SENSE OF TOUCH. 535 



or cold body is sufficient, the impressions being made after the manner 

 of those of odours, sounds, &c. It is worth remarking, with reference 

 to the question of the special nature of the sensory fibres, which are the 



Fig. 158. 



Distribution of the tactile nerves at the extremity of the Human Thumb, as seen in a thin perpendicular 



section of the skin. 



channel of these impressions, that no mechanical irritation of the nerves 

 of common sensation ever seems to excite sensations of heat or cold ; 

 these being apparently almost as distinct from the sense of contact as 

 they are from that of light or sound. 



941. The only idea communicated to our minds, when this sense is 

 exercised in its simplest form, is that of resistance; and we cannot 

 acquire a notion of the size or shape of an object, or of the nature of 

 its surface, through this sense alone, unless we move the object over our 

 own sensory organ, or pass the latter over the former. By the various 

 degrees of resistance which we then encounter, we form our estimate of 

 the hardness or softness of the body. By the impressions made upon 

 our sensory papillae, when they are passed over its surface, we form our 

 idea of its smoothness or roughness. But it is through the muscular 

 sense, which renders us cognizant of the relative position of the fingers, 

 the amount of movement the hand has performed in passing over the 

 object, and of other impressions of like nature, that we acquire our 

 notions of the size and figure of the object ; and hence we perceive that 

 the sense of touch, without the power of giving motion to the tactile 

 organ, would have been of comparatively little use. It is chiefly in the 

 variety of movements of which the hand of Man is capable, thus con- 

 ducive as they are, not merely to his prehensile powers, but to the 

 exercise of his sensory endowments, that it js superior to that of any 

 other animal ; and it cannot be doubted that this affords us a very 

 important means of acquiring information in regard to the external 

 world, and especially of correcting many vague and fallacious notions 

 which we should derive from the sense of Sight, if used alone. On the 

 other hand, it must be evident that our knowledge would have but a 

 very limited range, if this sense were the only medium through which 

 we could acquire ideas. Of this we have the clearest evidence in the- 

 very imperfect development of the mental powers in those unfortunate 

 persons who have suffered under the deprivation of sight and hearing 

 from their birth, and who have been consequently cut off from the most 

 direct means of profiting by the knowledge possessed by their fellow- 

 beings, through want of power to use the organs of speech. It is only 



