514 PRACTICE OF PHYSIC. 



in a manner we do not clearly understand, in every part of the 

 medullary substance of the brain and nerves, and which in a 

 living and healthy man is capable of being moved from every 

 one part to every other of the nervous system. 



MDXLII. With respect to this power, we have pretty clear 

 proof that it frequently has a motion from the sentient extremi- 

 ties of the nerves towards the brain, and thereby produces sen- 

 sation ; and we have the same proof that in consequence of voli- 

 tion, the nervous power has a motion from the brain into the 

 muscles or organs of motion. Accordingly, as sensation excites 

 our intellectual operations, and volition is the effect of these, 

 and as the connexion between sensation and volition is always 

 by the intervention of the brain and of intellectual operations ; 

 so we can hardly doubt, that these latter depend upon certain 

 motions, and the various modification of these motions in the 

 brain. 



MDXLIII. To ascertain the different states of these mo- 

 tions may be very difficult ; and physicians have commonly con- 

 sidered it to be so very mysterious that they have generally 

 despaired of attaining any knowledge with regard to it : but I 

 consider such absolute despair, and the negligence it inspires, 

 to be always very blamable ; and I shall now venture to go 

 some length in the inquiry, hoping that some steps made with 

 tolerable firmness may enable us to go still further. 



MDXLIV. To this purpose, I think it evident, that the 

 nervous power, in the whole as well as in the several parts 

 of the nervous system, and particularly in the brain, is at dif- 

 ferent times in different degrees of mobility and force. To 

 these different states, I beg leave to apply the terms of Excite- 

 ment and Collapse. To that state in which the mobility and 

 force are sufficient for the exercise of the functions, or when 

 these states are any way preternatiirally increased, I give the 

 name of Excitement ; and to that state in which the mobility 

 and force are not sufficient for the ordinary exercise of the func- 

 tions, or when they are diminished from the state in which they 

 had been before, I give the name of Collapse. I beg, however, 

 it may be observed, that by these terms I mean to express mat- 

 ters of fact only ; and without intending, by these terms, to 

 explain the circumstance or condition, mechanical or physi- 



