_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.
When you train the particular sense muscles that transmit external _impressions_ to a particular brain-mind unit (the same muscles that reflexively _express_ the ideas of that one part of your multiplex ego) you may be absolutely _sure_ of developing a particular related characteristic. For example, if you want to sharpen your _perceptive_ faculties so that you will see with the _eyes of your mind_ much more than the _ordinary_ man perceives, exercise your _physical_ eyes in taking snap-shots that you can see clearly in detail _with your imagination_ when you look away from an object after a glance at it. Try glancing at the furnishings of your room, then shut your eyes and construct a mental picture. When this is definitely clear to you, open your eyes. The reality will be very different from your imagined picture. But _sharpen your perceptive faculties_, develop a "camera eye;" then the reality will be exactly impressed on your mind. Witnesses in court often contradict one another, in all honesty, simply because their ability to perceive actualities is not highly developed. In consequence, they get false mental impressions of happenings or things they severally have seen.