550 PHYSIOLOGY 



which is determined by the central connections of the entering nerve 

 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 at 

 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 our- 

 selves, 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 con- 

 tact 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 well shown 

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

 Aristotle. If we cross the first and middle fingers and bring them 

 in this position in contact with a pea, if the eyes are shut, we should 

 at once say that two peas lay under the fingers. This is especially 

 marked if the pea be rolled between the fingers. The two sides of the 

 fingers which come in contact with the pea usually touch two different 

 objects, and these parts of the skin would have to be re-educated, i.e. 



