In the action of sucking, the lips of the infant are closed around the
nipple of its mother, till he has filled his stomach, and the pleasure
occasioned by the stimulus of this grateful food succeeds. Then the
sphincter of the mouth, fatigued by the continued action of sucking, is
relaxed; and the antagonist muscles of the face gently acting, produce the
smile of pleasure: as cannot but be seen by all who are conversant with
children.

Hence this smile during our lives is associated with gentle pleasure; it is
visible in kittens, and puppies, when they are played with, and tickled;
but more particularly marks the human features. For in children this
expression of pleasure is much encouraged, by their imitation of their
parents, or friends; who generally address them with a smiling countenance:
and hence some nations are more remarkable for the gaiety, and others for
the gravity of their looks.

5. _Of Anger._

The actions that constitute the mode of fighting, are the immediate
language of anger in all animals; and a preparation for these actions is
the natural language of threatening. Hence the human creature clenches his
fist, and sternly surveys his adversary, as if meditating where to make the
attack; the ram, and the bull, draws himself some steps backwards, and
levels his horns; and the horse, as he most frequently fights by striking
with his hinder feet, turns his heels to his foe, and bends back his ears,
to listen out the place of his adversary, that the threatened blow may not
be ineffectual.

6. _Of Attention._