CH. LII.] 



TACTILE LOCALISATION 



723 



locomotor ataxy, which is a disease of the sensory nerve-units, and 

 remain healthy in infantile paralysis, which is a disease of the 



s.e 



m.'n.b. 



FIG. 526. Neuro-muscular spindle, c., Capsule; n.tr., nerve trunk; m.n.b., motor nerve bundle; 

 pl.e., plate-ending; pr.e. t primary nerve-ending; s.e., secondary ending. (After Rufflni.) 



motor cells of the anterior horn of the cord (Batten). 



In addition to the special end-organs, sensory fibres may 

 terminate in plexuses of fibrils, as in 

 the sub-epithelial and the mtra-epithelial 

 plexus of the cornea (tig. 527) and 

 around the hair follicles in the skin 

 generally. In some cases the nerve-fibrils 

 within a stratified epithelium end in 

 crescentic expansions (tactile discs) which 

 are applied to the deeper epithelium 

 cells. These are woll seen in the skin 

 of the pig's snout. 



Localisation of Tactile Sensations. 



The ability to localise tactile sensa- 

 tions on different parts of the surface is 

 proportioned to the power which such 

 parts possess of distinguishing and iso- 

 lating 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 im- 

 pressions on different contiguous points 



will act on only one nerve-fibre, and FIG. 527. vertical section of rabbit's 

 hence produce but one sensation. The BSS?t5^?5S 

 experiments which have been made to 

 determine the spatial relationships of 

 the cutaneous sense consist in touching the skin, while the eyes 

 are closed, with the points of a pair of compasses, and in ascer- 



under and within the epithelial 

 layer, e. 



