226 OF SENSATION, AND THE ORGANS OF THE SENSES. 



manifest the same discomfort, even when the circulation is in a feeble, rather 

 than an excited state. It is remarkable that the sensibility of the mucous 

 membranes lining the internal organs, is less exalted by the state of inflamma- 

 tion, than is that of most other parts ; and in this arrangement we may trace 

 a wise and beneficent provision ; since, were it otherwise, the functions neces- 

 sary to life could not be performed without extreme distress, with a very mo- 

 derate amount of disorder in the viscera. If a joint is inflamed, we can give 

 it rest ; but to the actions of the alimentary canal we can give little voluntary 

 respite. 



305. The feelings of Pain or Pleasure, which are connected with particular 

 sensations, cannot, (for the most part at least) be explained upon any other 

 principle than that of the necessary association of these feelings, by an original 

 law of our nature, with the sensations in question. As a general rule it may 

 be stated, that the violent excitement of any sensation is disagreeable, even 

 when the same sensation in a moderate degree may be a source of extreme 

 pleasure. This is the case alike with those impressions which are communi- 

 cated through the organs of sight, hearing, smell, and taste, as with those that 

 are received through the nerves of common sensation ; and there can be no 

 doubt that the final cause of the association of painful feelings with such violent 

 excitement, is to stimulate the individual to remove himself from what would 

 be injurious in its effects upon the system. Thus, the pain resulting from 

 violent pressure on the cutaneous surface, or from the proximity of a heated 

 body, gives warning of the danger of injury, and excites mental operations 

 destined to remove the part from the influence of the injurious cause ; and 

 this is shown by the fact, that loss of sensibility is frequently the indirect occa- 

 sion of severe lesions, the individual not receiving the customary intimation 

 that an injurious process is taking place. Instances have occurred, in which 

 severe inflammation of the membrane lining the air-passages has resulted 

 from the effects of ammoniacal vapours, introduced into them during a state of 

 syncope, the patient not receiving that notice of the irritation which would, 

 in an active condition of his nervous system, have prevented him from inhaling 

 the noxious agent. It is a general rule, with regard to all sensations, however, 

 that their intensity is much affected by habit; being greatly diminished by 

 frequent and continual repetition. This is partly due to the different degree 

 of attention which the sensations excite in the mind ; but there are many facts 

 which lead to the conclusion, that it is chiefly to be attributed to a change in 

 the degree in which, after frequent repetition, they impress the consciousness 

 itself. Thus, most persons are readily awoke from a sound sleep by a trifling 

 noise, if the sound be of a kind which they are unaccustomed to hear ; but 

 after a few repetitions, the sound loses its effect, unless its intensity be increased. 

 Of this, every one has had experience, who has occasionally made use of an 

 alarum to arouse him for a few mornings in succession. It is curious also, 

 that the feelings of pain or pleasure, which unaccustomed sensations excite, 

 are often exchanged for each other, when the system is habituated to them ; 

 this is especially the case in regard to impressions communicated 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 it is at first unpleasant to most 

 persons ; and yet it first becomes 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. 



306. The general law, that sensations are blunted by frequent repetition, 

 may perhaps be connected with certain other general facts, which lie under 

 the observation of every one. It is well known that the vividness of sensa- 

 tions depends rather on the degree of change which they produce in the 



