The Anatomy of the Axilla
The axilla, or underarm (or, less delicately put, the armpit) is that indentation formed under the area where your arm attaches to your shoulder. Blood, lymph vessels, and nerves going to and from the arm pass through the axilla.
Forming the apex, the base, and the walls
The size and shape of the axilla is defined by six features:
Apex: Formed by the cervicoaxillary canal; bounded by the clavicle, the 1st rib, and the top of the scapula
Base: Formed by the skin that stretches from the arm to the thoracic cage; forms the indentation known as the axillary fossa
Anterior wall: Made up of the pectoralis major and minor, forming the anterior axillary fold
Posterior wall: Made up of the subscapularis, latissimus dorsi, and teres major muscles, forming the posterior axillary fold
Medial wall: Formed by the thoracic cage and the serratus anterior
Lateral wall: Made by the intertubercular sulcus (bicipital groove) of the humerus
The axillary arteries and veins
The axillary artery branches from the subclavian artery as it passes over the 1st rib. It ends near the bottom of the scapula. The axillary artery is divided into three parts: first, second, and third:
First part of the axillary artery: Goes from the lateral border of the 1st rib to the medial border of the pectoralis minor; has one branch, the superior thoracic artery
Second part of the axillary artery: Runs behind the pectoralis minor; has two branches: the thoracoacromial artery and the lateral thoracic artery
Third part of the axillary artery: Extends from the lateral border of the pectoralis minor to the lower border of the teres major; has three branches: the subscapular artery and the anterior and posterior circumflex humeral arteries
The axillary vein is formed by the union of the accompanying brachial veins and the basilic vein. It runs up alongside the axillary artery and becomes the subclavian vein around the level of the 1st rib.