THE SENSES. 513 



wounds caused by puncturing or cutting instruments may be felt, un- 

 accompanied by any sense of suffering. Similar observations have been 

 made in cases of paralysis, where the patient can sometimes perceive the 

 contact of foreign bodies without experiencing any painful sensation ; 

 or, on the other hand, the sense of pain may persist, while that of 

 touch is diminished or lost. Notwithstanding this apparent indepen- 

 dence of the necessary conditions for the sensation of pain, it is trans- 

 mitted by the same nerves which convey ordinary impressions ; and 

 those which, like the branches of the fifth pair, are endowed with the 

 most acute tactile sensibility, are also capable, in injury or disease, of 

 giving rise to the severest painful impressions. 



Mode of Action of the Senses in general. There are certain facts 

 connected with general sensibility, and common to the operation of all 

 the senses, which are of sufficient importance to be considered by them- 

 selves. 



In the first place, an impression of any kind, made upon a sensitive 

 organ, remains for a time after the removal of its exciting cause. 

 The excitement produced in the nerve fibres has a certain persistence, 

 which is longer in some cases than in others, but which exists to some 

 extent in all. The pressure of a foreign body upon the skin, especially 

 if somewhat forcible and continued, is felt for a perceptible interval after 

 the foreign body is removed. The sense of cold or warmth, from the 

 contact of ice or heated liquids, lasts more or less after their application 

 is discontinued. Even for the senses of sight and hearing, the same 

 fact may be verified; and the duration of the nervous impression, though 

 very short, has been found susceptible of measurement. 



Secondly, the organs of sense after a time become accustomed to a 

 continued impression, so that it is no longer perceived. If a uniform 

 pressure be exerted on any part of the body, it at last fails to attract 

 notice, and we become unconscious of its existence. In order to again 

 excite a sensation, the pressure must be increased or diminished, or 

 changed in locality or direction. 



The olfactory apparatus also becomes habituated to odors, whether 

 agreeable or disagreeable. A continuous and uniform sound, like the 

 rumbling of carriages, or the hissing of boiling water, becomes after a 

 time inaudible ; but when the sound ceases our attention is excited 

 by the change. The senses, accordingly, receive their stimulus as 

 much from the variation and contrast of external impressions as from 

 the impressions themselves. 



Sense of Taste. 



The sense of taste is, in some measure, intermediate in character 

 between general and special sensibility. The organ by which it is 

 exercised is furnished with vascular and nervous papillae analogous to 

 those of the general integument. Its mucous membrane is also endowed 

 with general sensibility. Although it is highly probable that certain 

 minute formations in its epithelial layer, known as " taste buds," may 



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