Axilla

The axilla, also known as the arm pit, is a pyramid-shaped space situated below the shoulder joint. Its apex, called the axillary inlet, faces upwards, while the base, known as the floor, points downward. Inside the axilla there are numerous vessels, nerves, lymph nodes, and adipose tissue. It serves as a pathway for nerves and vessels to travel from the neck to the arm.

The axillary inlet has a triangular shape and is bounded by the first rib on the medial side, the clavicle anteriorly, and the superior border of the scapula and coracoid process posteriorly. The trunks of brachial plexus, subclavian artery, and vein cross over the first rib as they pass through the axillary inlet into the axillary space. Inside the axilla, the trunks branch into divisions and cords, while the subclavian vessels transform into the axillary artery and vein.

The axillary space is surrounded by anterior, posterior, medial, and lateral walls, which are formed by muscles or bone. For example:

• The pectoralis major and minor muscles form the anterior wall, and the pectoralis major also creates the anterior axillary fold.

• The subscapularis, teres minor, teres major, latissimus dorsi, and long head of triceps muscles form the posterior wall.

• The serratus anterior muscle and upper ribs form the medial wall.

• The intertubercular sulcus of the humerus forms the lateral wall.

Please note that the short head of the biceps and coracobrachialis, both inserting into the coracoid process of the scapula, also pass through and ascend in the axilla.

The main artery in the axilla is the axillary artery, which is a continuation of the subclavian artery. It exits the axilla (distal to the teres major muscle) to enter the arm where it becomes the brachial artery.

The primary nerve structure within the axilla is the brachial plexus, formed by the C5-T1 roots (or anterior rami) of spinal nerves. Its branches provide innervation to the axillary, shoulder, and upper limb regions. The innervation of axillary muscles is as follows:

• The pectoralis major and minor muscles are innervated by the medial and lateral pectoral nerves (branches of the medial and lateral cords of the brachial plexus).

• The serratus anterior muscle is innervated by the long thoracic nerve (derived from the C5, C6, C7 roots of the brachial plexus).

• The coracobrachialis and short head of the biceps femoris muscles are innervated by the musculocutaneous nerve (a branch of the medial cord of the brachial plexus).

• The subscapularis muscle is Innervated by the upper and lower subscapular nerves (branches of the posterior cord of the brachial plexus).

• The teres major muscle is Innervated by the lower subscapular nerve (a branch of the posterior cord).

• The latissimus dorsi muscle is Innervated by the thoracodorsal nerve (a branch of the posterior cord).