Jt 
NERVOUS SYSTEM. “* 155 
In order to arrive at accurate conclusions on this subject, 
it would be of importance to examine the brain in a variety 
of species, with whose manners we are intimately ac- 
quainted. But as we know the characters or dispositions of 
few animals, with any degree of accuracy, except those 
which have been domesticated, our observations, for the 
present, must be imperfect, and our general conclusions pre- 
mature. 
Attempts have been made to determine the functions of 
the different parts of the brain, by attending to the effects 
of disease or injuries of its different parts*. There are, 
however, peculiar difficulties attending this method of in- 
vestigating the subject. Between the different parts of 
the nervous system, there is an intimate connection by 
means of what are termed sympathies. Hence, when we 
witness any derangement of the functions of the brain, it is, 
in many cases, difficult to determine the seat of the disease, 
or the various causes which combine in order to produce 
the effects. 
2. The Nerves.—In considering the action of the nerves, 
it is necessary to attend to their effectsin the production of 
sensation, of voluntary, and of involuntary motion. 
a. Sensation.—In general, sensation is produced by an 
impression made on the external organs, and conveyed by 
the nerves to the common sensorium. That this is the or- 
dinary course of sensation, is rendered obvious by experi- 
ment. We can protect the external organs from receiving 
the impression, or when received, stop its progress along 
the nerves by a ligature or section. It may therefore be 
asked, has any substance entered the nerves from the ex- 

™“ See an enumeration of a number of very remarkable cases of this 
kind, in Sir Everarp Homer’s ‘‘ Observations on the Functions of the Brain “ 
Phil. Trans, 1814, p. 46% 
