The digastric is a suprahyoid muscle with two bellies and an intermediate tendon that has dual nerve supply, the anterior belly from the trigeminal nerve and the posterior belly from the facial nerve, reflecting their different embryological origins from the first and second branchial arches respectively. It elevates the hyoid during the pharyngeal phase of swallowing and assists in mouth opening when the hyoid is stabilised by the infrahyoid muscles.
| Origin | Digastric fossa on the lower inner surface of the mandible; Mastoid notch of the temporal bone |
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| Insertion | Both bellies insert via an intermediate tendon into the hyoid bone, held there by a fibrous loop |
| Nerve Supply | Anterior belly: mylohyoid nerve from the inferior alveolar branch of V3; Posterior belly: facial nerve (CN VII) |
| Blood Supply | Submental artery (anterior); Occipital and posterior auricular arteries (posterior) |
| Actions | Elevates the hyoid bone during swallowing; Depresses the mandible when the hyoid is fixed; Retracts the mandible |
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The differential nerve supply of its two bellies from different cranial nerves is clinically important because facial nerve palsy from any cause can produce weakness of the posterior belly without affecting the anterior belly, an isolated pattern that is highly specific for facial nerve involvement.
The posterior digastric belly is a key landmark in parotid surgery as it defines the inferior border of the parotid gland and lies immediately adjacent to the main facial nerve trunk. The intermediate tendon creates a palpable landmark in the submandibular triangle. Digastric hypertrophy or spasm contributes to temporomandibular joint dysfunction and dysphagia in some patients.
The anterior belly is palpable in the midline submental triangle between the chin and the hyoid bone during mouth opening against resistance. The posterior belly can be felt as a firm cord running from the mastoid to the hyoid during forceful mouth opening.
Acute injury to one or both bellies from sudden forced mouth closure or a direct blow, producing submandibular pain with swallowing difficulty and restricted mouth opening.
