02 CONSCIOUSNESS AND SENSATION. 
extremity of the nerve along which the impression 
has been conducted. Further, let it be recollected 
that nervous impression is nothing but a physical 
condition, some of the peculiarities of which have 
been laid bare by experimenters, and which is 
capable of affecting any of the nervous elements, 
viz., nerves, both motor and sensory, and nerve 
corpuscles, 
In speaking of nervous impression thus defined, 
we deal with a matter of fact, although we are not 
thoroughly acquainted with its details; but in 
stating the doctrine of the modus operandi of 
sensation, we have merely to do with a theory. 
This theory is, however, a physiological as well 
as psychological theory, and involves the considera- 
tion of the functions of nervous structures, as well 
as the laws to which consciousness is subject ; and 
it is the more important to point this out, because 
the physiologist is liable to think that where con- 
sciousness is involved physiology cannot be con- 
cerned, whereas the doctrine of sensation, although 
it relates to a matter on the psychological frontier, 
is arrived at from physiological data; and it is 
because it is, as will be shewn, at variance with 
other anatomical and physiological data that it 
requires alteration. 
It may be taken as certain that when a nervous 
