Sternothyroid muscle
The sternothyroid muscle (or sternothyroideus) is an infrahyoid muscle of the neck. It acts to depress the hyoid bone. Structure 
The two muscles are in contact with each other proximally (close their origin), but diverge distally (towards their insertions). Origin 
The sternothyroid arises from the posterior surface of the manubrium of the sternum (inferior to the origin of the sternohyoid muscle), and the posterior margin of the first costal cartilage. Insertion 
It inserts onto the oblique line of the lamina of thyroid cartilage. Innervation 
The sternothyroid muscle receives motor innervation from branches of the ansa cervicalis (ultimately derived from cervical spinal nerves C1-C3). Relations 
The sternothyroid muscle is shorter and wider than the sternohyoid muscle and is situated deep to and partially medial to it. Variations
The muscle may be absent or doubled. It may issue accessory slips to the thyrohyoid muscle, inferior pharyngeal constrictor muscle, or the carotid sheath. Actions/movements 
The sternothyroid muscle depresses the hyoid bone. When the hyoid bone is fixed, it instead elevates the larynx (producing an increased voice pitch). Clinical significance
The upward extension of a thyroid swelling (goitre) is prevented by the attachment of the sternothyroid to the thyroid cartilage. A goitre can therefore only grow to the front, back or middle but no higher. Additional images

References

External links
 Photo of model at Waynesburg College musclehead/sternothyroid
  - "The Muscular triangle"
 PTCentral

Muscles of the head and neck