598 THE SENSES. 



fibres which are distinct from those endowed with other forms of general 

 sensibility, but it is certain that it may be preserved or lost independently 

 of the other varieties. The anaesthesia which is produced by the inha- 

 lation of ether or chloroform may be carried to such a point that the 

 capacity for feeling pain is abolished, while tactile sensibility remains ; 

 so that the wounds caused by puncturing or cutting instruments may 

 be felt, though unaccompanied by any sense of suffering. Similar ob- 

 servations have been made in cases of paralysis where, it is well known, 

 the patient may perceive the contact of foreign bodies or the prick of 

 a pin, but at the same time may not experience from them any painful 

 sensation ; or, on the other hand, the sense of pain may persist in the 

 affected parts, while that of touch is diminished or lost. 1 Notwith- 

 standing this apparent independence of the immediate conditions neces- 

 sary for the sensation of pain, it is transmitted by fibres of the same 

 nerves, belonging to the cerebro-spinal system, which convey ordinary 

 impressions ; and nerves which are endowed with the most acute tactile 

 sensibility, like the branches of the fifth pair, are also capable, when 

 irritated by injury or disease, of giving rise to the severest painful im- 

 pressions. 



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

 connected with the exercise of general sensibility which are also com- 

 mon to the operation of all the senses, and which are of sufficient im- 

 portance to be considered by themselves. 



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

 organ, remains for a short time after the removal of its immediate 

 cause. The state of excitement produced in the nervous expansions 

 and fibres has a certain degree of persistence, which is longer in duration 

 for some organs than for others, but which exists in some degree for 

 all. The sense of simple contact or pressure of a foreign body upon 

 the skin, especially if it be somewhat forcible and continued, remains 

 for a perceptible interval after the foreign body is removed. The feel- 

 ing of cold or warmth, from the application of ice or heated liquids, 

 lasts more or less after the application is discontinued. Even in the 

 case of sight and hearing it is easy to verify the same fact ; and the 

 duration of continuance of the nervous impression, though very short, 

 has been found in many instances susceptible of measurement. 



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

 tinued impression, so that the} 7 no longer perceive its existence. If a 

 uniform pressure be exerted upon any part of the body, the compressing 

 substance at last fails to excite sensation, and we remain unconscious of 

 its existence. In order to attract our notice, it is necessary to increase 

 or diminish the pressure or to change its locality or direction. 



The olfactory apparatus also becomes habituated to odors, whether 

 agreeable or disagreeable in their nature, in the confined air of a close 



1 Brown-Sequard, Physiology and Pathology of the Central Nervous System. 

 Philadelphia, 1860, pp. 97, 126. Hammond, Diseases of the Nervous System. 

 New York, 1871, p. 82. 



