,
your so-called "brain" is made up of innumerable, distinct "brain
centers" which function quite independently of one another. No
particular unit requires help from any of the others in order to do its
especial work with full efficiency. _Each center attends only to its
specific business in your life_. It rests, or relaxes from activity,
when it has nothing to do; or when the particular muscles it governs are
not in use. And, of course, when a certain _brain_ center rests or is
inactive, its associated _mind_ center also rests or is inactive.

As already has been stated, the mind of a man is built up, _through_ the
brain instrument, by the _sense impressions_ transmitted to his
consciousness. In other words, _all he knows with his mind first came
into his mental capacity from outside impressions of things and ideas_.
The fewer the impressions that come into the mind through the brain, the
less does a man know. And only the impressions that come into a
_particular_ mind center develop _that_ center. (For example, the
development of keenest eyesight by many _optical_ impressions would not
affect at all a man's ability to discriminate among the tones of music,
would not give him "a good _ear_.")

[Sidenote: Weak or Undeveloped Centers]

It is evident, therefore, that if a _particular brain center_
temporarily or permanently is deprived of right and sufficient exercise
in transmitting sense impressions, _its coordinated mind center_ will be
stunted in its growth or starved for lack of mental food. This is why a
man is awkward in using his native tongue when he returns to the country
of his birth after a long residence among people of a different nation
where that language was not spoken. But a little exercise of his brain
in transmitting again the sound of his native tongue will quickly
stimulate his mind with the renewed supply of this particular mental
food to which it formerly was accustomed. In a few weeks he will use
the old language naturally; whereas another man, who never had spoken
it, would require years to build up such full knowledge from a start of
complete ignorance of the language.

Evidently, too, a _weak_, undeveloped brain center would be incapable of
receiving _strong_ mental impulses from its coordinated mind center, and
of transmitting them in full strength to the particular muscles governed
by that mind center. This is why, if a man's _brain center_ of courage
is undeveloped, even the most courage