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A MANUAL OF PHYSIOLOGY 



hair aesthesiometer. This consists of a handle in which hairs of 

 different diameters can be fixed. The area of the cross section of each 

 hair is measured under the microscope, and the pressure necessary 

 to bend it is determined by pressing it upon the scale-pan of a 

 balance. The pressure in milligrammes, divided by the cross section 

 in square millimetres, gives the pressure per square millimetre, 

 which, according to v. Frey, permits hairs to be chosen so as to give 

 a uniform intensity of stimulation or a variable intensity, according 

 to the object of the investigation. Many observers, however, believe 

 that it is more accurate to take .no account of the pressure per unit 

 of area, but to graduate the hairs according to the total pressure 

 needed to bend them. When touch-spots ascertained in this way 

 are excited by an inadequate stimulus e.g., an alternating current 

 of minimal strength, applied by the unipolar method through the 

 head of a pin as an electrode they still respond by their character- 

 istic or specific reaction namely, a sensation of touch in the case 

 supposed, a vibrating sensation like that caused by a tuning-fork 

 in contact with the skin. In the spaces between the touch-spots 

 the sensation produced by the same strength of current, or even by 

 a weaker current, is not one of touch, but a painful pricking sensation 

 which has no vibratory character, but is permanent as long as the 

 current lasts. 



The spots most sensitive to touch lie close to the hairs on their 

 ' windward ' side i.e., on the side away from which they slope. 

 The minimum pressure necessary to evoke a sensation of contact 

 is not the same for every portion of the skin. The forehead and palm 

 of the hand are most sensitive. 



(Kiesow] . 



If two points of the skin are touched at the same time there is 

 a double sensation when tko distance between the points exceeds 

 a certain minimum, which varies for different parts of the sensitive 

 surface. 



Practice increases the acuity of touch for the two points test. Even 

 in a few hours it may be temporarily quadrupled on some parts of 

 the skin. Since at the same time it is increased in the corresponding 

 part of the opposite side of the body, it is argued that the modifica- 

 tion takes place in the central nervous system, not in the end-organs 

 themselves. 



