SENSE OF TEMPERATURE. 



intense being the impression produced. Thus, if the whole of one 

 hand be placed in water heated to a temperature of 102, and one 

 finger alone of the other hand, in water heated to 104, the tempera- 

 ture of the former will appear much higher than that of the latter. 

 Slight differences of temperature can be recognized by the whole hand, 

 which are not perceptible if a single finger be employed. 



The sensibility of the skin to differences of temperature, varies in 

 different individuals arid in different parts of the body ; this is un- 

 doubtedly in part dependent on differences in the degree of thickness 

 of the epidermis. The tip of the tongue, the face, the fingers, and 

 the soles of the feet, are the parts in which thermal sensations are 

 most easily and acutely felt. It is said that, with the tip of the 

 tongue, variations of temperature of even J- can be distinguished. 

 The sensibility of the left hand to temperature is more delicate than 

 that of the right. Weber found, that if both hands are immersed in 

 separate vessels of hot water, the left hand always appears the warmer, 

 even though the temperature of the water in which it is immersed, be 

 1 or 2 colder than that in which the right hand is placed. 



The sensations excited are more intense, when the alterations in the 

 temperature of the skin are rapidly effected. When a portion of the 

 skin is cooled by immersion in water at a low temperature, say 55, 

 and is then immersed in water at 68, a feeling of heat is experienced 

 for a few seconds, whilst the temperature of the skin rises, but a per- 

 manent sensation of cold then follows, because the temperature of the 

 water is still much lower than that of the skin. Cold bodies, which 

 are good conductors of heat, such as the metals, appear to us colder 

 than other bodies of the same temperature, which, like wood, are bad 

 conductors of heat, because the quantity of heat absorbed in a given 

 time from the skin is greater. The sensation of burning is communi- 

 cated to the hand by air at a temperature of 302, by wood at 212, 

 and by mercury at 144. Those bodies which have a high specific 

 heat, and which absorb and render latent large quantities of heat, also 

 act more powerfully on the thermal sense. 



It is supposed, that the giving up of heat by the skin, which takes 

 place when a cold body is brought into contact with it, causes a con- 

 traction of the cutis and its papillae, and that the taking up of heat, 

 which ensues when a warm body is brought into contact with the skin, 

 leads to the dilatation of those parts, and that, in this manner, the 

 nerves are acted upon, and the sensory impressions of cold and heat 

 are produced. This refers to cases, in which the heat is conducted 

 into or from the body, by some material substance, either solid, fluid, 

 vaporous, or gaseous, actually in contact with it. But the nerves 

 which receive and convey thermal impressions to the sensorium, are 

 also affected, as we know, by radiant heat coming to, or issuing from, 

 the body. In this case also, the heat is still conducted to or from the 

 extremities of the nerves, by material substance, viz., that of the skin 

 itself, the temperature of which is elevated, or depressed, by the re- 

 ception, or loss, of radiant heat. Accordingly, the nerves are prob- 

 ably not excited by the entering or departing radiant heat itself, but 

 by the heat conducted to, or from, them by the warmed or cooled skin. 



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