_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. THE HYOID MUSCLES