XVIII 



TOUCH 



The sense of touch is conveniently classified with the other 

 senses that lie in the skin, those of pain, heat and cold. All 

 these, like taste and smell, are contact senses, but they are 

 physico-receptors rather than chemo-receptors. Touch, like 

 smell and taste, can nevertheless to a certain extent act at 

 a distance, as we shall see. 



The nerve-endings in the skin are of several kinds, and 

 they are on the whole less complicated in structure than 

 those of the other senses already described. In the simplest 

 the nerve branches into a number of slender twigs close 

 beneath the outer skin, but in others it ends in a small speck 

 of jelly enclosed within concentric layers of supporting cells 

 like the skins of an onion. These more complicated end 

 organs are known as "corpuscles", and they are classified 

 into different sorts by the details of their structure. Touch 

 corpuscles lie close beneath the surface of the skin and are 

 particularly numerous under the bare skin of the palms of 

 our hands and soles of our feet. They respond to contact of 

 the skin with any object, that is to say, they respond to 

 light pressure ; other corpuscles situated deeper below the 

 surface respond to heavier or deep pressure. They are not 

 evenly spread over the surface of the body but are placed 

 most close to each other on the tip of the tongue, next 

 closely on the under side of the finger-tip, less closely on 

 the palm of the hand, the back of the hand, and much more 

 sparsely elsewhere. The spacing can easily be mapped by 

 moving a stiff bristle over the skin and marking each spot 

 where it can be felt. 



The sense of touch is served not only by the touch cor- 

 puscles which respond to things in contact with the skin, 

 but also by the nerve-endings associated with the roots of 



the hairs. These by responding to minute movements of 



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