584 THE HUMAN BODY. 



very different that we can no more compare them than lumi- 

 nous and auditory; and if we accept the modern modified 

 form of the doctrine of specific nerve energies (Chap. XIII), 

 in accordance with which the same sensory fibre when ex- 

 cited always arouses a sensation of the same quality, if any, 

 because it excites the same brain-centre, it is hard to conceive 

 how the same fibre could at one time arouse a tactile, and at 

 another a temperature sensation. It has, however, been 

 maintained that touch and temperature feelings sometimes 

 pass into one another insensibly. If a half dollar cooled to 5 

 C. (41 F.) be placed on a person's brow, and then two (one 

 on the other) warmed to 37 C. (98.5 F.), he commonly 

 thinks the weight in the two cases is equal ; i. e. , the tempera- 

 ture difference leads to errors in his pressure judgments. But 

 this does not prove an identity in the sensations; the cold 

 half-dollar may produce contraction of the cutaneous tissues, 

 leading to compression of the tactile end organs, which is 

 mistaken, in mental interpretation, for a heavier pressure. 

 When sensations are combined in other cases, as red and blue- 

 green to produce white, or partial tones to form a compound, 

 we either cannot, or can but with difficulty, recognize the 

 components ; in this case the person feels both the cold and 

 pressure distinctly when the half-dollar is laid on him. 



In certain cases a person mistakes the contact of a piece of 

 raw cotton with his skin, for the approach of a warm object; 

 this has been taken to prove that touch and temperature feel- 

 ings graduate into one another. However, the feeble touch 

 of the raw cotton might well be less felt than the increased 

 temperature of the skin, due to diminished radiation when it 

 was covered by this non-conducting substance ; and the con- 

 stancy with which, in the ordinary circumstances of life, we 

 feel and discriminate clearly, on the same skin region at the 

 same time, both temperature and touch sensations, is a strong 

 argument against any transition of one into the other. 



Moreover, there is direct evidence that three different ap- 

 paratuses in the skin or at least differently located apparatuses, 

 are concerned in arousing touch, heat and cold sensations. If 

 a metal point, lightly weighted, be slowly and evenly moved 

 along the skin by clockwork, it gives rise to sensations of 

 touch at some places and if hotter or cooler than the skin 

 to sensations of temperature at others; but never when in 

 contact with one point to more than one sensation. If the 



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