We must admit that the withdrawal of stimuli, or their monotonous repetition, are factors which do undoubtedly stand out as primary causes of sleep.  We may suppose, if we like, that consciousness depends upon a certain rate of vibration which takes place in the brain structure.  This vibration is maintained by the stimuli of the present, which awaken memories of former stimuli, and are themselves at the same time modified by these.  By each impulse streaming into the brain from the sense organs, we can imagine the structure of the cerebral cortex to be more or less permanently altered.  The impulses of the present, as they sweep through the association pathways, arouse memories of the past; but in what way this is brought about is outside the range of explanation.  Perhaps an impulse vibrating at a certain rate may arouse cells or fibrils tuned by past stimuli to respond to this particular rate of vibration.  Thus may be evoked a chain of memories, while by an impulse of a different rate quite another set of memories may be started.  Tracts of association are probably formed in definite lines through the nervous system, as during the life of a child repeated waves of sense impulses beat against and overcome resistances, and make smooth pathways here and there through the brain structure.  Thus may be produced growth of axons in certain directions, 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  We may suppose that when all external stimuli are withdrawn, or the brain soothed by monotony of gentle repetition, and when the body is placed at rest, and the viscera are normal and give rise to no disturbing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.