letely paralyze a finger, without impairing
your sight at all. Either the mind center that governs a finger, or the
set of muscles in that finger can be affected without necessarily
reacting upon any _other_ mind center or any _other_ set of muscles.

[Sidenote: Interrelation Of the Ego And Physical Man]

_But if the mind center that governs a certain set of muscles is
affected, that set of muscles also is directly affected and at once.
Likewise if anything happens to a particular set of muscles, the
reaction is instantly transmitted to its associated mind center through
the "direct wire" nerves and brain center which particularly serve that
part of the mind_.

Great scientists have studied mental and physical phenomena in
inter-relation and have learned certain facts. For example, it is known
that "the mind" not only affects the general functions of "the body,"
but also the rate of bodily activity and the chemistry of body tissues.
Long-continued hard thinking actually does "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 a