_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.