Digastric muscle

Origin and insertion

The digastric muscle is comprised of two parts; anterior and posterior bellies, joined by an intermediate tendon. Each of the digastric muscle bellies has a unique point of origin. 

• The posterior belly originates at the medial surface of mastoid notch of temporal bone. From here it travels anteroinferiorly towards the hyoid bone, piercing the stylohyoid muscle before attaching into the intermediate tendon of digastric muscle.
• The anterior belly of the digastric muscle originates from the digastric fossa of lower border of mandible, close to the midline near the mandibular symphysis (symphysis menti). This portion of the muscle extends posteroinferiorly from the mandible, joining with the intermediate tendon

Above the hyoid bone, these two muscle bellies unite as the intermediate tendon. The intermediate tendon of digastric muscle is encircled by a U-shaped fibrous tissue sling, formed by a thickening of the investing layer of the deep cervical fascia. This sling is anchored on the superior side of the body of hyoid bone. It acts as a pulley, allowing the intermediate tendon to slide anteriorly and posteriorly. 

Do you feel like you’re instantly forgetting all of these muscle facts? Then try out our neat muscle reference charts; the attachments, innervation and function of every muscle in the human body, all in one place!

Relations 

As the posterior belly of digastric descends towards the hyoid bone, it travels closely behind (posterior) the stylohyoid muscle. Later it passes through this same muscle. Also closely related to the posterior belly is the neurovascular bundle of the neck, consisting of the internal jugular vein, external and internal carotid arteries, and vagus, glossopharyngeal and hypoglossal nerves. These structures pass deep to the posterior belly of the digastric muscle.

The digastric muscle divides the anterior triangle of the neck into three smaller divisions: the carotid triangle, the submental triangle and the submandibular triangle.

• Carotid triangle: The posterior belly of digastric muscle forms the superior border of the carotid triangle. This paired triangle contains some very important structures, such as the common carotid artery, internal carotid artery and external carotid artery. 
• Submental triangle: This unpaired triangle is bordered laterally by the anterior bellies of digastric muscles on each side, and inferiorly by the body of the hyoid bone. This triangle contains submental lymph nodes, submental veins and jugular veins.
• Submandibular triangle: This paired triangle is also known as the digastric triangle, since its borders are formed by the anterior and posterior bellies of digastric muscle, along with the inferior border of the mandible. This triangle contains structures such as the submandibular gland and submandibular lymph nodes. 

Innervation

A characteristic feature of the digastric muscle is that its bellies have different embryologic origins, and hence different innervations. 

The anterior belly is derived from the first pharyngeal arch and is therefore innervated by the nerve to mylohyoid muscle, a branch of the inferior alveolar nerve that arises from the mandibular nerve. The posterior belly of the digastric muscle is derived from the mesoderm of the second pharyngeal arch and is therefore innervated by the digastric branch of the facial nerve (cranial nerve 7). 

Blood supply

The anterior belly of the digastric muscle is supplied mainly by the submental artery of the facial artery, while the posterior belly receives its arterial blood supply from the posterior auricular and occipital arteries.

Test what you've learned so far about the digastric muscle and the other muscles of the anterior compartment of the neck with the quiz below!

Functions

Digastric has two main functions:

• Depression of the mandible when the hyoid bone is fixed.
• Elevation of the hyoid bone and larynx when the mandible is fixed.

By depressing the mandible, the digastric muscle aids the act of opening the mouth, which is required when acting against resistance. This way the digastric muscle, together with the rest of the suprahyoid muscles, helps the action of chewing, swallowing and speech. This is why the suprahyoid muscles are sometimes referred to as accessory muscles of mastication. 

Conversely, when the mandible is fixed, the digastric muscle elevates the hyoid bone and the larynx with it. Superior movement of the hyoid and larynx results in lowering or closing of the epiglottis, blocking the entrance to the trachea and thereby preventing the inhalation of food or liquid during swallowing. 

Learn more about the digastric muscle and other suprahyoid muscles with our learning materials.