Simplify the following text for a Grade 9 native English-speaking person.
Original: In human anatomy , the trapezius is a large superficial muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae and laterally to the spine of the scapula ( shoulder blade ) .
Simplified: In human , trapezius muscle or trapezius is a large muscle that extends longitudinally on the upper portion of a human spine .