The Serratus Anterior Muscle is a flat, broad muscle that originates from the upper eight or nine ribs on the side of the human trunk and inserts onto the anterior surface of the medial (inner) border of the scapula (shoulder blade). It is innervated by the long thoracic nerve, which originates from the brachial plexus. The primary function of the serratus anterior is to pull the scapula forward around the thorax. This muscle plays a key role in upper extremity movements, particularly in raising and lowering the arm. Its name, derived from the Latin verb "serrare" (to cut), refers to the saw-toothed appearance of its edges. "Anterior" emphasizes its position at the front of the body.

Anatomy

Morphology and Settlement

The Serratus Anterior Muscle is a powerful muscle located on the lateral chest wall, with a broad surface and numerous muscle slips that insert on the ribs. The muscle usually originates from the first to the eighth or ninth ribs, forming nine or ten separate slips. Multiple slips can occur, particularly from the second rib, resulting in a total of more than the number of ribs from which they originate. The muscle fibers project to the scapula, where they insert on the medial surface of the medial border of the scapula, extending between its superior and inferior angles. This insertion divides the muscle into three functional divisions: the serratus anterior superior, which lies around the superior angle of the scapula. the serratus anterior intermedius, which lies along the medial border. the serratus anterior inferior, which lies near the inferior angle of the scapula.

Embryological Origin and Anatomical Relationships

The Serratus Anterior Muscle develops embryologically from the ventrolateral mesodermal muscle mass. During development, it forms part of the thoracic musculature along with the pectoralis major and minor muscles. Anatomically, it lies deep to the subscapularis muscle, with the supraserratus bursa located between these two structures. Conversely, the infraserratus bursa, located between the muscle and the ribs, reduces friction and facilitates movement. The muscle's wide distribution on the lateral chest wall and its attachment to the scapula provide a mechanical advantage for both thoracic and upper extremity movements.

Nerve Innervation and Vascular Supply

The Serratus Anterior Muscle is innervated exclusively by the long thoracic nerve (n. thoracicus longus). This nerve originates from the upper trunk of the brachial plexus (C5–C7) and courses down the surface of the muscle, directly reaching the muscle fibers. Due to its superficial course, the long thoracic nerve is vulnerable to trauma and surgical injuries. Vascular supply to the muscle is provided by branches of the lateral thoracic artery, the thoracodorsal artery, and the superior thoracic artery. These vessels branch according to the muscle's segmental structure, providing wide-area perfusion.

Function and Clinical Relationships

The primary function of the serratus anterior muscle is to pull the scapula forward and outward along the chest wall. This movement allows the arm to be extended forward (anteversion). In this respect, the muscle acts as an antagonist to the rhomboid muscles. However, when the upper and lower portions of the serratus anterior work together, it can also act synergistically with the rhomboids, compressing the scapula against the chest wall. The lower portion of the muscle pulls the inferior angle of the scapula forward and outward, contributing to the elevation of the arm overhead. When the shoulder girdle is stabilized, the serratus anterior fully contracts, elevating the ribs and acting as an accessory respiratory muscle. It is particularly activated during labored breathing. The muscle plays an active role in thrusting the arm forward or throwing a punch. For this reason, it is also known as the "boxer's muscle" or "big swing muscle." It also works with the upper and lower fibers of the trapezius muscle to rotate the scapula upward. This movement is essential for overhead arm activities (e.g., combing hair or reaching for shelves). Clinically, the most significant condition associated with the serratus anterior muscle is injury to the long thoracic nerve. Removal of axillary lymph nodes, particularly during breast cancer surgery, can damage this nerve. As a result of nerve damage, the muscle fails to function properly, and the scapula cannot remain attached to the body. This leads to a condition known as "winged scapula" (scapula alata), where the scapula is significantly splayed outward. In this clinical condition, the patient is unable to raise their arm overhead and experiences severely limited shoulder movement.