The axilla is the space between the upper arm and the side of the thorax, bounded in front and behind by the axillary folds, communicating above with the posterior triangle of the neck and containing neurovascular structures, including lymph nodes, for the upper limb and the side wall of the thorax. Its floor is the axillary fascia extending from the fascia over the serratus anterior to the deep fascia of the arm, attached in front and behind to the margins of the axillary folds, and supported by the suspensory ligament from the lower border of pectoralis minor. Its anterior wall is completed by pectoralis major, pectoralis minor, subclavius and the clavipectoral fascia. The posterior wall extends lower; it is formed by subscapularis and teres major, with the tendon of latissimus dorsi winding around the latter muscle. The medial wall is formed by the upper part of serratus anterior, the lower limit of the axilla being defined as the level of the fourth rib. The anterior and posterior walls converge laterally to the lips of the intertubercular groove of the humerus.
The apex is bounded by clavicle, scapula and the outer border of the first rib; it is the channel of communication between axilla and posterior triangle.
The axilla virtually does not exist when the arm is fully abducted. Its folds disappear as their muscles run tangentially along the humerus, and the axillary hollow is replaced by a bulge.