Anatomy of the axilla

I know what you’re thinking: Isn’t the axilla just a fancy name for the armpit?! Well, besides the axilla being notoriously ticklish, it’s also where many important neurovascular structures enter and exit to reach their target locations.

Think of it like a train station, with many trains passing through enroute to delivering electrochemical signals, blood, and lymphatics to their appropriate destinations.

Now, let’s start with the boundaries of the axilla. The axilla is located at the junction of the arm and thorax, and connects superiorly to the neck, anteriorly to the pectoral region, inferolaterally to the upper limb, and inferomedially to the thoracic wall.

The axilla is shaped like a pyramid that has an apex, a base, and four walls. The apex of the axilla is also called the cervico-axillary canal, which is the door between the neck and the axilla.

The cervico-axillary canal is bounded by the first rib, clavicle, and superior edge of the scapula. The base of the axilla is formed by skin, subcutaneous tissue, and axillary fascia, and forms what is called the axillary fossa, or what we know as the armpit.

Moving on to the four walls of the axilla, the anterior wall is made up by two muscles: the pectoralis major and pectoralis minor.

The inferior-most aspect of the anterior wall is called the anterior axillary fold, and it’s formed by the pectoralis major.

The posterior wall of the axillary is formed mainly by the scapula and overlying subscapularis muscle.

The inferior aspect of the posterior wall is formed by the teres major and latissimus dorsi muscles, inferiorly forming the posterior axillary fold. The medial wall is formed by serratus anterior muscle that overlies the 1st-4th ribs and intercostal muscles. Finally, the lateral wall is a bony wall formed by the intertubercular sulcus of the humerus.

Let’s move on to the contents of the axilla. Right below the skin, there’s a lot of fat and connective tissue. If you dissect deeper, you will see the axillary sheath that surrounds the axillary vein, which is the most superficial, as well as the axillary artery and the surrounding brachial plexus. The axilla also contains lymphatic vessels and axillary lymph nodes.

Let’s start with the axillary artery, which begins as a continuation of the subclavian artery at the lateral border of the first rib, and extends to the inferior border of the teres major before turning into the brachial artery. The axillary artery is divided into three parts as it passes posterior to the pectoralis minor. The first part is between the lateral border of the first rib and the medial border of the pectoralis minor, and is contained within the axillary sheath.

The second part is posterior to the pectoralis minor, and the third part extends from the lateral border of the pectoralis minor to the inferior border of teres major.

Each part of the axillary artery has a number of important branches. One memory trick is that the part number also tells you how many branches it has.