He was a person of very striking aspect, with a white, lofty, and impending brow, large, brown, melancholy eyes, and a mouth which, unless when he forcibly compressed it, was apt to be tremulous, expressing both nervous sensibility and a vast power of self-restraint. --Nathaniel Hawthorne (The Scarlet Letter)
Mood signals. 1. The muscular, fleshy, hairless folds surrounding the mouth opening, which may be moved a. to express emotions, b. to pronounce words, and c. to kiss. 2. The most emotionally expressive parts of the human body.Usage: Lips give off telling cues about inner feelings and moods. So connected are lips a. to our visceral nervous system and b. to companion muscles of our lower face, that we rarely keep them still. Like hands, lips are incredibly gifted communicators which always bear watching.
Anatomy I. Lip size (full or thin), curvature (sinuous or straight), and eversion (everted or inverted) vary in men and women, and in geographic populations as well. The principal lip muscle, orbicularis oris, is a sphincter consisting a. of pars marginalis (beneath the margin of the lips themselves), and b. pars peripheralis (around the lips' periphery from the nostril bulbs to the chin). (N.B.: P. marginalis is uniquely developed in humans for speech.) Contraction of orbicularis oris tenses the lips and reduces their eversion.
Anatomy II. Lips may be moved directly by orbicularis oris and by direct labial tractor muscles in the upper and lower lips. Contraction oflevator labii superioris alaeque nasi, levator labii superioris, and/or zygomaticus minor, e.g., elevate and/or evert the upper lip; whiledepressor labii inferioris and/or platysma par labialis depress and/or evert the lower lip. The complexity of muscle interactions thus reflects the complexity of emotion blends.
Anatomy III. Lips may also be moved indirectly by nine (or more) other facial muscles (e.g., by zygomaticus major in laughing) through attachments to a fibromuscular mass known as the modiolus. That so many facial muscles interlink via the modiolus makes our lips extremely expressive of attitudes, opinions, and moods.
Embryology. On day 22, pharyngeal arches form, and by 20 weeks, orbicularis oris (and other muscles of expression) form from the 2nd pharyngeal arch.
Infancy. From 3-to-6 months, babies bring objects to their lips to be explored, and make sounds with objects placed against their lips.
Lipreading. Functional magnetic resonance imaging (fMRI) studies show that the linguistic visual cues afforded by lip movements activate areas of auditory cortex in normal hearing individuals (Calvert et al. 1997).
Observation. Unconscious tension in lips reflects how we truly feel about, e.g., a boss's work assignment, a friend's off-hand comment, or a colleague's "helpful" idea. A slight drooping at the mouth corners (through unconscious contraction of depressor anguli oris) may be the first visible sign of (unvoiced) sadness or disappointment.
Primatology. Beginning with muscular contractions for suckling breast milk, the primate brain added the ability to grasp food items witheverted lips. Chimps, e.g., use prehensile lips to pluck termites from twigs. (N.B.: Humans use their own prehensile lips to pluck french fries from a bag.)
Neuro-notes I. The facial nerve's (i.e., cranial VII's) lower nucleus controls the pouted-, curled-, and tightened-lip expressions we unintentionally use to reveal our moods. Instructions for these signals come from limbic modules, such as the amygdala and cingulate gyrus, by way of the brain stem. Because there is little or no conscious control from higher brain centers, lip movements provide trustworthy cues.
Neuro-notes II. Our brain devotes an unusually large part of its surface area to lips (see HOMUNCULUS). In the mind's eye, as a result a. of the generous space they occupy on the sensory and motor strips of our neocortex, and b. of the older paleocircuits linking them to emotional,feeding, and grooming centers of the mammalian brain, almost anything a lip does holds potential as a sign.
Neuro-notes III. Our human brain added precision to lip movements through nerve fibers linked to the primary motor neocortex. Today, fiber links from this area descend through the corticobulbar tract to motor neurons of the facial nerve, whose branches take charge of specific muscle fibers of the lips. That we can whistle a tune (and that whistle languages are "spoken" in some areas of the world) testifies to our lips' extremely high IQ as neurological smart parts.
Detail of a male human's face, showing the muscular, fleshy, hairless lips (photo credit: unknown)