496 PHYSIOLOGY 



part of the skin. The temperature sense is also present, but of a low grade. 

 Thus heat over 38 C. and cold under 24 C. can be appreciated as such, 

 but the intervening temperatures produce no sensation. Sensations evoked 

 in the protopathic area are strongly endowed with what may be termed 

 ' affective ' character. Thus painful stimulation is much more unpleasant 

 when applied to this area than would a similar stimulation be when applied 

 to a normal area of skin. 



In contradistinction to the deep sensibility which is diffuse, protopathic 

 sensibility is distributed in spots, so that heat and cold spots, for instance, 

 may be distinguished as on the normal skin. It is interesting that the 

 glans penis is normally provided only with protopathic sensibility. 



EPICRITIC SENSIBILITY does not return to the desensitised area 

 until one to two years have elapsed since the division of the nerves. With 

 its return the affective character of the protopathic sensations at once 

 disappears and is replaced by an accurate discrimination of the nature and 

 extent of the stimulus ; the tactile sense proper, i.e. the appreciation of the 

 lightest touch applied to the skin and its accurate localisation, belonging 

 entirely to the epicritic sensations. The power of discriminating the 

 distance between two points applied to the skin simultaneously is also a 

 function of the epicritic sensibility. 



With the discriminating tactile sense returns also the power of appre- 

 ciating fine differences of temperature, i.e. differences between 26 and 

 37 C. 



This classification may be summed as follows : 



Deep sensibility . . . including ^Pressure sense 



(Pressure pain 



/Skin pain 



Protopathic sensibility J Heat over 38 C. 



(strongly affective) I Cold under 24 C. 



f Tactile sense proper 

 Pain localisation 

 Epicritic sensibility -{ Discrimination 



(accurately localised) j Heat and cold between 



26 and 37 C. 



Head and Thompson have shown that on entering the cord these various 

 sensations undergo a new grouping. Thus the pain impulses, which arise 

 in and are carried by the muscular nerves, the nerves of deep sensibility, 

 unite with those which run in the protopathic system, so that a lesion of the 

 cord which abolishes the sense of pain will abolish all forms of pain, whether 

 arising from the skin or from the underlying tissues. In the same way all 

 temperature sensations, whether the fine ones of the epicritic system or the 

 coarser ones of the protopathic system, run together in the cord. If the 

 heat sense is affected by a lesion of the cord all forms and all degrees of the 

 sensation are affected in like measure, and the same applies to the sensations 

 of cold. 



