Every child has made the attempt to hold the foot still while it is tickled, and has failed; and probably there is scarcely any one who has not vainly tried to avoid winking, when a hand has been suddenly passed before the eyes.  These examples of muscular movements which occur independently of the will, or in spite of it, illustrate what physiologists call reflex-action; as likewise do sneezing and coughing.  To this class of cases, in which involuntary motions are accompanied by sensations, has to be added another class of cases, in which involuntary motions are unaccompanied by sensations:—­instance the pulsations of the heart; the contractions of the stomach during digestion.  Further, the great mass of seemingly-voluntary acts in such creatures as insects, worms, molluscs, are considered by physiologists to be as purely automatic as is the dilatation or closure of the iris under variations in quantity of light; and similarly exemplify the law, that an impression on the end of an afferent nerve is conveyed to some ganglionic centre, and is thence usually reflected along an efferent nerve to one or more muscles which it causes to contract.

In a modified form this principle holds with voluntary acts.  Nervous excitation always tends to beget muscular motion; and when it rises to a certain intensity, always does beget it.  Not only in reflex actions, whether with or without sensation, do we see that special nerves, when raised to a state of tension, discharge themselves on special muscles with which they are indirectly connected; but those external actions through which we read the feelings of others, show us that under any considerable tension, the nervous system in general discharges itself on the muscular system in general:  either with or without the guidance of the will.  The shivering produced by cold, implies irregular muscular contractions, which, though at first only partly involuntary, become, when the cold is extreme, almost wholly involuntary.  When you have severely burnt your finger, it is very difficult to preserve a dignified composure:  contortion of face, or movement of limb, is pretty sure to follow.  If a man receives good news with neither change of feature nor bodily motion, it is inferred that he is not much pleased, or that he has extraordinary self-control—­either inference implying that joy almost universally produces contraction of the muscles; and so, alters the expression, or attitude, or both.  And when we hear of the feats of strength which men have performed when their lives were at stake—­when we read how, in the energy of despair, even paralytic patients have regained for a time the use of their limbs, we see still more clearly the relations between nervous and muscular excitements.  It becomes manifest both that emotions and sensations tend to generate bodily movements and that the movements are vehement in proportion as the emotions or sensations are intense.[2]