Anatomy, Head and Neck, Digastric Muscle

The digastrics are a pair of muscles individually made up of two distinct muscle bellies: the anterior and posterior digastrics. They derive embryonically from the first and second pharyngeal arches. Together, they function in swallowing, chewing, and speech, and serve as important surgical landmarks in neck dissections and are used routinely for reconstruction. Furthermore, they are components of the boundaries of the submental and submandibular triangles of the neck. There are numerous anatomical variants of the digastrics which can be misleading on MRI or CT. Careful consideration of these variations is critical in clinical assessment and surgical planning. 

The neck contains a pair of digastric muscles, each of which subdivides into an anterior and posterior belly. The two bellies connect by an intermediate tendon. The anterior belly of the digastric attaches near the midline of the base of the mandible on the digastric fossa and runs toward the hyoid. The posterior belly attaches to the temporal bone at the mastoid process and slopes to meet the intermediate tendon. The intermediate tendon typically courses through the stylohyoid muscle, but variations are found lying medial or lateral to it. The tendon runs through a fibrous loop attached to the hyoid bone at the body and greater cornu.[1] The anterior belly of the digastric divides the submental and submandibular triangles of the neck. The submandibular triangle, bordered by the anterior and posterior bellies of the digastric and the inferior border of the mandible, houses the submandibular gland, anterior facial vein, submental artery, mylohyoid nerve and vessels, and the external carotid. The submental triangle is bordered laterally by the anterior belly and houses lymph nodes that drain the floor of the mouth and part of the tongue.[2] The posterior belly of the digastric also serves as a boundary for the carotid triangle, which is where the facial artery branches from the external carotid.[3]

The digastric muscle functions during swallowing, chewing, and speech. The anterior belly of the digastric is one of the three suprahyoid muscles which stabilizes the hyoid during swallowing, an action critical in protecting the airway while eating. Furthermore, the digastrics work to depress the mandible for jaw opening, chewing, and speech.[4] The contraction of the posterior digastric muscle participates in the extension of the head.

The infrahyoid muscles are the antagonistic muscles to the digastric.

The embryologic period occurs during the first eight weeks following fertilization. During this time, neural crest cells migrate caudally to form the five pharyngeal arches. The first and second pharyngeal arches give rise to the anterior and posterior bellies of the digastrics as well as their respective nerve supplies. The anterior belly of the digastric and the mylohyoid nerve form from the first pharyngeal arch, also known as the mandibular arch. The posterior belly of the digastric and the facial nerve derive from the second pharyngeal arch or the hyoid arch. Other muscular structures derived from the first pharyngeal arch include the mylohyoid, tensor veli palatini, tensor tympani, and the muscles of mastication including the temporalis, masseter, and medial and lateral pterygoids. In addition to the posterior belly of the digastric, the second pharyngeal arch gives rise to the stapedius, buccinator, auricular, occipitofrontalis, facial muscles, platysma, and stylohyoid muscles.[5] Researchers postulate that the wide array of anatomical variants of the anterior belly of the digastric is mostly a result of complex morphogenesis of the first pharyngeal arch.[1]

The submental artery, a branch of the facial artery, supplies blood to the anterior belly of the digastric. It runs between the submandibular gland and mylohyoid muscle. The posterior auricular artery and occipital artery, branches of the external carotid artery, supply blood to the posterior belly of the digastric.[5]

The anterior belly of the digastric receives innervation from the mylohyoid nerve. The mylohyoid nerve is a branch of the inferior alveolar nerve, which arises from the mandibular branch of the trigeminal nerve.[5] It branches off before the inferior alveolar nerve penetrates the mandibular foramen and courses through the mylohyoid canal before giving off motor branches to both the mylohyoid muscle and anterior belly of the digastric muscle.[6] The facial nerve provides nerve supply to the posterior belly of the digastric.