392 IMPROVEMENT OF TOUCH BY EXERCISE. 



when threads were stretched in various directions across the 

 apartment. Hence some Naturalists were inclined to attribute 

 to the Bat the possession of a sixth sense unknown to Man ; 

 but Cuvier correctly pointed out that this idea becomes un- 

 necessary, if we attribute to the delicate membrane of the 

 wings (as we are justified in doing) a high degree of tactile 

 sensibility, so as to receive impressions from the pulses of the 

 air that are produced by the action of the wings and modified 

 by the neighbourhood of solid bodies. 



495. The only idea communicated to our minds by the sense 

 of Touch, when this is exercised in its simplest form, is that of 

 resistance; and we cannot form a notion either of the size or 

 shape of an object, or of the nature of its surface, by feeling 

 it, unless we move the object over our own sensory organ, or 

 move the latter over the former. By the various degrees of 

 resistance which we encounter, we estimate the hardness or 

 softness of the body ; and by the impressions made upon the 

 papillae, when they are moved over its surface, we form our 

 idea of its smoothness or roughness. It is by attention to the 

 muscular movements we execute, in passing our hands or 

 fingers over its surface, that we acquire our ideas of its size 

 and figure; and hence we perceive that the sense of touch, 

 without the power of moving the tactile organ over the object, 

 would have been- of comparatively little use. 



496. This sense is capable of improvement to a remarkable 

 degree ; as we see in persons who have become more dependent 

 upon it in consequence of the loss of their sight. This doubt- 

 less results, in part, from the increased attention which is 

 given to the sensations ; and partly from the greater acuteness 

 or impressibility of the organ itself, arising from the frequent 

 use of it. Amongst other remarkable instances of this kind 

 was that of Saunderson, who, though he lost his sight at two 

 years old, acquired such a reputation as a mathematician, that 

 he obtained a Professorship at Cambridge. He exhibited, in 

 several ways, an extraordinary acuteness in his touch ; but 

 one of his most remarkable faculties was the power of distin- 

 guishing genuine .medals from imitations, which he could do 

 more accurately than many connoisseurs in full possession of 

 their senses. 



497. The sense of temperature is of a different character 

 from common tactile sensibility; and either may be lost, 



