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 ( 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 

 a kind 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 

 of 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 distinguish 

 the 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 

 might hence be drawn that Electricity and Nervous agency 

 are identical is nevertheless premature, as will be shown 

 hereafter ( 585). 



