The physiological action is in this case clear.  The vibration of the nerve caused by the tickling travels from the foot to the appropriate centre in the spinal marrow, and here gives rise to, or is switched off as, a motor impulse travelling back to the muscles of the leg, causing them to contract.  In the injured patient the nervous impulse cannot reach the brain, the seat of consciousness, and hence this is not awakened.  Normally consciousness does result in a majority of such cases, but only after the beginning or completion of the appropriate action.  Yet the movements of our internal organs, intestine and heart, go on continually, and in health we remain entirely unconscious of their action.

But reflex actions may be anything but simple.  We walk and talk, and write or play the piano without ever thinking of a single muscle or organ.  Yet we had once to learn with much effort to take each step or frame each letter.  Thus actions, originally conscious and intended, easily become reflex; often repeated the brain leaves their control to the lower centres.  We often say, “I did not intend to do that; I could not help it.”  We forget that this excuse is our worst condemnation.  It is a confession that we have allowed or encouraged a habit to wear a groove from which the wheels of our life cannot escape.  The essential characteristic of reflex action is therefore that from beginning to completion it goes on independently of consciousness.

2.  Instinct.  This is a much-abused word.  It is frequently applied to all the mental actions of animals without much thought or care as to its meaning.  Let us gain a definition from the study of a typical case lest we use the word as a cloak for ignorance or negligent thoughtlessness.  Watch a spider building its wonderful geometrical web.  The web is a work of art, and every motion of the spider beautifully adapted to its purpose.  But the spider is not therefore necessarily an artist.  Let us see of how much the spider is probably conscious, remembering that our best judgment is but an inference.  We have good reason to believe that she is conscious of the stimulus to action, hunger.  She may be, probably is, conscious of the end to be attained—­to catch a fly for her dinner.  She seems conscious of what she is doing.  In all these respects this differs from reflex action.  But she is probably unconscious of the exact fitness of the means to the end.  We do not believe that she has adopted the geometrical pattern, because she has discovered or calculated that this will make the closest and largest net for the smallest outlay of labor and material.  Furthermore the young spider builds practically as good a web as the old one.  She has inherited the power, not developed or gained it by experience or observation.  And all the members of the species have inherited it in much the same degree of perfection.