- Continuing Education
Gaster means belly and di means two so naturally the digastric muscle is composed of two groups of muscle fibers connected by a central tendon. Unlike the occipitofrontalis which has somewhat the same structure this tendon, called the intermediate tendon, is round in cross section rather than flat. It passes through a loop of connective tissue attached to the hyoid bone. It does not actually insert into the hyoid. The posterior belly originates just anterior to the mastoid process and the anterior belly inserts into the inferior border of the mandible near the midline. The action of the muscle depends on the action of other muscles. If the elevators of the mandible contract the action will be to shorten the muscle which will result in the intermediate tendon moving the hyoid posteriorly and superiorly, so like the stylohyoid this muscle is involved in swallowing. However if the infrahyoids contract to stabilize the hyoid the action will be to depress the mandible. As it has two bellies and two actions naturally to make things difficult for the learner it has two separate nerves enervating it. The anterior belly is innervated by branches of cranial nerve V3 (trigeminal nerve – third division, the mandibular nerve) and the posterior belly is innervated by branches of cranial nerve VII (facial nerve).