The serratus anterior muscle is found on the sides and back of the torso on both sides. It is made up of several smaller muscles, giving it a serrated, or toothed appearance, and its primary function is to stabilize and affix the scapula, or shoulder blade, to the chest wall. If its function is impeded, the scapula may lift off the chest wall during certain maneuvers with an appearance of wings -- known medically as "winged scapula."
The serratus anterior muscle is actually a collection of separate muscles that arise from the anterior, or front, side of ribs one through eight or nine, depending on the individual. This attachment occurs just past what is known as the mid-axillary line, or the line running down from the center of your armpit, medically known as your axilla. At the other end, the serratus anterior muscle inserts into the anterior aspect of the medial border of the scapula -- or the front facing side of the middle edge of the shoulder blade.
The muscle is called serratus anterior due to its attachment, and due to its structure. As it is made up of eight segments in a typical person, it has a serrated appearance much like that of a serrated knife. It is possible to observe this looking in the mirror with your arms raised. The lateral thoracic arteries supply blood to the serratus anterior muscle, and the long thoracic nerve arising from cervical vertebrae C-5 through C-6 in the neck provides its nervous system connection.
The purpose of the serratus anterior muscle, like that of any muscle, can be derived directly from its attachments: Its action is to hold the scapula, or shoulder blade, against the back and to stabilize it while you use your shoulder. Specifically, it abducts and upwardly rotates the scapula -- pulling the medial aspect of the scapula forward and up. This serves to anchor the scapula against the chest wall during abduction of the arm, when the arm is pressed back as in a pushup.
If either the nervous supply or blood supply to the serratus anterior muscle is sufficiently interrupted, through either trauma or tumor invasion, this can cause a condition known as winged scapula. In order to test for this condition, a medical professional watches a patient perform a standing pushup maneuver against a wall. If there is dysfunction of the serratus anterior muscle, the medial edge of the scapula will lift off the back of the chest wall on the affected side, giving the appearance of a wing.