Final answer:
The long thoracic nerve serves the serratus anterior muscle, originating from the anterior rami of C5, C6, and C7 and is important for scapular movement.
Step-by-step explanation:
The nerve that serves the serratus anterior, a large and flat muscle originating on the ribs and inserting onto the scapula, is a lateral branch of the anterior rami of C5, C6, and C7. This nerve is known as the long thoracic nerve. It lies on the superficial side of the serratus anterior muscle and is crucial for movements of the scapula involved in pushing activities and overhead arm elevation.