140 OF THE FUNCTIONS OF 



ception or by means of sympathies (56) to give occasion 

 to reaction. 



221. Experiment and observation put these functions 

 of the nervous system beyond the reach of controversy. 

 To unfold their nature is difficult indeed. (H) 



222. Most opinions on this subject may be divided 

 into two classes. The one regards the action of the 

 nervous system as consisting in an oscillatory motion : 

 The other ascribes it to the motion of a certain fluid, 

 whose nature is a matter of dispute, by some called 

 animal spirits * and supposed to run in vessels, by 

 others conceived to be a matter analogous to fire, to 

 light, to a peculiar ether, to oxygen, to electricity, or 

 to magnetism. 



223. Although I would by no means assent to either 

 of these opinions, I may be allowed to observe, that most 

 arguments brought by one party against the hypothesis 

 of the other, must necessarily be rude in proportion to 

 the subtlety of the oscillations (if such exist) of the 

 nerves or the nervous fluid. 



224. These two hypotheses may, perhaps, be united 

 by supposing a nervous fluid thrown into oscillatory 

 vibrations by the action of stimulants. 



225. The analogy between the structure of the brain 

 and some secreting organs, favours the belief of the 

 existence of a nervous fluid. But tubes and canals are 

 evidently no more requisite for its conveyance, than 

 they are requisite in bibulous paper or any other matter 

 employed for filtering. 



The opinion receives much weight from the resem- 



* Sec Michclitz, Scrn-tinium Hypotheseos Spirit ttum Animalwm, I'rag. 

 1782. 8vo. 



