Part 26

_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.

MUSCLES OF THE NOSTRILS

1. =Levator nasolabialis.=—This has been described (p. 214).

2. =Dilatator naris lateralis= (M. caninus).—This thin, triangular muscle lies on the lateral nasal region, and passes between the two branches of the levator nasolabialis.

_Origin._—The maxilla, close to the anterior extremity of the facial crest.

_Insertion._—The outer wing of the nostril.

_Action._—To dilate the nostril.

_Structure._—The muscle has a flat tendon of origin, passes between the two branches of the levator nasolabialis, and spreads out in the external wing of the nostril. The lower fibers blend with the orbicularis oris.

_Relations._—Superficially, the skin, fascia, and the labial branch of the levator nasolabialis; deeply, the maxilla and the nasal branch of the levator nasolabialis.

_Blood-supply._—Facial artery.

_Nerve-supply._—Facial nerve.

3. =Dilatator naris transversus= (M. transversus nasi).—This is an unpaired, quadrilateral muscle, which lies between the nostrils. It consists of two layers.

_Attachments._—Superficial layer, the superficial faces of the laminæ of the alar cartilages; deep layer, the convex edges of the cornua of the same.

_Action._—To dilate the nostrils.

_Structure._—It is composed of transverse fleshy fibers, which blend below with the orbicularis.

_Relations._—Superficially, the skin, fascia, and tendinous expansion of the levator labii superioris proprius; deeply, the alar cartilages, the extremity of the septum nasi, and the palato-labial artery.

_Blood-supply._—Palato-labial artery.

_Nerve-supply._—Facial nerve.

4. =Dilatator naris superior= (Pars dorsalis m. lateralis nasi).—This very thin muscle occupies the angle between the nasal process of the premaxilla and the nasal bone.

_Origin._—The lateral border of the nasal bone.

_Insertion._—The inner wall of the false nostril and the parietal lamina of the septal cartilage.

_Action._—To dilate the vestibule of the nasal cavity.

_Structure._—Fleshy.

_Relations._—Superficially, the skin, fascia, and levator labii superioris proprius; deeply, the parietal cartilage and false nostril.

_Blood-supply._—Facial artery.

_Nerve-supply._—Facial nerve.

5. =Dilatator naris inferior= (Pars ventralis m. lateralis nasi).—This is a similar but thicker muscle, which lies on the nasal process of the premaxilla.

_Origin._—The maxilla and the nasal process of the premaxilla.

_Insertion._—The cartilaginous prolongations of the turbinal bones and the inner wall of the false nostril.

_Action._—To rotate the turbinal outward and dilate the nostril and the vestibule of the nasal cavity.

_Structure._—Fleshy. A division into two or more parts may be recognized. A small part posteriorly is inserted into the cartilage of the dorsal turbinal bone, while the bulk of the muscle is inserted into the cartilage of the ventral turbinal bone. A few fibers also pass between the cornu of the alar cartilage and the outer wing of the nostril.

_Relations._—Superficially, the levator labii superioris proprius, the levator nasolabialis, and the lateral nasal artery; deeply, the maxilla, the premaxilla, and the anterior nasal branch of the infraorbital nerve.

_Blood-supply._—Facial artery.

_Nerve-supply._—Facial nerve.

MUSCLES OF THE EYELIDS

1. =Orbicularis oculi.=—This is a flat, elliptical, sphincter muscle, situated in and around the eyelids, the portion in the upper lid being much broader than that in the lower. The chief attachment is to the skin of the lids, but some bundles are attached to the palpebral ligament at the inner canthus and to the lacrimal bone. Its action is to close the lids.

2. =Corrugator supercilii.=—This is a very thin, small muscle, which arises over the root of the supraorbital process and spreads out in the upper eyelid, blending with the orbicularis. Its action is to assist in raising the upper lid or, especially in pathological conditions, to wrinkle the skin.

3. =Malaris.=—This is a very thin muscle, which varies much in different subjects. It extends from the fascia in front of the orbit to the lower lid. Its action is to depress the lower lid.

The foregoing muscles receive their blood-supply from the facial, transverse facial, supraorbital, and infraorbital arteries; the nerve-supply is derived from the facial nerve.

4. =Levator palpebræ superioris.=—This slender, flat muscle is almost entirely within the orbital cavity. It arises on the pterygoid crest, passes forward above the rectus oculi superior and below the lacrimal gland, and terminates in a thin tendon in the upper lid.

_Action._—To elevate the upper lid.

_Blood-supply._—Ophthalmic artery.

_Nerve-supply._—Oculomotor nerve.

MUSCLES OF MASTICATION

The muscles of this group are six in number in the horse. They arise chiefly from the upper jaw and the base of the cranium, and are all inserted into the mandible.

1. =Masseter.=—This muscle extends from the zygomatic arch and facial crest over the broad part of the mandibular ramus. It is semi-elliptical in outline.

_Origin._—By a strong tendon from the zygomatic arch and the facial crest.

_Insertion._—The outer surface of the broad part of the ramus of the mandible.

_Action._—Its action is to bring the jaws together. Acting singly, it also carries the lower jaw toward the side of the contracting muscle.

_Structure._—The superficial face of the muscle in its upper part is covered by a strong, glistening aponeurosis, and several tendinous intersections partially divide the muscle into layers. The fibers of the superficial layer take origin from the malar and maxilla only, and diverge somewhat to their insertion close to the thick ventral border of the lower jaw. The fibers of the deep layer arise from the entire area of origin, and pass straight to the border of the mandible; it will be noted that a small part, near the temporo-maxillary joint, is not covered by the superficial layer. The two layers are separable only above and behind; elsewhere they are fused.

_Relations._—Superficially, the skin and panniculus, the parotid gland, the transverse facial and masseteric vessels, and the facial nerve; deeply, the ramus of the mandible, the buccinator, depressor labii inferioris, and mylo-hyoideus muscles, the superior buccal glands, the buccinator nerve, and two large varicose veins which join the facial vein at the anterior edge of the muscle. The facial vessels and parotid duct run along the anterior edge of the muscle; the duct, however, bends forward about the middle of the border and leaves the muscle.

_Blood-supply._—Transverse facial and masseteric arteries.

_Nerve-supply._—Mandibular nerve.

2. =Temporalis.=—This muscle occupies the temporal fossa.

_Origin._—The rough part of the temporal fossa and the crests which surround it.

_Insertion._—The coronoid process of the mandible, which it envelops.

_Action._—Chiefly to raise the lower jaw, acting with the masseter and internal pterygoid muscles.

_Structure._—The surface of the muscle is covered with a glistening aponeurosis, and strong tendinous intersections are found in its substance. The inner edge of the muscle is quite thin, but as the fibers converge toward the much smaller area of insertion, the muscle becomes nearly an inch thick. It fuses somewhat with the masseter.

_Relations._—Superficially, the scutiform cartilage and anterior muscles of the external ear and the orbital fat; deeply, the temporal fossa and the deep temporal vessels and nerves.

_Blood-supply._—Superficial and deep temporal, and mastoid arteries.

_Nerve-supply._—Mandibular nerve.

3. =Pterygoideus internus= (s. medialis).—This muscle occupies a position on the inner surface of the ramus of the mandible similar to that of the masseter on the outside.

_Origin._—The crest formed by the pterygoid process of the sphenoid and the palate bone.

_Insertion._—The concave inner surface of the broad portion of the ramus of the mandible, and the inner lip of the lower border.

_Action._—Acting together, to raise the lower jaw; acting singly, to produce also lateral movement of the jaw.

_Structure._—The muscle is capable of division into two parts. The principal part is superficial (internal), and its fibers are, for the most part, vertical in direction. It contains much tendinous tissue (septa). The smaller portion is external to the foregoing, and its fibers are directed downward and backward.

_Relations._—Externally, the ramus of the mandible, the external pterygoid muscle, the inferior alveolar vessels and nerve, and the lingual and mylo-hyoid nerves; internally, the great cornu of the hyoid bone, the tensor palati, pterygo-pharyngeus, palato-pharyngeus, mylo-hyoideus, digastricus, and stylo-hyoideus muscles, the guttural pouch, the external maxillary vessels, the ninth and twelfth nerves, the submaxillary salivary gland, the submaxillary and parotid ducts, and the submaxillary and pharyngeal lymph glands.

_Blood-supply._—Internal maxillary, masseteric, and inferior alveolar arteries.

_Nerve-supply._—Mandibular nerve.

4. =Pterygoideus externus= (=s. lateralis=).—This muscle is considerably smaller than the preceding one, and is situated external to its upper part.

_Origin._—The external surface of the pterygoid process of the sphenoid bone.

_Insertion._—The inner surface of the neck and the inner part of the anterior border of the condyle of the mandible.

_Action._—Acting together, to draw the lower jaw forward; acting singly, to move the jaw laterally, _i. e._, toward the side opposite to the muscle acting. The latter action is due to the fact that the origin is nearer to the median plane than the insertion.

_Structure._—The muscle is almost entirely fleshy, and the fibers are almost longitudinal in direction. Some of them are inserted into the edge of the interarticular meniscus.

_Relations._—Externally, the temporo-maxillary articulation and the temporalis muscle; internally, the internal pterygoid and tensor palati muscles. The internal maxillary artery crosses the lower face of the muscle and dips in between it and the tensor palati. The mandibular nerve lies on the ventral surface, and the buccinator nerve perforates the origin of the muscle.

_Blood-supply._—Internal maxillary and inferior alveolar arteries.

_Nerve-supply._—Mandibular nerve.

5. =Stylo-maxillaris= (s.-mandibularis) (M. jugulomandibularis).—This is a short muscle extending from the paramastoid or styloid process of the occipital bone to the posterior border of the lower jaw; it is covered by the parotid gland.

_Origin._—The paramastoid or styloid process of the occipital bone, in common with the posterior belly of the digastricus.

_Insertion._—The posterior border of the ramus of the mandible.

_Action._—To depress the lower jaw and open the mouth.

_Structure._—The muscle contains a good deal of tendinous tissue. It blends with the posterior belly of the digastricus.

_Relations._—Superficially, the parotid gland, the tendon of the sterno-cephalicus, and the fibrous expansion which connects it with the tendon of the mastoido-humeralis; deeply, the guttural pouch, the external carotid artery, the ninth and twelfth nerves, the pharynx, and the submaxillary gland.

_Blood-supply._—External carotid artery.

_Nerve-supply._—Facial nerve.

6. =Digastricus.=—This muscle is composed of two fusiform, flattened bellies, united by a round tendon.

_Origin._—The paramastoid or styloid process of the occipital bone, in common with the preceding muscle.

_Insertion._—The inner surface of the lower border of the mandible behind the symphysis.

_Action._—It assists in depressing the lower jaw and opening the mouth. If the mandible be fixed and both bellies contract, the hyoid bone and the base of the tongue are raised, as in the first phase of deglutition.

_Structure._—The posterior belly has the appearance of a branch detached from the inner surface of the stylo-maxillaris. It passes downward and forward, and is succeeded by a small rounded tendon. The latter perforates the tendon of insertion of the stylo-hyoideus, and is provided with a =synovial sheath=. The anterior belly is larger and terminates by thin tendinous bundles.

_Relations._—The posterior belly has practically the same relations as the stylo-maxillaris. The intermediate tendon is in contact externally with the internal pterygoid muscle, the submaxillary gland and duct, and the external maxillary artery. The anterior belly lies in the submaxillary space between the ramus of the jaw and the mylo-hyoideus muscle; the sublingual vessels run along its upper border.

_Blood-supply._—External carotid and sublingual arteries.

_Nerve-supply._—Facial and mandibular nerves.

THE HYOID MUSCLES

This group consists of eight muscles, one of which, the hyoideus transversus, is unpaired.

1. =Mylo-hyoideus.=—This muscle, together with its fellow, forms a sort of sling between the two rami of the lower jaw, in which the tongue is supported.

_Origin._—The inner surface of the alveolar border of the mandible.

_Insertion._—(1) A median fibrous raphé extending from the symphysis to the hyoid bone; (2) the lingual process and body of the hyoid bone.

_Action._—It raises the floor of the mouth, the tongue, and the hyoid bone.

_Structure._—Each muscle consists of a thin curved sheet, the fibers passing downward from their origin and then curving toward the median raphé. It is chiefly fleshy, and is thickest behind. The anterior superficial part of the muscle is termed the mylo-glossus.

_Relations._—On the superficial surface of the muscles are the ramus, the internal pterygoid and digastricus muscles, and the submaxillary lymph glands. The deep surface is in contact with the mucous membrane of the mouth, the stylo-glossus, hyo-glossus, and genio-hyoideus muscles, the sublingual gland and vessels, the submaxillary duct, and the lingual and hypoglossal nerves.

_Blood-supply._—Sublingual artery.

_Nerve-supply._—Mylo-hyoid branch of the mandibular nerve.

2. =Stylo-hyoideus.=—This is a slender, fusiform muscle, having a direction nearly parallel to that of the great cornu of the hyoid bone (Fig. 436).

_Origin._—The heel-like prominence on the proximal extremity of the great cornu of the hyoid bone.

_Insertion._—The anterior part of the thyroid cornu of the hyoid bone.

_Action._—It draws the base of the tongue and the larynx upward and backward.

_Structure._—It arises by a thin, short tendon, and has a fusiform belly. The tendon of insertion is perforated for the passage of the intermediate tendon of the digastricus, and at this point there is a small synovial sheath.

_Relations._—Superficially, the internal pterygoid muscle and the parotid gland; deeply, the guttural pouch, the pharynx, the external carotid and maxillary arteries, and the hypoglossal nerve.

_Blood-supply._—External carotid artery.

_Nerve-supply._—Facial nerve (stylo-hyoid branch).

3. =Occipito-hyoideus= (M. jugulo-hyoideus; occipito-styloideus).—This is a small triangular muscle, which lies in the space between the paramastoid process and the great cornu of the hyoid bone.

_Origin._—The paramastoid (styloid) process of the occipital bone.

_Insertion._—The proximal extremity and ventral edge of the great cornu of the hyoid bone.

_Action._—It carries the distal extremity of the great cornu backward and upward. Acting with the genio-hyoideus and digastricus, it raises the hyoid bone and the larynx.

_Structure._—The muscle is somewhat triangular, its fibers being longer as the ventral border is approached. It blends with the posterior belly of the digastricus.

_Relations._—Superficially, the parotid gland; deeply, the guttural pouch.

_Blood-supply._—Occipital artery.

_Nerve-supply._—Facial nerve.

4. =Genio-hyoideus.=—This is a long, spindle-shaped muscle, which lies under the tongue in contact with its fellow of the opposite side (Fig. 243).

_Origin._—The angle of union of the rami of the mandible.

_Insertion._—The extremity of the lingual process of the hyoid bone.

_Action._—It draws the hyoid bone and tongue forward.

_Structure._—The muscle arises by a short tendon, which is succeeded by the belly, composed of long bundles of parallel fibers.

_Relations._—Below, the mylo-hyoideus; above, the hyo-glossus, stylo-glossus, genio-glossus, the sublingual gland, submaxillary duct, and the lingual nerve.

_Blood-supply._—Sublingual artery.

_Nerve-supply._—Hypoglossal nerve.

5. =Kerato-hyoideus.=—This small triangular muscle lies in the space between the thyroid and small cornu, under cover of the hyo-glossus (Fig. 243).

_Origin._—The posterior edge of the small cornu and the adjacent part of the ventral border of the great cornu.

_Insertion._—The dorsal edge of the thyroid cornu.

_Action._—It raises the thyroid cornu and the larynx.

_Relations._—The muscle is crossed outwardly by the lingual artery.

_Blood-supply._—Lingual artery.

_Nerve-supply._—Glosso-pharyngeal nerve.

6. =Hyoideus Transversus.=—This is a thin, unpaired muscle, which extends transversely between the two small cornua of the hyoid bone.

_Attachments._—The small cornua close to the junction with the great cornua.

_Action._—When relaxed, its upper surface is concave; when it contracts, it elevates the root of the tongue.

_Structure._—Fleshy, composed of parallel transverse bundles.

_Blood-supply._—Lingual artery.

_Nerve-supply._—Glosso-pharyngeal nerve.

7. =Sterno-thyro-hyoideus=, and

8. =Omo-hyoideus.=—These are described with the muscles on the ventral surface of the neck.

THE FASCIÆ AND MUSCLES OF THE NECK

It is convenient to divide the muscles of the neck into ventral and lateral groups, the two lateral groups being separated from each other by the ligamentum nuchæ.

THE FASCIÆ OF THE NECK

The =superficial fascia= is in part two-layered, and contains the cervical portion of the panniculus. The fasciæ of the right and left sides are attached along the dorsal line of the neck to the ligamentum nuchæ, while along the ventral line they meet in a fibrous raphé. A deep layer is detached which passes underneath the panniculus, bridges over the jugular furrow, and crosses over the deep face of the mastoido-humeralis and omo-hyoideus to join the superficial layer. It again separates to pass under the cervical trapezius, and become attached to the ligamentum nuchæ. Along the ventral line a septum is detached which separates the sterno-cephalici. Two other layers in front of the shoulder inclose the prescapular lymph glands.