10.2 External Anatomy
Key Takeaways
- Breast margins run from clavicle/upper ribs superiorly to the inframammary fold inferiorly, sternum medially, and midaxillary line laterally, with pectoralis major posteriorly.
- Morgagni tubercles are the raised areolar papules; Montgomery glands are the modified sebaceous glands that open through them.
- On MLO the pectoral muscle should be convex and visible to the posterior nipple line or nipple level, and the inframammary fold must be included and open.
- Skin contains sebaceous glands, sweat glands, and hair follicles — the source of many lucent-centered dermal calcifications, not TDLU disease.
- Nipple in profile on at least one standard view is required to evaluate subareolar tissue; both CC and MLO is the goal.
External anatomy as a critique checklist
The September 1, 2025 ARRT outline lists external anatomy as breast margins, nipple, areola (Morgagni tubercles and Montgomery glands), angle of the pectoral muscle, skin (sebaceous glands, sweat glands, hair follicles), axillary tail, and inframammary fold. These are not trivia labels. They are how you decide whether a mammogram included the tissue where cancer lives.
Quick Answer: Know the margins (clavicle superior, sternum medial, midaxillary line lateral, inframammary fold inferior), the nipple-areolar complex, skin appendages, the axillary tail, and the pectoral angle. On MLO, show an open IMF, a convex pec to the posterior nipple line, and a nipple in profile whenever you can.
Breast margins
The adult female breast is a modified skin gland on the anterior chest wall. For positioning and documentation its margins are:
| Margin | Anatomic boundary | What failure looks like |
|---|---|---|
| Superior | Clavicle and about the second–third ribs | CC cuts off upper posterior tissue if the receptor is too low or the patient slumps |
| Inferior | Inframammary fold (IMF) at about the sixth–seventh rib | Closed or missing IMF on MLO hides inferior breast |
| Medial | Sternum / lateral sternal border | CC that misses cleavage tissue misses medial breast — still breast, still cancer territory |
| Lateral | Midaxillary line / latissimus region | Clipped lateral CC or skinny MLO drops UOQ and tail of Spence |
| Posterior | Pectoralis major and chest wall | No retroglandular fat means you did not reach the posterior third |
If a question asks which structure marks the inferior extent, the answer is the IMF. If the lateral CC is clipped, you clipped the lateral margin, often the UOQ.
Nipple
The nipple is the raised center of the nipple-areolar complex. About 15–20 lactiferous ducts open on its surface, one from each lobe. Smooth muscle in the nipple erects or flattens it with cold, anxiety, or stimulation — a change that can mimic inversion if you skip the history.
Nipple in profile means the nipple is seen in tangent, not superimposed on gland. Clinical image evaluation expects the nipple in profile on at least one of the two standard views; both CC and MLO is the goal. If the nipple is buried on both views, subareolar tissue is not fully evaluated — obtain a dedicated nipple-in-profile view before you call the study complete. A buried nipple can hide or mimic a subareolar mass.
Distinguish long-standing bilateral inversion (often a variant) from new unilateral inversion (a clinical red flag that belongs in the history, not only the image critique).
Areola: Morgagni tubercles and Montgomery glands
The areola is the pigmented disk around the nipple. Scattered over it are small raised papules — Morgagni tubercles. Those papules are the surface openings of Montgomery glands (areolar glands): modified sebaceous glands that lubricate the nipple-areolar complex, especially in pregnancy and lactation.
Keep the names separate because ARRT lists both:
- Morgagni tubercles — what you see (elevations on the areola)
- Montgomery glands — the glands that produce the lubricant
Many textbooks treat the terms as synonyms. The outline does not. Montgomery glands can enlarge, inflame, or form oil cysts that calcify as lucent-centered dermal calcifications — a skin process, not a TDLU cancer.
Angle of the pectoral muscle
On the MLO, pectoralis major should appear as a wide, convex-anterior muscle whose inferior extent reaches the posterior nipple line (PNL) — a line from the nipple perpendicular to the pectoral margin (or to the posterior image edge if muscle is short). Classic teaching: pec visible to the level of the nipple.
The angle of the pectoral muscle is the C-arm angle chosen so the beam travels along the muscle fibers rather than across them. Typical MLO angles are about 30°–60° from vertical, matched to habitus:
| Habitus | Typical MLO angle | Why |
|---|---|---|
| Tall, thin | Steeper, about 50°–60° | Pec lies more vertical |
| Average | About 45° | Common starting angle |
| Short, heavy, or kyphotic | Shallower, about 30°–40° | Pec lies more horizontal |
A concave anterior pec border usually means the muscle was not relaxed or the arm/angle failed to capture the mobile lateral breast. Wrong angle can miss posterior and UOQ tissue even when a sliver of pec is visible.
Compare PNL length on CC versus MLO: they should agree within about 1 cm. A much shorter CC PNL means posterior tissue was not pulled onto the CC detector.
On CC, a small convex strip of pec may appear in a minority of patients (more often muscular or thin). Absence of pec on CC is not a failure if retroglandular fat is included. Presence of pec on CC is a bonus, not the MLO rule.
Skin: sebaceous glands, sweat glands, hair follicles
Normal mammographic skin is a thin smooth line, typically about 0.5–2 mm thick, often thicker at the IMF. The skin contains three blueprint structures:
- Sebaceous glands — associated with hair follicles; source of oil cysts and lucent-centered dermal calcifications
- Sweat glands — eccrine plus apocrine (concentrated in axilla and areola)
- Hair follicles — IMF, areola, and axilla; can calcify or trap deodorant and powder
Skin calcifications live in the dermis. A tangential skin view or a BB marker on a raised lesion proves a finding is dermal. The exam will ask you to put sebaceous, sweat, and hair-follicle calcifications in the skin, not in the TDLU.
Diffuse skin thickening is pathology (edema, radiation, inflammatory carcinoma, mastitis). Do not confuse it with the normally thicker IMF.
Axillary tail
As external anatomy, the axillary tail is the palpable extension of breast into the axilla. The MLO must include it. Axillary skin also holds hair follicles and sweat glands — deodorant artifact lives here. Include the tail without calling every axillary density a breast mass; nodes live here too.
Inframammary fold
The IMF is the inferior junction of breast and chest wall. On a correct MLO it should be included and open — the fold unfolded, not skin-on-skin overlap. A closed IMF hides inferior tissue and creates a skin fold that can mimic or hide a mass. Lift the breast, open the fold, and keep inframammary skin from overlapping. Hair-follicle calcifications cluster here because the fold is a skin crease.
Positioning implications the items actually test
- Nipple in profile — at least one standard view; extra view if the nipple is buried and the question is subareolar.
- Pec angle on MLO — match habitus; convex pec to PNL / nipple level.
- IMF on MLO — visible and open.
- Margins — medial and posterior on CC; posterior, axillary tail, and IMF on MLO.
- Skin — thin and smooth; mark moles and scars so they are not read as intramammary masses.
If any of those five fail, you did not image the corresponding external anatomy, and a cancer in that margin is an avoidable miss.
Montgomery glands of the areola are best described as which of the following?
Which MLO criterion best confirms that the inframammary fold was positioned correctly?
A technologist sees small raised papules on the areola. Which structure is being observed, and which glands open through them?