Final answer:
The serratus anterior is a muscle originating on the first to eighth ribs and inserts onto the medial border of the scapula, its primary functions include protraction and upward rotation of the scapula.
Step-by-step explanation:
The serratus anterior is a large and flat muscle that originates on the first to eighth ribs on the side of the thorax and inserts primarily onto the medial border of the scapula.
Its primary functions include protracting the scapula, which means pulling it forward and around the rib cage, as in throwing a punch.
It also plays an important role in the upward rotation of the scapula, necessary when lifting the arm above the shoulder level. Additionally, it acts to hold the scapula against the thoracic wall.
This muscle, along with others like the subclavius, pectoralis minor, and posterior muscles such as the trapezius and rhomboids, work to position and stabilize the pectoral girdle.
The pectoral girdle consists of the clavicle and the scapula, providing a stable origin for other muscles, like the pectoralis major and latissimus dorsi, to move the humerus.