NERVES OF SPECIAL SENSIBILITY. 389 
_ the amount of blood they contain ; since this blood may be 
sent to them for other purposes. Thus, it is a condition 
necessary to the action of Muscles, that they should be 
copiously supplied with blood Ss 591); but they are not 
acutely sensible: and Glands, also, the substance of which 
has very little sensibility, receive a large amount of blood for 
their peculiar purposes. 
488. But besides the general or common sensibility which is 
diffused over the greater part of the body of most animals, 
there are certain parts which are endowed with the property 
of receiving impressions of a peculiar or special kind, such as 
sounds or odours, which would have no influence upon the 
rest ; and the sensations which these impressions excite, being of 
akind very different from those already mentioned, arouse ideas 
in our minds such as we should never have formed without 
‘them. Thus, although we can gain a knowledge of the shape 
and position of objects by the touch, we could form no notion 
_ 0f their colour without sight, of their sounds without hearing, 
_ or of their odours without smell. 
489. The nerves which convey these special impressions 
_are not able to receive those of a “common” kind: thus, the 
_ Eye, however well fitted for seeing, would not feel the touch 
of the finger, if it were not supplied with branches from the 
5th pair, as well as by the optic nerve. Nor can the different 
"nerves of special sensation be affected by impressions that are 
adapted to operate on others: thus, the ear cannot 
Fie slightest difference between a luminous and a dark object; 
‘nor could the eye distinguish a sounding body from a silent 
“one, except by seeing its vibrations. But Electricity possesses 
‘the remarkable power, when transmitted along the several 
“nerves of special sense, of exciting the sensations peculiar to 
‘each; and thus, by proper management, this single agent 
} may be made to produce flashes of light, distinct sounds, 
a phosphoric odour, a peculiar taste, and a pricking feeling, 
in the same individual at one time. The inference which 
ight hence be drawn—that Electricity and Nervous agency 
identical—is nevertheless premature, as will be shown 
r (§ 585). 
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