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 It extends antero-inferiorly from its osseous attachment toward the intermediate tendon.
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). ==Function==