CUTANEOUS AND INTERNAL SENSATIONS 1051 



It is a remarkable circumstance that during regeneration stimu- 

 la'ion of the n:rv>trunk itself below the section, by the application 

 of touch, cold, or pain stimuli to the skin over its course, produces 

 peripherally referred sensations of the corresponding kind. This 

 is the case even when the nerve is stimulated outside the formerly 

 anaesthetic area, and suggests that the nerve-trunk itself has ac- 

 quired the specific sensibility normally associated with the terminal 

 organs of its afferent fibres through the lowering of the threshold 

 for the fibres themselves. 



The work of Head, who was the pioneer in this method of investiga- 

 tion, must also be mentioned. He found that when the median nerve 

 was divided in his own arm, total loss of sensation was caused over the 

 greater part of the index and middle fingers, and over a portion of the 

 thumb in its palmar aspect. In addition, sensation was partially lost 

 over a larger area, where there was complete insensibility to certain 

 stimuli, such as light touch, moderate heat and cold, and where the 

 contact of the two points of a pair of compasses could not be discrimin- 

 ated. Recovery of sensation after complete division of a peripheral 

 nerve began with the restoration of sensibility to pain and to extreme 

 degrees of heat and cold; but the hand still remained for a time as 

 insensitive as before to such stimuli as slight touch. In the parts 

 which had regained their sensibility to severe stimuli, like pricking and 

 extremes of heat and cold, the sensation radiated widely, was referred 

 to remote parts, and could not be accurately localized. This form of 

 sensibility Head calls protopathic. As the nerve recovered further, a 

 second form of sensibility appeared, associated with accurate localiza- 

 tion of cutaneous stimuli and discrimination of two compass points. 

 Light touch and moderate degrees of heat and cold could now be 

 again appreciated. This form of sensibility he terms epicritic. A third 

 form of sensibility (deep sensibility] was investigated after complete 

 division of the radial and external cutaneous nerves at the elbow. 

 The radial half of the arm and back of the hand became totally insensi- 

 tive to cutaneous stimuli, but retained their sensibility to pressure 

 or to any stimulus which deformed the subcutaneous structures, as 

 well as their power of localization of such stimuli. The afferent fibres 

 upon which this deep sensibility depends must run with the motor 

 nerves. According to Head, the other two forms of sensibility (proto- 

 pathic and epicritic) also depend on two separate systems of nerves. 

 It is assumed that the protopathic fibres regenerate more easily and 

 speedily than the epicritic or than the motor nerves of voluntary 

 muscle. The protopathic fibres are supposed by Head to exert a trophic 

 influence. A part deprived of its nerve-supply is liable to inj uries, and 

 the sores so produced heal slowly. But as soon as ' protopathic ' 

 sensibility returns to the part, they heal rapidly, even in the absence 

 of all epicritic sensation. The intestine is described as possessing 

 ' protopathic/ but not ' epicritic/ sensibility i.e., it reacts to extremes 

 of heat and cold, but not to moderate heat and cold or light touch. 



Head's experimental results must be sharply distinguished from his 

 interpretation of them, and the student is warned that the distinction 

 of protopathic and epicritic sensibility has met with adverse criticism. 

 There does not seem to be any real necessity in the observed facts for 

 introducing so revolutionary a conception of the nervous system. 

 Nor is it possible to uphold the distinction in any thoroughgoing fashion 

 for all structures. For instance, in abdominal operations performed 



