Surface Anatomy of the Neck

Which muscle divides the neck into anterior and posterior triangles?

A 5-year-old boy is taken to his pediatrician for a laceration on his right knee. A mass on his neck is noticed; his mother states it has been there for several months and is slowly getting larger. The mass is slightly to the left of midline. What is the most likely diagnosis? Ultrasound of neck shows a midline hypoechoic mass anterior and superior to the thyroid gland.