CUTANEOUS SENSIBILITY 



41 



than in adults, because the touch spots lie closer together 

 (Landois). It varies considerably with different individuals even 

 within physiological limits; numerous observations show that it 

 can be developed and improved by practice. Czermak and 

 Gartner found that the power of localisation is more highly 

 developed in the blind than in normal people, and Volkmann 

 noted that the improvement takes place on both sides of the 

 body, although the sense of touch is nearly always better ap- 

 preciated by the right hand. One of the most striking proofs 

 that tactile discrimination is improved by practice is that in 

 compositors it is extraordinarily well developed in the finger- 

 tips. In the highly mobile parts of the limbs, a few hours of 

 practice are enough to increase tactile discrimination to a 



FIG. 22. Ponzo's new aesthesiometer, substituted for Weber's compass, consists in a handle and 

 two arms that can be adjusted by means of a screw at the end of the handle. The arms carry 

 two brass clips with two points at their ends. To vary the quality of the tactile, thermal, or 

 painful stimulus the blunt ivory points may be replaced I by blunt or sharp metal points. 



remarkable degree, almost to double it. In the immobile and 

 more protected regions, on the contrary (e.g. the skin of the trunk 

 where it is low), even prolonged exercises do not increase it 

 perceptibly. It is certain that education of any area of the skin 

 on one side increases sensibility, not only in the vicinity of that 

 area, but also in the corresponding area of the opposite side. 



Many conditions alter the delicacy of tactile localisation. If 

 a limb is raised so as to make it anaemic, or the veins are com- 

 pressed till there is congestion or venous stasis, spatial sensi- 

 bility is blunted. The same occurs when the attention is fatigued 

 by unduly protracted tests (Alsberg), and by the action of cold 

 (Goltz) ; after prolonged application of the anode of a galvanic 

 current (Spanke) ; on passive distension of the skin (Czermak) ; 

 by certain poisons atropine, daturine, morphine, strychnine, 

 cannabine, alcohol, chloral hydrate, potassium bromide (Sichtenfels 

 and others). 



