The Mammary Gland - Detailed Description

a. Extent

Fig. 1 - The axillary process of the breast passing around the pectoralis major into the axilla (Gray's Anatomy for Students)

b. Relations

Anterior

• Skin with overlying superficial fascia
• Nipple and areola at the 4th intercostal space level

Posterior (Deep surface)

• Retromammary space (loose connective tissue) - allows the breast to slide on underlying structures
• Deep (pectoral) fascia covering pectoralis major (medially), serratus anterior (laterally), and external oblique/rectus abdominis (inferiorly)

Superior

• Clavicle
• Branches of the supraclavicular nerves

Inferior

• Upper anterior abdominal wall

Medial

• Sternum

Lateral

• Axilla (with the axillary tail projecting into it)

Muscles related to the breast:

c. Structure

Fig. 2 - Morphology of the mature breast showing lobes, lobuli, alveoli, connective tissue septa, and Montgomery glands (Creasy & Resnik's Maternal-Fetal Medicine)

Parenchyma (Glandular tissue)

• 15-20 lobes arranged radially around the nipple, separated by connective tissue septa (Cooper's ligaments)
• Each lobe contains several lobules
• Each lobule contains 10-100 alveoli (secretory units)
• Each lobe drains via a lactiferous duct that opens independently at the nipple tip
• Just before opening at the nipple, each lactiferous duct dilates into a lactiferous sinus (ampulla), which acts as a temporary reservoir for milk

Stroma

• Dense irregular connective tissue (interlobular) and loose connective tissue (intralobular)
• Cooper's (Suspensory) Ligaments: Fibrous bands that extend from the dermis of the skin to the deep fascia, providing structural support. Distortion or shortening of these by carcinoma produces characteristic skin dimpling
• Adipose tissue: The predominant component in non-lactating women; fat surrounds the glandular elements and gives the breast its smooth, rounded contour

Nipple and Areola

• Nipple: Conical elevation at the center of the areola; contains smooth muscle fibers (erectile), richly innervated with sensory and pain fibers; contains 4-18 lactiferous duct openings (5-8 main ducts)
• Areola: Circular pigmented zone (avg. 15-16 mm diameter, enlarges in pregnancy); contains Montgomery glands (modified sebaceous/sweat glands) that hypertrophy during pregnancy and lactation to lubricate and protect the nipple
• Verrucous surface of nipple; has sebaceous and apocrine sweat glands but no hair

Histological Stages

In males

• Rudimentary breast consisting of small ducts (often cords of cells) that do not extend beyond the areola

d. Blood Supply

Fig. 3 - Full breast anatomy showing arterial supply (lateral thoracic, internal thoracic), lymphatic drainage to axillary and parasternal nodes, suspensory ligaments, and retromammary space (Gray's Anatomy for Students)

Arterial Supply

Venous Drainage

1. Internal thoracic (mammary) vein - anteromedially
2. Axillary vein - laterally and superiorly
3. Intercostal veins - posteriorly and inferiorly

Note: The intercostal veins connect with the vertebral venous plexus (Batson's plexus), providing a route for haematogenous metastasis of breast cancer to the vertebral column, skull, and pelvis - without going through the pulmonary circulation.

Innervation

• Anterior and lateral cutaneous branches of the 2nd-6th intercostal nerves
• Nipple: specifically innervated by the 4th intercostal nerve
• Efferent nerves are sympathetic adrenergic; no parasympathetic or cholinergic supply
• Sensory stimulation of nipple/areola triggers release of prolactin (adenohypophysis) and oxytocin (neurohypophysis)

e. Lymphatic Drainage

Fig. 4 - Right breast showing axillary lymph nodes, parasternal nodes, internal thoracic artery perforating branches, lateral thoracic artery, and lactiferous sinuses (Gray's Anatomy for Students)

Primary Drainage Routes

1. Axillary Route (~75% of total drainage)

• Drains the lateral, superior, and central parts of the breast
• Drains into the axillary lymph nodes, which are divided into 5 groups:

2. Parasternal (Internal Mammary) Route (~20-25%)

• Drains the medial and central parts of the breast
• Lymphatics accompany perforating branches of the internal thoracic artery
• Drain into parasternal (internal mammary) nodes along the internal thoracic vessels on the deep surface of the anterior thoracic wall
• Parasternal nodes drain into the bronchomediastinal trunks

3. Intercostal Route (minor)

• Some drainage from the posterior and inferior breast
• Via lymphatics that follow lateral branches of posterior intercostal arteries
• Drain into intercostal nodes near the heads and necks of ribs
• Intercostal nodes drain into the thoracic duct or bronchomediastinal trunks

4. Inferior (Subdiaphragmatic) Route

• Small amount of drainage from the inferior breast
• May communicate with subdiaphragmatic lymphatics and hepatic lymphatics (via the falciform ligament)

Summary Diagram (Lymphatic Routes)

                  BREAST LYMPHATIC DRAINAGE
                         |
         ┌───────────────┼───────────────┐
         ▼               ▼               ▼
   AXILLARY (~75%)  PARASTERNAL    INTERCOSTAL
   (lateral/sup.)   (~20-25%)      (minor, post.)
         |          (medial)            |
   Pectoral nodes    Parasternal     Intercostal
   → Subscapular     nodes           nodes
   → Humeral              |               |
   → Central    Bronchomediastinal  Thoracic duct /
   → Apical     trunk              Bronchomediastinal
         |                          trunk
   Subclavian trunk
         |
   Venous junction
   (subclavian + IJ vein)

f. Applied Anatomy

1. Breast Carcinoma

• Acini (terminal duct lobular units - TDLU) - lobular carcinoma
• Lactiferous ducts - ductal carcinoma (most common)

• Lymphatic: To axillary nodes (most common), parasternal nodes, supraclavicular nodes, and contralateral breast
• Haematogenous: Via Batson's vertebral venous plexus (through intercostal veins) → vertebral column, skull, pelvis; or via axillary/internal thoracic veins → lungs, liver, bone
• Direct invasion: Into pectoral muscles, chest wall, skin

• Size of primary tumor
• Exact site
• Number and sites of lymph node spread
• Distant metastasis (lungs, liver, bone - detected by CT, bone scan, PET-CT)

2. Skin Changes in Carcinoma

3. Lymphoedema After Mastectomy

• Axillary lymph node clearance (ALND) or radiotherapy to the axilla disrupts lymphatic drainage of the upper limb
• Results in chronic lymphoedema of the arm (brawny, non-pitting oedema)
• Clinicians now favor sentinel lymph node biopsy (SLNB) to reduce this complication

4. Sentinel Lymph Node Biopsy (SLNB)

• The sentinel node is the first lymph node to receive lymphatic drainage from the primary tumor
• Identified by injecting blue dye or radioactive tracer around the tumor
• If sentinel node is clear of metastasis, full axillary clearance is avoided
• Reduces risk of lymphoedema significantly

5. Axillary Tail (Spence's Tail)

• Clinicians must examine the axillary tail when assessing breast lumps; pathology (carcinoma, fibroadenoma) can occur in this extension
• It may be mistaken for an axillary lymph node

6. Mammography

• The primary imaging modality for breast cancer screening
• Uses low-dose X-rays; the breast is compressed between plates
• Can detect microcalcifications (early sign of ductal carcinoma in situ - DCIS) and soft tissue masses

7. Paget's Disease of the Nipple

• Eczema-like change of the nipple/areola due to underlying intraductal carcinoma
• Paget's cells (large clear cells) invade the nipple epidermis
• Associated with subjacent ductal carcinoma in situ (DCIS) or invasive ductal carcinoma

8. Gynaecomastia

• Benign proliferation of male breast glandular tissue
• Can occur physiologically (neonatal, pubertal, elderly) or due to hormone imbalance, drugs (spironolactone, digitalis, cimetidine, exogenous estrogens), liver cirrhosis, or testicular tumors (hCG-secreting)

9. Supernumerary Breasts / Nipples (Polymastia / Polythelia)

• Due to failure of involution of the milk line (mammary ridge - a bilateral ectodermal thickening extending from axilla to groin during fetal development)
• Accessory nipples most commonly appear along the milk line, particularly below and medial to the normal breast

10. Abscess of the Breast

• Most common during lactation (puerperal mastitis)
• Superficial abscess: In the subcutaneous tissue; drains anteriorly
• Deep (retromammary) abscess: In the retromammary space; presents with deep fluctuance; may point inferiorly

Anterior Compartment of the Arm

1. Muscles of the Front (Anterior Compartment) of the Arm

1. Biceps Brachii (most superficial)
2. Brachialis (deepest, directly on humerus)
3. Coracobrachialis (proximal/medial)

Fig. 1 - 3D CT/MRI rendering of anterior arm compartment. Top: Biceps (long head = green, short head = light blue) with distal tendon inserting into radial tuberosity. Middle: Brachialis (dark blue) inserting at ulnar tuberosity; Coracobrachialis (red) from coracoid. Bottom lateral view showing biceps overlying brachialis, with triceps posterior. (Imaging Anatomy, Bones, Joints, Vessels & Nerves)

Fig. 2 - Anterior compartment of arm: biceps brachii (long and short heads), brachialis, and coracobrachialis, with the transverse humeral ligament, bicipital aponeurosis, and insertions at radial and ulnar tuberosities. (Gray's Anatomy for Students)

Summary Table: Muscles of the Anterior Compartment

2. Origin and Insertion of the Biceps Brachii

Long Head

• Origin: Supraglenoid tubercle of the scapula (and blends with the superior glenoid labrum - forming the bicipitolabral complex)
• The tendon passes through the glenohumeral joint (intra-articular but extrasynovial), over the head of the humerus, through the rotator interval (between supraspinatus and subscapularis), then enters the intertubercular (bicipital) groove on the anterior humerus
• Held in the groove by the transverse humeral ligament (spanning between the greater and lesser tubercles)

Short Head

• Origin: Apex of the coracoid process of the scapula (shared with coracobrachialis as a conjoint tendon)
• Passes vertically through the axilla

Convergence and Insertion

• Both heads join in the distal arm, forming a single flattened tendon approximately 7 cm above the elbow
• The two tendons twist 90° as they descend (long head moves anteriorly, short head posteriorly), assisting in supination mechanics
• Insertion (primary): Radial tuberosity (posterior margin) of the proximal radius
• Insertion (secondary): Bicipital aponeurosis - a flat fibrous sheet that fans medially from the tendon to merge with the deep fascia of the forearm over the flexor muscles

Actions

Key point: The biceps is the most powerful supinator of the forearm (more powerful than supinator muscle itself), but only when the elbow is flexed. This is because with elbow flexion the tendon's lever arm is nearly perpendicular to the axis of pronation/supination.

3. The Musculocutaneous Nerve

Fig. 3 - Musculocutaneous nerve (yellow): arising from lateral cord, piercing coracobrachialis, running between biceps and brachialis, giving branches to all 3 anterior arm muscles, then emerging laterally as the lateral cutaneous nerve of the forearm. Median and ulnar nerves are shown medially. (Gray's Anatomy for Students)

Origin

• Arises from the lateral cord of the brachial plexus (C5, C6, C7)

Course

1. Exits the axilla by piercing through the coracobrachialis muscle (which it innervates)
2. Passes diagonally down the arm in the plane between the biceps brachii (anteriorly) and brachialis (posteriorly)
3. At the elbow, emerges laterally to the biceps tendon
4. Pierces the deep fascia and continues into the forearm as the lateral cutaneous nerve of the forearm

Distribution

4. Nerve Supply of Muscles of the Front of the Arm

Important: Brachialis is the only muscle in the anterior compartment with dual nerve supply - its inferolateral fibers (deep head) commonly receive branches from the radial nerve as it passes the radial tunnel. This is significant: even with musculocutaneous nerve palsy, some elbow flexion is retained via the radial nerve supply to brachialis.

Reflex Testing

• Biceps reflex (C5, C6): Tapping the biceps tendon in the cubital fossa
• Brachioradialis reflex (C5, C6): Tests the brachioradialis (not an anterior arm muscle, but commonly tested alongside)

5. Brachialis Muscle - Full Account

Origin

• Distal half of the anterior surface of the humerus (from deltoid tuberosity downward)
• Medial and lateral intermuscular septa
• Two heads:
  • Superficial head: originates near the deltoid tuberosity (larger, more proximal origin)
  • Deep head: from distal third of the humeral shaft

Insertion

• Superficial head: Ulnar tuberosity, distal to the coronoid process
• Deep head: Coronoid process of the ulna (all but its tip); via three units - medial aponeurosis, lateral aponeurosis, and direct muscle fibers into the ulna

Nerve Supply

• Primary: Musculocutaneous nerve (C5, C6) - runs between brachialis and biceps brachii, lying on the surface of brachialis
• Variable: Radial nerve (C7) to inferolateral deep head fibers (~81% of individuals)
• Occasional: Median nerve to lower quarter (~15% of individuals)

Action

• Powerful flexor of the elbow joint - regardless of forearm position (pronated or supinated)
• The workhorse of elbow flexion - it is the largest contributor to elbow flexion force
• Also provides anterior stability to the coronoid process, contributing to resistance against posterior dislocation of the elbow

Key Characteristics

• Lies directly on the humerus - it is a pure elbow flexor with no rotatory action (unlike biceps which also supinates)
• Often called the "workhorse" of elbow flexion because it acts in all forearm positions
• The coronoid process is one of the anterior constraints providing buttress against posterior dislocation; the deep head of brachialis reinforces this

6. Brachial Artery, Median Nerve, and Ulnar Nerve

Brachial Artery

Fig. 4 - Surface marking of brachial artery (red), median nerve, and ulnar nerve (yellow) along the medial aspect of the right arm. (Gray's Anatomy for Students)

• Begins as a continuation of the axillary artery at the lower border of teres major
• Travels through the anterior compartment in the medial neurovascular bundle
• Proximal arm: Lies on the medial side, compressible against the humerus
• Distal arm: Moves laterally to lie midway between the medial and lateral epicondyles
• At the elbow (cubital fossa): Lies medial to the biceps tendon; bifurcates into radial and ulnar arteries just distal to the elbow joint

• Profunda brachii artery (deep artery of arm) - largest branch, passes posteriorly with radial nerve through triangular interval, runs in the radial groove
• Superior ulnar collateral artery
• Inferior ulnar collateral artery
• Muscular branches and nutrient artery to humerus

Median Nerve

Fig. 5 - Antecubital region: brachial artery (red), median nerve, and ulnar nerve at the cubital fossa. Biceps brachii and pronator teres shown. (Roberts & Hedges' Clinical Procedures in Emergency Medicine)

• Lateral root from the lateral cord (C6, C7)
• Medial root from the medial cord (C8, T1)

1. Enters the arm at the lower border of teres major, initially lateral to brachial artery
2. In the middle of the arm: crosses over the brachial artery (anterior) to lie on its medial side
3. At the elbow: lies medial to the brachial artery anterior to the elbow joint
4. Has no branches in the arm (occasional branch to pronator teres may arise proximal to elbow)

Ulnar Nerve

1. Enters the arm medial to the axillary artery (posterior to the brachial artery)
2. In the proximal arm: medial to brachial artery in the anterior compartment
3. In the middle of the arm: pierces the medial intermuscular septum and enters the posterior compartment, where it lies anterior to the medial head of triceps
4. Passes posterior to the medial epicondyle (in the ulnar groove/cubital tunnel) - this is where it is palpable and vulnerable to injury
5. Enters the anterior compartment of the forearm between the two heads of flexor carpi ulnaris
6. Has no major branches in the arm

Summary Diagram: Neurovascular Bundle of the Anterior Arm

AXILLA (entering arm at lower border of teres major)
         │
         ├── BRACHIAL ARTERY (medial → moves laterally distally)
         │       ├─ Profunda brachii → enters posterior compartment
         │       ├─ Superior ulnar collateral
         │       └─ Inferior ulnar collateral
         │
         ├── MEDIAN NERVE (lateral → crosses to medial side at mid-arm)
         │       └─ No branches in arm
         │
         └── ULNAR NERVE (medial → pierces medial intermuscular septum)
                 └─ Enters posterior compartment at mid-arm
                         → wraps around medial epicondyle
                                 → enters forearm (cubital tunnel)

MUSCULOCUTANEOUS NERVE (separate - lateral cord):
     Pierces coracobrachialis → runs between biceps and brachialis
     → emerges as LATERAL CUTANEOUS NERVE OF FOREARM

Applied Points