SENSE OF TOUCH. 397 



are assisted by the muscular sense ( 433), which makes us conscious of the 

 degree of pressure we are employing. By the impressions made upon the 

 papillae, during the movement of the tactile surface over that which is being 

 examined, the roughness, smoothness, or other peculiar characters, of the lat- 

 ter are estimated. Our knowledge of form, however, is a very complex pro- 

 cess, requiring not merely the exercise of the sense of touch, but also great 

 attention to the muscular sensations. It is chiefly, as formerly remarked, in 

 the variety of movements of which the hand of Man is capable, that it is 

 superior to that of any other animal ; and it cannot be doubted that this affords 

 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 confessed, that our knowledge would have a very limited range, if 

 this sense were the only medium, through which we could acquire ideas. It 

 is probably on the sensations communicated through the touch, that the idea 

 of the material world, as something external to ourselves, chiefly rests ; but 

 this idea is by no means a direct result of these sensations, being rather an 

 instinctive or intuitive perception excited by them. Every person who directs 

 the least attention to the subject must perceive, how completely different are 

 those notions of the primary or elementary properties of matter, which we 

 base upon the information thus communicated to us, from the sensations them- 

 selves ; and, as Dr. Alison has justly remarked, "a decisive proof of this being 

 the true representation of this part of our mental constitution, is obtained by 

 attending to the idea of extension or space ; which is undoubtedly formed 

 during the exercise of the sense of touch ; and is no sooner formed, than it 

 'swells in the human mind to Infinity,' to which certainly no human sensation 

 can bear any resemblance." 



524. That the conditions under which certain of the modifications of com- 

 mon sensation operate, are in some respects different from those of ordinary 

 Touch, is very easily shown. Thus, the feeling of tickling is excited most 

 readily in parts, which have the least tactual sensibility, the armpits, flanks, 

 and soles of the feet ; whilst in the points of the fingers it cannot be excited. 

 Moreover, the nipple is very moderately endowed with ordinary sensibility ; 

 yet by a particular kind of irritation, a very strong feeling may be excited 

 through it. Again, in regard to temperature, it is remarked by Weber, that 

 the left hand is more sensitive than the right ; although the sense of touch is 

 undoubtedly the most acute in the latter. He states that, if the two hands, 

 previously of the same temperature, be plunged into separate basins of warm 

 water, that in which the left hand is immersed will be felt as the warmest, 

 even though its temperature is somewhat lower than that of the other. In 

 regard to the sensations of heat and cold, he points out another curious fact, 

 that a weaker impression made on a large surface, seems more powerful than 

 a stronger impression made on a small surface ; thus, if the forefinger of one 

 hand be immersed in water at 104, and the whole of the other hand be 

 plunged in water at 102, the cooler water will be thought the warmer; 

 whence the well-known fact, that water in which a finger can be held, will 

 scald the whole hand. Hence it also follows, that minute differences in tem- 

 perature, which are imperceptible to a single finger, are appreciated by plung- 

 ing the whole hand into the water ; in this manner, a difference of one-third 

 of a degree may readily be detected, when the same hand is placed succes- 

 sively in two vessels. The judgment is more accurate, when the temperature 

 is not much above or below the usual heat of the body; just as sounds are 

 best discriminated, when neither very acute nor very grave. 



525. The improvement in the sense of Touch, in those persons whose de- 

 pendence upon it is increased by the loss of other senses, is well known ; this 



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