Sternocleidomastoid

• Sternal head originates on the manubrium;
• Clavicular head originates on the medial 1/3rd of the clavicle.
• Both insert on the mastoid process of the temporal bone.
• Unilateral contraction laterally flexes the neck and rotates the head to turn the face to the opposite side.
• Bilateral contraction extends the neck at the base of the head (if the posterior fibers contract) or, flexes the neck so that the chin moves toward the sternum (if the anterior fibers contract).
• Sternocleidomastoid divides the cervical region into anterior and posterior triangles.

Trapezius

• Originates on the external occipital protuberance, the nuchal ligament over the cervical vertebrae, and the spinous processes of the thoracic vertebrae; it inserts along the lateral 1/3rd of the clavicle and also the acromion and spine of the scapula.
• Trapeziuseither adducts, rotates, elevates, or depresses the scapula based on which muscle fiber region contracts.

Platysma:

• Originates on the superficial fascia over the superior shoulder and chest, and inserts into the skin and muscles of the mandible; it depresses the skin of the neck and lower lip and angle of the mouth.

Geniohyoid:

• Originates on the mental spine of the mandible
• Inserts on the body of the hyoid
• Geniohyoid elevates the hyoid and the floor of the mouth.

Mylohyoid

• Originates on the mylohyoid line of the mandible
• Inserts on the body of the hyoid bone and at the median raphe
• Mylohyoid elevates the hyoid bone (and floor of the mouth) and depresses the mandible
• Mylohyoid contributes to the floor of the mouth.

Digastric has two bellies: anterior and posterior

• Anterior belly arises from the digastric fossa on the mandible
• Posterior belly arises from the mastoid process of the temporal bone
• Both bellies insert at the intermediate tendon, which passes through a sling attached to the body of the hyoid bone.
• Digastric elevates the hyoid bone (and floor of the mouth) and it depresses the mandible.

Stylohyoid

• Originates on the styloid process of the temporal bone
• Inserts on the body of the hyoid; splits over digastric
• Stylohyoid elevates the hyoid.

Because of their actions on the larynx and hyoid bone, the infrahyoid muscles are involved in swallowing and speech.

Deep muscles:

Sternothyroid

• Originates on the manubrium of the sternum and the first costal cartilage and inserts on the oblique line of the thyroid cartilage; it depresses the larynx. It overlies the trachea.

Thyrohyoid

• Originates on the oblique line of the thyroid cartilage
• Inserts on the body of the hyoid
• Depresses the hyoid and elevates the larynx

Superficial muscles:

Sternohyoid

• Originates on the manubrium and the medial end of the clavicle
• Inserts on the body of the hyoid
• Depresses the hyoid

Omohyoid (inferior belly) &
Omohyoid (superior belly)

• Originates on the superior border of the scapula and passes through a sling
• Inserts on the body of the hyoid
• Depresses, retracts, and steadies the hyoid