GENERAL AND SPECIAL SENSATIONS. 531 



of objects by the touch, we could form no notion of their colour with- 

 out sight, of their sounds without hearing, or of their odours without 

 smell. The nerves which convey these special impressions, as already 

 mentioned, 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 by branches from the Fifth pair, as well 

 as by the Optic. 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 when the vibrations can be seen. 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. Each kind 

 of sensation may also be excited, however, by mechanical irritation of 

 the nerve which is subservient to it. The feeling of pain may be in- 

 duced by impressions made upon the nerves of special sense, as well as 

 upon those of feeling, if these impressions be too violent or excessive. 

 Thus the dazzling of the eye by a strong light, and still more, the 

 action of a moderate light in an irritable state of the retina, sudden 

 loud sounds, or even sounds of moderate intensity but of peculiar harsh- 

 ness, powerful odours, even such as are agreeable in moderation, 

 produce feelings of uneasiness, which may be properly called painful, 

 even though they are different from those excited through .the nerves of 

 common sensation. 



933. As a general rule, it may be stated, that the violent excite- 

 ment of any sensation is disagreeable ; even when the same sensation, 

 experienced in a moderate degree, may be a source of extreme pleasure. 

 But the question of degree is relative rather than absolute : that is, a 

 sensation may be felt as extremely violent by one individual, whilst 

 another, who is more accustomed to sensations of the same kind, is not 

 disagreeably affected by it. Thus, our sensations of heat and cold are 

 entirely governed by the previous condition of the parts affected ; as is 

 shown by the well-known experiment of putting one hand in hot water, 

 the other in cold, and then transferring them both to tepid water, 

 which will seem cool to the one hand, and warm to the other. The 

 same is the case in regard to light and sound, gmell and taste. A per- 

 son going out of a totally dark room, into one moderately bright, is for 

 the time painfully impressed by the light, but ^oon becomes habituated 

 to it ; whilst another, who enters it from a room brilliantly illuminated, 

 will consider it dark and gloomy. 



934. The intensity with which sensations are felt, therefore, depends 

 upon the degree of change which they produce in the sensorium. The 

 more frequent the recurrence of any particular sensation, the more 

 does the system become adapted to it, and the less change does it pro- 

 duce. It is, therefore, perceived in a less and less degree, and at last 

 it ceases to excite attention. The stoppage of a constantly-recurring 

 sensation, however, will produce a change, which makes as strong an 



