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.

The first part has one branch, the superior thoracic artery, which supplies the subclavius muscle, the muscles of the 1st and 2nd intercostal spaces, as well as the superior aspect of the serratus anterior.

The second part usually has two branches, the thoracoacromial artery and the lateral thoracic artery. The thoracoacromial artery comes out medial to the pectoralis minor as a short wide trunk and divides into four branches deep to the the pectoralis major: the acromial branch, which goes towards the acromion process of the scapula; the deltoid branch, which passes between the deltoid and pectoralis major muscles in the deltopectoral groove; the pectoral branch, which passes between the pectoralis major and minor and supplies both muscles; and the clavicular branch which supplies the subclavius muscle and the sternoclavicular joint.

The lateral thoracic artery travels along the lateral border of pectoralis minor towards the thoracic wall to supply the pectoralis muscles, serratus anterior, intercostal muscles, axillary lymph nodes, and the lateral aspect of the breast.

The third part of the axillary artery has, you guessed it!, three branches: the subscapular artery, anterior circumflex humeral, and the posterior circumflex humeral arteries.

The subscapular artery is the largest branch of the axillary artery, which travels along the subscapularis muscle before quickly dividing into its terminal branches: the circumflex scapular and thoracodorsal arteries.

The circumflex scapular artery turns posteriorly and passes between the subscapularis and teres major muscles to reach the dorsal surface of the scapula, where it forms an important anastomosis with the suprascapular artery around the scapula.

The thoracodorsal artery travels with the thoracodorsal nerve to supply the latissimus dorsi muscle. The final two branches of the third part are the anterior and posterior circumflex humeral arteries, which can emerge from the axillary artery separately or as one trunk before dividing.

These arteries encircle the surgical neck of the humerus and anastomose with each other, like an arterial hug for the humerus.

The smaller anterior circumflex humeral artery passes deep to the coracobrachialis and biceps brachii, and gives an ascending branch to supply the shoulder joint.

The posterior circumflex humeral artery passes posteriorly between the teres major and minor muscles along with the axillary nerve to supply the glenohumeral joint, or the shoulder joint, and to the surrounding muscles - most importantly, the deltoid muscle.

Since this space between the teres major and minor muscles looks like a quadrilateral, it is called the quadrangular space.

The medial side of the quadrangular space is formed by the long head of the triceps muscle, and the lateral boundary is formed by the humerus.

Now that was a lot of info! Take a second to pause the video and see if you can label the branches of the axillary artery before we move on.

Alright, now let’s move on to the axillary vein. The axillary vein is formed by the union of the brachial veins and the basilic vein at the inferior border of the teres major.

It ends at the lateral border of the first rib, where it turns into the subclavian vein. The axillary fat also contains five major groups of lymph nodes, which are collectively called the axillary lymph nodes.Time for some more spatial geometry!

Imagine that four of these groups of lymph nodes are the corners of a pyramid with a triangular base. In the corners of the triangular base, there are the anterior, or pectoral; the posterior, or subscapular; and the lateral, or humeral nodes.

Next, we have one group in the center of the pyramid, called the central nodes, and one group at the apex, called the apical nodes.

The anterior nodes are located along the medial wall of the axilla, and receive lymphatic drainage from the anterior thoracic wall and most of the breast.

The posterior nodes are located along the posterior axillary fold, and receive lymph from the posterior thoracic wall and the scapular region.

The lateral nodes are located along the lateral wall of the axilla around the axillary vein, and receive lymph from most of the upper limb.

The lymph from the anterior, posterior, and lateral nodes passes through efferent lymphatic vessels towards the central nodes.

These central nodes are located deep to the pectoralis minor muscle near the base of the axilla. Lymph from the central nodes then passes to the apical nodes, which also receive lymph from the lymphatic vessels accompanying the cephalic vein.

Alright, as a quick recap, the axilla is a pyramid shaped space that lies between the thorax and the arm, and serves as a connection point between the arm, neck, thorax, and pectoral regions.

It contains a number of neurovascular structures that are embedded in a fatty matrix, including the three parts of the axillary artery and their branches, the axillary vein and its tributaries, and the five groups of axillary lymph nodes.