1. =Orbicularis oris.=—This is the sphincter muscle of the mouth; it is
continuous with the other muscles which converge to the lips. It lies
between the skin and the mucous membrane of the lips, and is intimately
adherent to the former. Most of the fibers run parallel to the free
edges of the lips and have no direct attachment to the skeleton.
_Action._—It closes the lips.
_Blood-supply._—Palato-labial, facial, and mental arteries.
_Nerve-supply._—Facial nerve.
[Illustration:
FIG. 172.—MUSCLES OF HEAD OF HORSE, LATERAL VIEW. THE PANNICULUS IS
REMOVED.
_a_, Levator labii superioris proprius; _b_, levator nasolabialis;
_c_, mastoido-humeralis; _d_, sterno-cephalicus; _d′_, tendon of
_d_; _e_, omo-hyoideus; _f_, dilatator naris lateralis; _g_,
zygomaticus; _h_, buccinator; _i_, depressor labii inferioris; _k_,
orbicularis oris; _l_, dilatator naris superior; _m_, masseter; _n_,
parotido-auricularis; _o_, zygomatico-auricularis; _p_,
interscutularis; _p′_, fronto-scutularis, pars temporalis; _q_,
cervico-auricularis profundus major; _r_, cervico-auricularis
superficialis; _s_, obliquus capitis anterior; _t_, splenius; v,
stylo-maxillaris; _y_, mastoid tendon of mastoido-humeralis; _2_,
posterior, _3_, anterior, border of external ear; _8_, scutiform
cartilage; _9_, zygomatic arch; _10_, depression behind supraorbital
process; _18_, temporo-mandibular articulation; _27_, facial crest;
_30′_, angle of jaw; _37_, external maxillary vein; _38_, jugular
vein; 39, facial vein; _40_, parotid duct; _41_, transverse facial
vein; _42_, masseteric vein; _43_, facial nerve; _44_, parotid
gland; _45_, chin; _x_, wing of atlas. (After Ellenberger-Baum,
Anat. für Künstler.)
]
2. =Levator nasolabialis= (Levator labii superioris alæque nasi).—This
thin muscle lies directly under the skin, and chiefly on the lateral
surface of the nasal region.
_Origin._—The frontal and nasal bones.
_Insertion._—(1) The upper lip and the outer wing of the nostril; (2)
the commissure of the lips.
_Action._—(1) To elevate the upper lip and the commissure; (2) to dilate
the nostril.
_Structure._—The muscle arises by a thin aponeurosis. The belly is also
thin, and divides into two branches, between which the lateral dilator
of the nostril passes. The dorsal branch reaches the nostril and upper
lip, blending with the lateral dilator; the ventral one is much smaller,
and blends at the labial commissure with the orbicularis and buccinator.
_Relations._—Superficially, the skin, fascia, and lateral dilator (in
part); deeply, the levator labii superioris proprius, lateral dilator
(in part), buccinator, branches of the facial vessels and nerve, and the
infraorbital artery and nerve.
_Blood-supply._—Facial and palato-labial arteries.
_Nerve-supply._—Facial nerve.
[Illustration:
FIG. 173.—MUSCLES OF HEAD OF HORSE, DORSAL VIEW. THE PANNICULUS IS
REMOVED.
_a_, Levator labii superioris proprius; _a′_, common tendon of a with
opposite muscle; _b_, levator nasolabialis; _f_, dilatator naris
lateralis; _g_, zygomaticus; _l_, dilatator naris superior; _n_,
parotido-auricularis; _o″_, scutulo-auricularis superficialis
superior; _p_, interscutularis; _p′_, fronto-scutularis, pars
temporalis; _r_, cervico-auricularis superficialis; _u_, corrugator
supercilii; _x_, transversus nasi; _2_, posterior, _3_, anterior,
border of external ear; _8_, scutiform cartilage; _9_, zygomatic
arch; _10_, supraorbital depression; _35_, inner wing of nostril,
containing lamina of alar cartilage; _39_, facial vein. (After
Ellenberger-Baum, Anat. für Künstler.)
]
3. =Levator labii superioris proprius.=—This lies on the dorso-lateral
aspect of the face, partly covered by the preceding muscle.
_Origin._—The lacrimal, malar, and maxillary bones at their junction.
_Insertion._—The upper lip, by a common tendon, with its fellow.
_Action._—Acting with its fellow, to elevate the upper lip. This action,
if carried to the fullest extent, results in eversion. In unilateral
action the lip is drawn upward and to the side of the muscle acting.
_Structure._—The muscle has a short, thin tendon of origin. The belly is
at first flattened, but becomes narrower and thicker, then tapers over
the false nostril, to terminate in a tendon. The tendons of the two
muscles unite over the alar cartilages of the nostrils, forming an
expansion which spreads out in the substance of the upper lip.
_Relations._—Superficially, the skin, the levator nasolabialis, and the
angular vessels of the eye; deeply, the superior and transverse dilators
of the nostril and the infraorbital artery and nerve.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
4. =Zygomaticus.=—This very thin muscle lies immediately under the skin
of the cheek.
_Origin._—The fascia covering the masseter muscle below the facial
crest.
_Insertion._—The commissure of the lips, blending with the buccinator.
_Action._—To retract and raise the angle of the mouth.
_Structure._—Fleshy, with a thin aponeurotic origin.
_Relations._—Superficially, the skin; deeply, the buccinator.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
5. =Incisivus superior= (Depressor labii superioris).—This lies under
the mucous membrane of the upper lip.
_Origin._—The alveolar border of the premaxilla from the second incisor
to the first cheek tooth.
_Insertion._—The upper lip.
_Action._—To depress the upper lip.
6. =Incisivus inferior.=—This is arranged in the lower lip like the
preceding muscle in the upper one.
_Origin._—The alveolar border of the mandible from the second incisor to
a point near the first cheek tooth.
_Insertion._—The skin of the lower lip and the prominence of the chin.
_Action._—To raise the lower lip.
7. =Mentalis= (Levator menti).—This is situated in the prominence of the
chin. Its fibers arise from each side of the body of the mandible and
are inserted into the skin of the chin. It is mingled with fat and
strands of connective tissue, in which the roots of the tactile hairs
are embedded. It raises and corrugates the skin to which it is attached.
8. =Depressor labii inferioris.=—This muscle lies on the outer surface
of the ramus of the mandible, along the ventral border of the
buccinator.
_Origin._—The alveolar border of the mandible near the coronoid process
and the maxillary tuberosity, in common with the buccinator.
_Insertion._—The lower lip.
_Action._—To depress and retract the lower lip.
_Structure._—The tendon of origin and the belly are fused with the
buccinator as far forward as the first cheek tooth. From this point
forward the belly is distinct and rounded, terminating in a tendon which
spreads out in the lower lip, blending with the orbicularis and the
muscle of the opposite side.
_Relations._—Superficially, the skin, masseter, facial vessels, and
parotid duct; deeply, the mandible and inferior labial artery.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
9. =Buccinator.=—This muscle lies in the lateral wall of the mouth,
extending from the angle of the mouth to the maxillary tuberosity.
_Origin._—The external surface of the maxilla above the interdental
space and the molar teeth; the alveolar border of the mandible at the
interdental space and also posteriorly where it turns upward to the
coronoid process; the pterygo-mandibular ligament.
_Insertion._—The angle of the mouth, blending with the orbicularis oris.
_Action._—To flatten the cheeks, thus pressing the food between the
teeth; also to retract the angle of the mouth.
_Structure._—Two layers may be recognized. The =superficial layer= (Pars
buccalis) extends from the angle of the mouth to the masseter. It is
incompletely pennate, having a longitudinal raphé on which most of the
muscle-fibers converge. The upper fibers are directed chiefly downward
and backward, the lower ones upward and backward. The =deep layer= (Pars
molaris) consists mainly of longitudinal fibers. It blends in part with
the superficial layer of the orbicularis; it has a small tendinous
attachment to the coronoid process behind, and is united below with the
depressor labii inferioris.
_Relations._—Superficially, the skin and fascia, the zygomaticus,
levator nasolabialis, lateral dilator of the nostril, the superior
buccal glands, the parotid duct, the facial vessels, and branches of the
facial nerve; deeply, the mucous membrane of the mouth and the inferior
buccal glands.
_Blood-supply._—Facial and buccinator arteries.
_Nerve-supply._—Facial nerve.