CUTANEOUS AND INTERNAL SENSATIONS 



1089 



we work inwards from the normal skin to yield the sensation of 

 cool instead of cold. Similarly, the outer limit of thermo-hypo- 

 aesthesia can be determined by using a high temperature (50 C ). 

 It is the line at which the sensation of hot yielded by the normal 

 skin gives place to the sensation of warm. The two boundaries 

 correspond closely when allowance is made for the separate grouping 

 of cold and warmth spots on the normal skin. 



Fig. 465. Middle Cutaneous (External Branch): Left Thigh (Trotter and Davies). 

 Twenty-three days after section. Results of examination with algometer (an 

 arrangement by which a needle is pressed against the skin by a hair whose 

 pressure value has been determined). Spots marked reacted by sensation of 

 pain to pressure of 1,860 milligrammes (normal threshold); spots marked o re- 

 quired 2,280 milligrammes. The continuous line marks the area within which 

 there was anaesthesia to the camel's-hair brush. 



The investigation of the sensibility of the skin areas for painful 

 stimuli is complicated by the fact that during a certain period, 

 from about the second to the sixth week after division of the nerve, 

 hyperalgesia (increased sensitiveness to painful impressions) may 

 appear. This, however, does not seem to be a consequence of any 

 sensory loss, but rather a complication due to an irritative change. 

 When this is taken account of, it is found that the defect of sensi- 

 bility to pain after nerve section resembles the defects of sensi- 



69 



