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The geniohyoid is a paired short muscle that arises from the inferior mental spine, on the back of the mandibular symphysis, and runs backward and slightly downward, to be inserted into the anterior surface of the body of the hyoid bone. [2]: 346 It lies in contact with its fellow of the opposite side. It thus belongs to the suprahyoid muscles.
The mylohyoid muscle is flat and triangular, and is situated immediately superior to the anterior belly of the digastric muscle. It is a pharyngeal muscle (derived from the first pharyngeal arch) and classified as one of the suprahyoid muscles. Together, the paired mylohyoid muscles form a muscular floor for the oral cavity of the mouth. [2]
Laterally, in between the hyoglossus muscle and the mylohyoid muscle, lay several important structures (from upper to lower): sublingual gland, submandibular duct, lingual nerve, vena comitans of hypoglossal nerve, and the hypoglossal nerve.
The four classical muscles of mastication elevate the mandible (closing the jaw) and move it forward/backward and laterally, facilitating biting and chewing. Other muscles are responsible for opening the jaw, namely the geniohyoid , mylohyoid , and digastric muscles (the lateral pterygoid may play a role).
Mylohyoid: by its own nerve, a branch of the inferior alveolar (from the mandibular division of trigeminal nerve), which arises just before the parent nerve enters the mandibular foramen, pierces the sphenomandibular ligament, and runs forward on the inferior surface of the mylohyoid, supplying it and the anterior belly of the digastric.
The mylohyoid line is a bony ridge on the internal surface of the body of the mandible. The mylohyoid line extends posterosuperiorly. The mylohyoid line continues as the mylohyoid groove on the internal surface of the ramus. The mylohyoid muscle originates from the anterior (front) part of the mylohyoid line. [1]
The mylohyoid muscles are thin, flat muscles that form a sling inferior to the tongue supporting the floor of the mouth. The geniohyoids are short, narrow muscles that contact each other in the midline. [2] The stylohyoids are long, thin muscles that are nearly parallel with the posterior belly of the digastric muscle. [2]
These muscles are grouped as the suprahyoid and infrahyoid muscles depending on if they are located superiorly or inferiorly to the hyoid bone. The suprahyoid muscles (stylohyoid, digastric, mylohyoid, geniohyoid) elevate the hyoid bone, while the infrahyoid muscles (omohyoid, sternohyoid, thyrohyoid, sternothyroid) depress it.