976 



A MANUAL OF PHYSIOLOGY 



hair brush, or more exactly by the v. Frey hairs the! "sensi- 

 bility to touch can be shown to be diminished (region of hypo- 

 aesthesia to touch) (Fig. 428). 



For a variable distance within the ' stroking outline ' the 

 hypoaesthesia for tactile stimuli is so slight that it cannot be 



FIG. 428. AREAS OF ALTERED SENSIBILITY PRODUCED BY SECTION OF ALL 

 THREE BRANCHES OF THE INTERNAL CUTANEOUS NERVE OF THE LEFT 

 FOREARM (TROTTER AND DAVIES). (REDUCED BY TWO-THIRDS.) 



The thick lines show the areas of anaesthesia to the brush. The thick continuous 

 lines enclose the areas of the anterior and posterior branches. The thick broken 

 line and heavy shading mark the area of the increase in anaesthesia which followed 

 section of the middle branch. The thin lines show the areas of minimal hypo- 

 aesthesia i.e., the ' stroking outline.' The complete oval outline is the ' stroking 

 outline ' which followed section of the posterior branch. The large addition to 

 the oval on the right of the diagram shows the increase in the ' stroking outline ' 

 which followed section of the anterior branch. The thin broken line and fine 

 shading show the additions to the ' stroking outline ' produced by division of the 

 middle branch. 



detected with the brush or with cotton-wool, or even with the 

 v. Frey hairs. Like those of normal skin, 90 per cent, of its 

 hair-bulbs respond to a hair exerting a pressure of 70 milli- 

 grammes, and the remaining 10 per cent, to hairs exerting a 

 pressure of 140 or 280 milligrammes. Inside this zone of minimal 

 hypoaesthesia the defect of sensibility rapidly increases as we 



