CH. L1II.] PKOTOPATHIC AND EPICRITIC SENSATIONS 775 



physiological zero is altered : they persist until a new zero is formed, i.e. until adapta- 

 tion is complete ; according to Rivers and Head, adaptation to temperature is 

 impossible when epicritic sensibility is absent. So, too, heavy weights feel unduly 

 heavy after light weights, and vice versa. When eyeglasses or false teeth are first 

 worn, their contact is well-nigh unbearable; yet later, through adaptation, the dis- 

 comfort vanishes. 



It is very difficult to draw any hard-and-fast line between the cutaneous sensa- 

 tions we have just described, and those which are grouped under the name ** common 

 or general sensibility (coencesthesia)." Sensations which are difficult to describe but 

 which are perfectly familiar, such as those accompanying tickling, shivering, shudder- 

 ing, and the like, are regarded as varieties of "common sensation." Pain may be 

 looked upon as an excessive form of "common sensation," but cutaneous pain is 

 so distinct a sensation that most psychologists agree to place it under a " special" 

 rather than a " common " heading. The term "common sensation " is most frequently 

 employed in reference to sensations from the interior of the body. 



Drugs. Cocaine applied locally depresses all forms of cutaneous sensibility, 

 but especially the true tactile sense ; carbolic acid acts similarly but less strongly. 

 Chloroform produces a temporary burning sensation, and then blunts sensibility, 

 especially to temperature changes. Menthol produces a feeling of local cold because 

 it first causes hyperaesthesia of the end-organs for cold ; this is followed by a depres- 

 sion of the activity of these organs, together with that subserving other forms of 

 cutaneous sensation. 



