The armpit, or axilla, is not defined by a single muscle. Instead, it is a complex anatomical region where several muscles converge and create boundaries. This space also serves as a crucial passageway for various essential structures, including nerves, blood vessels, and lymph nodes.
The Armpit’s True Nature
The axilla is a pyramidal-shaped space located at the junction of the upper arm and the chest wall. It functions primarily as a protected channel for structures extending between the neck, chest, and upper limb. This region is an anatomical hollow filled with loose connective tissue, including fat, which allows for the passage and movement of vital components. The axilla’s boundaries are formed by an intricate arrangement of bones and muscles, giving it its characteristic shape and allowing for extensive arm movement.
Key Muscles Defining the Armpit Region
Several muscles contribute to forming the walls and contents of the axilla. The anterior wall is primarily composed of the Pectoralis major, a large, fan-shaped muscle that forms the bulk of the chest and the visible anterior axillary fold, and the smaller Pectoralis minor, which lies beneath it. The posterior wall is formed by the Latissimus dorsi, a broad muscle of the back that wraps around to the front near the armpit, the Teres major, which runs along the lower border of the scapula, and the Subscapularis muscle, located on the anterior surface of the shoulder blade.
The medial wall is formed by the upper ribs and the Serratus anterior muscle, which lies over the chest wall and attaches to the scapula. The lateral boundary is a narrow groove on the humerus, the upper arm bone, where the Coracobrachialis muscle is found. Additionally, the proximal parts of the Biceps brachii and Triceps brachii muscles are closely related to the axilla, contributing to its overall structure.
The Role of These Muscles
The muscles surrounding the armpit play a cohesive role in the movement and stability of the shoulder joint and arm. The Pectoralis major is a powerful muscle that allows for movements such as flexing the arm forward, extending it from a flexed position, and rotating it inward. The Pectoralis minor helps to stabilize the shoulder blade and assists in drawing it downward and forward against the chest wall.
The Latissimus dorsi and Teres major work together to extend the arm backward, pull it toward the body (adduction), and rotate it inward. These muscles are particularly active in pulling motions. The Subscapularis muscle, one of the four rotator cuff muscles, is primarily responsible for internally rotating the arm and contributes to shoulder stability.
The Coracobrachialis muscle assists in flexing the arm at the shoulder and pulling it toward the body. The Biceps brachii, located on the front of the upper arm, is primarily involved in bending the elbow and rotating the forearm, while the Triceps brachii, on the back of the arm, is responsible for straightening the elbow. The coordinated action of these muscles enables a wide range of arm movements and ensures the stability of the shoulder.