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, 

 lowever, peculiar difficulties attending this method of in- 

 stigating the subject. Between the different parts of 

 ic nervous system, there is an intimate connection by 

 ins of what are termed sympathies. Hence, when we 

 fitness 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. 



& The Nerves. In considering the action of the nerves, 

 it is necessary to attend to their effects in 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 EVERARD HOME'S " Observations on the Functions of the Brain " 

 Phil. Trans. 1814, p. 469. 



