Attachments
Each suprahyoid muscle runs from the hyoid bone upwards to the mandible or the base of the skull. Together they form the floor of the mouth and the upper boundaries of the submandibular and submental triangles.
| Muscle | Upper attachment | Hyoid attachment | Nerve supply |
|---|---|---|---|
| Digastric, posterior belly | Mastoid notch on the medial side of the mastoid process | Intermediate tendon, held to the body and greater horn by a fibrous sling | Facial nerve |
| Digastric, anterior belly | Digastric fossa on the inner surface of the mandible near the midline | Intermediate tendon | Nerve to mylohyoid (V3) |
| Stylohyoid | Posterior border of the styloid process | Junction of body and greater horn | Facial nerve |
| Mylohyoid | Mylohyoid line on the inner surface of the mandible | Midline raphe and body of the hyoid | Nerve to mylohyoid (V3) |
| Geniohyoid | Inferior mental spine (genial tubercle) | Anterior surface of the body | C1 fibres carried by the hypoglossal nerve |
Distinctive features
The digastric has two bellies joined by an intermediate tendon that slides through a fibrous loop on the hyoid. Stylohyoid splits near its insertion to let this tendon pass through it. The two mylohyoids meet in a midline raphe running from the mandibular symphysis to the hyoid, forming a muscular sheet called the oral diaphragm. Its posterior border is free. Geniohyoid is a narrow strap lying on the upper surface of mylohyoid, beside its partner, beneath genioglossus.
Actions
The suprahyoid muscles raise the hyoid bone, and with it the larynx, during swallowing and speech. Elevating the floor of the mouth pushes the tongue against the palate to drive the bolus backwards, and lifting the larynx helps close the laryngeal inlet beneath the epiglottis.
- Mylohyoid raises the floor of the mouth and the hyoid.
- Geniohyoid pulls the hyoid upwards and forwards, widening the pharynx.
- Posterior digastric and stylohyoid pull the hyoid upwards and backwards.
- Anterior digastric pulls the hyoid forwards.
When the infrahyoid muscles fix the hyoid, the digastric, mylohyoid and geniohyoid help lateral pterygoid depress the mandible, particularly when opening the mouth against resistance.
Relations
Mylohyoid is the key relation in the floor of the mouth because it divides the region into two compartments. Above it, in the sublingual space, lie the sublingual gland, the deep part of the submandibular gland, the submandibular duct, the lingual nerve and the hypoglossal nerve, the last two lying on hyoglossus. Below it, in the submandibular triangle, lie the superficial part of the submandibular gland, the facial artery and vein, the submental vessels and the nerve to mylohyoid. The two compartments communicate around the free posterior border of mylohyoid, where the submandibular gland folds round the muscle.
Posterior belly of digastric
The posterior belly of digastric is a surgical landmark in the upper neck. The internal and external carotid arteries, internal jugular vein, and the vagus, accessory and hypoglossal nerves all pass deep to it. The hypoglossal nerve emerges below its lower border to cross the carotid arteries, and the occipital artery runs along its lower border. The facial nerve leaves the stylomastoid foramen just above and deep to its origin.
Blood supply and innervation
Nerve supply follows embryological origin. Anterior digastric and mylohyoid develop from the first pharyngeal arch and are supplied by the nerve to mylohyoid, a branch of the inferior alveolar nerve (mandibular division of the trigeminal). Posterior digastric and stylohyoid develop from the second arch and are supplied by the facial nerve. Geniohyoid is derived from occipital somites and is supplied by C1 fibres that travel with the hypoglossal nerve.
The anterior digastric and mylohyoid are supplied by the submental branch of the facial artery and the mylohyoid branch of the inferior alveolar artery. Geniohyoid is supplied by the lingual artery. The posterior digastric receives branches of the occipital and posterior auricular arteries.
Clinical relevance
Mylohyoid decides where dental infection spreads. The mylohyoid line slopes downwards as it runs back, so the root apices of the second and third mandibular molars usually lie below it. Infection from these teeth drains into the submandibular space, while infection from the incisors and premolars tends to enter the sublingual space.
Ludwig's angina is a spreading cellulitis of the submandibular and sublingual spaces on both sides. Swelling pushes the tongue upwards and backwards and can obstruct the airway. It may track along fascial planes into the parapharyngeal space and down to the mediastinum.
In submandibular gland excision and level I neck dissection, the anterior belly of digastric marks the medial limit of the submandibular triangle. Retracting the posterior border of mylohyoid forwards exposes the lingual nerve, the submandibular duct and the hypoglossal nerve lying on hyoglossus.