The digastricus (digastric muscle) consists of two fleshy bellies united by an intermediate rounded tendon. The two bellies of the digastric muscle have different embryological origins, and are supplied by different cranial nerves.
Anterior belly: The anterior belly arises from a depression on the inner side of the lower border of the mandible called the Digastric Fossa, close to the symphysis, and passes downward and backward. The anterior body is supplied by the trigeminal via the mylohyoid nerve, a branch of the inferior alveolar nerve, itself a branch of the mandibular division of the TRIGEMINAL NERVE. It originates from the FIRST PHARYNGEAL ARCH.
Posterior belly: The posterior belly, longer than the anterior belly, arises on the inferior surface of the skull, from the mastoid notch on the medial surface of the mastoid process of the temporal bone and a deep groove between the mastoid process and the styloid process called the digastric groove. The posterior belly is supplied by the digastric branch of FACIAL NERVE. The digastric muscle stretches between the mastoid process of the cranium to the mandible at the chin, and part-way between, it becomes a tendon which passes through a tendinous pulley attached to the hyoid bone. It originates from the SECOND PHARYNGEAL ARCH. Intermediate tendon: The two bellies end in an intermediate tendon which perforates the Stylohyoideus muscle, and is held in connection with the side of the body and the greater cornu of the hyoid bone by a fibrous loop, which is sometimes lined by a mucous sheath.
Action: When the digastric muscle contracts, it acts to elevate the hyoid bone. If the hyoid is being held in place (by the infrahyoid muscles), it will tend to depress the mandible(open the mouth).