PHYSIOLOGY. 25 



Note. This is an example of the most ordinary case ; but 

 we do not mean to say it is the only case of communication be- 

 tween the different parts of the nervous system. " I do not 

 assert that every communication between the parts of the nerv- 

 ous system is with the interposition of both sensation and voli- 

 tion ; for many cases occur in which we are conscious of neither ; 

 it may be merely by a mechanical communication, in conse- 

 quence of the peculiar organization of all the several parts of 

 the nervous system." 



XXXIII. As the impulse of bodies on the sentient ex- 

 tremities of a nerve does not occasion any sensation, unless the 

 nerve between the sentient extremity and the brain be free 

 (XXIX. 3.); and as, in like manner, volition does not produce 

 any contraction of muscles, unless the nerve between the brain 

 and muscle be also free, we conclude, from both these facts, 

 that sensation and volition, so far as they are connected with 

 corporeal motions, are functions of the brain alone ; and we pre- 

 sume, that sensation arises only in consequence of external im- 

 pulse producing motion in the sentient extremities of the nerves, 

 and of that motion's being thence propagated along the nerves 

 to the brain ; and, in like manner, that the will operating in 

 the brain only, by a motion begun there, and propagated along 

 the nerves, produces the contraction of muscles. 



" Sensation and volition, as far as they are connected with 

 corporeal motions, are functions of the brain alone. This is 

 put here with a view to a controversy, which has long subsisted 

 in the schools of physic, viz. What is the seat of the soul ? 

 With many it is considered as diffused over the whole system, 

 so that it perceives in the eye, and operates in the muscles ; while 

 others say that it is immediately and only connected with the 

 brain, which has therefore been called the sensorium commune. 

 Another statement of the question is, whether the soul occasion- 

 ally acts in every part of the body, or if there is a sensorium 

 commune with which alone it is connected ? I maintain that 

 there is a sensorium commune, and that this is the brain taken 

 in its largest extent, the cerebrum, cerebellum, &c. and I have 

 concluded this from the facts alleged. This may be illustrated 

 with regard to both sensory and motory nerves, but it is clearest 

 with respect to the latter. If you apply a ligature upon such a 



