The structure indicated is the serratus anterior muscle.

The serratus anterior muscle originates on the first to eighth ribs laterally on the chest and inserts medially on the scapula. The serratus anterior is divided into three parts:

1. Serratus anterior superior
2. Serratus anterior intermediate
3. Serratus anterior inferior

The serratus anterior is often called the “boxer’s muscle” as it is responsible for protraction of the scapula – a movement which occurs when throwing a punch.

Origin: fleshy slips from lateral parts of 1^st to 8^th ribs

Insertion: Medial margin of scapula on anterior surface

Innervation: Long thoracic nerve

Action: Protraction and upward rotation of scapula