Digastric muscle
The digastric muscle is a bilaterally paired suprahyoid muscle located under the jaw. Its posterior belly is attached to the mastoid notch of temporal bone, and its anterior belly is attached to the digastric fossa of mandible; the two bellies are united by an intermediate tendon which is held in a loop that attaches to the hyoid bone. The anterior belly is innervated via the mandibular nerve , and the posterior belly is innervated via the facial nerve . It may act to depress the mandible or elevate the hyoid bone.
The term "digastric muscle" refers to this specific muscle even though there are other muscles in the body to feature two bellies.
Anatomy
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.
Anterior belly
The anterior belly attaches at the digastric fossa of mandible.It extends postero-inferiorly from its origin toward the intermediate tendon.
Intermediate tendon
The two bellies meet at the intermediate tendon which perforates the stylohyoideus muscle. The tendon is embraced by a fibrous sling which attaches the body and greater cornu of hyoid bone. The tendon occasionally features a synovial sheath.Innervation
The anterior belly receives motor innervation from the mylohyoid nerve.The posterior belly is supplied by the digastric branch of facial nerve.
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 , the carotid triangle, the submental triangle , and the inferior carotid triangle .Variation
The intermediate tendon may be absent. The posterior belly may arise partly 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.