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 
papille, 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. d 
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 aceurately 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, © 
