580 TACTILE PERCEPTIONS. [Boon in. 



And an analogous distribution has been observed in reference to 

 the localisation of sensations of temperature. As a general rule it 

 may be said that the more mobile parts are those by which we can 

 thus discriminate sensations most readily. The lighter the pressure 

 used to give rise to the sensations, the more easily are two sen- 

 sations distinguished ; thus two points which, when touching the skin 

 lightly, appear as two, may, when firmly pressed, give rise to one 

 sensation only. The distinction between the sensations is obscured 

 by neighbouring sensations arising at the same time. Thus two 

 points brought to bear within a ring of heavy metal pressing on 

 the skin, are readily confused into one. And it need hardly be 

 said that these tactile perceptions, like all other perceptions, are 

 immensely increased by exercise. 



Our 'field of touch,' if we may be allowed the expression, 

 is composed of tactile areas or units, in the same way that our field 

 of vision is composed of visual areas or units. The tactile sensation 

 is, like the visual sensation, a symbol to us of some external event, 

 and we refer the sensation to its appropriate place in the field of 

 touch. All that has been said (p. 523) concerning the subjective 

 nature of the limits of visual areas, applies equally well, mutatis 

 mutandis, to tactile areas. When two points of the compasses are 

 felt as two distinct sensations, it is not necessary that two and only 

 two nerve-fibres should be stimulated ; all that is necessary is that 

 the two cerebral sensation-areas should not be too completely 

 fused together. The improvement by exercise of the sense of 

 touch must be explained not by an increased development of the 

 terminal organs, not by a growth of new nerve-fibres in the skin, 

 but by a more exact limitation of the sensational areas in the 

 brain, by the development of a resistance which limits the radiation 

 taking place from the centres of the several areas. 



By a multitude of simultaneous and consecutive tactile sensa- 

 tions thus converted into perceptions we are able to make ourselves 

 acquainted with the form of external objects. We can tell by 

 variations of pressure whether a surface is rough or smooth, plane 

 or curved, what variations of surface a body presents, and how far 

 it is heavy or light ; and from the information thus gained we 

 build up judgments as to the form and nature of objects, judgments 

 however which are most intimately bound up with visual judg- 

 ments, the knowledge derived by one sense correcting and com- 

 pleting that obtained by the other. As in other senses so in this, 

 our sensations may mislead us and cause us to form erroneous 

 judgments. This is well illustrated by the so-called experiment of 

 Aristotle. It is impossible in an ordinary position of the fingers to 

 bring the radial side of the middle finger and the ulnar side of the 

 ring finger to bear at the same time on a small object such 

 as a marble. Hence when with the eyes shut we cross one finger 

 over the other, and place a marble between them so that it touches 

 the radial side of the one and the ulnar side of the other, we 



