THE SENSES 979 



nerve, hyperalgesia (increased sensitiveness to painful impres- 

 sions) may appear. This, however, does not seem to be a con- 

 sequence 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 sensibility to pain after nerve section resembles 

 the defects of sensibility to touch and temperature, showing a 

 central area of absolute anaesthesia surrounded by a zone of 



FIG. 431. 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 pres- 

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

 to pressure of 1,860 milligrammes (normal threshold) ; Spots marked o required 

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

 was anaesthesia to the camel's-hair brush. 



partial loss, which is slight towards the outer boundary, but 

 increases as we pass inwards (Fig. 431). 



After section of a nerve function is recovered only as a result 

 of regeneration. This is true of all the sensory functions of the 

 skin and of the pilo-motor and sudo-motor functions. Vaso- 

 motor tone in the affected area is restored much sooner than 

 the other functions. This rapid recovery probably depends upon 



62 2 



