This generally, but not invariably, causes a slight angular bend
in the lower lip on both sides, near the corners of the mouth.
The result of the upper and lower lip being thus acted on is that
the mouth assumes a squarish outline. The contraction of the depressor
muscle is best seen in infants when not screaming violently,
and especially just before they begin, or when they cease to scream.
Their little faces then acquire an extremely piteous expression,
as I continually observed with my own infants between the ages
of about six weeks and two or three months. Sometimes, when they
are struggling against a crying-fit, the outline of the mouth
is curved in so exaggerated a manner as to be like a horseshoe;
and the expression of misery then becomes a ludicrous caricature.
The explanation of the contraction of this muscle, under the influence
of low spirits or dejection, apparently follows from the same
general principles as in the case of the obliquity of the eyebrows.
Dr. Duchenne informs me that he concludes from his observations,
now prolonged during many years, that this is one of the facial muscles
which is least under the control of the will.
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