_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.
[Illustration:
FIG. 175.—SAGITTAL SECTION OF HEAD OF HORSE, SHOWING DEEP
PTERYGO-MAXILLARY REGION, AND NASAL AND CRANIAL CAVITIES.
_1_, Cerebral compartment of cranial cavity; _2_, cerebellar
compartment of same; _3_, tentorium osseum; _4_, tentorium
cerebelli; _5_, sphenoidal sinus; _6_, hamulus of pterygoid
bone-tendon of tensor palati cut off short at anterior border of
hamulus; _7_, mylo-glossus. The olfactory mucous membrane is shaded.
]
_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.
[Illustration:
FIG. 176.—SUBMAXILLARY AND LARYNGEAL REGIONS OF HORSE, AFTER REMOVAL
OF SKIN AND PANNICULUS.
_c_, Mastoido-humeralis; _d_, sterno-cephalicus; _e_, omo-hyoideus and
sterno-hyoideus; _h_, buccinator; _i_, depressor labii inferioris;
_m_, masseter; _v_, stylo-maxillaris; _w_, mylo-hyoideus; _2_,
posterior, _3_, anterior, border of external ear; _30′_, angle of
jaw; _36_, submaxillary lymph glands; _37_, external maxillary vein;
_39_, facial continuation of _37_; _40_, parotid duct; _44_, parotid
gland; _45_, prominence of chin; _x_, wing of atlas. (After
Ellenberger-Baum, Anat. für Künstler.)
]
_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.