THE SENSE OF PRESSURE. 395 



skin can thus be marked cut in areas (tactile areas), within each of which the 

 compass-points are felt as a single object, but if they are separated so as to fall 

 beyond the borders of these areas, they are at once perceived to be two. 



The following figures 1 represent the distances at which the compass-points 

 can just be distinguished as double when applied to various parts of the body: 



Tip of tongue • • 1.1 mm. 



Palm of last phalanx of finger 2.2 



Palm of second phalanx of finger 4.4 



Tip of nose 6.6 " 



Back of second phalanx of finger 11.1 



Back of hand 29.8 " 



Forearm 39.6 



Sternum 44 " 



Back 66 " 



It will be observed that accuracy of localization and sensitiveness to pressure 

 find their most perfect manifestations in widely separate regions of the skin. 



Tactile areas are found to have a general oval form with the long axis 

 parallel with the long axis of the member investigated. If the compass-points, 

 separated, say, half an inch apart, be passed over the skin of the palm from 

 the middle of the hand to the finger-tips, the sensation will be that of a single 

 line gradually separating into two diverging lines. The result, of course, 

 depends on the compass-points passing successively through areas of finer 

 localization. If an area be marked out on a part of the skin where localiza- 

 tion is poor, within which area two points simultaneously applied appear to be 

 one, a single point moved within it is still perceived to change its place, and 

 two points successively applied may be perceived to occupy different positions. 

 The mental fusion or separation of the two compass-points, cannot depend 

 altogether on their being placed over the terminal twigs of the same or of two 

 adjoining nerve-fibres, for, were this the case, the points could be discriminated 

 when separated by a very small distance across the line drawn between the 

 endings of adjoining nerve-fibres, while on either side the points would have 

 to be much more widely separated in the area of distribution of a single fibre. 

 The important factor in the mental separation of two stimulated points is, that 

 between such points there shall be found a certain number of sensory elements 

 which are unstimulated. 2 Practice in such experiments greatly increases the 

 power to localize impressions. This improvement is evidently due not to 

 the establishment of new nerves, bul to a more perfeel discrimination of sen- 

 sations in the nerve-centres. Dressier" found thai after practice tor four 

 weeks, the compass-points. which at die beginning had to be separated is 

 millimeters on the skin of the forearm to be distinguished, could, at the end of 

 the period, !»e recognized as two \\ hen only aboul I millimeters apart. A Imosl 

 as great an improvement of localizing power was gained by the unexercised 



1 Foster's Physiology, ">tli ed., 1891. 



2 Weber: " Tastsinn und Gemeingefuhl," Wagner's Hamdworterbueh (!>;■ Physiologic, 1846. 



:1 Dressier : Loc. tit. 



