CHAPTER XV 



SKIN-SENSATIONS 



THE senses, according to a time-honoured classification, are 

 five in number smell, sight, taste, hearing, and common 

 sensation, or touch ; but such a classification of our sensations 

 and of the organs which originate them is too crude for modern 

 needs. Already we have shown that, whereas the nose and 

 the tongue afford the same kind of information, the ear affords 

 information of two, perhaps of three, different kinds. Within 

 the realm of common sensation we pick out three special senses 

 served by specialized sense-organs touch, cold and heat and, 

 possibly, a fourth, served by non-specialized nerves, to which 

 alone the epithet " common " properly applies. 



The skin is supplied with nerves naked fibrils in the 

 richest abundance. They are most easily demonstrated in the 

 layer which covers the cornea, thanks to its transparency ; 

 in this, as shown in Fig. 41, having branched on the front 

 of the fibrous tissue of which the cornea is composed, the 

 nerves pass towards the surface, forming connections with 

 every one of its cells, or, at any rate, with every cell of the 

 more superficial of the three or four layers of which the 

 epithelium is made up. Ramified nerve-twigs of this type 

 do nob, under ordinary conditions, convey any sensations to 

 consciousness. So long as the skin-cells with which they are 

 connected are healthy, the nerve- twigs establish for them con- 

 nections with the central nervous system by which their nutri- 

 tion is regulated ; but they carry no impulses to which we can 

 direct attention. The movement of blinking is accompanied 

 by no sensation until the edges of the eyelids come in contact. 

 A pencil pressed against the lid evokes touch-sensations from 

 the skin, but none from the cornea which underlies it. When 



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