This is a long, narrow muscle situated in front of the neck, along the entire length of which it runs from the breast below to the lower jaw above. Its inferior portion is united with its fellow on the opposite side. About the middle of the neck they separate and pass upward and outward to their destination. The jugular vein runs on the outer border of this muscle.

Origin. - From the cariniform cartilage in front of the sternum.

Insertion. - By a short, flattened tendon into the angle of the lower jaw.

Action. - When acting with its fellow it draws the head downward. Acting alone, it pulls it to one side.