546 MANUAL OF PHYSIOLOGY. 



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 teniperature 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 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 

 receiving 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 

 and the eyelids are especially sensitive to changes of tempera- 

 ture, a fact known by people who want a ready gauge of the heat 

 of a body thus, a barber approaches the curling-tongs to his 

 cheek to measure its temperature before applying it to the hair 

 of his client. The middle of the chest, moreover, is very sensi- 

 tive to heat, while it is dull in feeling tactile impressions. 



The hand is far from being the best gauge of temperature, for 

 heat appreciation is not developed in a due proportion to the 

 keenness of the tactile sensibility. The larger the surface ex- 

 posed to changes of temperature, the more accurate the judgment 

 at which we can arrive the slightest changes being at once 



