The Breast: Anatomy That Decides a Cancer's Course

A sweat gland that learned to make milk

Developmentally the breast is skin, and it never stops behaving like skin. The mammary gland is a modified apocrine sweat gland of the skin — which is why it sits in the superficial fascia described in skin and fascia, and not inside the chest. Its glandular tissue is arranged into fifteen to twenty LOBES radiating out from the nipple like the spokes of a wheel. Each lobe is drained by one LACTIFEROUS DUCT, and each duct opens independently on the surface of the nipple — so a nipple carries fifteen to twenty separate openings, not one. Just before it reaches the surface each duct widens into a LACTIFEROUS SINUS, a small reservoir where milk collects between feeds and which the infant's gums compress during suckling. Each lobe is subdivided into LOBULES, and each lobule is a cluster of ALVEOLI: the actual secretory sacs, wrapped in myoepithelial cells that contract under oxytocin to squeeze milk out.

All of this glandular machinery is embedded in fat, and the fat is the majority of the volume. Size and shape are therefore almost entirely a matter of adipose tissue, not of glandular capacity — which is the anatomical reason a small breast feeds a baby exactly as well as a large one. The proportions change through life: fat and connective tissue dominate before pregnancy, glandular tissue proliferates enormously during pregnancy and lactation, and after the menopause the gland involutes and is replaced by still more fat. That last change is quietly useful, because fat is radiolucent and glandular tissue is not — which is precisely why mammography reads a post-menopausal breast so much more easily than the dense breast of a woman of thirty.

Cooper's ligaments: the strands that betray a tumour

A gland hanging off the chest wall needs a scaffold, and the scaffold is a witness. Running through the breast is a lattice of fibrous septa: the SUSPENSORY LIGAMENTS OF COOPER. They pass from the dermis of the overlying skin, through the glandular tissue and the fat, to attach to the pectoral fascia beneath. Their honest job is mechanical — they divide the breast into compartments, support the glandular tissue against gravity, and give the breast its shape rather than letting it hang as a formless bag. Their unintended job is diagnostic. A carcinoma growing in the breast provokes a dense fibrous reaction; that scar tissue infiltrates the nearby ligaments and, as scar always does, it CONTRACTS. The ligaments shorten. Because their upper end is anchored in the dermis, shortening drags the skin inwards — and you see TETHERING: a fixed dimple, a puckered patch, a nipple pulled in or deviated towards the tumour. It is visible long before the mass is big enough to be obvious, and it is exaggerated by raising the arms or contracting pectoralis major, which is why every taught self-examination includes both manoeuvres.

Push the same process further and you get the most famous sign in breast disease: PEAU D'ORANGE, skin like the peel of an orange. It needs two things happening at once. First, the tumour blocks the dermal lymphatics, so lymph cannot leave the skin and the skin becomes waterlogged and swollen. Second, Cooper's ligaments hold the skin down at every point where they insert into the dermis. The oedematous skin therefore bulges between the tethering points and is pinned down at them, and the pits are the enlarged openings of the hair follicles and sweat glands — an orange peel exactly. The lesson is worth stating plainly: peau d'orange is not the tumour you are looking at. It is lymphatic obstruction seen through a fibrous grid.

Where it sits, and the tail that escapes into the armpit

The base of the breast is remarkably constant. Vertically it extends from the second to the sixth rib of the thoracic cage; horizontally from the lateral border of the sternum to the mid-axillary line. It lies on the PECTORAL FASCIA, and the bed beneath that fascia is mostly pectoralis major, with the lower lateral part of the breast overlying serratus anterior and a small slip of the external oblique aponeurosis. Between the posterior surface of the breast and the pectoral fascia lies the RETROMAMMARY SPACE, a thin plane of loose connective tissue and fat. That space is why a normal breast can be lifted and moved freely over the chest wall — and why loss of that mobility, a breast that will not slide when pectoralis major is tensed, means the tumour has invaded through the space into the muscle. One part of the gland breaks the rule: the AXILLARY TAIL OF SPENCE, a prolongation of the upper lateral quadrant that pierces the deep fascia at the foramen of Langer and enters the axilla. It is normal breast tissue lying in the armpit — which is why a "lump in the armpit" may be breast, may swell and become tender before a period, and may harbour a carcinoma like any other part of the gland.

At the summit, a small piece of specialised skin does more work than it looks. The NIPPLE is a conical projection of skin at roughly the fourth intercostal space in a young nulliparous woman, pierced by the lactiferous duct openings. It contains no fat and no hair, but it is packed with circular and longitudinal SMOOTH MUSCLE fibres and richly supplied with sensory nerve endings; the muscle contracts to make the nipple erect and firm, which both aids the infant's grip and empties the sinuses. Around it lies the AREOLA, a ring of pigmented skin that darkens permanently in pregnancy. Its surface carries small nodules — the AREOLAR GLANDS OF MONTGOMERY, large modified sebaceous glands that enlarge in pregnancy and secrete an oily film that lubricates and protects the nipple and areola from the mechanical trauma of feeding. The whole nipple–areolar complex is a small masterpiece of purpose-built skin: pigmented so the newborn can find it, muscular so it can be gripped, glandular so it can survive being gripped.

Blood in, blood out — and a vein that carries cancer to the spine

Arterial supply comes from three directions. Medially, and most importantly, PERFORATING BRANCHES of the INTERNAL THORACIC ARTERY (a branch of the subclavian) pierce the upper intercostal spaces beside the sternum — the second, third and fourth perforators are the dominant supply to the medial half of the gland. Laterally, the AXILLARY ARTERY contributes the LATERAL THORACIC artery and the pectoral branch of the THORACOACROMIAL artery, with the subscapular artery adding to the axillary tail. Posteriorly, lateral mammary branches of the second to fourth POSTERIOR INTERCOSTAL ARTERIES reach the gland through the intercostal spaces. This triple supply is generous and overlapping, which matters in reconstructive surgery: flaps can be raised on one pedicle because the others will keep the rest alive.

The veins matter less for blood and enormously for metastasis. Venous drainage mirrors the arteries: a superficial venous plexus under the areola drains into the internal thoracic vein medially and the axillary vein laterally, while posterior intercostal veins drain backwards into the AZYGOS system. That posterior route is the dangerous one. The posterior intercostal veins communicate freely with the BATSON VERTEBRAL VENOUS PLEXUS — a valveless network of veins surrounding the vertebral column, inside and outside the spinal canal. Because it has no valves and lies outside the thorax's normal pressure gradients, any rise in intra-thoracic or intra-abdominal pressure (a cough, a strain, lifting) pushes blood — and any tumour cells in it — backwards into the vertebrae rather than forwards to the heart. This is why breast carcinoma metastasises so characteristically to the thoracic vertebrae, and why a woman with known breast cancer and new back pain is investigated urgently rather than reassured. The same valveless plexus explains vertebral secondaries from prostate cancer, and it bypasses the lungs entirely, so spinal metastases can appear with clear lung fields.

The lymphatic map: 75% turns towards the armpit

This is the single most consequential paragraph in breast anatomy. Lymph from the breast begins in a subareolar plexus and drains through the general system described in lymphatics and the body cavities — but the proportions are what matter. About SEVENTY-FIVE PER CENT of all lymph from the breast, and essentially all of it from the lateral quadrants, goes to the AXILLARY NODES. Anatomists group these into five sets: the ANTERIOR (pectoral) nodes along the lower border of pectoralis minor, which receive the bulk of the breast lymph; the POSTERIOR (subscapular) nodes on the posterior axillary fold; the LATERAL (humeral) nodes along the axillary vein, which drain the upper limb; the CENTRAL nodes embedded in axillary fat; and the APICAL nodes at the apex of the axilla, which receive from all the others and drain onwards into the subclavian lymph trunk. Surgeons prefer a different, purely practical grouping based on PECTORALIS MINOR: LEVEL I is lateral to the muscle, LEVEL II is deep to it, and LEVEL III is medial to it, at the apex. The two schemes describe the same nodes; the anatomical one tells you where lymph comes from, and the surgical one tells you how far a dissection has gone.

The remaining quarter escapes in directions that make breast cancer a whole-body problem. Most of it, chiefly from the MEDIAL quadrants, passes through the intercostal spaces beside the sternum to the PARASTERNAL (internal thoracic) NODES — and those nodes drain to the bronchomediastinal trunks and communicate with their fellows on the OTHER SIDE. That single anatomical fact is why a tumour in the inner half of one breast can seed the opposite breast and the mediastinum without ever going near an axilla, and why a clinically "node-negative" axilla does not guarantee a node-negative patient. Beyond that, superficial lymphatics cross the midline to the OPPOSITE BREAST; lymph from the lower and inner part of the breast can follow the rectus sheath through the ABDOMINAL WALL to reach the subdiaphragmatic and hepatic nodes; a few vessels run posteriorly to the intercostal nodes; and the venous route through Batson's plexus delivers cells straight to the VERTEBRAE. Five escape routes from one gland — the anatomy of why this disease is staged so carefully.

Three nerves a surgeon must not cut

Operating in the axilla means working in a fat-filled space with three nerves running through it. The LONG THORACIC NERVE (C5, C6, C7 — "C5, 6, 7 keeps the scapula from heaven") descends on the surface of SERRATUS ANTERIOR on the medial wall of the axilla, and it is the sole supply to that muscle. Divide it and serratus anterior is paralysed: the scapula loses its anchor against the chest wall, and when the patient pushes against a wall the medial border lifts away as a WINGED SCAPULA, with weakness of overhead abduction because the scapula can no longer be rotated upwards — the mechanics are laid out in the shoulder and scapular muscles. The THORACODORSAL NERVE (C6, C7, C8) crosses the posterior wall with the subscapular vessels to supply LATISSIMUS DORSI; injury weakens extension, adduction and medial rotation of the arm, and it also destroys the latissimus dorsi flap that might otherwise have been used to reconstruct the breast. The INTERCOSTOBRACHIAL NERVE, the lateral cutaneous branch of the second intercostal nerve, crosses the axilla horizontally to supply the skin of the floor of the axilla and the medial side of the upper arm; it is frequently sacrificed in a clearance, leaving a permanent numb patch there — the commonest neurological consequence of breast surgery, and one that every patient should be warned about beforehand.