156 PHILOSOPHY OF ZOOLGGY. 
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 oF 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. 
The 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, imme 
