Digastric muscle

The digastric muscle consists of two muscular bellies united by an intermediate tendon with the posterior belly 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 thus differ in their innervation.

Structure

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 antero-inferiorly 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 postero-inferiorly 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 while the anterior belly is closer to the sub-mandibular salivary gland which is a content of the digastric triangle.

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 a 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).

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.

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.