and synapses of this cell with that. If the same stimulus be often
repeated, the synapses between groups of cells may become permanent. A
memory, a definite line of action which is manifested by a certain
muscular response, may thus become structurally fixed. If the stimulus
be not repeated, the synapses may be but temporary, and the memory
fade as the group of cells is occupied by a new memory of some more
potent sense stimulus. Many association tracts and synapses are laid
down in the central nervous system when the child is born. These are
the fruits of inheritance, and by their means, we may suppose,
instinctive reflex actions are carried out.

So long as the present stimuli are controlled by past memories and are
active in recalling them, so long does consciousness exist, and the
higher will be the consciousness, the greater the number and the more
intense the character of the memories aroused. We may suppose that
when all external stimuli are withdrawn, or the brain soothed by
monotony of gentle repetition, and when the body is placed at rest,
and the viscera are normal and give rise to no disturbing sensations,
consciousness is then suspended, and natural sleep ensues. Either
local fatigue of the muscles, or of the heart, or ennui, or exhaustion
of some brain center usually leads us to seek those conditions in
which sleep comes. The whole organism may sleep for the sake of the
part. To avoid sleeplessness, we seek monotony of stimulus, either
objective or subjective. In the latter case, we dwell on some
monotonous memory picture, such as sheep passing one by one through a
gap in the hedge. To obtain our object, we dismiss painful or exciting
thoughts, keep the viscera in health, so that they may not force
themselves upon our attention, and render the sense organs quiet by
seeking darkness, silence and warmth.--L.H., in Nature.