Omohyoid muscle

Origin and insertion

The narrow flat omohyoid muscle consists of two bellies, an inferior and a superior belly, joined by an intermediate tendon; similar to the digastric muscle. Each of these muscle bellies has their own unique origin points.

The inferior belly of omohyoid muscle originates from the superior border of the scapula, medial to the suprascapular notch. Some fibers may also arise from the transverse scapular ligament. From there, the muscle inclines anteriorly and superiorly towards the lower part of the neck, inserting into the omohyoid’s intermediate tendon at the level of the arch of cricoid cartilage in the lateral cervical region. The superior belly of omohyoid arises from the intermediate tendon, near the level of the internal jugular vein. Here, the muscle changes its direction and courses almost vertically to insert at the lower border of the body of the hyoid bone, lateral to the insertion of the sternohyoid muscle.

Relations

Upon originating at the scapula, the inferior belly of omohyoid passes deep to the sternocleidomastoid muscle to join the intermediate tendon located behind it. Further along its course towards the hyoid bone, the superior belly of omohyoid muscle converges with the sternohyoid muscle and inserts in close proximity to it. Occasionally, the superior belly can even be fused with the sternohyoid muscle and share a common attachment. 

The intermediate tendon of omohyoid is ensheathed by deep cervical fascia. This acts as a sling, binding the muscle to the inferiorly located clavicle and first rib, and helping the muscle to maintain its angulated pathway. The intermediate tendon is also connected to the carotid sheath, which surrounds the neurovascular bundle containing the common carotid artery, internal jugular vein and vagus nerve (CN X). This is why the intermediate tendon is sometimes used as a surgical landmark for the carotid artery or internal jugular vein. 

The omohyoid muscle forms the borders of two important triangles in the neck:

• Occipital triangle: the base of this triangle is formed by the superior margin of the inferior belly of omohyoid muscle. This triangle contains some important nerves, including the accessory nerve (CN XI) and branches of both the cervical plexus and brachial plexus. 
• Supraclavicular triangle: the superior border of this triangle is formed by the inferior margin of the inferior belly of omohyoid muscle. This triangle contains structures such as branches of the brachial plexus, subclavian artery, nerve to subclavius and various lymph nodes.

Don’t be alarmed if you haven’t mastered the triangles of the neck just yet. Yes, there are a lot of them but we’re here to help! Take a look at the following video.

Innervation

All infrahyoid muscles, including the omohyoid, are supplied by the ansa cervicalis which arises from the cervical plexus. The superior belly of omohyoid muscle is innervated by the superior root of ansa cervicalis (C1), while the inferior belly receives fibers from spinal nerves C1, C2 and C3.

Blood supply

The omohyoid muscle, just like the rest of the infrahyoid muscles, receives its arterial blood supply from the superior thyroid artery, a branch of the external carotid artery; and inferior thyroid artery, a branch of the thyrocervical trunk. 

Function

The infrahyoid muscles, along with the suprahyoid muscles, are responsible for the positioning of the hyoid bone and the larynx. Specifically, the infrahyoid muscles, and omohyoid, depress the hyoid bone following its elevation during the act of swallowing. This action reopens the laryngeal inlet, which is normally closed of during swallowing to prevent inhalation of the food bolus. Thus, the act of opening the laryngeal inlet reestablishes breathing after swallowing.

An additional function of the omohyoid is related to its attachment at the carotid sheath. By contracting, it pulls on the sheath and maintains a low pressure in the internal jugular vein, thereby increasing the venous return from the head to the superior vena cava. Likewise, it is thought that this fascial connection allows the omohyoid muscle to control the bodily response to extended inspiratory activity, in which it tenses the cervical fascia and thereby lessens the likelihood of inward suction of soft tissues that would compress the great vessels and lung apices. 

Learn more about the infrahyoid muscles and other muscles of the ventral neck in our learning materials on the topic.