On each side of the neck, digastric is one of the suprahyoid muscles. Its name is derived from the Greek for 'two bellies', indicative of its division into two muscular sections - anterior and posterior - by an intermediate tendinous region.
The posterior belly arises from the mastoid notch on the medial side of the base of the mastoid process. A slender muscular slip, it passes anteriorly, inferiorly and medially. About a centimetre from the hyoid bone it becomes tendinous - the intermediate tendon.
The tendon runs through a fibrous sling that is closely apposed and attached to the lesser horn of the hyoid bone. The tendon is separated from the inner surface of the sling by a synovial sheath that permits free movement.
On emergence from the apex of the sling, digastric becomes muscular once again as the anterior belly. This passes anteriorly and medially to insert into the digastric fossa on the internal surface of the mandible. It provides an anatomical landmark for division of the space inferior to the mandible into submental and submandibular triangles.
Innervation is derived from:
- anterior belly: mylohyoid nerve, a branch of the mandibular division of the trigeminal nerve
- posterior belly: nerve from posterior auricular branch of facial nerve
The actions of digastric are to:
- depress and retract the mandible, so assisting the lateral pterygoid muscles in opening the mouth
- elevation of the hyoid bone, utilised during swallowing and speech