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.