THE^SENSES 981 



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 investi- 

 gation, 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 com- 

 passes could not be discriminated. 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 a 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 localization 

 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 insensitive 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 (protopathic and epicritic) also depend on two separate 

 systems of nerves. It is assumed that the protopathic fibres re- 

 generate more easily and speedily than the epicritic or than the 

 motor nerves of voluntary muscle. The protopathic fibres are sup- 

 posed by Head to exert a trophic influence. A part deprived of its 

 nerve-supply is liable to injuries, 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 dis- 

 tinction 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 abdo- 

 minal operations performed under local anaesthesia it has been seen 

 that the parietal peritoneum is quite insensitive to touch, pressure, 

 and temperature stimuli, including extreme temperatures (Ram- 

 strom), while pain is caused by traction on it. Its sensibility is 

 therefore neither purely epicritic nor purely protopathic in Head's 

 sense. In like manner the mucous membrane of the mouth, in 



