544 PHYSIOLOGY 



to drink hot fluid, such as tea, at a temperature which would be painful 

 to the hand, and still more to any other part of the body. The scalp 

 is also very insensitive to changes of temperature. The acuteness of 

 the temperature sense varies considerably with the condition of the 

 skin and with the previous stimulation of the sense-organs. The sense 

 is most acute at about ordinary skin temperature, i.e. between 27 

 and 32 C. At this temperature the skin can appreciate a difference of 

 i C. When the skin is very cold or very hot the temperature sense is 

 not nearly so delicate. This sense presents the phenomenon of adapta- 

 tion in a marked degree. It is a familiar experience that on coming 

 from the external air on a cold day into a warm room a sensation of 

 warmth is experienced all over the body. In a few minutes this sensa- 

 tion wears off. On now leaving the room to go outside again, the 

 sensation of cold is at once appreciated, to disappear in its turn after 

 a few minutes. The effect of adaptation is still better shown by 

 the experiment of taking three basins of water a, b, and c : a con- 

 tains cold water, b tepid water, c hot water. The left hand is im- 

 mersed in the cold water and the right hand in the hot water for a 

 few minutes. On now placing both hands into the basin of tepid 

 water it feels hot to the left hand and cold to the right hand. Such 

 experiences as this led Weber to the conclusion that the essential 

 stimulus for the temperature sense was not the actual temperature to 

 which the sense-organs were subjected, but the fact of a change of 

 temperature. He imagined that while the temperature sense-organs 

 were being warmed a sensation of warmth was produced, and when 

 their temperature was being lowered, a sensation of cold. Such a 

 theory would not, however, account for the fact that, above a certain 

 temperature, water may feel warm and the feeling may continue so 

 long as the skin continues to be stimulated. On a cold day the air 

 may feel cold to the face and the feeling may last the whole time that 

 the face is exposed. Moreover we have in the temperature sense 

 conditions which remind one of the after-images which occur in the 

 eye and which will form the subject of a later section. If a penny 

 be pressed on the forehead and then removed the sensation of co]d 

 lasts some little time after the penny has been removed. In this 

 case a sensation of cold is produced although the end- organs are 

 being gradually warmed up after the removal of the penny. In 

 order to account for these facts Hering, at a time when the differen- 

 tiation of hot and cold spots had not yet been effected, suggested 

 that the temperature sense-organs could be regarded as having a 

 zero-point at which no sensation was produced. If their tempera- 

 ture was raised above this point a sensation of warmth was produced 

 and vice versa. The zero-point, however, was not a fixed one, but could 

 move upwards to a certain extent on prolonged exposure to high 



