The digastricus consists of two muscular 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. Each person has a right and left digastric muscle. In most anatomical discussions, the singular is used to refer to a muscle, even when each person actually has two of that muscle—one on the right side, and another on the left. For example, we speak of the deltoid, even though there is one deltoid in each shoulder. Likewise, we speak of the digastric even though there is a right and left digastric muscle.
Posterior belly
The posterior belly, longer than the anterior belly, arises from the mastoid notch which is on the inferior surface of the skull, medial to the mastoid process of the temporal bone. The mastoid notch is a deep groove between the mastoid process and the styloid process. The mastoid notch is also referred to as the digastric groove or the digastric fossa. 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.
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.
Variations
Variations are numerous. The posterior belly may arise partly or entirely from the styloid process, or be connected by a 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; anterior belly may be absent and posterior belly inserted into the middle of the jaw or hyoid bone. The tendon may pass in front, more rarely behind the Stylohoideus. The mentohyoideus muscle passes from the body of hyoid bone to chin.
the submandibular triangle, bounded above by the lower border of the body of the mandible, and a line drawn from its angle to the sternocleidomastoideus, below by the posterior belly of the digastricus and the stylohyoideus, in front by the anterior belly of the diagastricus;
the carotid triangle, bounded above by the posterior belly of the digastricus and stylohyoideus, behind by the sternocleidomastoideus, below by the omohyoideus;
the suprahyoid or submental triangle, bounded laterally by the anterior belly of the digastricus, medially by the middle line of the neck from the hyoid bone to the symphysis menti, and inferiorly by the body of the hyoid bone.
The inferior carotid triangle, is bounded, in front, by the median line of the neck from the hyoid bone to the sternum; behind, by the anterior margin of the sternocleidomastoideus; above, by the superior belly of the omohyoideus
Function
The digastric muscle is involved in any complex jaw action such as speaking, swallowing, chewing and breathing. When the digastric muscle contracts, it acts to elevate the hyoid bone. If the hyoid is being held in place, it will tend to depress the mandible.
Other animals
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.
