SECT. XIV. PHYSIOLOGY. 117 
to one subject, a sensation of fatigue is felt; and 
when the ideas are turned into another direction, 
a sense of satisfaction assumes the place of un- 
easiness. The inference to be drawn from this 
circumstance is, that when any particular thought 
occupies the mind for a length of time, the fibres 
of a certain part of it are in an artificial state, 
and become relieved by employing a different 
part of the brain on a different subject. Long 
continued thought is not confined to mere lassi- 
tude of the brain; it is not unfrequently attended 
with convulsive motions of the muscular system. 
Every thing, therefore, tends to prove, that in 
any particular train of thoughts, or fixed ideas, 
there is a strain on some particular fibres of the 
brain. Analogy consequently supports the idea, 
that the motions of the white substance of the 
brain resemble, in some degree, the irritability of 
the muscles; and beyond analogy I dare not 
advance. 
y, 
