the _mental impulse_ to
persist is backed by a _permanent firm muscle structure_ that can stand
up as long as the mind needs the physical support.

[Sidenote: A Slump in Determination]

In contrast, the man who is only characteristically _determined_, but
who lacks _persistence_ in his determination, has developed just the
habit of hardening his muscles _for the time_ he is determined on doing
a particular thing. That does not exercise his muscles sufficiently to
make them firm _all_ the time, whether under tension or not.
Consequently his determination is likely to slump if his resolution is
subjected to a long strain. He does not possess muscular structure
sufficiently strong to support persistence in his determination.

_Habitual lack_ of firmness in the jaw muscles, as you know, results in
a sagging chin; which detrimentally affects the brain-mind center of
persistence. A man whose jaw habitually hangs loose may be capable of
great _determination_ for a while, but he is not _persistent in
character_. He might clench his teeth, stiffen his body, and plunge into
the surf to rescue a drowning person; but his first resolution to
effect the rescue would be weakened by the cold water and by fear. He
lacks the quality of the bulldog that will die rather than loose its
teeth from another dog's throat.

[Sidenote: Muscles Express and Impress Ideas]

The coordinated muscles _express_ the mental attitude, as we have
perceived; and equally they _impress_ the mind with _their_ attitude. If
you have a sagging chin, you are incapable of the mental bulldog grip of
persistence. So _tighten up your jaw muscles, and never let them hang
utterly loose_, if you are resolved to develop the characteristic of
"stick-to-it-iveness." _Begin_ with _muscle_ training, for your muscles
must be utilized to start the process of building up your brain-mind
center of persistence.

[Sidenote: Developing Perception]

When you train the particular sense muscles that transmit external
_impressions_ to a part