SENSE OF TOUCH. 235 



their place in regard to each other, we obtain the additional notion of Exten- 

 sion or Space. By the various degrees of resistance which the sensory surface 

 encounters, we estimate the hardness or softness of the body ; but in this we 

 are assisted by the muscular sense ( 257), 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 latter 

 are estimated. Our knowledge of form, however, is a very complex process, 

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

 tion 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 mate- 

 rial 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 intui- 

 tive 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 informa- 

 tion thus communicated to us from the sensations themselves ; and, as Dr. 

 Alison has justly remarked, "a decisive proof of this being the true repre- 

 sentation 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 resem- 

 blance." 



318. 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 temperature, 

 which are imperceptible to a single finger, are appreciated by plunging 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 successively in two 

 vessels. The judgment is more accurate, when the temperature is not much 



