OF SENSATION IN GENERAL. 851 



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 unpleasant feelings, when the individual 

 is again subjected to the impression. Again, it is a well-known fact that im- 

 pressions made upon the organs of sense continue to affect the consciousness 

 for a time after the cause of the impression has ceased : it is in this man- 

 ner 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. And there seems reason to believe that sensorial changes of frequent 

 recurrence produce a modification in the nutrition of the Sensorium itself, which 

 grows to them, as it were, just as the Cerebrum may be considered as growing 

 to the mode in which it is habitually exercised ( 807); for not only would the 

 production of such a modification be quite in accordance with the general phe- 

 nomena of Nutrition, 1 but we can scarcely otherwise explain the progressive form- 

 ation of that connection between sensorial changes and motor actions which 

 gives rise to the "secondarily automatic" movements ( 749). Hence it seems 

 reasonable to attribute that diminution in the force of Sensations which is the 

 consequence of their habitual recurrence, to the want of such a change in the 

 condition of the Sensorium as is needful to produce an impression on the con- 

 sciousness ; the effects which they at first induced being no longer experienced m 

 the same degree, when the structure of the part has accommodated itself to 

 them. 



854. It is curious, also, that the feelings of Pain or Pleasure, which unac- 

 customed sensations excite, are often exchanged for each other, when the system 

 is habituated to them ; this is especially the case, in regard to impressions com- 

 municated through the organs of Smell and Taste. There are many articles in 

 common use among mankind such as tobacco, fermented liquors, &c., the use 

 of which cannot be said to produce a natural enjoyment, since they are at first 

 unpleasant to most persons; and yet they first become tolerable, then agreeable, 

 and at last the want of them is felt as a painful privation, and the stimulus must 

 be applied in an increasing degree, in order to produce the usual effect . 



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

 the material world around us, by the psychical operations which its changes 

 excite 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 (786). 

 It is well known that instances exist, in which, from some imperfection of the 

 organization, there is an incapacity for distinguishing colors or musical tones, 

 whilst there is no want of sensibility to light or sound ; and that some persons 

 are naturally endowed with a much greater range of the sensory faculties than 

 others possess. Hence it does not seem at all improbable that, there are pro- 

 perties of matter, of which none of our senses can take immediate cognizance ; 

 and which other beings might be formed to perceive, in the same manner as we 

 are sensible to light, sound, &c. Thus many animals are affected by atmo- 

 spheric changes in such a manner that their actions are regarded by Man as 

 indications of the probable state of the weather ; and the same is the case in a 

 less degree with some of our own species, who are peculiarly susceptible of the 

 same influences. Now the most universal of all the qualities or properties of 



' We have a remarkable exemplification of this in the tolerance which may be gradually 

 established in the system for various toxic agents, especially for such as particularly affect 

 the Nervous substance, such as Opium or Alcohol. It seems impossible to explain this 

 tolerance on any other hypothesis than that of the alteration of the nutrition of the 

 tissue by repeated doses, so that no further change can be produced by the quantity ori- 

 ginally taken. 



