The Axilla-Walls of the Axilla-Contents of the Axilla-Key Muscles in the Axilla-Pectoralis Minor-Clavipectoral Fascia-Absent Pectoralis Major-

The Axilla

The axilla, or armpit, is a pyramid-shaped space between the upper part of the arm and the side of the chest. It forms an important passage for nerves, blood, and lymph vessels as they travel from the root of the neck to the upper limb. The upper end of the axilla, or apex, is directed into the root of the neck and is bounded in front by the clavicle, behind by the upper border of the scapula, and medially by the outer border of the first rib. The lower end, or base, is bounded in front by the anterior axillary fold (formed by the lower border of the pectoralis major muscle), behind by the posterior axillary fold (formed by the tendon of latissimus dorsi and the teres major muscle), and medially by the chest wall

Walls of the Axilla

The walls of the axilla are made up as follows:

■■ Anterior wall: By the pectoralis major, subclavius, and pectoralis minor muscles
■■ Posterior wall: By the subscapularis, latissimus dorsi, and teres major muscles from above down

■■ Medial wall: By the upper four or five ribs and the intercostal spaces covered by the serratus anterior muscle

■■ Lateral wall: By the coracobrachialis and biceps muscles in the bicipital groove of the humerus
The base is formed by the skin stretching between the anterior and posterior walls.

Contents of the Axilla

The axilla contains the axillary artery and its branches, which supply blood to the upper limb; the axillary vein and its tributaries, which drain blood from the upper limb; and lymph vessels and lymph nodes, which drain lymph from the upper limb and the breast and from the skin of the trunk, down as far as the level of the umbilicus. Lying among these structures in the axilla is an important nerve plexus, the brachial plexus, which innervates the upper limb. These structures are embedded in fat.

Key Muscles in the Axilla

Pectoralis Minor

The pectoralis minor is a thin triangular muscle that lies beneath the pectoralis major. It arises from the3rd, 4th, and 5th ribs and runs upward and laterally to be inserted by its apex into the coracoid process of the scapula. It crosses the axillary artery and the brachial plexus of nerves. It is used when describing the axillary artery to divide it into three parts
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Clavipectoral Fascia

The clavipectoral fascia is a strong sheet of connective tissue that is attached above to the clavicle. Below, it splits to enclose the pectoralis minor muscle and then continues downward as the suspensory ligament of the axilla and joins the fascial floor of the armpit.

Absent Pectoralis Major

Occasionally, parts of the pectoralis major muscle may be absent. The sternocostal origin is the most commonly missing part, and this causes weakness in adduction and medial rotation of the shoulder joint.