564 THE HUMAN BODY. 



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 temperature difference leads to errors in 

 his pressure judgments. But this does not prove an iden- 

 tity 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 in- 

 terpretation, 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 can- 

 not, or but with difficulty, recognize the components; in 

 this case the person feels both the cold and pressure dis- 

 tinctly 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 tem- 

 perature feelings gradate 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 dimin- 

 ished radiation when it was covered by this non-conducting 

 substance; and the constancy 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. 



That not merely touch and temperature sensations are 

 distinct and have different nerves, but that hot and cold 

 sensations also depend on the excitation of different nerve- 

 fibres, has recently been proved. If a metal point, lightly 

 weighted, be slowly and evenly moved along the skin, it 

 gives rise to sensations of touch at some places, and to 

 sensations of temperature at others. If it be a little 

 warmer than the skin, at certain places it causes a sensa- 

 tion of heat. These "heat-points" remain the same in 

 the same person from day to day. If the travelling point 

 be a little colder than the skin, it gives rise to a sensation 



