Which structure forms the medial boundary of the axilla?

The serratus anterior muscle is indeed the structure that forms the medial boundary of the axilla. This muscle plays a significant role in the movement and stabilization of the scapula during actions such as reaching and pushing. It is positioned laterally to the thorax and extends from the first to the eighth or ninth ribs, attaching to the medial border of the scapula.

The importance of the serratus anterior muscle in the context of the axilla is highlighted by its proximity to major neurovascular structures, including the brachial plexus and axillary artery and vein, which traverse this region. Its position and function contribute to the overall anatomy of the axilla and the upper limb, making it a critical landmark in surgeries and interventions involving the shoulder and upper thorax.

In contrast, the humeral head, pectoralis major muscle, and subscapularis muscle are related to the shoulder and axillary region but do not define the medial boundary of the axilla like the serratus anterior does. The humeral head is part of the shoulder joint and does not contribute to the axillary boundary, while the pectoralis major forms the anterior boundary of the axilla, and the subscapularis lies on the posterior aspect.