CHAP, vi.] SOME OTHER SENSATIONS. 1039 



In a similar manner small consecutive variations of pressure, 

 as in counting a pulse, are more readily appreciated by certain 

 parts of the skin, such as the tip of the finger, than by others. 

 In all cases variations of pressure are more easily distinguished 

 when they are successive than when they are simultaneous. 



646. 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 finger 

 or on the back we refer the sensations to the finger 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 refer- 

 ence 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' anal- 

 ogous 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 

 simultaneous 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 concerning 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 sensory im- 

 pulses 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 



