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AN9.1-3 | Pectoral region — Part 1
CLINICAL SCENARIO
A 35-year-old woman presents to the outpatient clinic after finding a lump in her left breast during self-examination. Her doctor palpates the lump and notes it is hard, irregular, and tethered to the overlying skin, causing a classic 'dimple.' She is referred for surgical assessment and a mammogram.
This scenario is not unusual — breast cancer is the most common cancer in women worldwide. But here is the question that separates the doctor who can help from the one who merely worries: Where exactly does the breast drain its lymph? Which lymph node group is first involved in spread? Through which muscles does a surgeon cut to reach the lesion?
The answers lie entirely in the anatomy of the pectoral region — what you will master in this guide.
WHY THIS MATTERS
Why does a medical student need to know the pectoral region in detail?
- Clinical procedures: Chest drains, central venous lines, and cardiac resuscitation all involve pectoral anatomy.
- Surgery: Mastectomy, axillary lymph node dissection, and breast reconstruction require precise knowledge of pectoral muscles, fascia, and vessels.
- Breast cancer: The lymphatic drainage of the breast determines the pattern of metastatic spread — this is not theory, it is examined in every surgical finals paper.
- Examination: Palpating and describing a breast lump correctly is a clinical skill you will demonstrate in OSCEs.
- NMC 2024: Competencies AN9.1–AN9.3 are assessed in Written exams and Viva voce.
In Biochemistry this week, you are studying calcium homeostasis — the same calcium that governs muscle contraction in every pectoral muscle fibre. In Physiology, you are learning about nerve and muscle physiology — the same principles explain why pectoralis major is innervated by both medial and lateral pectoral nerves.
RECALL
Before we begin, let us check what you already know from NCERT Biology and your first weeks of MBBS.
What you already know:
• Muscle tissue is made of fibres that contract when stimulated by a nerve
• Tendons attach muscles to bones; ligaments attach bones to bones
• The thorax (chest) contains the heart and lungs, protected by ribs and the sternum (breastbone)
• The shoulder connects the upper limb to the trunk
• The breast in females contains mammary glands for milk production
Quick recap — where is the pectoral region?
Place your hand flat on your upper chest, just below your collarbone (clavicle). The bulging muscle you feel under your palm — when you push your palms together hard — is pectoralis major. This is the pectoral region: the front of the chest wall, from the collarbone down to the lower ribs, home to two muscles, a specialised fascia, and the breast.
Pectoralis Major — The Powerful Fan
Pectoralis major (Latin: pectus = chest, major = larger) is the large, fan-shaped muscle that forms the front wall of the axilla (armpit) and covers much of the anterior thorax.
Attachments (Origin to Insertion):
Pectoralis major has a broad origin from three parts:
• Clavicular head — medial half of the anterior surface of the clavicle (collarbone)
• Sternocostal head — anterior surface of the sternum, costal cartilages of ribs 1–6, and the aponeurosis of the external oblique muscle
• Abdominal head — anterior layer of the rectus sheath (small contribution)
All three heads converge to insert on the lateral lip of the bicipital groove (intertubercular sulcus) of the humerus (upper arm bone). The insertion is bilaminar — the clavicular fibres fold posteriorly over the sternocostal fibres.
Actions:
Pectoralis major is a powerful mover of the shoulder:
1. Adduction — brings the arm toward the body
2. Medial rotation — rotates the arm inward
3. Flexion — raises the arm forward (clavicular head; up to 90 degrees)
4. Extension from flexed position — pulls arm back down (sternocostal head)
5. Accessory muscle of inspiration — when the arm is fixed, expands the thorax
Nerve Supply:
• Clavicular head — lateral pectoral nerve (C5, C6, C7)
• Sternocostal head — medial pectoral nerve (C8, T1) AND lateral pectoral nerve
Mnemonic: "Lat serves the little head, Med serves the main mass" — but both nerves cross over, so always preserve both during surgery.
The lateral and medial pectoral nerves arise from the brachial plexus. Injury during axillary surgery can cause weakness or wasting of pectoralis major — a visible clinical deformity.
IMAGE PLACEHOLDER
Anterior view of pectoralis major showing clavicular and sternocostal heads, with origin and insertion labelled
Pectoralis Minor — The Deeper Stabiliser
Pectoralis minor (Latin: minor = smaller) lies deep to pectoralis major. It is a thin, flat, triangular muscle that stabilises the scapula against the thoracic wall.
Attachments:
• Origin: Outer surfaces of ribs 3, 4, and 5 near their costal cartilages
• Insertion: Coracoid process of the scapula (the beak-shaped projection on the front of the shoulder blade)
Mnemonic: "3, 4, 5 keep the minor alive" — ribs 3, 4, and 5.
Actions:
• Depresses the scapula (pulls shoulder forward and down)
• Protracts the scapula (draws it forward around the chest wall)
• Rotates the glenoid cavity downward
• Accessory muscle of forced inspiration — when scapula is fixed, elevates ribs 3–5
Nerve Supply:
• Medial pectoral nerve (C8, T1) — primarily
• Some contribution from the lateral pectoral nerve
Clinical relevance — the landmark for axillary lymph node levels:
• Level I nodes: lateral to pectoralis minor
• Level II nodes: behind pectoralis minor
• Level III nodes: medial to pectoralis minor
In breast cancer surgery (modified radical mastectomy), the surgeon uses pectoralis minor as a guide to the level of lymph node dissection.
Clavipectoral Fascia — The Gateway to the Axilla
Clavipectoral fascia is a sheet of dense connective tissue that fills the gap between the clavicle (above) and pectoralis minor (below). It is sandwiched between the deep surface of pectoralis major and the chest wall.
Structure and boundaries:
• Superiorly: splits to enclose subclavius muscle and attaches to the clavicle on both sides
• Inferiorly: encloses pectoralis minor and continues below it as the suspensory ligament of the axilla
• The part above pectoralis minor is called the costocoracoid membrane
What passes through it — Mnemonic LCTA:
1. Lateral pectoral nerve — to pectoralis major
2. Cephalic vein — draining the upper limb superficially into the axillary vein
3. Thoracoacromial artery (pectoral branches)
4. Axillary lymphatics — from the breast toward the axilla
Why it matters clinically:
• The cephalic vein is commonly used for central venous catheter insertion
• The lymphatics from the upper outer quadrant of the breast pass through this fascia — blockage after mastectomy causes lymphoedema of the arm
Cross-reference: In Biochemistry, you are studying collagen and extracellular matrix. Fascia is primarily Type I collagen — the same protein that gives both bone and fascia their tensile strength.