Anatomy and Structure
The muscle consists of two parts connected by an intermediate tendon:
- Anterior belly (venter anterior) — originates from the digastric fossa on the mandible (fossa digastrica). The investing layer of the deep cervical fascia splits to form fascial sheaths for the anterior bellies of the m. digastricus.
- Posterior belly (venter posterior) — originates from the mastoid notch of the temporal bone (incisura mastoidea).
- Intermediate tendon — connects both muscular bellies and attaches to the body of the hyoid bone via a fibrous loop.
Functions and Biomechanics
The action of the muscle depends on which bony structure is fixed:
- Mandibular depression: when the hyoid bone is fixed, the muscle depresses the mandible (opens the mouth).
- Hyoid elevation: when the mandible is fixed, the muscle elevates the hyoid bone.
During mouth opening, the mandibular condyles perform a rotational movement, followed by gliding anteriorly and inferiorly down the slope of the articular tubercle. At maximal opening, the distance between the incisal edges of the upper and lower incisors averages 4–5 cm.
Blood Supply and Innervation
Due to its dual embryological origin, the two bellies have different innervation:
- Anterior belly innervation: mylohyoid nerve (n. mylohyoideus), a branch of the inferior alveolar nerve derived from the mandibular nerve (V3 of the trigeminal nerve, n. trigeminus).
- Posterior belly innervation: facial nerve (n. facialis).
- Blood supply: submental artery (a. submentalis) and occipital artery (a. occipitalis).
Topography and Surrounding Structures
The bellies of the m. digastricus form key anatomical boundaries for triangles of the neck:
- Submandibular triangle (trigonum submandibulare): bounded anteriorly by the anterior belly and posteriorly by the posterior belly of the digastric muscle. Contains the submandibular salivary gland, facial vessels (a. et v. facialis), and lymph nodes.
- Submental triangle (trigonum submentale): located between the anterior bellies of the right and left digastric muscles. Contains 1–2 submental lymph nodes, which drain lymph from the tip of the tongue, the central floor of the mouth, and the central part of the lower lip.
- Jugulodigastric lymph node (nodus lymphoideus jugulodigastricus): palpable near the angle of the mandible, located at the crossing of the internal jugular vein (v. jugularis interna) and the posterior belly of the digastric muscle.
Clinical Significance
Understanding the topography of the digastric muscle is essential when evaluating neck masses:
- Branchial cleft cysts (second cleft anomalies): the upper pole is frequently located near or deep to the posterior border of the m. digastricus.
- Dermoid and epidermoid cysts: may be localized in the submental or submandibular regions in close relation to the digastric muscle.