[Sidenote:  Brain Development]

A particular brain center, of course, will be strengthened both by the food of sense impressions it is given, and by the exercise of handling messages to and from the mind.  The brain, or physical instrument of the mind, is like an intermediary or go-between of the ego and the body.  It is of the utmost importance that it should do its work efficiently.  Otherwise the full capability of neither the outer nor the inner man can be utilized.

If Brown passes something to Jones, who passes it along to Smith; then Smith passes it back to Jones to be re-passed to Brown—­Jones, the middle agent of transmission or handling instrument, whom we are comparing to the brain, might be so awkward, slow, and inefficient as a go-between that the possible ability of Brown and Smith in passing would be nullified or greatly hampered.  But if the inefficiency of Jones is blamable to his inexperience, it evidently can be changed to efficiency by sufficient right exercise in passing.  The more of that sort of work he does, in either direction, the better passer will Jones become.

His exercise, however, must be in passing things, if passing capability is to be developed.  He would not become a better and quicker passer by any amount of exercise in taking things apart, or in inspecting things—­wholly dissimilar functions.

[Sidenote:  Training in Passing]

Moreover, Jones would not become an expert passer of glassware as a result of practice in passing bricks, for the two kinds of things are not handled alike.  Indeed, the man accustomed to passing bricks might be more likely to break glassware than another man who previously had no particular skill in passing anything.  The expert brick-passer would be apt to forget sometimes that he was passing glass.  His muscles might treat the fragile ware with the rough habit acquired in passing bricks.

Plainly, discriminative-restrictive methods of training are required to perfect capability in any particular kind of physical passing; however much skill in general passing may have been developed.  If Jones should become expert in passing pails of liquid, he would nevertheless need to train himself anew in order to pass frozen liquid efficiently in the form of cakes of ice.  And, to particularize still more, it would be necessary for him to learn how to pass different liquids.  Water and thick molasses in pails should not be handled alike.

Similarly the various brain centers, as passers of different sense impressions and mental reflexes in and out, require, each of them—­like Jones—­the specific exercises that will develop their several particular abilities.  The individual brain unit (as of courage, memory, judgment, etc.) is strengthened only by handling the in and out business of its coordinated muscles and mind center.  Also, while a particular set of muscles and coordinate mind center are strengthening their brain center by the exercise they give it, they are both being developed by the same exercise of passing along sense impressions and thoughts to each other through the brain—­like Smith and Brown.