sought in this way to estimate the degree of 

 thermal sensitivity of the skin regions. The method is not the best 

 adopted for that purpose ; but for the questions under consideration 

 his results are of interest. They were as follows : — 



Thermal discrimination is at its maximum in the face, especially in the eye- 

 lids and cheeks. The eyelids are more sensitive at the canthi than between. 

 The lips, less sensitive than eyelids and cheeks, are, like the former, more 

 sensitive toward their angles of junction than in the middle. This is the 

 reverse of the arrangement of tactual spatial sense. The thermal sensitivity of 

 the nose is least at its tip, and greatest along the lower part of the outer edge 

 of the nostril. It is very poor inside the nostril : in the external auditory 

 meatus it is, on the contrary, well developed. The skin just in front of the 

 tragus has greater thermal sensitivity than the lip, — again the exact reverse of 

 the distribution of the space-sense of touch. Along the median line of the 

 body, thermal sensitivity is everywhere poor, alike in face, chest, abdomen, and 

 back. Weber found the proximal part of the proximal phalanx of the palmar 

 face of the index finger more sensitive than the corresponding surface of the 

 other fingers ; the ball of the thumb more sensitive than the hypothenar 

 region. The skin over the olecranon he found more sensitive than that of the 

 rest of the arm ; the skin over the trochanter major more sensitive than that 

 over the crista ilii. 



In considering these results, it must be remembered that Weber 

 does not seem to have paid attention to what Hering 2 terms " the 

 adequate temperature." There is much evidence that the excitability 

 of a sense organ is at any time partly determined by antecedent 

 stimulation. As the visual organ adjusts itself with a different pitch of 

 excitability, becomes differently attuned, under influence of light and of 

 dark respectively, so the thermal sense organs at different temperatures 

 of the environment. When a skin surface under its living conditions 

 of continual accession of heat and loss of heat generates sensation 

 neither of " cold " or " warmth," it is described as " fully adapted " 



1 Loc. tit., S. 554. 



2 Hermann's " Handbuch," 1880, Bd. iii. S. 430. 



