Anatomy: Regional, surface, imaging and procedural anatomy

Detailed anatomy — Upper limb

• Framework: Overview and Surface Anatomy | The Shoulder Girdle and Joint | Rotator Cuff and Shoulder Movements | The Axilla and Breast Lymphatics | The Arm and Elbow | The Forearm | The Wrist and Hand
• Nerves and vessels: The Brachial Plexus | Median Nerve | Ulnar Nerve | Radial, Axillary and Musculocutaneous Nerves | Root Lesions — Myotomes and Dermatomes | Blood Supply and Venous Drainage | Clinically Important Injuries
• Revision: Upper Limb at a Glance | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

💪 Upper limb anatomy — page 3 of the Anatomy and Physiology chapter. Upper limb anatomy is tested constantly — in nerve lesions (wrist drop, claw hand, carpal tunnel), fractures that damage nerves and vessels, the brachial plexus, and practical procedures (cannulation, blood gases, nerve blocks). Learn the nerves by what they supply and what happens when they are cut, and the anatomy becomes clinical medicine.
Your four questions for every upper limb problem: Is this a root, plexus, or peripheral nerve lesion? Which muscles are weak and which skin is numb? Which bone or joint injury puts which nerve or vessel at risk? What does the examination need to test to prove it?

Overview and Surface Anatomy

• Regions — shoulder (pectoral girdle), arm (brachium — shoulder to elbow), forearm (antebrachium), wrist (carpus), hand.
• Bones — clavicle, scapula, humerus, radius (lateral — thumb side), ulna (medial), 8 carpals, 5 metacarpals, 14 phalanges.
• Compartments:
  • Arm — anterior (flexors — musculocutaneous nerve) and posterior (extensors — radial nerve).
  • Forearm — anterior (flexors and pronators — mostly median, some ulnar) and posterior (extensors and supinator — radial / posterior interosseous).
• Surface landmarks:
  • Clavicle, acromion, coracoid process (below the lateral clavicle), spine of the scapula.
  • Medial and lateral epicondyles, olecranon — form a straight line in extension and a triangle in flexion (lost in elbow dislocation; preserved in supracondylar fracture).
  • Cubital fossa — from lateral to medial: biceps tendon, brachial artery, median nerve ("TAN") — antecubital veins overlie it (venepuncture).
  • Anatomical snuffbox — between extensor pollicis longus and the abductor/extensor brevis tendons — scaphoid lies in its floor; radial artery crosses it.
  • Radial pulse — lateral to flexor carpi radialis at the wrist (arterial blood gas sampling — check collateral flow with Allen's test).
• Anatomical position — palms facing forward; "lateral" is the thumb side.

The Shoulder Girdle and Joint

• Shoulder girdle — clavicle and scapula; the only bony attachment of the upper limb to the trunk is the sternoclavicular joint; the scapula moves on the chest wall (scapulothoracic "joint").
• Joints:
  • Sternoclavicular — posterior dislocation can compress the great vessels and trachea.
  • Acromioclavicular — injured by falls on the tip of the shoulder (AC joint separation — "step").
  • Glenohumeral — ball-and-socket synovial joint; very mobile but unstable (shallow glenoid, deepened by the labrum).
• Stabilisers of the glenohumeral joint — rotator cuff (dynamic), glenoid labrum, capsule and glenohumeral ligaments, long head of biceps.
• Dislocation — anterior (95% — abduction and external rotation) — risks axillary nerve injury; Bankart lesion (labral tear), Hill–Sachs lesion (humeral head impaction); posterior (seizures, electric shock).
• Clavicle fractures — commonest in the middle third; the lateral fragment is pulled down by the weight of the arm and the medial fragment up by sternocleidomastoid; subclavian vessels and brachial plexus lie below (rarely injured).
• Scapula — fractures imply high-energy trauma (check chest); winging — long thoracic nerve palsy (serratus anterior).
• Frozen shoulder (adhesive capsulitis) — loss of external rotation (active and passive); associated with diabetes.

Rotator Cuff and Shoulder Movements

• Rotator cuff — "SITS":

  Muscle	Action	Nerve	Test
  Supraspinatus	Initiates abduction (first 15°)	Suprascapular (C5–6)	Empty can (Jobe's) test
  Infraspinatus	External rotation	Suprascapular (C5–6)	Resisted external rotation
  Teres minor	External rotation	Axillary (C5–6)	Resisted external rotation (hornblower's)
  Subscapularis	Internal rotation	Upper and lower subscapular (C5–6)	Lift-off and belly-press tests

• Abduction — supraspinatus (0–15°), deltoid (15–90° — axillary nerve), then trapezius and serratus anterior rotate the scapula (above 90°).
• Other shoulder muscles — pectoralis major (adduction, internal rotation — medial and lateral pectoral nerves), latissimus dorsi (extension, adduction, internal rotation — thoracodorsal nerve), teres major, biceps and coracobrachialis (flexion).
• Subacromial impingement — supraspinatus tendon squeezed under the acromion — painful arc between about 60° and 120° of abduction.
• Rotator cuff tears — degenerative (over 40) or traumatic; weakness of abduction or rotation; ultrasound or MRI; massive tears in older people — "pseudoparalysis".
• Calcific tendinitis — acute severe pain, calcium deposit in supraspinatus on X-ray.
• Long head of biceps rupture — "Popeye" bulge in the arm; little functional loss.

The Axilla and Breast Lymphatics

• Axilla — a pyramid-shaped space between the arm and chest wall, transmitting vessels and nerves to the limb.
• Walls — anterior (pectoralis major and minor), posterior (subscapularis, teres major, latissimus dorsi), medial (serratus anterior and ribs), lateral (humerus — intertubercular groove); apex at the clavicle–first rib–scapula.
• Contents — axillary artery and vein, brachial plexus (cords and branches), axillary lymph nodes, fat; intercostobrachial nerve (numbness of the inner arm after axillary surgery).
• Axillary artery — first rib to the lower border of teres major; divided into 3 parts by pectoralis minor.
• Axillary lymph nodes — drain the upper limb, breast (about 75% of breast lymph) and chest wall above the umbilicus:
  • Groups — anterior (pectoral), posterior (subscapular), lateral (humeral), central, apical.
  • Surgical levels (relation to pectoralis minor) — level I (below/lateral), level II (behind), level III (above/medial — apical).
  • Sentinel lymph node biopsy — first node to drain the tumour (dye and radioisotope) — avoids full clearance if negative.
• Nerves at risk in axillary surgery:
  • Long thoracic nerve (C5–7 — serratus anterior) — winged scapula.
  • Thoracodorsal nerve (latissimus dorsi) — weak extension and adduction.
  • Intercostobrachial nerve — numb inner arm.
• Lymphoedema of the arm after axillary clearance or radiotherapy.
• Other breast drainage — internal mammary (parasternal) nodes (medial tumours), supraclavicular nodes.

The Arm and Elbow

• Humerus — nerves at risk:
  • Surgical neck — axillary nerve and posterior circumflex humeral artery.
  • Mid-shaft (spiral groove) — radial nerve → wrist drop.
  • Supracondylar — brachial artery and median nerve (anterior interosseous branch) — children.
  • Medial epicondyle — ulnar nerve.
• Muscles of the arm:
  • Anterior — biceps brachii (elbow flexion and supination — the main supinator when flexed), brachialis (main elbow flexor), coracobrachialis — all musculocutaneous nerve (C5–6).
  • Posterior — triceps (elbow extension) — radial nerve (C7).
• Elbow joint — hinge joint (humero-ulnar and humero-radial) plus the proximal radio-ulnar joint (pronation and supination — radial head turns in the annular ligament).
• Elbow problems:
  • Lateral epicondylitis ("tennis elbow") — common extensor origin — pain on resisted wrist extension.
  • Medial epicondylitis ("golfer's elbow") — common flexor origin.
  • Olecranon bursitis — gout, infection, trauma.
  • Pulled elbow — radial head subluxes through the annular ligament (children aged 1–4).
  • Radial head fracture — fall on outstretched hand — "fat pad sign" on X-ray.
  • Cubital tunnel syndrome — ulnar nerve compression at the elbow.
• Carrying angle — the forearm angles slightly outwards in extension (valgus — greater in women); altered after supracondylar malunion (cubitus varus — "gunstock" deformity).

The Forearm

• Anterior compartment — flexors of the wrist and fingers and pronators (in layers):
  • Superficial — pronator teres, flexor carpi radialis, palmaris longus (absent in about 15%), flexor carpi ulnaris (ulnar nerve) — from the medial epicondyle.
  • Intermediate — flexor digitorum superficialis (flexes PIP joints).
  • Deep — flexor digitorum profundus (flexes DIP joints — lateral half median/AIN, medial half ulnar), flexor pollicis longus (thumb IP — AIN), pronator quadratus (AIN).
• Nerve supply — all anterior forearm muscles by the median nerve except flexor carpi ulnaris and the medial half of FDP (ulnar).
• Posterior compartment — extensors of the wrist, fingers and thumb, supinator, brachioradialis — all radial nerve (or its deep branch, the posterior interosseous nerve).
• Testing flexor tendons — FDP (hold PIP straight, ask to bend the fingertip), FDS (hold other fingers straight, ask to bend the finger at the PIP) — after hand lacerations.
• Forearm fractures:
  • Monteggia — fracture of the proximal ulna with dislocation of the radial head.
  • Galeazzi — fracture of the distal radius with dislocation of the distal radio-ulnar joint ("MUGR" — Monteggia Ulna, Galeazzi Radius).
  • Both-bone fractures in adults usually need surgical fixation.
• Forearm compartment syndrome — after supracondylar fractures, crush injuries, tight casts — pain on passive finger extension — fasciotomy; untreated → Volkmann's ischaemic contracture (clawed, flexed fingers and wrist).

The Wrist and Hand

• Carpal bones (lateral to medial) — proximal row: scaphoid, lunate, triquetrum, pisiform; distal row: trapezium, trapezoid, capitate, hamate ("Some Lovers Try Positions That They Can't Handle").
• Scaphoid — commonest carpal fracture (fall on outstretched hand — young adults); snuffbox tenderness; blood supply enters distally → proximal pole at risk of avascular necrosis and non-union — immobilise and image (MRI) if suspected even with a normal X-ray.
• Lunate dislocation — can compress the median nerve; "spilled teacup" on lateral X-ray.
• Carpal tunnel — roof is the flexor retinaculum; contains the median nerve and 9 tendons (4 FDS, 4 FDP, FPL); flexor carpi radialis runs in its own compartment; ulnar nerve and artery pass outside (Guyon's canal).
• Intrinsic hand muscles:
  • Thenar (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) — median nerve (recurrent branch).
  • Hypothenar — ulnar nerve.
  • Lumbricals — flex the MCP and extend the IP joints — lateral two (index and middle) median, medial two ulnar.
  • Interossei — dorsal abduct (DAB), palmar adduct (PAD) the fingers — ulnar nerve.
  • Adductor pollicis — ulnar nerve (Froment's sign — thumb IP flexes to compensate when gripping paper).
• "LOAF" — the median nerve supplies the Lateral two lumbricals, Opponens pollicis, Abductor pollicis brevis and Flexor pollicis brevis; the ulnar nerve supplies all other intrinsic hand muscles.
• Hand conditions — Dupuytren's contracture (palmar fascia — ring and little fingers), trigger finger, de Quervain's tenosynovitis (APL and EPB — Finkelstein's test), ganglion, mallet finger (extensor tendon avulsion at the DIP), boutonnière and swan-neck deformities (rheumatoid arthritis), flexor sheath infection (Kanavel's signs).

The Brachial Plexus

• Formation — anterior rami of C5–T1; "Real Teachers Drink Cold Beer" — Roots (5) → Trunks (3 — upper C5–6, middle C7, lower C8–T1) → Divisions (6 — anterior and posterior) → Cords (3 — lateral, posterior, medial — named by relation to the axillary artery) → Branches.
• Course — between scalenus anterior and medius (with the subclavian artery), behind the clavicle, into the axilla.
• Terminal branches:

  Cord	Main branches	Roots
  Lateral	Musculocutaneous; lateral root of median	C5–7
  Medial	Ulnar; medial root of median; medial cutaneous nerves of arm and forearm	C8–T1
  Posterior	Axillary, radial; subscapular and thoracodorsal nerves	C5–T1

• Branches from roots — long thoracic nerve (C5–7 — serratus anterior; "C5, 6, 7 — raise your wings to heaven"), dorsal scapular (C5 — rhomboids), contribution to the phrenic nerve (C5).
• Branches from the upper trunk — suprascapular nerve (C5–6).
• Plexus lesions:
  • Erb's palsy (upper trunk — C5–6) — traction separating the head and shoulder (shoulder dystocia, motorbike falls) — "waiter's tip" — arm adducted and internally rotated, elbow extended, forearm pronated; loss of abduction, elbow flexion and supination; numb lateral arm.
  • Klumpke's palsy (lower trunk — C8–T1) — traction with the arm abducted above the head (grabbing a branch while falling, breech delivery) — claw hand (all intrinsics weak), numb medial forearm; may have Horner's syndrome (T1).
  • Pancoast tumour (lung apex) — invades the lower trunk and sympathetic chain — shoulder and arm pain, hand wasting, Horner's.
  • Cervical rib / thoracic outlet syndrome — compresses the lower trunk or subclavian vessels.
  • Neuralgic amyotrophy (Parsonage–Turner) — severe shoulder pain then patchy weakness and wasting.
• Regional anaesthesia — interscalene (shoulder — almost always blocks the phrenic nerve), supraclavicular, infraclavicular and axillary blocks.

Median Nerve

• Roots C5–T1 (mainly C6–8) — lateral and medial cords.
• Course — with the brachial artery in the arm (no branches in the arm) → cubital fossa (medial to the artery) → between the heads of pronator teres → deep to FDS → anterior interosseous nerve branch → carpal tunnel → recurrent motor branch (thenar) and digital nerves.
• Supplies — most forearm flexors and pronators; thenar muscles and lateral two lumbricals (LOAF); skin of the lateral palm and the palmar surface of the thumb, index, middle and lateral half of the ring finger (and their nail beds); palmar cutaneous branch arises before the carpal tunnel (palm sensation spared in carpal tunnel syndrome).
• Lesions by level:

  Level	Cause	Features
  Wrist (carpal tunnel)	Carpal tunnel syndrome, wrist lacerations, lunate dislocation	Thenar wasting, weak thumb abduction, numbness of the lateral 3½ fingers (palm spared); worse at night; Tinel's and Phalen's tests
  Anterior interosseous nerve	Supracondylar fracture, forearm trauma	Can't make an "OK" sign (weak FPL and index FDP — pinch with flat fingertips); no sensory loss
  Elbow and above	Supracondylar fracture, dislocation	All of the above plus weak forearm pronation and flexion — "hand of benediction" (index and middle fingers stay extended when trying to make a fist)

• Carpal tunnel syndrome — causes: idiopathic, pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, obesity, amyloid (dialysis), wrist fractures — treatment: night splints, steroid injection, surgical decompression (division of the flexor retinaculum); nerve conduction studies confirm.

Ulnar Nerve

• Roots C8–T1 (medial cord).
• Course — medial arm → behind the medial epicondyle ("funny bone" — cubital tunnel) → between the heads of FCU → forearm (supplies FCU and medial half of FDP) → Guyon's canal (superficial to the flexor retinaculum, beside the pisiform) → deep motor branch and superficial sensory branch.
• Supplies — FCU, medial half of FDP, most intrinsic hand muscles (hypothenar, interossei, medial two lumbricals, adductor pollicis, deep head of FPB); skin of the medial 1½ fingers (front and back) and the medial hand.
• Ulnar claw hand — hyperextension of the MCP joints and flexion of the IP joints of the ring and little fingers (lumbricals lost; FDP unopposed).
• The ulnar paradox — the claw is worse with a distal (wrist) lesion than a proximal (elbow) lesion, because FDP to the ring and little fingers is still working and pulls the fingers into flexion.
• Signs — wasting of the first dorsal interosseous and hypothenar eminence, weak finger abduction and adduction (card test), Froment's sign, numb little finger; Wartenberg's sign (little finger abducted).
• Causes — compression at the elbow (cubital tunnel — leaning on the elbow, arthritis, valgus deformity — "tardy ulnar palsy"), medial epicondyle fractures, wrist lacerations, cyclists (Guyon's canal), leprosy.
• Ulnar vs C8–T1 root lesion — C8 root also weakens median-supplied thenar muscles and long finger flexors, and numbness extends up the medial forearm.

Radial, Axillary and Musculocutaneous Nerves

• Radial nerve (C5–T1, posterior cord):
  • Course — posterior arm in the spiral (radial) groove of the humerus → anterior to the lateral epicondyle → divides into the posterior interosseous nerve (motor — through supinator) and the superficial radial nerve (sensory).
  • Supplies — triceps, brachioradialis, all wrist, finger and thumb extensors, supinator; skin of the posterior arm and forearm and the dorsal web space between thumb and index finger.
  • Lesions:
    • Axilla (crutches) — triceps weak, wrist drop.
    • Spiral groove (mid-shaft humerus fracture, "Saturday night palsy") — wrist drop, triceps spared, numb dorsal web space.
    • Posterior interosseous nerve — finger drop (can't extend fingers at the MCP joints) but wrist extension largely preserved (ECRL spared) and no sensory loss.
    • Superficial radial nerve — tight watch straps or handcuffs (Wartenberg's syndrome) — numb dorsal thumb only.
  • Wrist drop makes grip appear weak (flexors can't work well with the wrist flexed) — test grip with the wrist supported.
• Axillary nerve (C5–6, posterior cord):
  • Winds around the surgical neck of the humerus (with the posterior circumflex humeral artery) through the quadrangular space.
  • Supplies deltoid and teres minor; skin over the lower deltoid ("regimental badge" area).
  • Injured in anterior shoulder dislocation and surgical neck fractures — weak abduction (15–90°), deltoid wasting — check sensation before and after reduction.
• Musculocutaneous nerve (C5–7, lateral cord) — pierces coracobrachialis; supplies biceps, brachialis and coracobrachialis; continues as the lateral cutaneous nerve of the forearm (skin of the lateral forearm) — rarely injured alone; weak elbow flexion and supination; absent biceps jerk.
• Long thoracic nerve — winging of the scapula (pushing against a wall) after axillary surgery, carrying heavy loads.

💡 Clinical Pearl: Three questions sort most hand nerve injuries: can the patient extend the wrist (radial), make an "OK" sign and abduct the thumb (median), and spread the fingers against resistance (ulnar)? Then map the numb skin.

Root Lesions — Myotomes and Dermatomes

• Cervical radiculopathy — root compression by a disc or osteophyte (spondylosis) — neck and arm pain in a dermatomal pattern, weakness, reduced reflex; C6 and C7 most common; Spurling's test (neck extension and rotation reproduces arm pain).
• Root vs nerve — C6 root vs median nerve (thumb numbness in both; C6 also weakens biceps and brachioradialis and reduces their reflexes); C8 root vs ulnar nerve (see above); C7 root vs radial nerve (C7 weakens triceps and wrist flexion too).
• Inverted supinator jerk — C5–6 cord compression — tapping brachioradialis causes finger flexion (brisk reflexes below the level) instead of elbow flexion.
• Cervical myelopathy — UMN signs in the legs, clumsy hands, Hoffmann's sign, Lhermitte's sign — urgent MRI.

Blood Supply and Venous Drainage

• Arteries — subclavian (left from the aortic arch; right from the brachiocephalic trunk) → axillary (outer border of the first rib) → brachial (lower border of teres major) → divides in the cubital fossa into radial and ulnar arteries → superficial (mainly ulnar) and deep (mainly radial) palmar arches.
• Pulses — brachial (medial to the biceps tendon — BP measurement), radial (wrist), ulnar (often hard to feel).
• Allen's test — occlude both radial and ulnar arteries, release the ulnar — colour should return within about 5–10 seconds — checks ulnar collateral supply before radial artery puncture or cannulation.
• Brachial artery injury — supracondylar fracture in children — pale, pulseless hand (or pink pulseless) — urgent reduction and vascular review.
• Subclavian steal syndrome — proximal subclavian stenosis → reversed flow in the vertebral artery when the arm is used → dizziness; BP difference between arms over 15–20 mmHg.
• Veins:
  • Superficial — cephalic (lateral — up the arm into the deltopectoral groove → axillary vein), basilic (medial — pierces deep fascia mid-arm → joins brachial veins → axillary vein), median cubital (links them in the cubital fossa — venepuncture).
  • Deep — venae comitantes alongside the arteries → axillary → subclavian vein.
  • PICC lines — usually the basilic vein; AV fistulas for dialysis (radiocephalic, brachiocephalic) — protect the non-dominant arm in CKD.
  • Upper limb DVT — lines, pacing wires, thoracic outlet (Paget–Schroetter — effort thrombosis).
• Lymphatics — superficial vessels follow the veins → epitrochlear nodes (medial elbow) and axillary nodes.

Clinically Important Injuries

• Colles' fracture — distal radius, dorsal displacement ("dinner fork" deformity) — fall on outstretched hand in older women (osteoporosis); Smith's fracture — volar displacement (fall on a flexed wrist); Barton's — intra-articular with carpal subluxation.
• Fragility fractures (distal radius, proximal humerus) — assess for osteoporosis (FRAX, DEXA) and falls.

Upper Limb at a Glance

Spot the Diagnosis

Exam-style Scenarios

1. A 7-year-old fell from a climbing frame. His elbow is swollen; X-ray shows a displaced supracondylar fracture. The hand is pale with a weak radial pulse.
Answer: Supracondylar fracture with brachial artery compromise.

• Urgent orthopaedic referral — splint in a comfortable position (slight flexion), analgesia, urgent closed reduction and fixation.
• Check and document the median (AIN — "OK" sign), radial and ulnar nerves.
• Monitor for compartment syndrome (pain on passive finger extension) — risk of Volkmann's contracture.

2. A 52-year-old woman has tingling in her right thumb, index and middle fingers that wakes her at night. She has weight gain and constipation.
Answer: Carpal tunnel syndrome — possibly secondary to hypothyroidism.

• Examine — thenar wasting, weak thumb abduction, Tinel's and Phalen's tests; palm sensation spared.
• Check TFTs (and glucose); nerve conduction studies if diagnosis unclear or before surgery.
• Night splint, steroid injection, or surgical decompression; treat the underlying cause.

3. A 40-year-old man wakes after a heavy night out with his arm hanging over a chair. He can't lift his wrist or fingers; there is numbness over the back of the thumb web space. Triceps is normal.
Answer: Radial nerve compression at the spiral groove ("Saturday night palsy").

• Neurapraxia — usually recovers over weeks to a few months.
• Wrist splint (cock-up) to maintain function; physiotherapy.
• Test grip with the wrist supported (grip appears weak otherwise); NCS if not recovering.

4. A 30-year-old cyclist has weakness of finger spreading and wasting of the hand muscles, but no numbness.
Answer: Ulnar nerve compression in Guyon's canal (deep motor branch).

• The deep branch is purely motor — sensory loss spared (lesion distal to the sensory branch).
• Advice — padded gloves, change hand position on handlebars.
• NCS; imaging if a ganglion is suspected; surgical decompression if persistent.

5. A 64-year-old smoker has pain in the right shoulder and inner arm, wasting of the small muscles of the hand, and a small right pupil with ptosis.
Answer: Pancoast tumour (right lung apex) invading the lower brachial plexus (C8–T1) and sympathetic chain.

• Chest X-ray (apical opacity) and CT chest.
• MRI to assess plexus and spinal involvement; biopsy.
• Refer urgently via the lung cancer pathway.

Key Points

• Arm — anterior compartment musculocutaneous, posterior radial; forearm — anterior median (except FCU and medial FDP), posterior radial.
• Rotator cuff — SITS; supraspinatus initiates abduction; painful arc means impingement.
• Brachial plexus — C5–T1; roots, trunks, divisions, cords, branches; Erb's (C5–6) vs Klumpke's (C8–T1).
• Axillary nerve — surgical neck and shoulder dislocation; radial nerve — spiral groove (wrist drop).
• Median nerve — carpal tunnel (LOAF muscles, lateral 3½ fingers, palm spared); AIN — "OK" sign, no sensory loss.
• Ulnar nerve — claw hand (worse with distal lesions), Froment's sign, medial 1½ fingers.
• Know C5–T1 myotomes, dermatomes and reflexes.
• Supracondylar fractures threaten the brachial artery and median nerve; scaphoid fractures threaten the proximal pole.
• Do Allen's test before radial artery puncture.

Guidelines and Further Reading

• Gray's Anatomy for Students
• Last's Anatomy — Regional and Applied
• Moore: Clinically Oriented Anatomy
• McMinn's Clinical Atlas of Human Anatomy
• Medical Research Council: Aids to the Examination of the Peripheral Nervous System
• British Orthopaedic Association Standards for Trauma (BOAST): supracondylar fractures in children; wrist fractures
• NICE CKS: Carpal tunnel syndrome
• BNF — for current doses

Detailed anatomy — Lower limb

• Framework: Overview and Surface Anatomy | The Hip Joint and Pelvis | The Gluteal Region | The Thigh and Femoral Triangle | The Knee | The Leg and Compartments | The Ankle and Foot
• Nerves and vessels: Lumbar and Sacral Plexuses | Sciatic, Tibial and Common Peroneal Nerves | Femoral, Obturator and Other Nerves | Root Lesions — Myotomes and Dermatomes | Arterial Supply and Pulses | Veins and Lymphatics | Gait and Clinically Important Injuries
• Revision: Lower Limb at a Glance | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🦵 Lower limb anatomy — page 4 of the Anatomy and Physiology chapter. Lower limb anatomy underpins hip fractures, knee injuries, foot drop, sciatica, peripheral arterial disease, varicose veins and DVT — and procedures such as femoral access, IM injections and nerve blocks. Learn the blood supply of the femoral head, the knee ligaments, the leg compartments and the nerves around the fibula and buttock.
Your four questions for every lower limb problem: Is it a root, plexus or peripheral nerve lesion? Which bone or joint injury threatens which nerve or vessel? Is the blood supply adequate (arterial) and is the return adequate (venous)? How does it change the patient's gait?

Overview and Surface Anatomy

• Regions — gluteal region, hip, thigh, knee, leg (knee to ankle), ankle, foot.
• Bones — hip bone (ilium, ischium, pubis — fuse at the acetabulum), femur (the longest bone), patella (largest sesamoid), tibia (weight-bearing, medial), fibula (lateral — muscle attachment, little weight), 7 tarsals, 5 metatarsals, 14 phalanges.
• Compartments:
  • Thigh — anterior (extensors of the knee — femoral nerve), medial (adductors — obturator nerve), posterior (hamstrings — sciatic nerve).
  • Leg — anterior (dorsiflexors — deep peroneal), lateral (evertors — superficial peroneal), posterior superficial and deep (plantarflexors — tibial).
• Surface landmarks:
  • Anterior superior iliac spine (ASIS), iliac crest (highest point at L4 — lumbar puncture), pubic tubercle, ischial tuberosity, greater trochanter.
  • Inguinal ligament — ASIS to pubic tubercle.
  • Mid-inguinal point (halfway between ASIS and pubic symphysis) — femoral artery; the midpoint of the inguinal ligament (halfway between ASIS and pubic tubercle) — deep inguinal ring.
  • Head of the fibula — the common peroneal nerve winds around its neck.
  • Medial malleolus (tibia) and lateral malleolus (fibula — lower and more posterior).
• Safe IM injection — upper outer quadrant of the buttock (or ventrogluteal site / vastus lateralis) — avoids the sciatic nerve.

The Hip Joint and Pelvis

• Hip joint — ball-and-socket synovial joint between the femoral head and acetabulum (deepened by the labrum); very stable — strong ligaments (iliofemoral — the strongest ligament in the body, pubofemoral, ischiofemoral) and deep socket.
• Blood supply of the femoral head:
  • Mainly the medial circumflex femoral artery (from the profunda femoris) — retinacular vessels run up the femoral neck within the capsule.
  • Lateral circumflex femoral artery; the artery of ligamentum teres (small in adults).
  • Intracapsular (femoral neck) fractures disrupt the retinacular vessels → avascular necrosis and non-union — displaced fractures usually need hemiarthroplasty or total hip replacement; extracapsular (intertrochanteric) fractures keep the blood supply — fixation with a dynamic hip screw or nail.
• Hip fracture signs — shortened and externally rotated leg; pain on movement; inability to weight-bear.
• Hip dislocation — posterior (90% — dashboard injury, or after hip replacement) — leg shortened, flexed, adducted and internally rotated; sciatic nerve at risk; reduce urgently (AVN).
• Hip movements and muscles:
  • Flexion — iliopsoas (femoral nerve and L1–3), rectus femoris, sartorius.
  • Extension — gluteus maximus (inferior gluteal nerve), hamstrings.
  • Abduction — gluteus medius and minimus (superior gluteal nerve).
  • Adduction — adductor longus, brevis and magnus, gracilis (obturator nerve).
• Pelvis — a ring formed by the two hip bones and the sacrum (sacroiliac joints and pubic symphysis); pelvic ring fractures in major trauma can cause life-threatening haemorrhage (venous plexus) — pelvic binder; pubic rami fractures in older people after falls (fragility).
• Hip conditions by age — DDH (infants), transient synovitis and Perthes' (children), SCFE (adolescents), osteoarthritis and fracture (older adults); hip pain may refer to the knee (obturator nerve).

The Gluteal Region

• Muscles:
  • Gluteus maximus — hip extension (climbing stairs, rising from sitting) — inferior gluteal nerve (L5–S2).
  • Gluteus medius and minimus — hip abduction — stabilise the pelvis when standing on one leg — superior gluteal nerve (L4–S1).
  • Piriformis and the short lateral rotators (obturator internus, gemelli, quadratus femoris).
• Trendelenburg sign — standing on the affected leg, the pelvis drops on the opposite (unsupported) side — weak hip abductors (superior gluteal nerve injury, hip OA, DDH, after hip surgery, L5 radiculopathy) → Trendelenburg gait (waddling if bilateral).
• Greater sciatic foramen — transmits structures from the pelvis to the buttock; piriformis passes through it:
  • Above piriformis — superior gluteal nerve and vessels.
  • Below piriformis — sciatic nerve, inferior gluteal nerve and vessels, pudendal nerve, posterior cutaneous nerve of the thigh.
• Sciatic nerve in the buttock — runs through the lower inner quadrant, midway between the greater trochanter and ischial tuberosity — at risk from misplaced injections, posterior hip dislocation and hip surgery.
• Piriformis syndrome — buttock pain and sciatica from compression by piriformis (diagnosis of exclusion).
• Greater trochanteric pain syndrome (trochanteric bursitis/gluteal tendinopathy) — lateral hip pain, tender greater trochanter, pain lying on that side.
• Pudendal nerve (S2–4) — perineal sensation and external sphincters — pudendal block in childbirth (ischial spine).

The Thigh and Femoral Triangle

• Anterior compartment — quadriceps femoris (rectus femoris, vastus lateralis, medialis, intermedius) → patellar tendon → tibial tuberosity — knee extension (rectus femoris also flexes the hip); sartorius (longest muscle — "tailor's" position); iliopsoas — all femoral nerve (L2–4).
• Medial compartment — adductors (longus, brevis, magnus), gracilis, pectineus — obturator nerve (L2–4); adductor strain in sport.
• Posterior compartment — hamstrings (biceps femoris, semitendinosus, semimembranosus) — knee flexion and hip extension — sciatic nerve (tibial part, except the short head of biceps — common peroneal); hamstring tears in sprinters.
• Femoral triangle:
  • Boundaries — inguinal ligament (superior), sartorius (lateral), adductor longus (medial).
  • Contents (lateral to medial) — "NAVY" — femoral Nerve, Artery, Vein, (empty space — femoral canal containing lymphatics — Cloquet's node), Y-fronts (midline).
  • The femoral artery, vein and canal lie in the femoral sheath; the nerve lies outside it.
• Clinical uses — femoral arterial puncture and access (angiography, ECMO), femoral venous lines (vein medial to the pulse), femoral nerve block (lateral to the artery).
• Femoral hernia — through the femoral canal, below and lateral to the pubic tubercle; narrow neck → high risk of strangulation; more common in women (inguinal hernias are still commoner overall).
• Adductor (subsartorial/Hunter's) canal — femoral artery and vein, saphenous nerve and nerve to vastus medialis → artery passes through the adductor hiatus to become the popliteal artery; adductor canal block for knee surgery.
• Femoral shaft fracture — high-energy; can lose 1–1.5 L of blood into the thigh; traction splint; fat embolism risk.

The Knee

• Knee joint — modified hinge joint (femur, tibia and patella); the largest synovial joint; flexion, extension and a little rotation ("screw-home" mechanism locks the knee in full extension — popliteus unlocks it).
• Stabilising structures:

  Structure	Function	Injury and test
  Anterior cruciate ligament (ACL)	Prevents anterior tibial translation	Twisting injury with a "pop", rapid swelling (haemarthrosis), instability — Lachman's test (most sensitive), anterior drawer
  Posterior cruciate ligament (PCL)	Prevents posterior tibial translation	Dashboard injury, fall on flexed knee — posterior drawer, posterior sag
  Medial collateral ligament (MCL)	Resists valgus stress	Blow to the lateral knee — valgus stress test; attached to the medial meniscus
  Lateral collateral ligament (LCL)	Resists varus stress	Varus stress test; may injure the common peroneal nerve
  Menisci (fibrocartilage)	Load distribution, stability	Twisting on a flexed knee — joint line pain, delayed swelling, locking (bucket-handle tear); McMurray's test; medial tears more common

• "Unhappy triad" — ACL, MCL and medial meniscus injury together (lateral blow to a planted knee).
• Acute haemarthrosis (swelling within 2 hours) — ACL tear, fracture (lipohaemarthrosis), patellar dislocation; slower swelling (hours to a day) suggests a meniscal tear.
• Extensor mechanism — quadriceps tendon, patella, patellar tendon — rupture or patellar fracture → can't straight-leg raise.
• Bursae — prepatellar ("housemaid's knee" — kneeling), infrapatellar ("clergyman's knee"); Baker's (popliteal) cyst — posterior knee; rupture mimics DVT.
• Popliteal fossa — diamond-shaped; contents (superficial to deep) — tibial nerve, popliteal vein, popliteal artery (deepest); popliteal artery aneurysm (pulsatile mass — often bilateral, with AAA); posterior knee dislocation can tear the popliteal artery (check ABPI).
• Ottawa knee rules — X-ray if age 55 or more, isolated patellar tenderness, fibular head tenderness, can't flex to 90°, or can't weight-bear 4 steps.

The Leg and Compartments

• Four compartments (enclosed by tight fascia — compartment syndrome risk):

  Compartment	Muscles	Action	Nerve and artery
  Anterior	Tibialis anterior, extensor hallucis longus, extensor digitorum longus	Dorsiflexion, toe extension, inversion (TA)	Deep peroneal; anterior tibial artery
  Lateral	Peroneus (fibularis) longus and brevis	Eversion	Superficial peroneal; peroneal artery branches
  Superficial posterior	Gastrocnemius, soleus (→ Achilles tendon), plantaris	Plantarflexion	Tibial
  Deep posterior	Tibialis posterior, flexor digitorum longus, flexor hallucis longus	Inversion, toe flexion, plantarflexion	Tibial; posterior tibial artery

• Compartment syndrome — raised pressure within a compartment → ischaemia of muscle and nerve:
  • Causes — tibial fractures (commonest), crush injury, tight casts, reperfusion after vascular surgery, burns, prolonged immobilisation (long lie), IV drug injection.
  • Features — pain out of proportion to the injury, pain on passive stretch of the muscles (earliest sign), tense swelling, paraesthesia (first web space numbness — deep peroneal nerve in anterior compartment syndrome); pulses are usually present — pulselessness is very late.
  • Management — remove dressings and split casts to skin, limb at heart level, measure compartment pressures if unclear (delta pressure — diastolic BP minus compartment pressure — under 30 mmHg), urgent fasciotomy of all four compartments.
• Achilles tendon rupture — sudden pain "like being kicked" during sport, weak plantarflexion, palpable gap — Simmonds' (Thompson's) test (squeezing the calf doesn't plantarflex the foot); risk with fluoroquinolones and steroids.
• Tibial fractures — the tibia is subcutaneous anteromedially — open fractures are common; poor blood supply to the distal third (delayed union).
• Shin splints (medial tibial stress syndrome) and stress fractures in runners and military recruits.

The Ankle and Foot

• Ankle (talocrural) joint — hinge between the tibia, fibula and talus (mortise) — dorsiflexion and plantarflexion; most stable in dorsiflexion.
• Subtalar and midtarsal joints — inversion and eversion.
• Ligaments:
  • Lateral — anterior talofibular (ATFL — the most commonly sprained ligament — inversion injury), calcaneofibular, posterior talofibular.
  • Medial — deltoid ligament (strong — avulses the medial malleolus rather than tearing).
  • Syndesmosis — links the distal tibia and fibula (high ankle sprain; Weber C fractures).
• Ottawa ankle rules — X-ray if bone tenderness at the posterior edge or tip of either malleolus (distal 6 cm) or unable to weight-bear 4 steps; foot X-ray if tenderness at the base of the 5th metatarsal or navicular.
• Weber classification (lateral malleolus fractures — relative to the syndesmosis) — A (below — stable), B (at the level — may be unstable), C (above — unstable, syndesmosis torn — surgery); Maisonneuve — proximal fibular fracture with ankle syndesmosis injury.
• Tarsal tunnel (behind the medial malleolus) — contents from front to back: Tibialis posterior, flexor Digitorum longus, posterior tibial Artery (and veins), tibial Nerve, flexor Hallucis longus — "Tom, Dick And Nervous Harry"; posterior tibial pulse midway between the medial malleolus and the heel; tarsal tunnel syndrome (tibial nerve compression).
• Arches of the foot — medial longitudinal (tibialis posterior — tibialis posterior dysfunction → acquired flat foot), lateral longitudinal, transverse.
• Foot conditions — plantar fasciitis (heel pain worse with first steps), Morton's neuroma (between 3rd and 4th metatarsal heads), hallux valgus (bunion), gout (1st MTP), Charcot foot (neuropathic — diabetes), 5th metatarsal base fracture (peroneus brevis avulsion) vs Jones fracture (poor healing), calcaneal fractures (fall from height — check spine).
• Diabetic foot — neuropathy (10 g monofilament), ischaemia and infection — daily checks; urgent referral for ulcers or a hot, swollen foot.

Lumbar and Sacral Plexuses

• Lumbar plexus (L1–4) — forms within psoas major:
  • Iliohypogastric and ilioinguinal (L1) — lower abdominal wall, groin, scrotum or labia (injury in hernia repair — groin numbness and pain).
  • Genitofemoral (L1–2) — cremaster muscle (cremasteric reflex) and skin of the upper thigh and scrotum.
  • Lateral cutaneous nerve of the thigh (L2–3) — skin of the lateral thigh — meralgia paraesthetica (compressed under the inguinal ligament near the ASIS — obesity, tight belts, pregnancy).
  • Femoral nerve (L2–4) — posterior divisions.
  • Obturator nerve (L2–4) — anterior divisions.
• Sacral plexus (L4–S4) — on the posterior pelvic wall (lumbosacral trunk L4–5 joins S1–4):
  • Superior gluteal (L4–S1) and inferior gluteal (L5–S2) nerves.
  • Sciatic nerve (L4–S3) — the largest nerve in the body.
  • Posterior cutaneous nerve of the thigh (S1–3).
  • Pudendal nerve (S2–4) — "S2, 3, 4 keeps the penis off the floor" — and continence.
  • Pelvic splanchnic nerves (S2–4 — parasympathetic to the bladder, rectum and genitals).
• Lumbosacral plexopathy — diabetic amyotrophy (painful proximal weakness and wasting in the thigh — older people with type 2 diabetes), pelvic tumours, retroperitoneal haematoma (anticoagulants — femoral neuropathy), radiotherapy, obstetric injury.
• Psoas abscess — fever, back or groin pain, hip held flexed (pain on extension — psoas sign); TB or Staph. aureus.

Sciatic, Tibial and Common Peroneal Nerves

• Sciatic nerve (L4–S3):
  • Leaves the pelvis through the greater sciatic foramen below piriformis → buttock → posterior thigh (supplies hamstrings) → divides (usually above the knee) into the tibial and common peroneal nerves.
  • Complete lesion — weak knee flexion and all muscles below the knee (flail foot); sensory loss below the knee except the medial leg (saphenous); absent ankle jerk.
  • Causes — posterior hip dislocation, hip replacement, misplaced IM injection, pelvic fractures, tumours.
• Common peroneal (fibular) nerve (L4–S2):
  • Winds around the neck of the fibula (superficial — very vulnerable) → divides into deep peroneal (anterior compartment — dorsiflexion; sensation to the first web space) and superficial peroneal (lateral compartment — eversion; sensation to the dorsum of the foot).
  • Lesion — foot drop (weak dorsiflexion and eversion; inversion preserved — tibialis posterior is tibial nerve), numb dorsum of foot, high-stepping gait; ankle jerk normal.
  • Causes — fibular neck fracture, tight plaster casts, prolonged leg crossing, squatting, weight loss, lithotomy position, kneeling, knee dislocation.
• Tibial nerve (L4–S3):
  • Through the popliteal fossa → deep posterior compartment → behind the medial malleolus (tarsal tunnel) → medial and lateral plantar nerves (sole).
  • Supplies — calf muscles (plantarflexion, inversion via tibialis posterior, toe flexion), intrinsic muscles of the sole; sensation of the sole.
  • Lesion — weak plantarflexion and inversion (can't stand on tiptoe), numb sole, absent ankle jerk.
• Sural nerve (tibial and common peroneal branches) — lateral foot and ankle; used for nerve biopsy and grafts.
• Foot drop — differential:

  Lesion	Weakness	Other clues
  Common peroneal nerve	Dorsiflexion, eversion; inversion normal	Dorsum of foot numb; reflexes normal
  L5 radiculopathy	Dorsiflexion, eversion and inversion, hip abduction	Back pain, sciatica, L5 dermatome
  Sciatic nerve	All below the knee plus hamstrings	Absent ankle jerk
  Upper motor neurone	Pyramidal pattern	Brisk reflexes, extensor plantar (stroke, MS)
  Peripheral neuropathy / MND	Bilateral, symmetrical or patchy	Glove and stocking loss; fasciculations

Femoral, Obturator and Other Nerves

• Femoral nerve (L2–4):
  • Forms in psoas → under the inguinal ligament (lateral to the artery) → femoral triangle.
  • Supplies — quadriceps (knee extension), iliacus, sartorius, pectineus; sensation of the anterior thigh and, via the saphenous nerve, the medial leg and medial foot (runs with the great saphenous vein — injured in varicose vein surgery and vein harvest).
  • Lesion — weak knee extension (knee gives way, difficulty climbing stairs), absent knee jerk, numb anterior thigh and medial leg.
  • Causes — retroperitoneal or iliacus haematoma (anticoagulation, haemophilia), femoral catheterisation, hip surgery, pelvic surgery (retractors), diabetic amyotrophy.
  • Femoral nerve vs L3–4 radiculopathy — L4 also weakens hip adduction (obturator) and tibialis anterior.
• Obturator nerve (L2–4):
  • Through the obturator foramen (canal) → medial thigh.
  • Supplies — adductors; small area of skin on the medial thigh; branches to the hip and knee (hip pain referred to the knee).
  • Lesion — weak adduction (leg swings outwards — circumduction); causes — pelvic surgery, obstetric injury, obturator hernia (elderly thin women — Howship–Romberg sign — medial thigh pain).
• Lateral cutaneous nerve of the thigh — meralgia paraesthetica — burning numbness of the lateral thigh, purely sensory; weight loss, loosen belts.
• Superior gluteal nerve — Trendelenburg gait; inferior gluteal — weak hip extension (difficulty rising from a chair or climbing stairs).
• Saphenous nerve block (adductor canal) and femoral and fascia iliaca blocks — knee and hip surgery and hip fractures.

Root Lesions — Myotomes and Dermatomes

• Lumbar disc prolapse — usually L4/5 (compresses L5) or L5/S1 (compresses S1) — posterolateral prolapse compresses the traversing root (the lower one); far-lateral prolapse compresses the exiting root.
• Sciatica — leg pain worse than back pain, below the knee, dermatomal; straight leg raise positive (L5, S1); femoral stretch test (L2–4).
• Cauda equina syndrome — compression of the lumbosacral roots below L1/2 (central disc, tumour, abscess, haematoma) — bilateral sciatica, saddle anaesthesia, bladder dysfunction (retention or reduced sensation), bowel dysfunction, sexual dysfunction, reduced anal tone — emergency MRI and decompression.
• Spinal cord ends at L1/2 in adults (conus medullaris) — LP below L3; lesions above cause UMN signs, below LMN signs (cauda equina); conus lesions — mixed signs, early sphincter problems.
• Spinal stenosis — neurogenic claudication (leg pain on walking, relieved by sitting or bending forward — "shopping trolley sign"; pulses normal) vs vascular claudication (relieved by standing still; pulses reduced).

⚠️ Safety Pearl: Back pain with bilateral leg symptoms, saddle numbness or any new bladder or bowel disturbance is cauda equina syndrome until an urgent MRI proves otherwise. Waiting for retention to develop means waiting for permanent damage.

Arterial Supply and Pulses

• Course — abdominal aorta (bifurcates at L4) → common iliac → external iliac (internal iliac supplies the pelvis and buttock) → femoral artery (under the inguinal ligament at the mid-inguinal point) → gives off the profunda femoris (deep thigh; circumflex femoral arteries) → superficial femoral artery (adductor canal — commonest site of atherosclerotic occlusion) → popliteal artery → divides below the knee into the anterior tibial (→ dorsalis pedis) and the tibioperoneal trunk (→ posterior tibial and peroneal arteries).
• Pulses:
  • Femoral — mid-inguinal point.
  • Popliteal — deep in the fossa — knee slightly flexed, press with fingertips of both hands (easily felt — consider aneurysm).
  • Posterior tibial — behind the medial malleolus.
  • Dorsalis pedis — lateral to extensor hallucis longus on the dorsum of the foot (absent in about 10% of healthy people).
• Peripheral arterial disease:
  • Intermittent claudication — calf pain on walking relieved by rest (superficial femoral disease); buttock and thigh claudication with aorto-iliac disease (Leriche syndrome — with erectile dysfunction).
  • Critical (chronic limb-threatening) ischaemia — rest pain (worse at night, relieved by hanging the leg down), ulcers, gangrene.
  • ABPI (ankle–brachial pressure index) — normal 0.9–1.3; claudication about 0.5–0.9; critical ischaemia under 0.5; falsely high (over 1.3) with calcified arteries (diabetes, CKD); Buerger's test (pallor on elevation, rubor on dependency).
• Acute limb ischaemia — "6 Ps" — Pain, Pallor, Pulseless, Perishingly cold, Paraesthesia, Paralysis (late — irreversible) — embolism (AF) or thrombosis — heparin and urgent vascular surgery.
• Popliteal artery — at risk in knee dislocation and supracondylar femoral fractures.

Veins and Lymphatics

• Superficial veins:
  • Great (long) saphenous vein — starts anterior to the medial malleolus (cutdown site) → up the medial leg and thigh (with the saphenous nerve) → joins the femoral vein at the saphenofemoral junction (about 3–4 cm below and lateral to the pubic tubercle); used for coronary bypass grafts.
  • Small (short) saphenous vein — behind the lateral malleolus → up the back of the calf (with the sural nerve) → popliteal vein (saphenopopliteal junction).
• Deep veins — accompany the arteries (venae comitantes below the knee) → popliteal → femoral → external iliac; soleal and gastrocnemius venous sinuses (where DVT often starts).
• Perforating veins — connect superficial to deep, with one-way valves (blood flows superficial → deep).
• Calf muscle pump — calf contraction squeezes deep veins; valves stop backflow — fails with immobility (DVT) and with valve damage.
• Chronic venous disease:
  • Varicose veins — incompetent valves (often at the saphenofemoral junction) — duplex ultrasound; endovenous ablation.
  • Venous insufficiency — oedema, haemosiderin staining, lipodermatosclerosis ("inverted champagne bottle"), venous eczema, venous ulcers (gaiter area — medial malleolus — compression bandaging after checking ABPI).
• Deep vein thrombosis — Virchow's triad (stasis, endothelial injury, hypercoagulability) — Wells score, D-dimer, proximal leg vein ultrasound; May–Thurner syndrome (left common iliac vein compressed by the right common iliac artery — left-sided DVT).
• Lymphatics — superficial vessels follow the great saphenous vein → superficial inguinal nodes (also drain the lower abdominal wall below the umbilicus, buttock, perineum, external genitalia, anal canal below the pectinate line, and lower vagina) → deep inguinal → external iliac nodes; testis drains to para-aortic nodes (not inguinal); popliteal nodes drain the lateral foot (small saphenous territory).

Gait and Clinically Important Injuries

• Gait cycle — stance phase (about 60% — heel strike to toe-off) and swing phase (40%).
• Abnormal gaits:

  Gait	Features	Causes
  Antalgic	Short stance on the painful leg	Pain — arthritis, fracture
  Trendelenburg	Pelvis drops on the swing side; trunk lurches	Weak hip abductors — hip OA, superior gluteal nerve, L5
  High-stepping	Lifts the knee high; foot slaps	Foot drop — common peroneal, L5, neuropathy
  Spastic / hemiplegic	Stiff leg circumducts	Stroke, MS
  Scissoring	Legs cross	Cerebral palsy, cord lesions
  Parkinsonian	Shuffling, reduced arm swing, festination, turning en bloc	Parkinson's
  Ataxic (cerebellar)	Broad-based, unsteady; can't tandem walk	Cerebellar disease, alcohol
  Sensory ataxic	Stamping; worse in the dark; Romberg positive	Dorsal column loss, neuropathy
  Waddling	Bilateral Trendelenburg	Proximal myopathy, bilateral DDH, pregnancy
  Apraxic ("magnetic")	Feet stuck to the floor	Normal pressure hydrocephalus, vascular dementia

• Key injuries and what they threaten:
  • Intracapsular neck of femur fracture — femoral head blood supply (AVN).
  • Posterior hip dislocation — sciatic nerve; femoral head blood supply.
  • Pelvic ring fracture — internal iliac vessels and venous plexus (haemorrhage), bladder and urethra.
  • Femoral shaft fracture — blood loss, fat embolism.
  • Knee dislocation — popliteal artery and common peroneal nerve.
  • Fibular neck fracture — common peroneal nerve.
  • Tibial fracture — compartment syndrome.
  • Calcaneal fracture (fall from height) — associated spinal fractures.

Lower Limb at a Glance

Spot the Diagnosis

Exam-style Scenarios

1. An 84-year-old woman fell at home. Her right leg is shortened and externally rotated. X-ray shows a displaced intracapsular fracture of the femoral neck.
Answer: Displaced intracapsular hip fracture.

• The retinacular vessels (from the medial circumflex femoral artery) are disrupted — high risk of avascular necrosis and non-union.
• Treatment — arthroplasty (hemiarthroplasty or total hip replacement), not fixation; surgery on the day of or day after admission.
• Analgesia including a fascia iliaca block; orthogeriatric care; delirium prevention; falls and bone health assessment.

2. A 35-year-old man has back pain and pain radiating down the back of the leg to the sole. The ankle jerk is absent and plantarflexion is weak.
Answer: S1 radiculopathy — likely L5/S1 posterolateral disc prolapse.

• Check for red flags — cauda equina (saddle anaesthesia, bladder or bowel dysfunction), cancer, infection, progressive weakness.
• Most improve with analgesia and staying active; MRI if not improving or if surgery is considered.
• Urgent MRI and referral if progressive neurological deficit or cauda equina symptoms.

3. A 22-year-old footballer was tackled from the side. His knee swelled within an hour. Lachman's test shows increased laxity and the knee is tender medially.
Answer: ACL tear, probably with MCL and medial meniscal injury ("unhappy triad").

• Haemarthrosis — rapid swelling suggests ACL tear or fracture; X-ray to exclude fracture (Segond fracture).
• MRI; RICE, analgesia, physiotherapy (prehabilitation).
• Orthopaedic or sports medicine referral — ACL reconstruction for active people with instability.

4. A 30-year-old man's leg was in a below-knee cast for a tibial fracture. He now has severe pain, especially when his toes are moved, and numbness between the first and second toes. Pulses are present.
Answer: Acute compartment syndrome (anterior compartment — deep peroneal nerve).

• Split the cast and padding to the skin; keep the leg at heart level (not elevated).
• Urgent orthopaedic review; compartment pressure measurement if uncertain.
• Emergency four-compartment fasciotomy — palpable pulses don't exclude it.

5. A 68-year-old smoker has calf pain after walking 100 metres that settles with rest. Foot pulses are absent; ABPI is 0.6.
Answer: Intermittent claudication — superficial femoral artery disease likely.

• Stop smoking; supervised exercise programme; statin and antiplatelet (clopidogrel); manage BP and diabetes.
• Duplex ultrasound; vascular referral if lifestyle-limiting despite exercise.
• Warn about red flags — rest pain, ulcers, gangrene (chronic limb-threatening ischaemia).

Key Points

• Thigh — anterior femoral, medial obturator, posterior sciatic; leg — anterior deep peroneal, lateral superficial peroneal, posterior tibial.
• The femoral head is supplied mainly by the medial circumflex femoral artery — displaced intracapsular fractures need arthroplasty.
• Femoral triangle — NAVY; femoral hernias are below and lateral to the pubic tubercle and strangulate.
• Know the knee ligaments and tests — Lachman's for ACL; rapid swelling means haemarthrosis.
• Compartment syndrome — pain on passive stretch; pulses present until late; fasciotomy.
• Common peroneal nerve at the fibular neck — foot drop with normal inversion; L5 — foot drop with weak inversion.
• L4/5 disc compresses L5; L5/S1 disc compresses S1; cauda equina needs emergency MRI.
• Learn pulse positions, ABPI values and the 6 Ps of acute ischaemia.
• Great saphenous vein — anterior to the medial malleolus to the saphenofemoral junction.
• Superficial inguinal nodes drain the leg, perineum and lower abdominal wall — not the testis.

Guidelines and Further Reading

• Gray's Anatomy for Students; Last's Anatomy — Regional and Applied
• Medical Research Council: Aids to the Examination of the Peripheral Nervous System
• NICE CG124: Hip fracture — management; NICE NG59: Low back pain and sciatica
• NICE CG147: Peripheral arterial disease; NICE CG168: Varicose veins
• BOAST: Diagnosis and management of compartment syndrome of the limbs; open fractures
• GIRFT / NICE: Cauda equina syndrome national pathway
• Ottawa ankle and knee rules
• BNF — for current doses

Detailed anatomy — Head, neck and back

• Head: The Skull and Scalp | Meninges and Intracranial Haemorrhage | Blood Supply of the Brain | The Cranial Nerves | The Face and Trigeminal Nerve | The Facial Nerve
• Neck: Triangles of the Neck | Larynx, Thyroid and Neck Structures | Vessels and Lymph Nodes of the Neck | Lower Cranial Nerves in the Neck
• Back: The Vertebral Column | The Spinal Cord and Its Blood Supply | Cord Lesions and Back Pain | Lumbar Puncture and Epidural Anatomy
• Revision: Head, Neck and Back at a Glance | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🧠 Head, neck and back anatomy — page 6 of the Anatomy and Physiology chapter. This region holds the brain, the airway, the great vessels and the spinal cord, packed into a small space — so small lesions cause big problems. Head injuries, strokes, cranial nerve palsies, neck lumps, airway procedures, cord compression and lumbar puncture all depend on knowing what lies where.
Your four questions for every head, neck or back problem: Which cranial nerve, vessel or spinal level is involved? What lies next to it — and could be compressed or damaged? Is there a danger to the airway, the brain or the cord? Which landmark guides the procedure safely?

The Skull and Scalp

• Skull — neurocranium (vault and base — protects the brain) and viscerocranium (facial skeleton); 22 bones (excluding ossicles).
• Sutures — coronal (frontal–parietal), sagittal (between parietals), lambdoid (parietal–occipital); fontanelles in infants — anterior (closes by about 18 months — bulging in raised ICP, sunken in dehydration) and posterior (closes by about 2 months); craniosynostosis (premature fusion).
• Pterion — the H-shaped junction of the frontal, parietal, temporal and sphenoid bones (about 3 cm above the zygomatic arch) — thinnest part of the skull; the middle meningeal artery runs beneath it → extradural haematoma after a blow to the side of the head.
• Cranial fossae (base) — anterior (frontal lobes; cribriform plate — olfactory nerves), middle (temporal lobes; pituitary in the sella turcica), posterior (cerebellum, brainstem; foramen magnum).
• Key foramina:

  Foramen	Contents
  Cribriform plate	Olfactory nerves (I) — fracture → anosmia, CSF rhinorrhoea
  Optic canal	Optic nerve (II), ophthalmic artery
  Superior orbital fissure	III, IV, V1, VI, ophthalmic veins
  Foramen rotundum	V2 (maxillary)
  Foramen ovale	V3 (mandibular)
  Foramen spinosum	Middle meningeal artery
  Internal acoustic meatus	VII, VIII (vestibular schwannoma)
  Jugular foramen	IX, X, XI, internal jugular vein
  Hypoglossal canal	XII
  Foramen magnum	Medulla, vertebral arteries, spinal accessory roots — tonsillar herniation (coning)

  "Standing Room Only" — superior orbital fissure, rotundum, ovale for V1, V2, V3.
• Basal skull fracture signs — periorbital bruising ("panda eyes"), Battle's sign (mastoid bruising — late), haemotympanum, CSF rhinorrhoea or otorrhoea — CT head; avoid nasal tubes.
• Scalp layers — "SCALP" — Skin, Connective tissue (dense — vessels held open → scalp wounds bleed heavily), Aponeurosis (galea), Loose areolar tissue (the "danger layer" — emissary veins can carry infection inside the skull; subgaleal haematoma), Pericranium.

Meninges and Intracranial Haemorrhage

• Meninges — dura mater (tough, outer — two layers that separate to form the venous sinuses; folds — falx cerebri, tentorium cerebelli), arachnoid mater, pia mater (on the brain surface).
• CSF — produced by the choroid plexus (about 500 mL a day; about 150 mL present) → lateral ventricles → foramina of Monro → third ventricle → cerebral aqueduct → fourth ventricle → foramina of Luschka and Magendie → subarachnoid space → absorbed by arachnoid granulations into the dural venous sinuses.
• Hydrocephalus — obstructive (non-communicating) (aqueduct stenosis, tumours, colloid cyst) vs communicating (impaired absorption — after SAH or meningitis; normal pressure hydrocephalus — triad of gait apraxia, urinary incontinence and dementia).
• Intracranial haemorrhage by space:

  Type	Vessel	CT appearance	Clinical features
  Extradural	Middle meningeal artery (pterion fracture)	Biconvex (lentiform); doesn't cross sutures	Young; head injury, lucid interval, then rapid deterioration — urgent evacuation
  Subdural	Bridging veins (cortex to sinuses)	Crescent-shaped; crosses sutures, not the midline	Older people, alcohol, anticoagulants; acute (trauma) or chronic (weeks — fluctuating confusion, headache)
  Subarachnoid	Berry aneurysm (circle of Willis) or trauma	Blood in the sulci and basal cisterns	Thunderclap headache, neck stiffness, collapse
  Intracerebral	Small penetrating arteries (hypertension), amyloid angiopathy	Blood within the brain (basal ganglia, lobar)	Stroke symptoms, headache, vomiting

• Dural venous sinuses — superior sagittal → transverse → sigmoid → internal jugular vein; cavernous sinus (beside the pituitary) contains the internal carotid artery and CN VI in the sinus, with III, IV, V1 and V2 in its lateral wall → cavernous sinus thrombosis (from facial infection — "danger triangle" of the face) → ophthalmoplegia, proptosis, chemosis.
• Venous sinus thrombosis — headache, seizures, papilloedema, focal signs — OCP, pregnancy, dehydration, thrombophilia — CT or MR venogram — anticoagulation.
• Brain herniation — subfalcine (under the falx), uncal (transtentorial) (temporal lobe compresses CN III — fixed dilated pupil — and the cerebral peduncle), tonsillar (cerebellar tonsils through the foramen magnum — compresses the medulla — respiratory arrest; why LP is dangerous with raised ICP).

Blood Supply of the Brain

• Anterior circulation — internal carotid arteries → ophthalmic artery (amaurosis fugax), anterior cerebral artery (ACA) and middle cerebral artery (MCA).
• Posterior circulation — vertebral arteries (through the transverse foramina of C6–C1) → join to form the basilar artery (on the pons) → posterior cerebral arteries (PCA); branches to the cerebellum (PICA, AICA, SCA) and brainstem.
• Circle of Willis — anastomosis at the base of the brain linking anterior and posterior circulations (anterior and posterior communicating arteries) — collateral flow; berry aneurysms at junctions (anterior communicating commonest; posterior communicating aneurysm compresses CN III — painful third nerve palsy with a dilated pupil).
• Territories and stroke syndromes:

  Artery	Territory	Stroke features
  ACA	Medial frontal and parietal lobes	Contralateral leg weakness and sensory loss more than arm; personality change, incontinence
  MCA	Lateral hemisphere, internal capsule	Contralateral face and arm more than leg weakness and sensory loss, homonymous hemianopia; aphasia (dominant — usually left) or neglect (non-dominant)
  PCA	Occipital lobe, thalamus	Contralateral homonymous hemianopia with macular sparing; visual agnosia
  PICA (or vertebral)	Lateral medulla	Lateral medullary (Wallenberg) syndrome — ipsilateral facial pain/temperature loss, Horner's, ataxia, dysphagia; contralateral body pain/temperature loss; vertigo
  Basilar	Pons, brainstem	Locked-in syndrome, coma, quadriplegia, cranial nerve palsies
  Lenticulostriate	Internal capsule, basal ganglia	Lacunar — pure motor, pure sensory, ataxic hemiparesis (hypertension, diabetes)

• Venous drainage — cerebral veins → dural venous sinuses → internal jugular veins.
• Autoregulation — cerebral blood flow kept constant (about 50 mL/100 g/min) over a MAP of about 60–150 mmHg; lost in brain injury; CO₂ dilates cerebral vessels (hypercapnia raises ICP).
• Blood–brain barrier — tight junctions of capillary endothelium with astrocyte foot processes — limits entry of drugs (lipid-soluble pass); absent in circumventricular organs (area postrema — vomiting centre).

The Cranial Nerves

• Brainstem levels — midbrain (III, IV), pons (V, VI, VII, VIII), medulla (IX, X, XI, XII) — "2 above the pons, 4 in the pons, 4 in the medulla".
• Eye movements — "LR6 SO4" — lateral rectus by VI, superior oblique by IV, all others by III.
• Bulbar vs pseudobulbar palsy — bulbar (LMN — IX, X, XII — wasted fasciculating tongue, absent gag, nasal speech — MND, GBS, myasthenia) vs pseudobulbar (UMN — bilateral corticobulbar — spastic tongue, brisk jaw jerk, emotional lability — bilateral strokes, MS, MND).
• Multiple cranial nerve palsies — cavernous sinus (III, IV, V1, V2, VI), cerebellopontine angle (V, VII, VIII — vestibular schwannoma), jugular foramen (IX, X, XI), basal meningitis, sarcoidosis, nasopharyngeal carcinoma.

The Face and Trigeminal Nerve

• Trigeminal nerve (V) — the main sensory nerve of the face:
  • V1 Ophthalmic — forehead, upper eyelid, cornea (corneal reflex), tip of the nose (Hutchinson's sign — nasal tip vesicles in shingles predict eye involvement).
  • V2 Maxillary — cheek, lower eyelid, upper lip, upper teeth, maxillary sinus.
  • V3 Mandibular — lower lip, chin, lower teeth, anterior two-thirds of the tongue (general sensation), temple; motor to the muscles of mastication (masseter, temporalis, pterygoids).
  • The angle of the jaw is supplied by C2–3 (great auricular nerve), not V — non-organic facial sensory loss often includes it.
• Trigeminal neuralgia — brief, severe, electric shock-like pain (usually V2/V3) triggered by touching, chewing or shaving — carbamazepine first line; MRI (compression by a vessel; MS in young people).
• Muscles of facial expression — supplied by the facial nerve (VII); orbicularis oculi (eye closure), orbicularis oris, buccinator, frontalis, platysma.
• Parotid gland — in front of the ear; the facial nerve passes through it (divides into 5 branches — "To Zanzibar By Motor Car" — Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical); parotid duct (Stensen's) opens opposite the upper second molar; secretomotor supply via CN IX.
• Parotid tumours — mostly benign (pleomorphic adenoma); facial nerve palsy suggests malignancy; facial nerve at risk in parotidectomy.
• Submandibular gland — duct (Wharton's) opens beside the frenulum — stones (swelling with eating); marginal mandibular nerve at risk in surgery (lip weakness).
• Facial arteries and veins — the facial artery crosses the mandible in front of masseter (palpable pulse); facial veins connect with the cavernous sinus — the "danger triangle" (nose and upper lip) — infection can spread intracranially.
• Temporomandibular joint — dislocation (anterior — can't close the mouth), TMJ dysfunction (clicking, pain); jaw jerk (V3 — brisk in UMN lesions above the pons).
• Orbit — blowout fracture of the orbital floor (inferior rectus entrapment — can't look up; infraorbital nerve numbness — V2).

The Facial Nerve

• Course — pons → cerebellopontine angle → internal acoustic meatus (with VIII) → facial canal in the temporal bone (geniculate ganglion — branches: greater petrosal — lacrimation; nerve to stapedius; chorda tympani — taste to the anterior two-thirds of the tongue and submandibular/sublingual salivation) → stylomastoid foramen → parotid gland → 5 terminal motor branches.
• UMN vs LMN facial weakness:
  • UMN lesion (stroke, tumour) — forehead spared (upper face has bilateral cortical supply) — contralateral weakness of the lower face.
  • LMN lesion (Bell's palsy, parotid tumour, Ramsay Hunt) — whole half of the face on the same side, including the forehead; can't close the eye (corneal exposure).
• Localising a LMN lesion — loss of taste (chorda tympani) and hyperacusis (stapedius) mean a lesion within the facial canal; parotid lesions spare these.
• Bell's palsy — idiopathic LMN facial palsy, onset over 72 hours or less — prednisolone within 72 hours; eye protection (lubricants, tape at night); most recover within 3–6 months.
• Ramsay Hunt syndrome — herpes zoster of the geniculate ganglion — facial palsy, painful vesicles in the ear canal and on the palate, hearing loss and vertigo — aciclovir and steroids; worse prognosis.
• Other causes — Lyme disease (bilateral), Guillain–Barré (bilateral), sarcoidosis (bilateral — with parotid swelling), cholesteatoma and middle ear infection, temporal bone fracture, parotid tumours, HIV, pregnancy, diabetes.
• Bilateral facial palsy — think Lyme, GBS, sarcoid (rarely bilateral Bell's).

Triangles of the Neck

• Sternocleidomastoid (SCM) divides the neck into the anterior and posterior triangles; it turns the head to the opposite side and is supplied by CN XI.
• Anterior triangle — boundaries: anterior border of SCM, midline of the neck, lower border of the mandible.
  • Contents — carotid arteries (carotid sheath), internal jugular vein, CN IX, X, XII, larynx, trachea, thyroid and parathyroids, submandibular gland, lymph nodes.
  • Sub-triangles — submental, submandibular (digastric), carotid (carotid bifurcation — at the upper border of the thyroid cartilage, C3/4), muscular.
• Posterior triangle — boundaries: posterior border of SCM, anterior border of trapezius, middle third of the clavicle.
  • Contents — accessory nerve (XI) (superficial — injured in lymph node biopsy → weak shoulder shrug, winging), roots and trunks of the brachial plexus, subclavian artery (third part), external jugular vein, cervical plexus branches (great auricular, lesser occipital, transverse cervical, supraclavicular nerves — emerge at Erb's point), phrenic nerve on scalenus anterior, lymph nodes.
• Carotid sheath — contains the common/internal carotid artery (medial), internal jugular vein (lateral) and vagus nerve (posterior, between them); deep cervical lymph nodes along the IJV; the sympathetic chain lies behind the sheath.
• Neck lumps by site:

  Site	Causes
  Midline	Thyroglossal cyst (moves up on tongue protrusion), thyroid isthmus nodule (moves on swallowing), dermoid cyst, lymph node (submental)
  Anterior triangle	Lymph nodes (commonest), branchial cyst (anterior border of SCM, young adult), carotid body tumour (pulsatile, moves side to side not up and down), salivary gland, thyroid lobe, laryngocele
  Posterior triangle	Lymph nodes (lymphoma, TB, metastases, EBV), cystic hygroma (infants — transilluminates), cervical rib
  Supraclavicular	Virchow's node (left — gastric and abdominal malignancy), lung, breast and lymphoma

• Red flags in a neck lump — age over 45, persistent over 3 weeks, hard, fixed, growing, with hoarseness, dysphagia, weight loss or night sweats — 2-week-wait head and neck referral (or haematology).

Larynx, Thyroid and Neck Structures

• Vertebral levels in the neck:
  • C2/3 — hyoid bone (C3).
  • C4 — upper border of the thyroid cartilage — carotid bifurcation (C3/4).
  • C6 — cricoid cartilage — start of the trachea and oesophagus; middle cervical ganglion; vertebral artery enters the transverse foramen.
  • C7 — vertebra prominens.
• Larynx — thyroid cartilage (Adam's apple), cricoid cartilage (complete ring — cricoid pressure), arytenoids, epiglottis; cricothyroid membrane between the thyroid and cricoid cartilages — emergency front-of-neck access (cricothyroidotomy) — below the vocal cords and above the thyroid isthmus.
• Vocal cord nerves — recurrent laryngeal nerve (all intrinsic laryngeal muscles except cricothyroid — unilateral palsy → hoarse, breathy voice; bilateral → stridor) and the external branch of the superior laryngeal nerve (cricothyroid — high-pitched voice, e.g. singers after thyroidectomy).
• Thyroid gland:
  • Two lobes joined by an isthmus over the 2nd–4th tracheal rings; enclosed in pretracheal fascia — moves up on swallowing.
  • Blood supply — superior thyroid artery (external carotid — near the external branch of the superior laryngeal nerve), inferior thyroid artery (thyrocervical trunk — closely related to the recurrent laryngeal nerve), occasional thyroidea ima (midline — tracheostomy risk).
  • Development — from the foramen caecum at the base of the tongue, descending along the thyroglossal duct (ectopic lingual thyroid, thyroglossal cyst, pyramidal lobe).
  • Thyroidectomy complications — bleeding (neck haematoma — airway compression), recurrent laryngeal nerve palsy, hypocalcaemia (parathyroid damage — tingling, tetany within 24–48 hours), hypothyroidism.
• Parathyroid glands — usually 4, on the posterior thyroid; inferior glands develop from the 3rd pharyngeal pouch (with the thymus — may be ectopic in the mediastinum), superior from the 4th.
• Trachea — C6 to T4/5 (carina); tracheostomy through the 2nd–4th rings (below the cricoid).
• Oesophagus — begins at C6 behind the trachea; Zenker's (pharyngeal pouch) diverticulum through Killian's dehiscence (above cricopharyngeus) — dysphagia, regurgitation, halitosis, gurgling neck lump.
• Fascial spaces of the neck — infections can spread from the teeth and pharynx through the retropharyngeal and parapharyngeal spaces into the mediastinum (descending mediastinitis); Ludwig's angina (floor of the mouth — airway emergency).

Vessels and Lymph Nodes of the Neck

• Common carotid artery — divides at C3/4 (upper border of the thyroid cartilage) into:
  • Internal carotid — no branches in the neck; enters the skull through the carotid canal; carotid sinus (baroreceptors — CN IX) at its origin; carotid body (chemoreceptors).
  • External carotid — branches supply the neck and face: "Some Anatomists Like Freaking Out Poor Medical Students" — superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, maxillary (→ middle meningeal), superficial temporal (temporal arteritis).
• Carotid disease — atherosclerosis at the bifurcation → TIA and stroke, amaurosis fugax — carotid endarterectomy for symptomatic stenosis of 50–99% (NASCET); carotid bruit; carotid dissection (neck pain, Horner's, stroke — after neck trauma or manipulation).
• Carotid sinus hypersensitivity — syncope with neck pressure (shaving, tight collars) — older men; carotid sinus massage (contraindicated with bruits or recent stroke).
• Subclavian artery — branches: vertebral, internal thoracic (mammary — used for CABG), thyrocervical trunk, costocervical trunk.
• Veins — internal jugular vein (from the jugular foramen in the carotid sheath — JVP is assessed from the right IJV between the heads of SCM; central lines) and external jugular vein (superficial over SCM — visible, but less reliable for the JVP).
• JVP — vertical height above the sternal angle (normal under 3–4 cm) with the patient at 45°; raised in right heart failure, fluid overload, tricuspid regurgitation (giant v waves), constrictive pericarditis (Kussmaul's sign), SVC obstruction (non-pulsatile).
• Cervical lymph node levels (I–VI) — I submental and submandibular, II–IV along the IJV (upper, middle, lower deep cervical — jugulodigastric node at level II drains the tonsil), V posterior triangle, VI central (around the thyroid and larynx).
• Lymphatic drainage — all head and neck lymph eventually passes through the deep cervical nodes along the IJV → jugular trunk → thoracic duct (left) or right lymphatic duct.

Lower Cranial Nerves in the Neck

• Glossopharyngeal (IX) — leaves through the jugular foramen; sensation to the pharynx and posterior tongue (afferent limb of the gag reflex), taste to the posterior third, carotid sinus and body, parotid secretion.
• Vagus (X) — jugular foramen → in the carotid sheath → thorax and abdomen:
  • Branches — pharyngeal (palate and pharynx), superior laryngeal (internal — sensation above the cords; external — cricothyroid), recurrent laryngeal (right — around the subclavian artery; left — around the aortic arch), cardiac, pulmonary, oesophageal, gastric.
  • Lesion — palate drops on the affected side; uvula deviates to the healthy side; hoarse voice; dysphagia; reduced cough.
  • Vagal stimulation — vasovagal syncope, bradycardia (carotid sinus massage, Valsalva), Arnold's (auricular) branch — cough on ear syringing.
• Accessory (XI) — spinal roots rise through the foramen magnum and leave through the jugular foramen → SCM → crosses the posterior triangle superficially → trapezius.
• Hypoglossal (XII) — hypoglossal canal → loops forward below the digastric muscle, crossing the carotid arteries → tongue muscles; injured in carotid endarterectomy and submandibular surgery.
• Sympathetic chain — behind the carotid sheath; superior (C1–2 — Horner's from carotid dissection), middle (C6) and inferior (stellate with T1) cervical ganglia; stellate ganglion block (complex regional pain syndrome, refractory arrhythmias).
• Phrenic nerve (C3–5) — on the anterior surface of scalenus anterior (deep to the prevertebral fascia) → thorax — blocked by interscalene brachial plexus blocks (hemidiaphragm paralysis).
• Cervical plexus (C1–4) — sensory to the neck and back of the scalp (great auricular — angle of jaw and ear; lesser occipital); motor — ansa cervicalis (strap muscles), phrenic contribution.
• Swallowing — oral (voluntary — V, VII, XII) → pharyngeal (reflex — IX, X; larynx elevates, epiglottis closes, cords adduct) → oesophageal (peristalsis — X); dysphagia after stroke (bulbar weakness) — formal swallow assessment before oral intake (aspiration risk).

The Vertebral Column

• Vertebrae — 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused — sacrum), 4 coccygeal (fused); curvatures — cervical and lumbar lordosis, thoracic and sacral kyphosis.
• Typical vertebra — body, vertebral arch (pedicles and laminae), spinous process, transverse processes, articular processes (facet joints); intervertebral foramina transmit spinal nerves.
• Regional features:
  • Cervical — small bodies, transverse foramina (vertebral arteries — C6 to C1), bifid spinous processes; C1 atlas (no body — "yes" nodding), C2 axis (dens/odontoid peg — "no" rotation — atlanto-axial joint).
  • Thoracic — articulate with ribs (costal facets); long downward-sloping spinous processes; relatively immobile.
  • Lumbar — large bodies; most disc prolapses and degeneration at L4/5 and L5/S1.
• Intervertebral discs — outer annulus fibrosus (fibrocartilage) and inner nucleus pulposus (gel) — prolapse is usually posterolateral (posterior longitudinal ligament is narrow and strongest in the midline).
• Ligaments — anterior and posterior longitudinal ligaments, ligamentum flavum (between laminae — "pop" felt during epidural insertion), interspinous and supraspinous ligaments.
• Spinal nerves — 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal):
  • Cervical nerves exit above their vertebra (C1–C7), C8 exits below C7; thoracic and lumbar nerves exit below their vertebra.
  • A C5/6 disc compresses the C6 root; an L4/5 disc (posterolateral) compresses L5.
• Atlanto-axial instability — rheumatoid arthritis, Down syndrome — risk during intubation (neck extension) — flexion–extension X-rays.
• Cervical spine injury — NEXUS or Canadian C-spine rule decide imaging (CT for adults per NICE NG232); C1 (Jefferson — burst), C2 (hangman's, odontoid peg) fractures.
• Ankylosing spondylitis — fused, brittle spine ("bamboo spine") — fractures after minor trauma; osteoporotic vertebral fractures (thoracic — kyphosis, height loss); spondylolisthesis (slip of one vertebra on another — L5/S1).

The Spinal Cord and Its Blood Supply

• Spinal cord — from the medulla at the foramen magnum to the conus medullaris at L1/2 in adults (L3 at birth); enlargements at C5–T1 (arms) and L1–S3 (legs); below the conus, the nerve roots form the cauda equina; filum terminale anchors it to the coccyx.
• Vertebral vs cord segments — the cord is shorter than the canal, so cord segments lie higher than the matching vertebrae (e.g. a T10 vertebral fracture may damage lumbar cord segments).
• Cross-section — central grey matter (H-shaped — dorsal horns sensory, ventral horns motor, lateral horns T1–L2 sympathetic) surrounded by white matter tracts.
• Main tracts:
  • Dorsal columns — vibration and proprioception (ipsilateral in the cord).
  • Lateral corticospinal tract — voluntary movement (ipsilateral in the cord — crossed in the medulla).
  • Spinothalamic tract (anterolateral) — pain and temperature (contralateral — crossed near entry).
• Blood supply:
  • One anterior spinal artery (from the vertebral arteries) — supplies the anterior two-thirds of the cord (corticospinal and spinothalamic tracts, anterior horns).
  • Two posterior spinal arteries — supply the posterior third (dorsal columns).
  • Segmental (radicular) arteries reinforce them — the largest is the artery of Adamkiewicz (usually left, T9–T12) — at risk in thoracoabdominal aortic aneurysm repair and dissection → anterior spinal artery syndrome (paraplegia).
  • Watershed zone — mid-thoracic cord (T4–T8) — vulnerable to hypotension.
• Venous drainage — internal and external vertebral venous plexuses (Batson's plexus — valveless, linking pelvic and vertebral veins → spread of prostate, breast and other cancers to the spine).

Cord Lesions and Back Pain

• Spinal cord syndromes:

  Syndrome	Cause	Features
  Complete transection	Trauma, tumour, transverse myelitis	Loss of all motor and sensory function below the level; bladder and bowel; spinal shock initially
  Anterior cord	Anterior spinal artery occlusion (aortic surgery, dissection), flexion injury	Paralysis and loss of pain and temperature below the level; dorsal columns spared (vibration and proprioception preserved)
  Central cord	Hyperextension in older people with cervical spondylosis; syringomyelia	Arms weaker than legs (central fibres for the arms); variable sensory loss; "cape" loss in syrinx
  Brown-Séquard	Penetrating injury, tumour, MS	Ipsilateral weakness and loss of vibration/proprioception; contralateral pain and temperature loss
  Posterior cord	B12 deficiency, syphilis (tabes), nitrous oxide	Loss of vibration and proprioception — sensory ataxia, positive Romberg
  Conus medullaris	Disc, tumour, trauma at L1	Early sphincter dysfunction, saddle anaesthesia, mixed UMN and LMN signs
  Cauda equina	Central disc prolapse, tumour, abscess	Asymmetrical LMN leg weakness, saddle anaesthesia, sciatica, sphincter dysfunction

• Metastatic spinal cord compression — known cancer (breast, lung, prostate, kidney, myeloma) with new back pain (especially thoracic, night pain, worse lying flat), then weakness, sensory level, sphincter problems — MRI whole spine within 24 hours (immediately if neurological signs), dexamethasone 16 mg, flat if unstable, radiotherapy or surgery (NICE NG234).
• Spinal cord injury levels — C3–5 (diaphragm — ventilation), above T6 (autonomic dysreflexia), neurogenic shock (hypotension and bradycardia — loss of sympathetic outflow above T6).
• Back pain red flags — age under 20 or over 55 (new), history of cancer, weight loss, fever and IV drug use or immunosuppression (infection — discitis, epidural abscess), trauma (or minor trauma with osteoporosis), thoracic pain, night pain, progressive neurological deficit, cauda equina symptoms, steroid use.
• Mechanical back pain — most common; managed with simple analgesia, staying active, exercise, physiotherapy (NICE NG59 — no paracetamol alone or opioids routinely; no imaging without red flags).
• Inflammatory back pain — young, morning stiffness over 30 minutes, improves with exercise, night pain — axial spondyloarthritis.

Lumbar Puncture and Epidural Anatomy

• Site — L3/4 or L4/5 interspace (below the end of the cord at L1/2); Tuffier's (intercristal) line — between the top of the iliac crests — crosses the L4 spinous process or L4/5 interspace.
• Position — lateral (knees to chest — opens the interspaces; opening pressure measured) or sitting.
• Layers traversed (lumbar puncture) — skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum ("give") → epidural space (fat and venous plexus) → dura mater → arachnoid mater → subarachnoid space (CSF).
• Epidural — the needle stops in the epidural space (loss of resistance after the ligamentum flavum); spinal (intrathecal) — the needle passes through the dura and arachnoid into CSF.
• Dermatome levels for blocks — T4 (nipple) for caesarean section, T10 (umbilicus) for labour and lower limb, sacral for perineal surgery.
• Caudal epidural — through the sacral hiatus (children's analgesia).
• Contraindications to LP — raised ICP (risk of coning — tonsillar herniation), coagulopathy, local infection, spinal abnormality (relative).
• Post-dural puncture headache — CSF leak lowers intracranial pressure — postural headache (worse upright) — reduced by atraumatic (pencil-point) needles — blood patch if persistent.
• Normal CSF — clear; opening pressure 10–20 cmH₂O; cells under 5/mm³ (lymphocytes); protein 0.15–0.45 g/L; glucose over 60% of plasma glucose.

Head, Neck and Back at a Glance

Spot the Diagnosis

Exam-style Scenarios

1. A 19-year-old is hit on the side of the head by a cricket ball. He is briefly knocked out, then talks normally for an hour, before becoming drowsy with a dilated right pupil.
Answer: Right extradural haematoma from middle meningeal artery bleeding (pterion fracture) with uncal herniation.

• ABCDE; protect the airway (intubate if GCS 8 or less); avoid hypoxia and hypotension.
• Urgent CT head — biconvex haematoma.
• Emergency neurosurgical evacuation; temporising measures (head up, mannitol or hypertonic saline) while transferring.

2. A 45-year-old woman wakes with drooping of the left side of her face. She can't raise her left eyebrow or close her left eye. Food tastes odd and noise seems loud on the left.
Answer: Left LMN facial nerve palsy (Bell's palsy) — lesion within the facial canal (taste and stapedius affected).

• Check ears and palate for vesicles (Ramsay Hunt), parotid, and other neurology.
• Prednisolone within 72 hours.
• Eye protection — lubricants and taping at night; review if not improving at 3 weeks (or red flags).

3. A 50-year-old woman has a total thyroidectomy. The next day her voice is hoarse and breathy, and she has perioral tingling.
Answer: Recurrent laryngeal nerve injury and hypocalcaemia (parathyroid injury).

• Check calcium urgently; IV calcium gluconate if severe or symptomatic; oral calcium and alfacalcidol.
• ENT laryngoscopy to assess cord movement; most RLN neurapraxias recover.
• Watch for stridor (bilateral cord palsy) and neck haematoma (airway emergency).

4. A 68-year-old man with prostate cancer has 2 weeks of thoracic back pain and now has weak legs, a sensory level at T10 and urinary hesitancy.
Answer: Metastatic spinal cord compression.

• Immediate MRI of the whole spine; dexamethasone 16 mg with PPI cover.
• Flat bed rest with spinal alignment if mechanical instability suspected.
• Urgent oncology and spinal surgery — radiotherapy or decompression; catheter, VTE prophylaxis, pressure care.

5. You are performing a lumbar puncture for suspected meningitis in a 30-year-old with normal GCS and no papilloedema.
Answer: Lumbar puncture anatomy.

• Lateral position, knees to chest; identify Tuffier's line (iliac crests — L4) and use the L3/4 or L4/5 space (cord ends at L1/2).
• Needle passes skin, supraspinous and interspinous ligaments, ligamentum flavum, epidural space, dura and arachnoid into CSF.
• Measure opening pressure; send samples with a paired plasma glucose; don't delay antibiotics.

Key Points

• Pterion — middle meningeal artery — extradural (lucid interval); bridging veins — subdural.
• CSF flows from choroid plexus through the ventricles to the arachnoid granulations — obstruction causes hydrocephalus.
• Know ACA, MCA, PCA and PICA stroke syndromes and the circle of Willis.
• Learn all 12 cranial nerves; LR6 SO4; tongue and jaw deviate towards, uvula away from, the lesion.
• UMN facial weakness spares the forehead; LMN doesn't — Bell's needs steroids and eye protection.
• Neck — carotid bifurcation C3/4, cricoid C6; cricothyroid membrane for emergency airway.
• Thyroidectomy risks — recurrent laryngeal nerve, parathyroids, haematoma.
• Midline lumps — thyroglossal cyst (tongue protrusion) vs thyroid (swallowing).
• Cord ends at L1/2; LP at L3/4 or L4/5; anterior spinal artery supplies the anterior two-thirds.
• Metastatic cord compression — urgent MRI and dexamethasone; cauda equina — emergency MRI.

Guidelines and Further Reading

• Gray's Anatomy for Students; Last's Anatomy — Regional and Applied
• Crossman and Neary: Neuroanatomy — An Illustrated Colour Text
• NICE NG232: Head injury — assessment and early management (2023)
• NICE NG128: Stroke and TIA in over 16s
• NICE NG234: Spinal metastases and metastatic spinal cord compression (2023)
• NICE NG59: Low back pain and sciatica in over 16s
• NICE CKS: Bell's palsy; NICE NG36: Cancer of the upper aerodigestive tract
• BNF — for current doses

Detailed anatomy — Thorax, abdomen and pelvis

• Thorax: The Chest Wall and Surface Anatomy | Intercostal Spaces and the Diaphragm | The Mediastinum | Surface Anatomy of the Heart and Lungs | The Great Vessels and Thoracic Duct
• Abdomen: The Abdominal Wall and Regions | The Inguinal Canal and Hernias | The Peritoneum | Blood Supply of the Gut | Abdominal Organs — Position and Relations | Portal Venous System and Portosystemic Anastomoses | Retroperitoneum, Kidneys and Ureters
• Pelvis and perineum: The Pelvis and Pelvic Organs | The Perineum and Genital Anatomy
• Revision: Thorax, Abdomen and Pelvis at a Glance | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🫁 Thorax, abdomen and pelvis — regional and surface anatomy — page 5 of the Anatomy and Physiology chapter. Trunk anatomy is used every day — placing chest drains and central lines, finding the apex beat, localising abdominal pain, understanding hernias, reading CT scans, and knowing why a bleeding varix, a referred shoulder-tip pain or a ureteric stone behaves as it does. The organ-system pages cover physiology; this page covers where things are and what lies next to them.
Your four questions for every trunk problem: Which surface landmark marks the structure? What lies next to it — and could be damaged? What is its blood supply and lymphatic drainage? Where does its pain refer?

The Chest Wall and Surface Anatomy

• Bony thorax — 12 thoracic vertebrae, 12 pairs of ribs, sternum (manubrium, body, xiphoid).
  • True ribs (1–7) attach directly to the sternum; false ribs (8–10) join the costal margin; floating ribs (11–12).
  • First rib — short, flat; subclavian vessels and lower brachial plexus cross it; a fracture implies high-energy trauma.
• Sternal angle (angle of Louis) — junction of the manubrium and body — the key landmark at the level of T4/5:
  • 2nd costal cartilage attaches here — count ribs and intercostal spaces from it.
  • Marks the tracheal bifurcation (carina), the beginning and end of the aortic arch, the azygos vein joining the SVC, the division between superior and inferior mediastinum, the pulmonary trunk bifurcation, and where the thoracic duct crosses to the left.
• Other landmarks:
  • Jugular (suprasternal) notch — T2/3; check the trachea is central.
  • Xiphisternal joint — T9.
  • Nipple — 4th intercostal space in men (T4 dermatome).
  • Inferior angle of the scapula — 7th rib or space (arms at sides).
  • Vertebra prominens — C7 spinous process.
• Lines — midclavicular, anterior, mid- and posterior axillary lines — used to describe drain sites and the apex beat.
• Chest wall injuries — rib fractures (pain limits breathing → pneumonia — older people), flail chest (2 or more fractures in 2 or more adjacent ribs — paradoxical movement — underlying lung contusion), sternal fracture (cardiac contusion — ECG).

Intercostal Spaces and the Diaphragm

• Intercostal muscles — external (inspiration — "hands in pockets" direction), internal and innermost (forced expiration).
• Neurovascular bundle — runs in the costal groove under each rib in the order (top to bottom) Vein, Artery, Nerve ("VAN") — between the internal and innermost layers.
• Practical consequence — insert chest drains, pleural aspiration needles and thoracentesis just above the upper border of the rib below to avoid the bundle; intercostal nerve blocks are placed at the lower border.
• Safe triangle for chest drains — lateral border of pectoralis major, anterior border of latissimus dorsi, a line through the 5th intercostal space (nipple level), apex below the axilla.
• Intercostal nerves (T1–11) — supply the chest and abdominal wall muscles and skin (dermatomes); shingles follows a dermatome; intercostal neuralgia.
• Diaphragm:
  • The main muscle of inspiration (about 70% of quiet breathing) — dome-shaped; contraction flattens it and enlarges the thorax.
  • Phrenic nerve (C3, 4, 5) — "C3, 4 and 5 keep the diaphragm alive" — motor and central sensory supply → shoulder-tip pain from diaphragmatic irritation (subphrenic abscess, ruptured spleen — Kehr's sign, ectopic pregnancy, laparoscopy gas).
  • Openings — "I 8 10 Eggs At 12" — IVC at T8 (with right phrenic nerve), Oesophagus at T10 (with vagal trunks), Aorta at T12 (with thoracic duct and azygos vein — behind the diaphragm, between the crura).
  • Right dome higher than the left (liver); domes rise to about the 5th rib in expiration.
• Diaphragmatic problems — hiatus hernia (sliding — commonest; rolling/para-oesophageal), congenital diaphragmatic hernia (Bochdalek — posterolateral left — neonates), traumatic rupture (usually left), phrenic nerve palsy (raised hemidiaphragm — tumour, surgery, interscalene block, MND).

The Mediastinum

• Mediastinum — the central compartment between the pleural sacs, from the thoracic inlet to the diaphragm.
• Divisions — superior (above the plane of the sternal angle — T4/5) and inferior (anterior, middle, posterior).
• Contents:

  Region	Contents	Masses
  Superior	Aortic arch and branches, SVC and brachiocephalic veins, trachea, oesophagus, thymus, thoracic duct, vagus and phrenic nerves, left recurrent laryngeal nerve	Retrosternal goitre, thymoma, lymphoma
  Anterior	Thymus (remnant), fat, lymph nodes, internal thoracic vessels	"4 Ts" — Thymoma, Teratoma (germ cell), Thyroid, "Terrible" lymphoma
  Middle	Heart and pericardium, ascending aorta, pulmonary trunk, main bronchi, phrenic nerves	Lymphadenopathy (sarcoid, TB, lymphoma, cancer), pericardial and bronchogenic cysts
  Posterior	Descending aorta, oesophagus, azygos and hemiazygos veins, thoracic duct, sympathetic chains, vagus nerves	Neurogenic tumours (schwannoma, neuroblastoma in children), oesophageal lesions, aneurysms

• Nerves in the thorax:
  • Phrenic nerves — pass anterior to the lung roots (with the pericardiacophrenic vessels, on the pericardium).
  • Vagus nerves — pass posterior to the lung roots; form the oesophageal plexus.
  • Left recurrent laryngeal nerve — hooks under the aortic arch (ligamentum arteriosum) → hoarseness from lung cancer (left hilum), aortic aneurysm, mitral stenosis (Ortner's), mediastinal nodes; the right hooks under the right subclavian artery (higher — less often affected).
• Pleura — visceral (no pain fibres) and parietal (sensitive — pleuritic pain; intercostal nerves, central diaphragmatic part via the phrenic nerve); the pleural cavity is a potential space with a small amount of fluid.
• Pleural reflections — "2, 4, 6, 8, 10, 12" — pleura from the apex (2–3 cm above the medial clavicle) to the sternal angle (2nd costal cartilage), runs down to the 4th (left diverges for the heart — cardiac notch) and 6th costal cartilages, crosses the 8th rib in the midclavicular line, 10th in the midaxillary line, and reaches the 12th rib posteriorly; the lungs lie about 2 ribs higher (6, 8, 10).
• Lung apex — rises above the clavicle — pneumothorax risk with subclavian and internal jugular lines and supraclavicular blocks.

Surface Anatomy of the Heart and Lungs

• Heart borders (on chest X-ray):
  • Right border — right atrium.
  • Left border — left ventricle (and left atrial appendage, pulmonary trunk and aortic knuckle above).
  • Inferior border — right ventricle (mainly).
  • Anterior surface — mostly right ventricle (most often injured by stab wounds; behind the sternum).
  • Posterior surface — left atrium — lies against the oesophagus (transoesophageal echo; enlarged LA in mitral stenosis can cause dysphagia).
• Apex beat — normally 5th intercostal space, midclavicular line; displaced with LV dilatation, mediastinal shift, chest deformity.
• Auscultation areas — aortic (2nd right intercostal space, parasternal), pulmonary (2nd left), tricuspid (4th–5th left, lower sternal edge), mitral (apex) — "All Physicians Take Money".
• Lung fissures:
  • Oblique fissure (both lungs) — from about T3 spinous process posteriorly, around to the 6th costal cartilage anteriorly — follows the line of the medial border of the scapula with the arm raised.
  • Horizontal fissure (right only) — along the 4th costal cartilage to meet the oblique fissure in the midaxillary line.
• Lobes — right lung — 3 lobes (upper, middle, lower); left — 2 lobes (upper with the lingula, lower).
  • Examine the lower lobes from behind, the right middle lobe and lingula from the front (and in the axilla).
  • Right middle lobe consolidation obscures the right heart border; lingular consolidation obscures the left heart border; lower lobe disease obscures the hemidiaphragm (silhouette sign).
• Bronchial anatomy — the right main bronchus is shorter, wider and more vertical → aspiration (supine — posterior segment of the right upper lobe and apical segment of the right lower lobe; upright — right lower lobe) and inhaled foreign bodies go right; endobronchial intubation is usually right-sided.
• Lung roots (hila) — pulmonary artery, two pulmonary veins, bronchus, bronchial vessels, lymph nodes, nerves; the left hilum is usually slightly higher than the right.

The Great Vessels and Thoracic Duct

• Aorta — ascending (coronary arteries arise from the aortic sinuses) → arch (at the sternal angle) → descending thoracic → through the diaphragm at T12 → abdominal aorta.
• Arch branches (right to left) — brachiocephalic trunk (→ right subclavian and right common carotid), left common carotid, left subclavian.
• Veins — internal jugular + subclavian → brachiocephalic vein (behind the sternoclavicular joint); both brachiocephalic veins → SVC (the left crosses in front of the arch branches) → right atrium; the azygos vein drains the posterior chest wall into the SVC.
• Superior vena cava obstruction — lung cancer, lymphoma — facial swelling and plethora, distended neck and chest veins, breathlessness, headache worse bending forward — steroids, stenting, treat the cause.
• Central venous access:
  • Internal jugular — at the apex of the triangle between the two heads of sternocleidomastoid, lateral to the carotid — ultrasound-guided (preferred).
  • Subclavian — below the clavicle — higher pneumothorax risk, lower infection risk.
  • Femoral — medial to the femoral artery.
  • Catheter tip at the cavoatrial junction (about at the level of the carina on X-ray).
• Aortic dissection — Stanford type A (involves the ascending aorta — surgery) vs type B (descending only — usually medical); may occlude branches (stroke, MI, limb, gut, kidneys).
• Traumatic aortic injury — deceleration — tears at the isthmus (just distal to the left subclavian, at the ligamentum arteriosum) — widened mediastinum on X-ray.
• Thoracic duct — drains lymph from the whole body except the right upper quadrant (right head, neck, arm and chest — right lymphatic duct); starts at the cisterna chyli (L1–2), passes through the aortic hiatus, ascends behind the oesophagus, crosses to the left at T4/5 and drains into the junction of the left internal jugular and subclavian veins → injury causes chylothorax (milky effusion — triglycerides high); Virchow's node (left supraclavicular — Troisier's sign — gastric cancer).

The Abdominal Wall and Regions

• Nine regions — defined by two vertical midclavicular lines and two horizontal planes (subcostal and intertubercular):

  Right hypochondrium (liver, gallbladder)	Epigastrium (stomach, pancreas, duodenum, aorta)	Left hypochondrium (spleen, stomach)
  Right flank (lumbar) Right kidney, ascending colon	Umbilical Small bowel, aorta, transverse colon	Left flank (lumbar) Left kidney, descending colon
  Right iliac fossa Appendix, caecum, right ovary	Suprapubic (hypogastrium) Bladder, uterus	Left iliac fossa Sigmoid colon, left ovary

• Important planes:
  • Transpyloric plane (L1) — halfway between the jugular notch and pubic symphysis — pylorus, neck of pancreas, duodenojejunal flexure, hila of kidneys, fundus of gallbladder (9th costal cartilage), end of the spinal cord, origin of the SMA, renal hila.
  • Subcostal plane (L3); umbilicus (L3/4); supracristal plane (L4) — aortic bifurcation and LP level; intertubercular plane (L5).
• Layers of the anterolateral wall — skin → superficial fascia (Camper's fatty and Scarpa's membranous layers) → external oblique → internal oblique → transversus abdominis → transversalis fascia → extraperitoneal fat → parietal peritoneum.
• Rectus abdominis — in the rectus sheath; the linea alba in the midline (laparotomy — little bleeding); the arcuate line (below which the posterior sheath is absent); linea semilunaris (lateral border — Spigelian hernia).
• Vessels — inferior epigastric artery (from the external iliac — runs up behind the rectus; injured with laparoscopic ports or paracentesis — use the left iliac fossa lateral to the rectus for ascitic drains) and the superior epigastric (internal thoracic).
• Nerves — T7–T12 and L1 (thoracoabdominal, iliohypogastric, ilioinguinal); dermatomes — T10 umbilicus; TAP block (between internal oblique and transversus abdominis).
• Incisions — midline laparotomy, Kocher's (right subcostal — gallbladder), Lanz and gridiron (appendix — McBurney's point: one-third of the way from the ASIS to the umbilicus), Pfannenstiel (caesarean), Rutherford Morison, laparoscopic ports.

The Inguinal Canal and Hernias

• Inguinal canal — about 4 cm long, running obliquely above the medial half of the inguinal ligament from the deep ring (midpoint of the inguinal ligament, lateral to the inferior epigastric vessels) to the superficial ring (above and medial to the pubic tubercle).
• Walls — "MALT: 2Ms, 2As, 2Ls, 2Ts":
  • Roof — 2 Muscles (internal oblique and transversus abdominis).
  • Anterior wall — 2 Aponeuroses (external oblique, reinforced laterally by internal oblique).
  • Floor — 2 Ligaments (inguinal and lacunar).
  • Posterior wall — 2 Ts (transversalis fascia and conjoint tendon).
• Contents — spermatic cord (men) or round ligament of the uterus (women), and the ilioinguinal nerve (passes through the superficial ring, not the deep ring).
• Spermatic cord — vas deferens, testicular artery, pampiniform plexus, artery to vas, cremasteric artery, genital branch of the genitofemoral nerve, sympathetic fibres, lymphatics, processus vaginalis remnant — covered by three fascial layers.
• Hernias:

  Hernia	Anatomy	Clinical features
  Indirect inguinal	Through the deep ring, lateral to the inferior epigastric vessels, along the canal (patent processus vaginalis)	Commonest hernia (both sexes); young men and children; can reach the scrotum; controlled by pressure over the deep ring
  Direct inguinal	Through the posterior wall (Hesselbach's triangle), medial to the inferior epigastric vessels	Older men; weak abdominal wall; rarely strangulates
  Femoral	Through the femoral canal, below and lateral to the pubic tubercle	Women; high risk of strangulation — repair urgently
  Umbilical / paraumbilical	Through or beside the umbilicus	Congenital (infants — most close by age 4–5) or acquired (obesity, ascites)
  Incisional	Through a surgical scar	After laparotomy — obesity, infection
  Spigelian, obturator, epigastric	Linea semilunaris; obturator canal; linea alba above the umbilicus	Uncommon; obturator — elderly thin women, bowel obstruction

• Hesselbach's triangle — inferior epigastric vessels (lateral), lateral border of rectus (medial), inguinal ligament (inferior).
• Complications — incarceration (irreducible), obstruction, strangulation (compromised blood supply — tender, red, painful — emergency surgery); Richter's hernia (part of the bowel wall — strangulation without obstruction).
• Scrotal swelling — can you get above it? (No — inguinoscrotal hernia); transilluminates (hydrocele); separate from the testis (epididymal cyst).

The Peritoneum

• Peritoneum — a serous membrane — parietal (lines the wall — somatic nerves — sharp, localised pain) and visceral (covers organs — autonomic — dull, poorly localised pain).
• Intraperitoneal vs retroperitoneal organs:
  • Intraperitoneal (on a mesentery) — stomach, first part of the duodenum, jejunum, ileum, transverse and sigmoid colon, liver, spleen, gallbladder, appendix, caecum (variably).
  • Retroperitoneal — "SAD PUCKER" — Suprarenal (adrenal) glands, Aorta and IVC, Duodenum (2nd–4th parts), Pancreas (except the tail), Ureters, Colon (ascending and descending), Kidneys, Esophagus (lower), Rectum (middle and lower).
• Mesenteries and omenta:
  • Greater omentum — hangs from the greater curvature of the stomach — "abdominal policeman" (walls off inflammation — appendix mass); site of cancer deposits (omental cake — ovarian cancer).
  • Lesser omentum — liver to the lesser curvature — its free edge (the hepatoduodenal ligament) contains the portal triad: portal vein (posterior), hepatic artery (left), common bile duct (right) — the Pringle manoeuvre compresses it to control liver bleeding.
  • Epiploic foramen (of Winslow) — behind the free edge — entrance to the lesser sac (behind the stomach — pancreatic pseudocysts, posterior gastric ulcer perforation).
• Peritoneal spaces:
  • Hepatorenal pouch (Morison's pouch) — the lowest point of the peritoneal cavity when lying supine — fluid collects here (FAST scan).
  • Rectouterine pouch (of Douglas) in women / rectovesical pouch in men — lowest point when upright — pelvic abscesses, ectopic blood, metastases (Blumer's shelf).
  • Subphrenic spaces — abscesses after surgery (shoulder-tip pain, swinging fever).
  • Paracolic gutters — fluid tracks from the upper abdomen to the pelvis (perforated duodenal ulcer can mimic appendicitis).
• Peritonitis — guarding, rigidity (board-like in perforation), rebound tenderness, lying still; pain from inflamed visceral peritoneum moves to parietal peritoneum as inflammation spreads (appendicitis — periumbilical then right iliac fossa).

Blood Supply of the Gut

• Coeliac trunk branches — left gastric (oesophageal branches), splenic (short gastrics, left gastro-omental, pancreatic branches), common hepatic (→ proper hepatic, gastroduodenal artery, right gastric).
• Gastroduodenal artery — runs behind the first part of the duodenum — posterior duodenal ulcers erode it → major upper GI bleeding.
• Left gastric artery — bleeding from lesser curvature gastric ulcers.
• Watershed areas — splenic flexure (Griffiths' point — SMA/IMA junction) and the rectosigmoid (Sudeck's point) — vulnerable to ischaemic colitis in low-flow states.
• Marginal artery of Drummond — connects SMA and IMA along the colon.
• Acute mesenteric ischaemia — SMA embolus (AF) or thrombosis — pain out of proportion to the signs, raised lactate (late) — CT angiography — emergency revascularisation and resection.
• Rectum and anal canal — superior rectal (IMA), middle rectal (internal iliac), inferior rectal (internal pudendal) arteries.
• Pectinate (dentate) line — marks the embryological junction:
  • Above — columnar epithelium, visceral innervation (painless — internal haemorrhoids), drains to the portal system and internal iliac/inferior mesenteric nodes — adenocarcinoma.
  • Below — squamous epithelium, somatic innervation (painful — external haemorrhoids, fissures), drains to systemic veins and superficial inguinal nodes — squamous cell carcinoma.

Abdominal Organs — Position and Relations

• Oesophagus — 25 cm; begins at C6 (cricopharyngeus); three constrictions (upper sphincter, aortic arch/left main bronchus, diaphragm — about 15, 25 and 40 cm from the incisors) — where foreign bodies and strictures lodge; lower third drains to portal and systemic veins (varices).
• Stomach — cardia, fundus, body, antrum, pylorus (transpyloric plane, L1); lesser and greater curvatures; posterior relations (the "stomach bed") — pancreas, spleen, left kidney and adrenal, splenic artery.
• Duodenum — C-shaped around the head of the pancreas: 1st (superior — duodenal cap — commonest ulcer site; intraperitoneal), 2nd (descending — major papilla (ampulla of Vater) — common bile and pancreatic ducts enter), 3rd (horizontal — crossed by the SMA — superior mesenteric artery syndrome), 4th (ascending → duodenojejunal flexure — ligament of Treitz — boundary between upper and lower GI bleeding).
• Small bowel — jejunum (proximal, thicker wall, fewer arcades, long vasa recta, plicae circulares) and ileum (more fat, more arcades, Peyer's patches; terminal ileum — B12 and bile salt absorption — Crohn's); Meckel's diverticulum (rule of 2s — 2 feet from the ileocaecal valve, 2% of people, 2 inches long, may contain gastric or pancreatic tissue).
• Large bowel — caecum, appendix (base at the confluence of the taeniae coli; tip position varies — retrocaecal commonest), ascending, transverse, descending and sigmoid colon, rectum; taeniae coli, haustra, appendices epiploicae distinguish colon from small bowel (on X-ray — haustra don't cross the full width).
• Liver — right upper quadrant under ribs 7–11; divided into 8 functional segments (Couinaud) by hepatic and portal veins; dual blood supply — portal vein (75%) and hepatic artery (25%); falciform ligament (ligamentum teres — obliterated umbilical vein).
• Gallbladder — fundus at the tip of the right 9th costal cartilage (Murphy's sign); Calot's triangle (cystic duct, common hepatic duct, inferior border of the liver) contains the cystic artery — identified in cholecystectomy ("critical view of safety").
• Biliary tree — right and left hepatic ducts → common hepatic duct + cystic duct → common bile duct → joins the pancreatic duct → ampulla of Vater (sphincter of Oddi).
• Pancreas — head (in the duodenal C-loop), neck (in front of the portal vein/SMV — L1), body, tail (in the splenic hilum); retroperitoneal — pain radiates to the back; head tumours obstruct the bile duct → painless jaundice with a palpable gallbladder (Courvoisier's law).
• Spleen — "1, 3, 5, 7, 9, 11" — about 1 × 3 × 5 inches, weighs about 7 oz, lies under the 9th–11th ribs on the left; enlarges towards the right iliac fossa; notch on the anterior border; can't get above it; injured with left lower rib fractures.

Portal Venous System and Portosystemic Anastomoses

• Hepatic portal vein — formed behind the neck of the pancreas by the superior mesenteric vein and the splenic vein (the inferior mesenteric vein usually joins the splenic vein); carries nutrient-rich blood from the gut, spleen and pancreas to the liver; about 8 cm long.
• Hepatic veins (3 — right, middle, left) drain into the IVC — obstruction → Budd–Chiari syndrome (pain, hepatomegaly, ascites).
• Portal hypertension (cirrhosis, portal vein thrombosis, schistosomiasis) — blood diverts through portosystemic anastomoses:

  Site	Portal side	Systemic side	Clinical result
  Lower oesophagus	Left gastric vein	Oesophageal veins → azygos	Oesophageal varices (life-threatening bleeding)
  Umbilicus	Paraumbilical veins	Superficial epigastric veins	Caput medusae
  Anorectal junction	Superior rectal vein	Middle and inferior rectal veins	Rectal varices (not ordinary haemorrhoids)
  Retroperitoneum	Colic veins	Retroperitoneal and renal veins	Retroperitoneal varices; spontaneous splenorenal shunts

• Other consequences — splenomegaly (hypersplenism — low platelets), ascites (with low albumin and sodium retention), hepatic encephalopathy (toxins bypass the liver).
• TIPSS (transjugular intrahepatic portosystemic shunt) — a stent between a hepatic vein and portal vein branch to lower portal pressure (refractory varices or ascites) — risk of encephalopathy.
• First-pass metabolism — drugs absorbed from the gut pass through the liver before reaching the systemic circulation (oral morphine, GTN) — bypassed by sublingual, rectal (partly), transdermal and IV routes.

Retroperitoneum, Kidneys and Ureters

• Abdominal aorta — enters at T12; bifurcates at L4 (level of the umbilicus/iliac crests) into the common iliac arteries.
  • Branches — coeliac (T12), SMA (L1), renal arteries (L1/2), gonadal arteries (L2), IMA (L3), lumbar arteries, median sacral.
  • AAA — usually infrarenal; screening of men at 65 in the UK (ultrasound); pulsatile, expansile mass above the umbilicus; rupture — back or flank pain, collapse.
• IVC — formed at L5 (right of the aorta); left renal vein crosses in front of the aorta (below the SMA — "nutcracker" compression); passes through the diaphragm at T8.
• Kidneys — retroperitoneal at T12–L3; right lower than the left (liver); hila at L1; move with respiration; surrounded by perirenal fat and Gerota's fascia.
  • Hilum structures (anterior to posterior) — renal vein, renal artery, ureter (pelvis) — "VAU".
  • Relations — posteriorly the 12th rib, diaphragm, psoas and quadratus lumborum; right kidney — liver, duodenum, hepatic flexure; left kidney — stomach, spleen, pancreas tail, splenic flexure.
  • Left gonadal vein drains into the left renal vein (right into the IVC) → left varicocele; a sudden left varicocele may signal a left renal tumour.
• Adrenal glands — on the upper poles; right adrenal vein drains into the IVC, left into the left renal vein.
• Ureters — about 25–30 cm; run down on psoas, cross the pelvic brim at the bifurcation of the common iliac artery (sacroiliac joint), then into the pelvis to the bladder.
  • Three narrowings where stones lodge — pelviureteric junction, pelvic brim (crossing the iliac vessels), vesicoureteric junction (narrowest).
  • On X-ray (KUB) — run along the tips of the lumbar transverse processes, over the sacroiliac joints, to the ischial spines.
  • "Water under the bridge" — the ureter passes under the uterine artery (women) and under the vas deferens (men) — at risk in hysterectomy.
  • Pain — renal colic radiates "loin to groin" (T11–L2) as the stone descends.
• Psoas major — retroperitoneal; lumbar plexus forms within it; psoas abscess; psoas sign (appendicitis — retrocaecal).

The Pelvis and Pelvic Organs

• Bony pelvis — two hip bones, sacrum and coccyx; the pelvic inlet (brim) divides the greater (false) pelvis from the lesser (true) pelvis.
• Male vs female pelvis — female — wider, rounder (gynaecoid) inlet, wider subpubic angle (over 80°), more everted ischial spines — adapted for childbirth.
• Pelvic floor — levator ani (pubococcygeus, puborectalis, iliococcygeus) and coccygeus — supports pelvic organs; puborectalis sling maintains the anorectal angle (continence); nerve supply S3–4 and pudendal nerve; damaged in childbirth → stress incontinence and prolapse.
• Bladder — behind the pubic symphysis; rises into the abdomen when full (palpable; suprapubic catheter site — 2–3 cm above the symphysis in the midline with a full bladder); trigone (between the ureteric orifices and internal urethral meatus); detrusor (parasympathetic S2–4 — voiding).
• Male:
  • Prostate — below the bladder, around the prostatic urethra; zones — peripheral (70% — most cancers; felt on DRE), transition (BPH), central; venous plexus connects with vertebral veins (bone metastases to the spine).
  • Male urethra — prostatic, membranous (narrowest; external sphincter; injured in pelvic fractures — blood at the meatus, high-riding prostate — don't catheterise urethrally), spongy (penile).
• Female:
  • Uterus — usually anteverted and anteflexed; supported by the pelvic floor and ligaments (uterosacral, cardinal (transverse cervical), round ligaments — the round ligament has little support role); broad ligament contains the uterine tubes, ovarian and uterine vessels.
  • Uterine artery (internal iliac) crosses above the ureter near the cervix.
  • Ovaries — on the lateral pelvic wall; ovarian artery from the aorta (via the suspensory ligament); lymph to para-aortic nodes; ovarian pain may refer to the medial thigh (obturator nerve).
  • Female urethra — short (about 4 cm) — UTIs more common.
• Rectum — about 12–15 cm; no taeniae; supported by the mesorectum (total mesorectal excision for rectal cancer); digital rectal examination — prostate (men) or cervix (women) anteriorly; rectouterine/rectovesical pouch above.
• Internal iliac artery — supplies the pelvic organs, perineum, gluteal region; ligated or embolised for pelvic haemorrhage (PPH, trauma).

The Perineum and Genital Anatomy

• Perineum — the diamond-shaped region below the pelvic floor, between the thighs — divided by a line between the ischial tuberosities into the urogenital triangle (anterior) and the anal triangle (posterior).
• Perineal body — fibromuscular central point — supports the pelvic floor; damaged in childbirth (perineal tears — 1st to 4th degree; 3rd and 4th involve the anal sphincter); episiotomy is mediolateral to avoid it.
• Anal canal — about 4 cm; internal sphincter (smooth muscle — involuntary — sympathetic) and external sphincter (skeletal — voluntary — pudendal nerve, S2–4); pectinate line (see above).
• Anorectal conditions:
  • Haemorrhoids — vascular cushions (at 3, 7 and 11 o'clock in the lithotomy position) — internal (painless bleeding) and external.
  • Anal fissure — tear below the pectinate line (usually posterior midline) — painful defecation.
  • Perianal abscess and fistula — anal glands; Goodsall's rule.
  • Ischioanal fossa — fat-filled space beside the anal canal (ischiorectal abscess).
• Pudendal nerve (S2–4) — passes around the ischial spine through Alcock's canal — external anal and urethral sphincters, perineal skin, penis/clitoris — pudendal block in labour.
• Testis:
  • Develops in the abdomen and descends through the inguinal canal (gubernaculum) — undescended testis (orchidopexy — cancer and fertility risk).
  • Testicular artery from the aorta (L2); venous drainage via the pampiniform plexus → testicular vein (left into the left renal vein — varicoceles commoner on the left); lymph drains to para-aortic nodes (not inguinal — scrotal skin drains to inguinal nodes).
  • Coverings from the abdominal wall; tunica vaginalis (hydrocele).
  • Testicular torsion — "bell-clapper" deformity; pain may refer to the lower abdomen (T10); cremasteric reflex (L1–2 — genitofemoral) absent.
• Penis — corpora cavernosa (erectile — parasympathetic, nitric oxide) and corpus spongiosum (urethra); priapism (sickle cell, drugs).
• Female external genitalia — vulva; Bartholin's glands (4 and 8 o'clock — cysts and abscesses); vulval lymph to the superficial inguinal nodes.

Thorax, Abdomen and Pelvis at a Glance

Spot the Diagnosis

Exam-style Scenarios

1. You need to insert a chest drain for a large pleural effusion. Describe the site.
Answer: The safe triangle, just above the upper border of a rib.

• Borders — lateral pectoralis major, anterior latissimus dorsi, a line through the 5th intercostal space; apex below the axilla.
• The neurovascular bundle runs in the costal groove under each rib — go over the top of the rib below.
• Ultrasound to mark the site; avoid below the 5th space (diaphragm, liver, spleen).

2. A 70-year-old man has a tender, irreducible right groin lump and bowel obstruction. The lump is above and medial to the pubic tubercle.
Answer: Incarcerated (possibly strangulated) inguinal hernia.

• Inguinal hernias lie above and medial to the pubic tubercle; femoral hernias below and lateral.
• Resuscitation, NG tube, analgesia, bloods (lactate).
• Emergency surgical repair — assess bowel viability.

3. A 55-year-old man with alcohol-related cirrhosis vomits a large volume of blood.
Answer: Likely variceal bleed from portal hypertension.

• Oesophageal varices form at the portosystemic anastomosis between the left gastric (portal) and oesophageal/azygos (systemic) veins.
• Resuscitate (restrictive transfusion), terlipressin and antibiotics, urgent endoscopy with band ligation.
• Balloon tamponade as a bridge; TIPSS for refractory bleeding.

4. A 40-year-old woman has severe left loin pain radiating to the groin. CT KUB shows a 5 mm stone.
Answer: Ureteric colic — stone likely at a ureteric narrowing.

• Narrowings — pelviureteric junction, pelvic brim (iliac vessels), vesicoureteric junction.
• Analgesia (NSAID), medical expulsive therapy (alpha-blocker) for distal stones over 5 mm in some cases.
• Urgent decompression (nephrostomy or stent) if infection with obstruction or AKI.

5. After a road traffic collision, a 25-year-old man has abdominal pain and left shoulder-tip pain. BP 85/50, HR 130. FAST scan shows fluid in Morison's pouch.
Answer: Intra-abdominal haemorrhage — probable splenic injury.

• Shoulder-tip pain — diaphragmatic irritation by blood (phrenic C3–5 — Kehr's sign).
• Morison's pouch (hepatorenal) is the most dependent space when supine — free fluid collects there.
• Major haemorrhage protocol; unstable — laparotomy (or interventional radiology if responding).

Key Points

• The sternal angle (T4/5) is the master landmark of the thorax.
• Neurovascular bundles run under the ribs — needles and drains go above the rib below, in the safe triangle.
• Diaphragm — IVC T8, oesophagus T10, aorta T12; phrenic nerve C3–5 refers pain to the shoulder.
• Left recurrent laryngeal nerve loops under the aortic arch — hoarseness is a lung cancer red flag.
• Right main bronchus — aspiration and foreign bodies.
• Transpyloric plane (L1) and aortic bifurcation (L4) are key abdominal levels.
• Indirect hernias are lateral and direct hernias medial to the inferior epigastric vessels; femoral hernias strangulate.
• Foregut, midgut and hindgut pain refers to the epigastrium, umbilicus and suprapubic region.
• Portosystemic anastomoses explain varices, caput medusae and rectal varices.
• Ureteric stones lodge at three narrowings; the ureter runs under the uterine artery and vas.
• Testis and ovary drain to para-aortic nodes; scrotum, vulva and lower anal canal to inguinal nodes.

Guidelines and Further Reading

• Gray's Anatomy for Students; Last's Anatomy — Regional and Applied
• Moore: Clinically Oriented Anatomy
• Abrahams: McMinn's Clinical Atlas of Human Anatomy
• Weir and Abrahams: Imaging Atlas of Human Anatomy
• British Thoracic Society: Pleural disease guideline (2023) — chest drain insertion
• NHS Abdominal Aortic Aneurysm Screening Programme
• Medical Schools Council: MLA content map — applied basic sciences
• BNF — for current doses