Digastric muscle

The digastric muscle (also digastricus) (named digastric as it has two 'bellies') is a bilaterally paired suprahyoid muscle located under the jaw. Its posterior belly is attached to the mastoid notch of temporal bone, and its anterior belly is attached to the digastric fossa of mandible; the two bellies are united by an intermediate tendon which is held in a loop that attaches to the hyoid bone. The anterior belly is innervated via the mandibular nerve (cranial nerve V), and the posterior belly is innervated via the facial nerve (cranial nerve VII). It may act to depress the mandible or elevate the hyoid bone.

The term "digastric muscle" refers to this specific muscle even though there are other muscles in the body to feature two bellies.

Anatomy
The digastric muscle consists of two muscular bellies united by an intermediate tendon. The posterior belly is longer than the anterior belly. The two bellies of the digastric muscle have different embryological origins - the anterior belly is derived from the first brachial arch and the posterior belly from the second brachial arch - and consequently differ in their innervation (the former being innervated via CN V and the latter via CN VII).

Posterior belly
The posterior belly attaches at the mastoid notch of the temporal bone (which is located upon the inferior surface of the skull, medial to the mastoid process of the temporal bone - between the mastoid process and the styloid process of the temporal bone).

It extends anteroinferiorly from its osseous attachment toward the intermediate tendon.

Anterior belly
The anterior belly attaches at the digastric fossa of mandible (situated at the base of the mandible near the midline).

It extends inferoposteriorly from its origin toward the intermediate tendon.

Intermediate tendon
The two bellies meet at the intermediate tendon which perforates the stylohyoideus muscle. The tendon is embraced by a fibrous sling which attaches the body and greater cornu of hyoid bone. The tendon occasionally features a synovial sheath.

Innervation
The anterior belly receives motor innervation from the mylohyoid nerve (a branch of the inferior alveolar nerve, which is in turn a branch of the mandibular division of the trigeminal nerve (CN V3)).

The posterior belly is supplied by the digastric branch of facial nerve.



Relations
The posterior belly is situated posterior to the parotid gland and the facial nerve.

Triangles of the neck
The digastric muscle divides the anterior triangle of the neck into four smaller triangles: the submandibular triangle (digastric triangle), the carotid triangle, the submental triangle (suprahyoid triangle), and the inferior carotid triangle (muscular triangle).

Variation
The intermediate tendon may be absent. The posterior belly may arise partly (by an supplemental strip of muscle) or entirely from the styloid process of the temporal bone. It may be connected by a muscle slip to the middle or inferior constrictor.

The anterior belly may be double, or extra slips from this belly may pass to the jaw or mylohyoideus or decussate with a similar slip on opposite side. It may be absent and posterior belly inserted into the middle of the jaw or hyoid bone. It may fuse with the mylohyoid muscle.

The tendon may pass in front, more rarely behind the stylohyoideus. The mentohyoideus muscle passes from the body of hyoid bone to chin.

Actions/movements
The muscle depresses the mandible, and may elevate the hyoid bone.

It depresses the mandible when the hyoid bone is held in place (by the infrahyoid muscles).

Function
The digastric muscle is involved in any complex jaw action such as speaking, swallowing, chewing, and breathing. The posterior belly is particularly functionally involved in swallowing and chewing.

Other animals
The digastric muscles are present in a variety of animals, specific attachment sites may vary. For example, in the orangutan, the posterior digastric attaches to the mandible rather than the hyoid.