oes "wear a man out." It consumes
blood and brain tissue. It "slows him up." It may impair his digestion
and appetite. We all know these things, but the scientists know just
_why_ we feel _physically_ tired after using only our _minds_.

They have learned also that every activity of the _mind_ has a direct
effect on the _brain substance._ 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. The _brain_ does not _think_. It
is as purely physical as any other part of the body. It just _handles_,
or transmits in and out, to and from the _mind_, the various impressions
sent _in_ by different sense muscles, and the mental reflexes or
impulses sent _out_ by the innumerable mind centers. Your mind works
_through_ your brain. Of course, therefore, the quality and quantity of
mental work _you_ are capable of doing are limited by the degree of
handling-or-transmitting _efficiency_ characteristic of _your_
particular brain structure.

[Sidenote: Value of Practical Psychology]

Any interference with the _brain_ quality or quantity of an individual
naturally interferes with his normal _mental_ functioning. If a
particular part of a man's brain is injured, the associated mind center
is harmed likewise and his mental _quality_ is affected in proportion.
Should a certain portion of his brain be cut out, the total _quantity_
of his ment