OF SENSATION IN GENERAL. 227 



system than on the absolute amount of the impressing cause ; and this is alike 

 the case with regard to the special and the ordinary sensations. 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 both to tepid water, 

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

 too, how much more we are affected by a warm day at the commencement of 

 summer, than by an equally hot day later in the season. The same is the 

 case in regard to light and sound, smell and taste. A person going out of a 

 totally dark room into one moderately bright, is for the time painfully impressed 

 by the light, but soon becomes habituated to it ; whilst another, who enters it 

 from a room brilliantly illuminated, will consider it dark and gloomy. Those 

 who are constantly exposed to very loud noises, become almost unconscious of 

 them, and are even undisturbed by them in illness ;* and the medical student 

 well knows, that even the effluvia of the dissecting-room are not perceived, 

 when the organ of smell is habituated to them ; although an intermission of 

 sufficient length would, in either instance, occasion a renewal of the first un- 

 pleasant feelings, when the individual is again subjected to the impression. 



807. Again, it is a well-known fact, that impressions made upon the organs 

 of sense continue for a time, after the cause of the impression has ceased. It 

 is in this manner that a musical tone, which seems perfectly continuous, results 

 from a series of consecutive vibrations, following each other with a certain 

 rapidity : and that a line or circle of light is produced by a luminous body 

 moving with a certain velocity. Now there is reason to believe that changes, 

 of which the effects thus transiently remain upon the nerves of sense, are more 

 permanently impressed upon the sensorium; since, as formerly shown ( 291), 

 we can only in this manner account for the phenomena of Memory, and for the 

 effects produced upon this power, by material changes in the brain. Hence 

 the diminution in the force of sensations, which is the consequence of their 

 habitual recurrence, may be considered as resulting from these two general 

 facts, the persistence of the impression made by them upon the sensorium, 

 and the consequent absence of a change in its state, when a sensory impression 

 is brought to it, which is of the same nature with one already registered there: 

 the degree in which the consciousness is excited, being dependent, as just 

 stated, not upon the absolute degree of the impressing cause, but upon the 

 amount of change which it produces in the sensorial apparatus. In this 

 respect there is a perfect conformity between the law of sensation, and that of 

 muscular contraction ; for stimuli which excite the latter, usually lose their 

 force in proportion to the frequency of their repetition. Indeed, both may be 

 considered as results of the more general laws of vitality; for the actions of 

 other tissues follow the same rule, as is shown by the tolerance that may be 

 gradually established in the system of medicinal agents, poisons, &c., which 

 would have at first produced the most violent effects, when given in the same 

 amount. 



308. It is through the medium of Sensation that we acquire a knowledge 

 of the material world around us ; and that its changes excite mental operations 

 in ourselves. The various kinds or modes of Sensation excite in us various 

 ideas regarding the properties of matter ; and these properties are known to 

 us only through the changes which they produce in the several organs. 

 Thus a man totally blind from birth can form no idea of the nature of light or 

 colours ; nor could one completely deaf have any just conception of musical 



* This fact is very well known in the manufacturing districts; where it is not at all 

 uncommon for a family to live in the immediate vicinity of a forge-hammer; and those 

 who are accustomed to the noise are unable to sleep anywhere else. 



