n, that there are various humiliations to others which
produce in us anything but laughter; and, in the second place, it does
not apply to the many instances in which no one's dignity is implicated:
as when we laugh at a good pun. Moreover, like the other, it is merely a
generalisation of certain conditions to laughter; and not an explanation
of the odd movements which occur under these conditions. Why, when
greatly delighted, or impressed with certain unexpected contrasts of
ideas, should there be a contraction of particular facial muscles, and
particular muscles of the chest and abdomen? Such answer to this
question as may be possible can be rendered only by physiology.

* * * * *

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