Together with adjacent tissue they form the floor of the mouth.
Anatomy and supply
The digastric muscle consists of two parts which are separated by an intermediate tendon. The anterior belly originates from the digastric fossa of mandible, the posterior belly from the mastoid notch of temporal bone. Together they insert on the intermediate tendon.
One particular feature of the digastric is that both bellies have different embryological origins. The anterior belly derives from the first pharyngeal arch and is therefore innervated by a branch of the mandibular nerve (mylohyoid nerve). In contrast the posterior belly arises from the 2nd pharyngeal arch, which is why its nerve supply comes from the facial nerve.
The mylohyoid muscle runs from the mylohyoid line of mandible to a median tendon, known as the mylohyoid raphe, where both parts of the muscle meet. The mylohyoid raphe continues its course and inserts on the body of hyoid bone. The muscle is supplied by the mylohyoid nerve from the mandibular nerve.
The geniohyoid muscle has its origin on the mental spine at the internal surface of the mandible from where it radiates to the body of hyoid. It is innervated by branches of the cervical plexus (C1-C2) accompanied by the hypoglossal nerve.
The stylohyoid muscle extends from the styloid process of temporal bone to the body of hyoid bone. In its distal part the muscle divides into two tendons. The muscle is supplied by the facial nerve.
All suprahyoid muscles contribute to the floor of the mouth but the actual muscle plate which bridges between the two rami of mandible is formed by the mylohyoid muscles (oral diaphragm).
From above the mouth floor is reinforced by the geniohyoid muscles and from below by the anterior bellies of the digastric muscles. The oral diaphragm separates the sublingual region from the submandibular region. The only link between those two spaces is the connective tissue on the posterior part of the mylohyoid muscles.
The suprahyoid muscles do not only form the floor of the mouth but play an important role in chewing, swallowing and speech. In combination with the infrahyoid muscles, they are responsible for the positioning of the hyoid bone.
In detail, the digastric and stylohyoid elevate the hyoid during swallowing and help keep the mouth open. The geniohyoid moves the hyoid forward and supports the opening and lateral movement of the mandible. Even though the main function of the mylohyoid is to form the oral diaphragm and elevate the floor of the mouth, it can also assist in jaw opening and chewing movements. All in all, due to their contribution during mastication, the suprahyoid muscles are also referred to as accessory muscles of mastication.
Phlegmon of the floor of the mouth (Ludwig’s angina) constitutes a rare, but dangerous complication from caries, gingivitis and tonsillitis.
Hereby, bacteria spread within the connective tissue of the floor of the mouth and throat causing high fever, painful swelling and difficulties in swallowing. Through the connective tissue of the mylohyoid muscles, the pathogens can easily migrate from the sublingual region to the submandibular region. The spread through the para- and retropharyngeal spaces is particularly feared as from there bacteria can enter the skull (cranially) or the mediastinum (caudally).