CUTANEOUS SENSATIONS 1 17 



sesthesiometer. In place of these, an instrument is sometimes used 

 consisting of a fine blunt needle set in a handle, within which is a 

 delicate spiral spring furnished with an index to show the amount of 

 pressure required before a sensation 

 is caused. But in general this is 

 found more difficult to apply than 

 the bristles. 



Graham Brown's sesthesiometer 

 consists of a convex piece of steel 

 with a polished surface, a part of 

 which can be made to project beyond 

 the rest by turning a truly cut micro- 

 meter screw. The surface is passed FlG - 87. v. Frey's hair 

 lightly over the surface of skin to 

 be tested, or vice versa. The relative 



delicacy of touch is gauged by the power of feeling different degrees 

 of projection. 



Discrimination of two points. For this purpose a pair of compasses 

 with blunted points is used ; their distance apart is measured upon 

 a scale after each observation. Or the points may be permanently 

 connected with the scale, one being fixed at its zero and the other 

 sliding along it (Sieveking's sesthesiometer). Test in this manner in 

 another person various parts of the integument (back and front of arm, 

 fingers, lips, tip of tongue, etc.) and record the distances at which the 

 two points are discriminated as separate, causing them always to 

 touch simultaneously, and without lateral movement. 



Accuracy of localisation. This is investigated by lightly touching 

 any part of the skin and immediately causing the subject to place 

 his finger upon the part touched. 



In all the above experiments the subject should be blindfolded. 



