156 PHILOSOPHY OF ZOOLOGY. 



ternal object ? The answer is obtained from an attention to 

 a well established fact, that people who have had their 

 limbs amputated, often experience sensations indicating the 

 existence of the lost organs, and the presence of external 

 objects making an impression upon these. Here is a sen- 

 sation conveyed to the sensorium without an impression on 

 the external organ, and arising, obviously, from a change 

 in the condition of the remaining part of the nerve. The 

 same fact demonstrates, that sensation is not produced by 

 a vibration of the nerves, arising from the concussion of 

 external objects, for it is excited without the external im- 

 pression or concussion. 



A sensation may likewise be produced, in certain cases, 

 by means widely different from those by which, in ordi- 

 nary cases, it is excited. Thus a blow on the eye, or the 

 contact of two pieces of metal, zinc and copper for instance, 

 one piece being placed under the upper lip, and another 

 under the tongue, make us perceive a flash, in the same 

 manner as if light had really struck the eye. This can only 

 take place in consequence of a change in the optic nerve, 

 similar to that which light itself produces. 



Tlie susceptibility of the sensitive faculty for receiving 

 impressions, is liable to considerable variations, depending 

 on the influence exercised over it by different circumstan- 

 ces. Thus, different medicines increase or diminish its sen- 

 sibility, and inflammation frequently heightens it to a painful 

 degree. In these cases, it appears obvious, that the causes 

 which are considered as in operation, produce a change in 

 the substance of the nerve, or its connections. 



But the most obvious changes which take place in the 

 sensitive faculty, are indicated by diminished energy, in 

 consequence of continued action. This state of fatigue or 

 exhaustion, must be known to every one who has attended 

 to the sensation produced by a feeble impression, mime- 



