_Origin._—In two portions: the first, by a broad and thin tendon from the entire length of the bony ridge above the external auditory meatus, from the squamosal process, and from the outer portion of the quadrate bone; the second, from the side and under border of the zygoma. _Insertion._—The first portion: The fibers pass downward and forward beneath the zygoma; a few of the fibers blend with those of the temporal; one tendon inserts to a small tubercle on the upper border of the jaw behind the coronoid process, and by fleshy insertion to the outer side of the ramus of the lower jaw, quite as far forward as the horny portion of the beak. The second portion: by a small tendon to the mandible on its upper border immediately in front of the articular portion. _Shape._—Flat, and elongate. Somewhat fleshy. _Relations._—Superiorly with the temporal, and inferiorly with the biventer maxillæ and stylo-hyoideus. Externally with the skin, and internally with the jaw bone. _Action._—Aids in closing the jaw. =Biventer Maxillæ= (Fig. 19, No. 3). _Location._—Covers the outer posterior portion of the mandible. _Origin._—It arises in two portions: The first, from a ridge bounding the posterior part of the auditory canal; the second, from a depression of the mesial side. These two parts blend and extend downward and forward. _Insertion._—To the posterior part of the articular end of the mandible. _Shape._—A curved fleshy mass. _Relations._—Superiorly with the temporal and stylo-hyoideus. Posteriorly with the digastricus and genio-hyoideus. Internally with the jaw bone, and externally with the skin. _Action._—It aids in opening the jaw. =Entotympanic.= _Location._—Posterior to the pterygoid. _Origin._—From the side of the basi-sphenoid and from the base of the rostrum immediately beyond it. _Insertion._—The fibers pass downward and forward, and insert by a double tendinous slip. One slip inserts to a spine-like process on the upper side of the pterygoid, and the other to the quadrato-pterygoidean articulation. _Shape._—Spindle-shaped. _Relations._—Anteriorly with the pterygoid. Internally and posteriorly with the basi-sphenoid. Externally with the pterygoideus internus. _Action._—Aids in raising the upper mandible by pulling forward the quadratus and pterygoidean against the palatines. =Pterygoideus Internus= (Fig. 17, No. _B_, 5). (Synonym.—Pterygoideus medialis.[4]) Footnote 4: According to international veterinary nomenclature. _Location._—A muscular mass at the roof of the mouth. _Origin._—From the major part of the surface of the palatine bone and the distal head and shaft of the pterygoid and the sphenoidal projection. _Insertion._—By a tendon to the antero-internal part of the articular part of the mandible. _Shape._—Fusiform with a thick fleshy belly. _Relations._—Superiorly with the palatine, the pterygoid, and the sphenoid bones. Inferiorly with the skin. _Action._—It aids in closing the jaw.