7oo 



TOUCH. 



[CH. LI. 



2. The sense of pressure. 



3. The sense of temperature. 



Fig. 524. Vertical section of rab- 

 bit's cornea, stained with gold 

 chloride. The nerves , ter- 

 minate in a plexus under and 

 within the epithelial layer, e. 



When any object rests on the 

 skin, it is possible by tactile sense 

 to ascertain its shape and the part 

 of the skin which it touches (localisa- 

 tion) ; to estimate its weight even if 

 it is not lifted (sense of pressure) ; 

 if it is lifted the muscular sense is 

 called into play ; and, thirdly, by 

 the temperature sense we determine 

 whether it is hot or cold. The end- 

 organs in the skin are numerous, and 

 it is quite possible that these sensations 

 are received by different kinds of end- 

 organs, though we are not acquainted 

 with which corresponds to which. It 

 is also not possible to draw a hard- 

 and-fast line between touch proper on 

 the one hand and general sensibility 

 and pain on the other. The facts 

 of disease, especially in that disease 

 of the sensory tracts called loco- 

 motor ataxy, point to the conclu- 

 sion that these varieties of sensation 

 are transmitted to the central nervous 

 system by different tracts (see p. 658), 

 and it may be that they are received 

 and interpreted there by different areas. 



Localisation of Tactile Sensations. 



The ability to localise tactile sensations on different parts of the 

 surface is proportioned to the power which such parts possess 

 of distinguishing and isolating the sensations produced by two 

 points placed close together. This power depends in part on the 

 number of nerve-fibres distributed to the part ; for the fewer the 

 fibres which any part receives, the more likely is it that several 

 impressions on different contiguous points will act on only one 

 nerve-fibre, and hence produce but one sensation. Experiments 

 have been made to determine the tactile properties of different 

 parts of the skin, as measured by this power of distinguishing 

 distances. These consist in touching the skin, while the eyes are 



