544 MANUAL OF PHYSIOLOGY. 



we have at first the sensation of cold ; to which, however, the 

 skin of the hand soon becomes accustomed so as no longer to 

 excite the sensation of cold ; if now the hand be placed in water 

 somewhat warmer but not higher in temperature than the atmos- 

 phere we have a feeling of warmth. If the hand be now placed 

 in as hot water as the skin can bear, it feels at first unpleasantly 

 hot, but this feeling soon passes away and the sensation is com- 

 fortable. If now from this hot water it be placed again in the 

 water of the air temperature, this which before felt warm now 

 feels very cold. 



An important item in the estimation of the temperature of an 

 object by the sensations derived from the skin depends upon 

 whether it be a good or a bad conductor of heat. Those sub- 

 stances which are good conductors, and therefore, when colder 

 than the body, quickly rob the skin of its heat, are said to feel 

 cold, whilst badly-conducting bodies, of exactly the same tem- 

 perature, do not feel cold. It is then the rapid loss of heat that 

 gives rise to the sensation of cold. 



The power of the skin in 'recognizing changes of temperature 

 is very accurate, although the power of judging of the absolute 

 degree of temperature is very slight. 



By dipping the finger rapidly into water of varying tempera- 

 ture it has been found that the skin can distinguish between tem- 

 peratures which differ by only i Cent, or i Fahr. The time 

 required for the arrival of temperature impressions at the brain 

 is remarkably long when compared with the rate at which ordi- 

 nary tactile impulses travel. To judge satisfactorily of the tem- 

 perature of an object we must feel it for some time. 



There must be special nerve-endings which are capable of re- 

 ceiving heat impressions, because warmth applied to the nerve 

 fibres themselves is not capable of giving rise to the sensation of 

 heat. Thermic stimuli, no doubt, do affect nerve fibres, but only 

 cause the sensation of pain when applied to them. 



These nerve-endings are not the same as those that receive 

 touch and pressure impressions, because the appreciation of tem- 

 perature differences is not most delicately developed in the parts 

 where the tactile sensations are most acute. Thus the cheeks 



