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AN10.1-13 | Axilla, Shoulder and Scapular region — Part 4

Serratus Anterior — The Boxer's Muscle (AN10.12)

Place your hand flat against the side of your chest, just below the armpit. Now push forward against a wall. The muscle you feel is the serratus anterior — a saw-toothed muscle (serratus = saw-like) that attaches from the ribs to the medial border of the scapula.

• Origin: outer surfaces of upper 8 ribs (the saw-tooth pattern)
• Insertion: costal surface of the medial border of scapula
• Action: protracts the scapula (pulls it forward around the chest wall) and rotates the scapula upward (essential for raising the arm above the head)
• Nerve supply: Long thoracic nerve (C5, C6, C7) — also called the nerve of Bell

Clinical connection — winged scapula:
The long thoracic nerve is vulnerable during surgery (mastectomy, axillary lymph node clearance) because it runs along the chest wall superficially. If it's damaged, serratus anterior is paralysed, and the scapula 'wings' — the medial border sticks out like a wing when the patient pushes against a wall.

This is called a winged scapula, and it's a classic clinical sign that examiners love to test. The patient can't raise their arm above the head because the scapula can't rotate upward.

Serratus Anterior Video Thumbnail Featuring a Serratus Anterior muscle illustration showing rib attachments and medial scapular insertion for shoulder protraction and stability. Brave (treadwelldpt.com). Used for educational purposes.

The Shoulder Joint — Maximum Mobility (AN10.11)

The glenohumeral (shoulder) joint is a synovial ball-and-socket joint — the most mobile joint in your body. It allows movements in all planes:

• Flexion (arm forward) — mainly by anterior deltoid and pectoralis major
• Extension (arm backward) — mainly by posterior deltoid and latissimus dorsi
• Abduction (arm out to the side) — initiated by supraspinatus (0–15°), then deltoid (15–90°), then trapezius rotates the scapula (90–180°)
• Adduction (arm to the body) — pectoralis major and latissimus dorsi
• Medial rotation — subscapularis, pectoralis major, latissimus dorsi
• Lateral rotation — infraspinatus and teres minor
• Circumduction — a combination of all movements

Why is the shoulder so mobile yet vulnerable?

• The glenoid cavity is small and shallow (covers only 1/3 of the humeral head) — like a golf ball on a tee
• The glenoid labrum (fibrocartilage ring) deepens the socket slightly but doesn't prevent dislocation
• The joint capsule is loose and redundant (has folds to allow movement)
• Stability depends almost entirely on the rotator cuff muscles — not on bony architecture

The shoulder dislocates more than any other joint. The most common direction is anteroinferior (the humeral head slips forward and downward) — because the rotator cuff is weakest inferiorly and the capsule is thinnest anteroinferiorly.

Illustration of glenohumeral ligaments lateral view Primal.png. Brave (physio-pedia.com). Used for educational purposes.

Scapular Anastomosis — The Backup Blood Supply (AN10.13)

What happens if the axillary artery is blocked? The shoulder region has a built-in backup: the scapular anastomosis — a network of arteries around the scapula that connects the subclavian artery system (above the block) with the third part of the axillary artery (below the block).

The key arteries in this anastomosis are:

• Suprascapular artery (from the thyrocervical trunk of the subclavian) — runs above the scapula
• Deep branch of the transverse cervical artery (also from the subclavian) — runs along the medial border of the scapula (also called the dorsal scapular artery)
• Subscapular artery and its branch, the circumflex scapular artery (from the 3rd part of the axillary artery) — runs behind the scapula

These arteries anastomose freely around the scapula, creating a detour route. If the axillary artery is slowly blocked (e.g., by atherosclerosis), the scapular anastomosis enlarges gradually and can maintain blood flow to the arm.

Spiral forward: We'll revisit arterial anastomoses as a general concept when we study the cardiovascular system (Week 20–24). The principle is the same everywhere: the body builds in backup routes near vulnerable points.