TACTILE PEKCEPTIONS AND JUDGMENTS. 817 



parts of the body. The following table, from Weber, gives the distance at 

 which two points of a pair of compasses must be held apart, so that when 

 the two points are in contact with the skin the two consequent sensations 

 can be localized with sufficient accuracy to be referred to two points of the 

 body, and not confounded as one : 



Tip of tongue l.lmm. 



Palm of last phalanx of finger 2.2 



Palm of second " 4.4 



Tip of nose 6.6 



White part of lips 8.8 



Back of second phalanx of finger 11.1 



Skin over malar bone 15.4 



Back of hand 29.8 



Forearm 39.6 



Sternum 44.0 



Back 66.0 



And an analogous distribution has been observed in reference to the locali- 

 zation 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 sensations 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 

 neighboring sensations arising at the same time. Thus, two points brought 

 to bear within a ring of heavy metal pressing on the skin, are readily con- 

 fused into one. And it need hardly be said that these tactile perceptions, 

 like all other perceptions, are immensely increased by exercise. 



748. 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. 768) 

 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 cere- 

 bral sensation-areas should not be too completely fused together. The im- 

 provement 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. 



749. By a multitude of simultaneous and consecutive tactile sensations 

 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 in- 

 formation thus gained we build up judgments as to the form and nature of 

 objects, judgments, however, which are most intimately bound up with visual 

 judgments, the knowledge derived by one sense correcting and completing 

 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 



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