_ That is, each mind operation _through_
the brain _changes_ its physical structure in some degree. Mental effort
or relaxation increases or decreases the amount of blood in the brain.
When you have been using your mind very hard, your head "feels heavy,"
and it _is_ unusually heavy then on account of the extra amount of blood
weight. Even the temperature of the brain, particularly of that portion
of the brain which is especially functioning at a given moment, is
changed with every mental effort.
[Sidenote: Slow Muscles Slow Mind]
There is abundant scientific proof that the quality and quantity of
muscle, brain, and nerve (_physical_) activity in a particular
individual are accompanied by corresponding qualities and quantities of
_mental_ activity. That is, when a person's muscle action, nerve
response, and brain action are sluggish, his _mind_ also develops a
characteristic of slow action. And vice versa.
We say of a certain acquaintance that he has an alert mind. But his
"ego," or mental self, could not act quickly and alertly if his _brain_,
the physical instrument of his _mind_, did not receive and transmit
impressions swiftly to his mentality.
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