of cells latent and
functional in the central system is early fixed. At any age this number
is accordingly represented by the granules as well as by the cells which
have already undergone further development. During growth the proportion
of developed cells increases, and sometimes, owing to the failure to
recognize potential nerve cells in the granules, the impression is
carried away that this increase implies the formation of new elements.
As has been shown, such is not the case."[1]

DEVELOPMENT OF NERVE FIBERS.--The nerve _fibers_, no less than the
cells, must go through a process of development. It has already been
shown that the fibers are the result of a branching of cells. At birth
many of the cells have not yet thrown out branches, and hence the fibers
are lacking; while many of those which are already grown out are not
sufficiently developed to transmit impulses accurately. Thus it has been
found that most children at birth are able to support the weight of the
body for several seconds by clasping the fingers around a small rod, but
it takes about a year for the child to become able to stand. It is
evident that it requires more actual strength to cling to a rod than to
stand; hence the conclusion is that the difference is in the earlier
development of the nerve centers which have to do with clasping than of
those concerned in standing. Likewise the child's first attempts to feed
himself or do any one of the thousand little things about which he is so
awkward, are partial failures not so much because he has not had
practice as because his nervous machinery connected with those movements
is not yet developed sufficiently to enable him to be accurate. His
brain is in a condition which Flechsig calls "unripe." How, then, shall
the undeveloped cells and system ripen? How shall the undeveloped cells
and fibers grow to full maturity and efficiency?

2. DEVELOPMENT OF NERVOUS SYSTEM THROUGH USE

IMPORTANCE OF STIMULUS AND RESPONSE.--Like all other tissues of the
body, the nerve cells and fibers are developed by judicious use. The
sensory and association centers require the constant stimulus of nerve
currents running in from the various end-organs, and the motor centers
require the constant stimulus of currents running from them out to the
muscles. In other words, the conditions upon which both motor and
sensory development depend are: (1) A rich environment of sights and
sounds and tastes and smells, and everythin