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CUTANEOUS SENSATIONS 633 



persons the power of sight. It is found that at first there is no power of 

 localising visual impressions. The * local sign ' is developed only in re- 

 sponse to experience, by comparing simultaneous visual, tactile, and motor 

 sensations. By analogy we might ascribe the local sign of cutaneous 

 sensations to a similar causation. Our study of the spinal animal has 

 indeed given us a physical or histological conception of local sign. We 

 know that stimulation of any part of the body evokes an appropriate reaction, 

 the nature of which is determined by the central connections of the entering 

 erve fibres. A fibre entering at one segment must therefore come into 

 relation with a different set of motor cells from those which are set into 

 action by a fibre entering one segment lower down. Every nerve fibre 

 from the. skin will therefore have an appropriate complex of motor paths 

 in functional connection with its central endings, and when the activity of 

 these reflex paths comes to be represented in consciousness, it is evident 

 that the sensation derived from each point must differ from that derived 

 from any other point of the skin by virtue of the differing motor events 

 actually or potentially excited from the two points. In ascribing therefore 

 ' local sign ' to coincident muscular sensations, and to the memory and 

 experience of past movements, we are giving but an imperfect explanation ; 

 since the difference between the sensations from different parts, which are 

 t the bottom of our powers of localisation, has its origin in the structure 

 of the central nervous system itself and is present from the very beginning 

 of the evolution of a reactive nervous system. 



PROJECTION OF TOUCH. Since the alterations in the surface of the skin 

 which give rise to tactile sensations are habitually caused by contact with 

 external objects, we come to regard the sensations themselves, not as 

 changes in the skin, but as qualities of the object which touch the skin, i.e. 

 we project the sensation. 'The projection is however not so great as in 

 the case of visual sensations. Cutaneous sensations we always consider 

 as qualities of an object immediately affecting and altering the condition 

 of ourselves, whereas the visual sensations are referred at once to objects 

 lying right away from ourselves, so that we are not aware that any change 

 has taken place in our bodies as a result of the entering of rays of light into 

 the eye. 



It is remarkable to what extent projection of touch sensation may 

 occur. Thus a surgeon actually lengthens his fingers by using a probe. 

 When he is probing for dead bone he feels the grating of the bone, not at 

 his finger-tips, but he projects the sensation to the end of the probe. In 

 the same way tactile sensations evoked by the contact of bodies with the 

 insentient endings of hair are referred* to the ends of the hairs rather than 

 to the hair follicles where the nerve impulses actually come into being. 



The dependence of local sign on habitual experience is shown by 

 the various tactile illusions, such as the well-known experiment of Aris- 

 totle. If with the eyes shut we cross the first and middle fingers and bring 

 them in this position in contact with a pea, we should at once say that two 

 peas lay under the fingers. This is especially marked if the pea be rolled 



