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AN10.1-13 | Axilla, Shoulder and Scapular region — Part 2

Contents of the Axilla — What's Inside the Pyramid (AN10.2)

Now that you know the walls and lymph nodes, here's the complete inventory of what's packed inside the axilla:

Axillary artery and its branches (the blood supply highway going to the arm)
Axillary vein and its tributaries (the return highway, lying medial to the artery)
Brachial plexus — the network of nerves supplying the entire upper limb (we'll cover this in Part 2)
Axillary lymph nodes — 5 groups as described above
Axillary fat — filling the spaces between these structures
Axillary tail of the breast — a projection of breast tissue that extends into the axilla (important: lumps here can be breast tissue, not just lymph nodes)

All these structures are wrapped in the axillary sheath, an extension of the prevertebral fascia from the neck that accompanies the vessels and nerves through the cervicoaxillary canal.

IMAGE PLACEHOLDER

Overview of axillary contents: axillary artery, axillary vein, cords of brachial plexus, lymph nodes, and axillary fat, all within the axillary sheath

SELF-CHECK

The axillary artery is divided into three parts by a muscle crossing it. Which muscle is this, and how many branches does the 2nd part give off?

A. Pectoralis major; 2 branches

B. Pectoralis minor; 2 branches

C. Pectoralis minor; 3 branches

D. Subscapularis; 1 branch

Reveal Answer

Answer: B. Pectoralis minor; 2 branches

The pectoralis minor crosses the axillary artery and divides it into three parts. The 2nd part (behind pectoralis minor) gives off 2 branches: the thoracoacromial artery and the lateral thoracic artery. Remember: the number of branches matches the part number — 1, 2, 3.

The Brachial Plexus — A Highway System for the Arm (AN10.3)

Every movement of your arm — from waving to writing to performing surgery — is controlled by nerves that originate from your spinal cord. These nerves don't travel to the arm individually; they first merge, split, and reorganise into a complex network called the brachial plexus.

Think of it as a highway interchange system: five entrance ramps (spinal nerve roots) merge into highways (trunks), pass through an interchange (divisions), reorganise into new highways (cords), and finally exit as individual routes (terminal branches) to specific muscles and skin areas.

The brachial plexus has five levels, remembered by the mnemonic "Robert Taylor Drinks Cold Beer":

1. Roots — C5, C6, C7, C8, T1 (five spinal nerve ventral rami)
2. Trunks — three: upper (C5+C6), middle (C7 alone), lower (C8+T1)
3. Divisions — each trunk splits into anterior and posterior divisions (6 total)
4. Cords — three, named by their position around the axillary artery:
- Lateral cord — anterior divisions of upper + middle trunks (C5,6,7)
- Posterior cord — all three posterior divisions (C5,6,7,8,T1)
- Medial cord — anterior division of lower trunk (C8,T1)
5. Branches — terminal nerves that supply the arm

The key terminal branches are:
• Musculocutaneous nerve (C5,6,7) — from the lateral cord → supplies anterior arm muscles (biceps)
• Median nerve (C5–T1) — from lateral + medial cords → supplies forearm flexors and hand
• Ulnar nerve (C8,T1) — from the medial cord → the 'funny bone' nerve
• Radial nerve (C5–T1) — from the posterior cord → supplies all extensors
• Axillary nerve (C5,6) — from the posterior cord → supplies deltoid and teres minor

IMAGE PLACEHOLDER

Diagram of the brachial plexus showing five levels: roots (C5-T1), trunks (upper, middle, lower), divisions (anterior/posterior), cords (lateral, posterior, medial) around axillary artery, and terminal branches (musculocutaneous, median, ulnar, radial, axillary)

Erb's Palsy — The Waiter's Tip Position (AN10.5)

When the upper trunk of the brachial plexus (C5, C6) is injured, it's called Erb-Duchenne palsy (commonly just Erb's palsy). This is the most common brachial plexus injury.

How does it happen?
• In newborns: during a difficult delivery when the baby's head is pulled to one side while the shoulder is stuck (shoulder dystocia)
• In adults: a fall onto the shoulder with the head pushed to the opposite side, or a motorcycle accident

The injury damages nerves that supply the shoulder abductors (deltoid), external rotators (infraspinatus), elbow flexors (biceps, brachialis), and supinators (supinator, biceps).

The result — the "waiter's tip" position:
• The arm hangs limply at the side (adducted — deltoid paralysed)
• The arm is medially rotated (external rotators paralysed, internal rotators unopposed)
• The elbow is extended (biceps paralysed)
• The forearm is pronated (supinators paralysed)

The hand looks like the person is waiting for a tip — palm facing backward and outward. The hand grip is preserved because the lower roots (C8, T1) are intact.

Anatomical diagram of the brachial plexus with C5 and C6 nerve roots and upper trunk highlighted — click to enlarge