120 PHYSIOLOGY. 



to labour under actual disease ; but their imaginations have 

 been excited by the influence of fear and superstition, and 

 the brain is accordingly excited to act in the same manner 

 as it would by the actual sight of some frightful object. 

 Thus a post will appear in the night, to be a robber ; a black 

 stump of a tree, a bear, or a tomb-stone, a ghost ; according 

 to the idea or apprehension at the time uppermost in the 

 mind. 



29. But though the brain is the seat of sensation, yet it 

 is very remarkable that it is not itself sensible. Wounds 

 in its substance do not seem to excite pain* The whole of 

 the cerebrum as well as the cerebellum has been pared away, 

 and yet the animal appeared to suffer no pain ; but as soon 

 as that portion of the base of the brain which seems to be 

 the commencement of the spinal marrow, is touched, con- 

 vulsions immediately follow. The posterior surface of the 

 spinal marrow is also highly sensible. This is the substance 

 of what is yet known on the subject of sensation. 



30. Voluntary motion, like sensation, has also its seat in 

 the brain. Though the muscles are the instruments of 

 motion, yet alfytheir voluntary motions are performed through 

 the influence of the brain, by an act of the will/ If the brain 

 is in a state of repose the body is at rest ; Jif the brain by 

 an act of violition sends a portion of nervous influence to a 

 voluntary muscle, it immediately contracts, and the limb or 

 organ to which it is attached, moves. All we are conscious 

 of in" this process, is, the act of the will and the motion, which 

 that act causes. The volition and the motion seem to be 

 at the same instant ; at least there is no perceptible interval 

 between them. 



31. But though the voluntary muscles are excited to action 

 through the agency of the brain, yet they are not always 

 under the control of the will. 'In many diseases of the 

 brain : in lock jaw, hydrophobia,;and other disorders where 

 the whole nervous system is in a state of irritation, the 

 muscles are excited to violent spasmodic contraction, alto. 



