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.