Dr Graves on the Sense of Touch, TT 



Weber has discovered a very remarkable fact, that the left 

 hand is more sensible of heat or cold than the right in most 

 persons. Thus, when the hands of a person lying in bed, and 

 of exactly the same temperature, were plunged each in a sepa- 

 rate vessel of hot water, the left hand was believed by the per- 

 son to bo in the hotter medium, even though the water it was in 

 was really one or two degrees colder than the other. Weber 

 has rendered it highly probable, that the greater sensibility 

 which the left hand undoubtedly possesses in perceiving changes 

 of temperature, is owing to the circumstance of its being cover- 

 ed, particularly on its palm, by a thinner epidermis, in conse- 

 quence of being less used. Nothing is more striking than the 

 accuracy of the skin in giving notice of changes of temperature, 

 for a difference of one-third of a degree is detected clearly 

 when the hand is immersed repeatedly and successively in two 

 vessels of water, differing only so much in temperature. The 

 skin detects best very minute changes of temperature when 

 the medium examined does not fall short of, or exceed very con- 

 siderably, the usual temperature of the body. Water at 98^ 

 can be much more certainly distinguished by the hand from 

 water at 100°, than can water at 120° from water at 118.° As 

 the ears perceive best a difference of tone in sounds, neither too 

 acute or too bass, or immoderately loud, so the skin judges with 

 most accuracy of medium temperatures, which produce no very 

 violent or painful effect on its nerves. Weber is of opinion that 

 the perception of temperature imparted to each nervous extre- 

 mity in the skin, goes to unite itself to, and strengthen simulta- 

 neous impressions in the other ramifications of the same nerve, 

 thus producing, by the conflux of a great number of impres- 

 sions, a much stronger result and effect. This, at least, is cer- 

 tain, that a large conveys much stronger impressions than a small 

 surface, and estimates changes of temperature with greater deli- 

 cacy. Thus, if we place the fore-finger of one hand in water 

 at 104°, and plunge the whole of the other hand into water at 

 102°, the latter will appear to us to be the warmer. If we plunge 

 the finger successively into vessels containing hot water, we are 

 unable to perceive very minute differences of temperature, which 

 at once become perceptible when we use the whole hand. Nay, 

 water, which can enei'y be bcrne by a singb finger, will appear 

 intolerably scalding to the whoW hand. With regard to the 



