276 ON CUTANEOUS AND [Boon in. 



all cases variations of pressure are more easily distinguished when 

 they are successive than when they are simultaneous. 



878. The localization of tactile sensations. When anything 

 touches a spot of our skin, we not only experience a ' pressure 

 sensation ' of greater or less intensity according to the amount of 

 pressure exerted and the particular region of skin pressed upon, 

 we are also at the same time aware that the sensation has been 

 started in that spot, that the spot in question and not another has 

 been touched. When we are touched on the ringer or on the back 

 we refer the sensations to the ringer or to the back respectively, 

 and when we are touched at two places on the same finger at the 

 same time we refer the sensations to two parts of the finger. We 

 localize our touch sensations with reference to the surface of our 

 body after the same fashion that we localize our visual sensations 

 with reference to the external world. Our whole skin serves us as 

 a ' field of touch ' analogous to the ' visual field ' of the eye ; and as 

 when experiencing a visual sensation, we refer it to its presumed 

 cause and say we perceive a light in some part or other of the 

 field of sight, so when we experience a tactile sensation we say we 

 perceive that something has touched this or that part of our skin ; 

 the tactile sensation has become a tactile perception. As the 

 accuracy of our visual perceptions is largely dependent on the 

 smallness of the retinal interval which must separate two simul- 

 taneous retinal stimulations in order that these shall give rise to 

 two separate sensations, vision being most distinct in the fovea 

 centralis where this interval is smallest, so also the accuracy of 

 our tactile perceptions is dependent on the smallness of a like 

 cutaneous interval. Where, as in the tip of the finger, the interval 

 is small, contact with even a small area of surface may give rise 

 to several simultaneous but distinct sensations, each of which we 

 localize ; and we thus obtain by means of one contact several 

 perceptions affording a considerable amount of information con- 

 cerning the nature of the surface. Where, as in the skin of the 

 back, the interval is great, contact with even a large area of 

 surface may give rise to one sensation, which we do not resolve 

 into its components, all the several senscry impulses from the 

 skin fusing into one common sensation ; we only localize this one 

 sensation, we have only one perception of something touching 

 that part of our back, and the information which we thus acquire 

 concerning the nature of the surface in contact with the skin is 

 limited. 



As the above remark indicates, the interval in question varies 

 very widely in different parts of the surface of the body ; our power 

 of localization is much .finer in certain parts than in others. 

 Moreover the distribution of the fineness of localization is not 

 identical with that of the mere appreciation of pressure ; some 

 parts may be very sensitive and yet possess imperfect localization. 

 The magnitude of the interval of space which must separate two 



