Digastric muscle: origin, insertion, action and innervation

The digastric is a small muscle under the jaw made of two fleshy bellies joined by a round tendon that is slung to the hyoid bone. It opens the mouth by pulling the mandible down, and each belly has its own nerve.

Where it is

The digastric lies just below the lower border of the mandible, in the upper part of the front of the neck. It is one of the suprahyoid muscles, the group that sits above the hyoid bone, together with the mylohyoid, the stylohyoid and the geniohyoid.

Its shape is a shallow V or a hammock:

• The posterior belly starts behind the ear and runs down and forward.
• The two bellies meet at an intermediate tendon just above the hyoid bone. On its way, the tendon passes through a split in the stylohyoid muscle.
• The anterior belly runs from that tendon forward and up to the inside of the chin.

With the lower border of the mandible, the two bellies outline the submandibular triangle, the area that holds the submandibular salivary gland.

Origin and insertion

The digastric has no single origin and insertion in the usual sense. Its fixed points are both ends, and the middle tendon is anchored to the hyoid:

• Posterior belly: the mastoid notch, a groove on the inner side of the mastoid process of the temporal bone, part of the skull.
• Anterior belly: the digastric fossa, a shallow pit on the inner surface of the mandible, close to the midline.
• Intermediate tendon: tied to the body and greater horn of the hyoid by a loop of fibrous tissue, which lets it slide a little.

What it does

1. Opens the mouth: when the infrahyoid muscles hold the hyoid still, the digastric pulls the chin down and back. It works with the lateral pterygoid, especially when you open wide or against resistance.
2. Raises the hyoid: when the jaw is closed and fixed, the two bellies lift the hyoid bone. This happens at the start of swallowing.
3. Pulls the hyoid backward or forward: the posterior belly alone draws it back and up; the anterior belly alone draws it forward.

Innervation and blood supply

The two bellies come from different pharyngeal arches and keep different nerves:

• Anterior belly: the nerve to mylohyoid, a branch of the inferior alveolar nerve from the mandibular division of the trigeminal nerve (CN V3). It also supplies the mylohyoid.
• Posterior belly: the digastric branch of the facial nerve (CN VII), given off soon after the nerve leaves the skull through the stylomastoid foramen.

The anterior belly receives blood from the submental artery, a branch of the facial artery. The posterior belly is supplied by the occipital and posterior auricular arteries.

How to remember it

• Di-gastric = two bellies: from the Greek for "two stomachs". Each belly has its own nerve.
• Front five, back seven: the anterior belly takes cranial nerve V (via V3), the posterior belly takes cranial nerve VII.
• Follow the neighbors: the anterior belly shares its nerve with the mylohyoid it lies on; the posterior belly shares its nerve with the stylohyoid it runs alongside.
• A tendon through a buttonhole: the intermediate tendon threads through the stylohyoid before reaching the hyoid.

Compare it with the jaw-closing temporalis, then test yourself in the neck quiz.

Frequently asked questions

What is the main action of the digastric muscle?

It depresses the mandible and opens the mouth when the hyoid is fixed. With the jaw fixed, it lifts the hyoid bone.

Why does the digastric have two nerves?

Each belly develops from a different pharyngeal arch. The anterior belly keeps the nerve of the first arch (CN V3) and the posterior belly keeps the nerve of the second arch (CN VII).

Where does the posterior belly of the digastric attach?

To the mastoid notch, a groove on the inner side of the mastoid process of the temporal bone.

Is the digastric a suprahyoid muscle?

Yes. It is one of the four suprahyoid muscles, with the mylohyoid, the stylohyoid and the geniohyoid.

Sources

• OpenStax, Anatomy and Physiology 2e, 11.3 Axial muscles of the head, neck and back
• OpenStax, Anatomy and Physiology 2e, 7.2 The skull
• Z-Anatomy, the libre 3D atlas of anatomy