92 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 



