Unit 1: Musculoskeletal Foundations

The Skeletal System

The skeleton is divided into two major regions. The axial skeleton comprises the bones of the skull, vertebral column, sternum, and ribs — the central axis of the body. The appendicular skeleton includes the bones of the upper and lower limbs, together with the pectoral and pelvic girdles that attach them to the axial skeleton.

Bones are classified by shape into five categories:

• Long bones: greater length than width, with a shaft (diaphysis) and two ends (epiphyses). Examples: humerus, femur, tibia, phalanges.
• Short bones: roughly cube-shaped. Examples: carpal and tarsal bones.
• Flat bones: thin and curved, providing protection and broad muscle attachment surfaces. Examples: scapula, sternum, calvaria.
• Irregular bones: complex shapes that fit none of the other categories. Examples: vertebrae, hip bones, facial bones.
• Sesamoid bones: embedded within tendons to protect them and alter the angle of pull. The patella (kneecap) is the largest sesamoid, sitting within the quadriceps tendon. The pisiform is a sesamoid within the tendon of flexor carpi ulnaris.

Joint Classifications

A joint (articulation) is where two bones come together. Joints are classified by the amount of movement they allow.

Fibrous joints are united by fibrous connective tissue and permit little or no movement. Subtypes include sutures (skull), syndesmoses (distal tibiofibular joint), and gomphoses (teeth in their sockets).

Cartilaginous joints are united by cartilage. Primary cartilaginous joints (synchondroses) use hyaline cartilage and typically fuse with age (e.g., epiphyseal plates, the first sternocostal joint). Secondary cartilaginous joints (symphyses) use fibrocartilage and allow limited motion; examples include the pubic symphysis and the intervertebral discs.

Synovial joints are the most common and most mobile type. All synovial joints share a set of defining features:

• Articular cartilage (hyaline) covering the opposing bone surfaces
• A joint (articular) capsule with an outer fibrous layer and inner synovial membrane
• Synovial fluid produced by the synovial membrane to lubricate and nourish the joint
• Ligaments reinforcing the capsule, classified as intrinsic (thickenings of the fibrous capsule itself) or extrinsic (separate ligaments outside or inside the capsule)

Synovial joints are further categorized by shape and movement:

The Muscular System

Skeletal muscles produce movement by contracting. The key landmarks for each muscle are its proximal attachment (origin, the relatively fixed end) and distal attachment (insertion, the moving end). An important principle: a muscle can only produce movement at joints it crosses.

Muscle actions are named relative to the anatomical position. For a joint, a muscle crossing anterior to the axis of rotation in the sagittal plane will generally flex it (bring segments together); one crossing posterior will extend it. For rotation, the relationship of the muscle’s attachment to the axis of rotation determines medial or lateral rotation.

Unit 2: Back and Pectoral Girdle

Vertebral Column

The vertebral column consists of 33 vertebrae organized into five regions. It has two types of curvatures: primary (kyphotic) curvatures in the thoracic and sacral regions are present from birth (concave anteriorly). Secondary (lordotic) curvatures in the cervical and lumbar regions develop after birth as we hold our heads up and stand (concave posteriorly).

Typical Vertebra

A typical vertebra consists of:

• Body: the weight-bearing anterior portion
• Pedicles: two short processes projecting posteriorly from the body
• Laminae: connecting pedicles to the spinous process
• Vertebral arch: pedicles + laminae
• Spinous process: midline posterior projection for muscle/ligament attachment
• Transverse processes: lateral projections for muscle/ligament attachment
• Superior and inferior articular processes: with articular facets for zygapophyseal joints
• Vertebral foramen: opening through which the spinal cord passes
• Intervertebral foramen: formed between adjacent vertebrae; transmits spinal nerves

Intervertebral Discs

Intervertebral discs sit between adjacent vertebral bodies and are secondary cartilaginous joints. Each disc has an outer annulus fibrosus (concentric rings of fibrocartilage that resist tension) and an inner nucleus pulposus (gelatinous core that distributes compressive load). Herniation of the nucleus pulposus through a weakened annulus can compress adjacent spinal nerve roots.

Ligaments of the Vertebral Column

Major longitudinal ligaments include the anterior longitudinal ligament (anterior surface of vertebral bodies; prevents hyperextension), the posterior longitudinal ligament (posterior surface of vertebral bodies inside the vertebral canal; limits hyperflexion and posterior disc herniation), and the ligamentum flavum (connecting adjacent laminae; limits flexion).

Deep Back Muscles (Erector Spinae)

The erector spinae group lies in the groove between the spinous and transverse processes of the vertebrae. From lateral to medial, the three columns are:

Mnemonic: “I Like Standing” (Iliocostalis, Longissimus, Spinalis), from lateral to medial.

Bones of the Pectoral Girdle

Clavicle

The clavicle is an S-shaped long bone, lying roughly horizontal at the base of the neck. It is the only bony attachment of the entire upper appendicular skeleton to the axial skeleton (via the sternoclavicular joint). The medial two-thirds is convex anteriorly; the lateral third is concave anteriorly. It transmits compressive forces from the upper limb to the sternum and acts as a strut that holds the shoulder away from the thorax.

Scapula

The scapula is a triangular flat bone on the posterior thorax, between ribs 2–7. Key landmarks:

• Medial (vertebral) border: closest to the spine
• Lateral (axillary) border: faces the axilla
• Superior border with suprascapular notch
• Inferior angle and superior angle
• Subscapular fossa: large anterior concavity (faces the ribs)
• Supraspinous fossa: posterior, above the spine of the scapula
• Infraspinous fossa: posterior, below the spine of the scapula
• Spine of the scapula: prominent posterior ridge, continuous laterally with the acromion
• Acromion: the “roof” of the shoulder joint; articulates with the clavicle
• Coracoid process: hook-shaped anterior projection (“crow’s beak”); attachment for muscles and ligaments
• Glenoid cavity (fossa): shallow socket that accepts the head of the humerus
• Supraglenoid tubercle: attachment for the long head of biceps brachii
• Infraglenoid tubercle: attachment for the long head of triceps brachii

Proximal Humerus

• Head: large, smooth hemisphere articulating with the glenoid cavity
• Anatomical neck: the groove between the head and the tubercles
• Surgical neck: the narrowing below the tubercles; common fracture site
• Greater tubercle: lateral prominence; attachments for supraspinatus, infraspinatus, teres minor (rotator cuff — SI TM)
• Lesser tubercle: anterior prominence; attachment for subscapularis
• Intertubercular (bicipital) groove: between the tubercles; contains the tendon of the long head of biceps brachii

Mnemonic for the bicipital groove: “The Lady between Two Majors” — the floor of the groove has the tendon of latissimus dorsi, the lateral lip has pectoralis major, and the medial lip has teres major.

• Deltoid tuberosity: lateral shaft; attachment for deltoid

Joints of the Pectoral Girdle

Sternoclavicular (SC) Joint

The SC joint is structurally a saddle joint but functions like a ball-and-socket joint. It is the only true synovial joint between the upper limb and the axial skeleton. It contains an articular disc that improves congruency. Movements include elevation/depression, protraction/retraction, and rotation.

Acromioclavicular (AC) Joint

A plane joint between the acromion of the scapula and the lateral end of the clavicle. An articular disc is often present. Supported by the acromioclavicular ligament and the more important coracoclavicular ligament (consisting of the trapezoid and conoid parts).

Glenohumeral (GH) Joint

A ball-and-socket joint — the most mobile joint in the body. The glenoid labrum, a fibrocartilaginous rim, deepens the shallow glenoid cavity. Intrinsic ligaments include the superior, middle, and inferior glenohumeral ligaments and the coracohumeral ligament. Despite these, the primary stabilizers are the rotator cuff muscles. The long head of biceps brachii tendon passes through the joint capsule from the supraglenoid tubercle, contributing to superior stability.

Scapulothoracic Joint

A physiological joint (not a true synovial joint) — simply the gliding surface between the anterior scapula and the posterior thorax. Movement here is entirely dependent on the SC and AC joints.

Scapulohumeral Rhythm

When the arm is elevated, movement occurs at both the GH joint and the scapulothoracic joint. For every 3° of arm elevation: 2° occur at the GH joint and 1° occurs as scapular rotation on the thorax. This ratio is the scapulohumeral rhythm and is essential for pain-free shoulder function.

Axio-Appendicular Muscles

Anterior Axio-Appendicular Muscles (Chest to Upper Limb)

Clinical note: Damage to the long thoracic nerve (e.g., from axillary surgery or carrying heavy loads on the shoulder) paralyzes serratus anterior, causing the medial border of the scapula to lift away from the thorax — winged scapula.

Posterior Axio-Appendicular Muscles (Back to Upper Limb)

Unit 3: Upper Limb

Scapulohumeral (Intrinsic Shoulder) Muscles

Rotator Cuff — SITS

The rotator cuff consists of four muscles whose tendons blend with the joint capsule of the GH joint, forming a dynamic cuff that stabilizes the joint by compressing the humeral head into the glenoid. Mnemonic: SITS (Supraspinatus, Infraspinatus, Teres minor, Subscapularis).

Deltoid and Teres Major

Teres major runs parallel to latissimus dorsi and performs similar actions — they are “little buddies.”

Axilla

The axilla is a pyramid-shaped space between the arm and the thorax, serving as the passageway for neurovascular structures entering and leaving the upper limb.

Boundaries:

• Anterior wall: pectoralis major, pectoralis minor, subclavius
• Posterior wall: subscapularis, latissimus dorsi, teres major
• Medial wall: ribs 1–4 and serratus anterior
• Lateral wall: bicipital groove of the humerus

Contents: axillary artery (continuation of subclavian artery), axillary vein, brachial plexus (cords and terminal branches), lymphatics.

Brachial Plexus

The brachial plexus arises from the anterior primary rami of C5–T1. It supplies the entire upper limb. The organization follows a strict sequence:

5 Roots → 3 Trunks → 6 Divisions → 3 Cords → 5 Terminal Branches

Mnemonic: “Real Texans Drink Cold Beer” (Roots, Trunks, Divisions, Cords, Branches).

Trunks: Superior (C5–C6), Middle (C7), Inferior (C8–T1)

Cords — named relative to the axillary artery:

• Lateral cord: from anterior divisions of superior and middle trunks
• Posterior cord: from posterior divisions of all three trunks
• Medial cord: from the anterior division of the inferior trunk

Terminal branches (5):

1. Musculocutaneous nerve (C5–C7) — from lateral cord
2. Median nerve (C5–T1) — from lateral and medial cords
3. Ulnar nerve (C8–T1) — from medial cord
4. Axillary nerve (C5–C6) — from posterior cord
5. Radial nerve (C5–T1) — from posterior cord

Important non-terminal branches:

• Long thoracic nerve (C5–C7): directly from roots; innervates serratus anterior
• Dorsal scapular nerve (C5): from C5 root; innervates rhomboids and levator scapulae
• Nerve to subclavius (C5–C6): from superior trunk
• Suprascapular nerve (C5–C6): from superior trunk; innervates supraspinatus and infraspinatus
• From lateral cord: lateral pectoral nerve (C5–C7)
• From medial cord: medial pectoral nerve (C8–T1), medial cutaneous nerve of the arm (C8–T1), medial cutaneous nerve of the forearm (C8–T1)
• From posterior cord: upper subscapular nerve (C5–C6), thoracodorsal nerve (C6–C8, to latissimus dorsi), lower subscapular nerve (C5–C6)

Arm (Brachium)

Distal Humerus Landmarks

The distal humerus widens into medial and lateral epicondyles. Between them on the anterior surface: the capitulum (lateral, articulates with head of radius) and the trochlea (medial, articulates with trochlear notch of ulna). Above the trochlea anteriorly is the coronoid fossa; posteriorly is the olecranon fossa. The lateral epicondyle is the common extensor origin; the medial epicondyle is the common flexor origin.

Radius and Ulna

The ulna is the medial bone of the forearm. Proximally: the trochlear notch articulates with the trochlea of the humerus; the olecranon forms the bony prominence of the elbow; the coronoid process is the anterior lip of the trochlear notch. Distally: the head of the ulna and the styloid process (medial).

The radius is the lateral bone. Proximally: the head (disc-shaped, articulates with capitulum); the radial tuberosity is the insertion of biceps brachii. Distally: the styloid process (lateral); the radial notch accepts the distal radioulnar joint.

Anterior Compartment of the Arm

The anterior compartment contains flexors of the elbow and supinators, all innervated by the musculocutaneous nerve (C5–C7).

Posterior Compartment of the Arm

The posterior compartment contains extensors of the elbow, innervated by the radial nerve (C6–C8).

Neurovascular Structures of the Arm

The brachial artery is the direct continuation of the axillary artery below the teres major muscle. It gives off the profunda brachii artery (deep brachial artery), which accompanies the radial nerve in the radial groove of the humerus. At the cubital fossa, the brachial artery bifurcates into the radial artery (lateral) and ulnar artery (medial).

Superficial veins: The cephalic vein ascends along the lateral (thumb) side of the arm and drains into the axillary vein via the deltopectoral triangle. The basilic vein ascends medially, pierces the brachial fascia at about mid-arm, and joins the brachial vein to form the axillary vein. The median cubital vein connects cephalic to basilic at the cubital fossa and is the preferred site for venipuncture.

Nerve pathways in the arm:

• Musculocutaneous nerve: penetrates the coracobrachialis, passes between biceps and brachialis, and emerges lateral to the biceps tendon as the lateral cutaneous nerve of the forearm.
• Median nerve: descends with the brachial artery; crosses the artery at the mid-arm; enters the forearm between the two heads of pronator teres.
• Ulnar nerve: descends medially, passes posterior to the medial epicondyle through the cubital tunnel, then enters the forearm between the two heads of flexor carpi ulnaris.
• Radial nerve: accompanies the profunda brachii in the radial groove (posterior to the humerus); wraps around the lateral side and passes anterior to the lateral epicondyle; divides at the elbow into the deep branch (motor) and superficial branch (sensory).

Cubital Fossa

The cubital fossa is a triangular depression anterior to the elbow — the anatomical equivalent of the popliteal fossa in the lower limb.

Boundaries:

• Superior (base): an imaginary line between the medial and lateral epicondyles
• Medial border: pronator teres
• Lateral border: brachioradialis
• Floor: brachialis (medial) and supinator (lateral)
• Roof: deep fascia (reinforced by the bicipital aponeurosis) + skin

Contents (lateral to medial) — mnemonic: TAN:

1. T — Tendon of biceps brachii
2. A — Brachial Artery
3. N — Median Nerve

The radial nerve lies just lateral to the tendon of biceps, in the groove between brachioradialis and brachialis. It divides here into its deep and superficial branches.

Elbow Joint and Radioulnar Joints

Elbow Joint

The elbow joint is a compound joint comprising two articulations sharing one capsule:

1. Humeroulnar articulation: trochlea of humerus + trochlear notch of ulna — the primary hinge
2. Humeroradial articulation: capitulum of humerus + head of radius — allows rotation

The joint type is hinge, permitting flexion and extension. Key ligaments:

• Radial (lateral) collateral ligament: lateral epicondyle to annular ligament
• Ulnar (medial) collateral ligament: medial epicondyle to coronoid process and olecranon. Torn in “Tommy John” surgery; tested in overhead-throwing athletes.
• Annular ligament: a collar wrapping around the head of the radius, keeping it in contact with the radial notch of the ulna

“Pulled elbow” (nursemaid’s elbow) occurs when the annular ligament slips over the head of the radius in young children.

Proximal Radioulnar Joint

A pivot joint between the head of the radius and the radial notch of the ulna, reinforced by the annular ligament. The radius rotates around the ulna during supination and pronation.

Distal Radioulnar Joint

A pivot joint between the head of the ulna and the ulnar notch of the radius, reinforced by an articular disc (triangular fibrocartilage). The articular disc excludes the ulna from the wrist joint — only the radius articulates with the carpals.

Wrist and Hand — Bones and Joints

Carpal Bones

Eight carpal bones are arranged in two rows:

Proximal row (lateral to medial): Scaphoid, Lunate, Triquetrum, Pisiform

Distal row (lateral to medial): Trapezium, Trapezoid, Capitate, Hamate

Mnemonic: “Stop Letting Those People Touch The Cadaver’s Hand”

The scaphoid is the most commonly fractured carpal (fall on outstretched hand). The pisiform is sesamoid within the FCU tendon. The hamate has a distinctive hook.

Joints of the Wrist and Hand

Radiocarpal joint (wrist joint): A condyloid joint between the distal radius + articular disc on one side, and the scaphoid, lunate, and triquetrum on the other. Movements: flexion/extension and radial/ulnar deviation (abduction/adduction). The ulna does not participate.

Midcarpal joint: Between the proximal and distal rows of carpals; contributes to wrist flexion/extension.

Carpometacarpal (CMC) joint of the thumb: A saddle joint — the most mobile CMC joint. Allows opposition, the key movement that enables precision grip.

CMC joints of fingers 2–5: Plane joints with limited gliding only.

Metacarpophalangeal (MCP) joints: Condyloid joints — flexion/extension + abduction/adduction.

Interphalangeal (IP) joints: Hinge joints — flexion/extension only. Each finger has a proximal (PIP) and distal (DIP) IP joint; the thumb has only one IP joint.

Muscles of the Anterior Forearm

The muscles of the anterior forearm are organized in three layers. Most arise from the common flexor origin at the medial epicondyle. Most are innervated by the median nerve, with the ulnar nerve innervating FCU and the medial half of FDP.

Superficial Layer

Middle Layer

Deep Layer

Carpal Tunnel

The carpal tunnel is an osseofibrous canal bounded by the carpal bones posteriorly and the flexor retinaculum (transverse carpal ligament) anteriorly. It contains:

• Tendon of FPL (1 tendon)
• Tendons of FDS (4 tendons)
• Tendons of FDP (4 tendons)
• Median nerve

The ulnar nerve and artery pass superficial (anterior) to the flexor retinaculum through Guyon’s canal, not through the carpal tunnel.

Carpal tunnel syndrome: compression of the median nerve within the tunnel causes pain, paraesthesia (pins and needles) in the lateral 3½ digits, and weakness of thenar muscles. Common in pregnancy, repetitive wrist use, and rheumatoid arthritis.

Muscles of the Posterior Forearm

Posterior forearm muscles extend the wrist and fingers, and supinate the forearm. Most arise from the common extensor origin at the lateral epicondyle and are innervated by the radial nerve (brachioradialis, ECRL) or its deep branch (all others).

Superficial Layer

Deep Layer

“Outcropping” Muscles of the Thumb

Three muscles emerge through the floor of the posterior forearm to form the boundaries of the anatomical snuff box:

Anatomical Snuff Box

The anatomical snuff box is a triangular depression visible on the dorso-lateral wrist when the thumb is extended.

• Lateral border: APL and EPB tendons
• Medial border: EPL tendon
• Floor: scaphoid and trapezium
• Contents: radial artery (used to palpate radial pulse here)

Tenderness in the snuff box following a fall on an outstretched hand suggests a scaphoid fracture.

Extensor Expansion (Dorsal Hood)

The extensor expansion (extensor hood) is a triangular aponeurosis on the dorsum of each finger. The central band (medial band) inserts into the base of the middle phalanx; two lateral bands converge to insert into the base of the distal phalanx. Intrinsic muscles (lumbricals, interossei) attach to the expansion and use it to extend the IP joints while flexing the MCP joints.

Vessels and Nerves of the Forearm and Hand

Arterial Supply

The radial artery runs lateral in the forearm (alongside FCR), enters the anatomical snuff box, then crosses the palm to form the deep palmar arch.

The ulnar artery runs medial (alongside FCU), passes superficial to the flexor retinaculum, and forms the superficial palmar arch (more distal). Digital arteries to the fingers arise from both arches.

Nerve Pathways in the Forearm and Hand

Median nerve (C5–T1): Enters the forearm between the two heads of pronator teres. Runs deep to FDS. Gives off the anterior interosseous nerve (to FPL, FDP digits 2–3, pronator quadratus). Passes through the carpal tunnel. In the palm, gives off the recurrent branch to thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) and digital branches to the lateral 3½ digits. Lumbricals 1–2.

Ulnar nerve (C8–T1): Passes posterior to the medial epicondyle (vulnerable to trauma here — “funny bone”). Innervates FCU and medial half of FDP in the forearm. Passes through Guyon’s canal at the wrist. Innervates hypothenar muscles, all interossei, lumbricals 3–4, and adductor pollicis in the hand. Sensory to medial 1½ digits (palm and dorsum).

Radial nerve (C5–T1): Superficial branch provides sensation to dorsum of lateral 3½ digits (but not fingertips). Deep branch (posterior interosseous nerve) innervates all posterior forearm muscles except brachioradialis and ECRL.

The Hand

Palmar Aponeurosis

The palmar aponeurosis is a thick, triangular sheet of fibrous tissue deep to the palmar skin. It anchors skin to underlying structures (important for grip) and creates the five longitudinal compartments of the hand.

Compartments

The hand has five compartments:

1. Thenar compartment: abductor pollicis brevis, flexor pollicis brevis, opponens pollicis — all innervated by the recurrent branch of the median nerve.
2. Adductor compartment: adductor pollicis (transverse and oblique heads) — innervated by the deep branch of the ulnar nerve.
3. Hypothenar compartment: abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi — all innervated by the deep branch of the ulnar nerve.
4. Central compartment: FDS tendons, FDP tendons, 4 lumbricals.
5. Interosseous compartment: 4 dorsal interossei + 3 palmar interossei.

Lumbricals

The lumbricals (4 muscles) originate on the tendons of FDP and insert into the extensor expansion on the radial side of each finger. They flex the MCP joint and extend the IP joints — the “bye-bye” movement. Lumbricals 1–2: median nerve; Lumbricals 3–4: deep branch of ulnar nerve.

Interossei

Dorsal interossei (4, bipennate): originate from adjacent metacarpals; insert into extensor expansions of digits 2–4. They ABduct fingers from the midline (middle finger). Mnemonic: DAB (Dorsal = ABduct).

Palmar interossei (3, unipennate): originate from the metacarpal of each finger; insert into extensor expansion. They ADduct fingers toward the midline. Mnemonic: PAD (Palmar = ADduct).

All interossei: deep branch of the ulnar nerve.

The midline of the hand is the middle finger — abduction/adduction is defined relative to it.

Unit 4: Abdomen and Pelvis

Muscles of the Abdominal Wall

Anterolateral Abdominal Wall

Three flat muscles wrap around the abdomen in layers, with fibers running in different directions. Their aponeuroses interdigitate to form the rectus sheath and the midline linea alba.

The inguinal ligament is the inferior free edge of the external oblique aponeurosis, stretching from the anterior superior iliac spine (ASIS) to the pubic tubercle.

Rectus Abdominis

The rectus abdominis runs vertically in the midline, enclosed within the rectus sheath formed by the aponeuroses of the three flat muscles.

The tendinous intersections (3 visible horizontal bands in the “six-pack”) divide the muscle into segments.

The Diaphragm

The diaphragm is the dome-shaped musculotendinous sheet separating the thoracic and abdominal cavities. It is the primary muscle of inspiration. The central part is the central tendon (a trefoil-shaped aponeurosis). Peripheral muscle fibers arise from the sternal, costal, and lumbar portions.

Three major openings transmit structures between the thorax and abdomen:

Mnemonic: “I ate 10 eggs at noon” — IVC at T8, esophagus at T10, aorta at T12.

Arcuate ligaments bridge gaps in the diaphragm’s muscular origin: median arcuate ligament (over the aorta), medial arcuate ligaments (over psoas major), and lateral arcuate ligaments (over quadratus lumborum).

Innervation: primarily the phrenic nerves (C3–C5 — “C3, 4, 5 keep the diaphragm alive”); peripheral portions also innervated by lower intercostal nerves.

Posterior Abdominal Wall Muscles

Lumbar Plexus

The lumbar plexus arises from the anterior primary rami of L1–L4 within the psoas major muscle. Key branches:

• Femoral nerve (L2–L4): innervates anterior compartment of the thigh
• Obturator nerve (L2–L4): innervates medial compartment of the thigh
• Lumbosacral trunk (L4–L5): joins the sacral plexus

Pelvic Girdle

Hip Bone

Each hip bone (os coxae) consists of three fused bones: the ilium, ischium, and pubis. They fuse at the acetabulum (the socket of the hip joint) around puberty.

Ilium landmarks:

• Iliac crest: superior curved rim; connects ASIS to PSIS
• Anterior superior iliac spine (ASIS): attachment for sartorius and inguinal ligament
• Anterior inferior iliac spine (AIIS): attachment for rectus femoris
• Posterior superior iliac spine (PSIS): marks the sacroiliac joint (produces the “dimple” at the base of the back)
• Iliac fossa: anterior concave surface; origin of iliacus
• Auricular surface: articulates with the sacrum
• Gluteal lines (posterior, anterior, inferior): divide the gluteal surface for muscle attachments

Ischium landmarks:

• Ischial spine: separates greater and lesser sciatic notches
• Ischial tuberosity: the “sitting bones”; origin of hamstrings
• Ischial ramus: joins the inferior pubic ramus
• Greater sciatic notch → greater sciatic foramen (with ligaments)
• Lesser sciatic notch → lesser sciatic foramen (with ligaments)

Pubis landmarks:

• Body of pubis; pubic tubercle (attachment for inguinal ligament)
• Superior and inferior pubic rami
• Pubic symphysis: secondary cartilaginous joint with fibrocartilaginous disc

Sacrum and Coccyx

The sacrum is formed by five fused sacral vertebrae. The sacral canal continues the vertebral canal and contains the cauda equina. Sacral foramina (4 pairs, anterior and posterior) transmit sacral spinal nerves.

Pelvic Joints

The sacroiliac joint is a compound joint — synovial anteriorly (plane joint type), fibrous/syndesmosis posteriorly (very strong). It transmits the weight of the upper body to the lower limbs.

The sacrotuberous ligament (sacrum to ischial tuberosity) and the sacrospinous ligament (sacrum to ischial spine) convert the sciatic notches into foramina and resist posterior rotation of the sacrum.

The pubic symphysis is a secondary cartilaginous (symphysis) joint with a fibrocartilaginous interpubic disc.

Sex Differences in the Pelvis

Sacral Plexus and Sciatic Nerve

The sacral plexus arises from L4–S4. The sciatic nerve (L4–S3) is the largest nerve in the body. It exits the pelvis through the greater sciatic foramen, typically below piriformis, and descends through the posterior thigh. It is really two nerves wrapped together:

• Tibial nerve (innervates most of posterior thigh, all of posterior leg, and most of foot)
• Common fibular nerve (innervates short head of biceps femoris, then divides into deep fibular nerve and superficial fibular nerve in the leg)

Unit 5: Lower Limb

Proximal Femur and Hip Joint

Femur Landmarks

Proximal: head (spherical, articulates with acetabulum); fovea (small pit on the head for the ligament of the head); neck (connecting head to shaft); greater trochanter (lateral); lesser trochanter (posteromedial); intertrochanteric line (anterior, between trochanters); intertrochanteric crest (posterior); trochanteric fossa (medial to greater trochanter; insertions of obturator internus and gemelli).

Shaft: linea aspera (posterior ridge; attachment for many muscles); proximally splits into gluteal tuberosity (lateral) and pectineal line (medial).

Distal: medial and lateral condyles and epicondyles; adductor tubercle (on medial epicondyle); intercondylar fossa (posterior, between condyles; cruciate ligament attachments); patellar surface (anterior, for patella).

Hip Joint

A ball-and-socket joint between the head of the femur and the acetabulum. The acetabular labrum (fibrocartilaginous rim) deepens the socket. The lunate surface is the C-shaped articular portion; the acetabular fossa is the non-articular central depression. The transverse acetabular ligament bridges the gap in the labrum.

The ligament of the head of the femur (ligamentum teres) runs from the fovea to the acetabular fossa; it carries a small artery to the head of the femur.

Intrinsic ligaments of the hip (all tighten during extension):

• Iliofemoral ligament (Y-ligament of Bigelow): strongest; from AIIS to intertrochanteric line
• Pubofemoral ligament: from pubic bone to inferior femoral neck
• Ischiofemoral ligament: from ischium to greater trochanter, spiraling across the posterior capsule

Iliotibial Band and Distal Thigh

The iliotibial band (IT band) is a thickening of the fascia lata (deep fascia of the thigh) running down the lateral thigh. It receives the insertions of gluteus maximus (part) and tensor fasciae latae and inserts on Gerdy’s tubercle on the lateral condyle of the tibia. It provides lateral stability to the knee.

The fascia lata surrounds the thigh musculature and sends intermuscular septa to the femur, dividing the thigh into anterior, medial, and posterior compartments.

Tibia and Fibula Landmarks

Tibia: medial and lateral condyles proximally; intercondylar eminence between them (attachment for cruciate ligaments); tibial tuberosity anteriorly (attachment of patellar tendon/quadriceps mechanism); soleal line (oblique posterior line just below condyles; origin of soleus); medial malleolus distally.

Fibula: head proximally (articulates with lateral tibial condyle — plane joint); lateral malleolus distally (forms lateral wall of ankle mortise). The fibula is non-weight-bearing.

Superficial Veins of the Lower Limb

The great (long) saphenous vein is the longest vein in the body. It ascends along the medial aspect of the leg and thigh, draining into the femoral vein in the femoral triangle — the equivalent of the cephalic vein.

The small (short) saphenous vein ascends along the posterolateral leg and drains into the popliteal vein at the popliteal fossa — the equivalent of the basilic vein.

Muscles of the Anterior Thigh

The fascia lata divides the thigh into three compartments. The anterior compartment contains flexors of the hip and extensors of the knee, innervated by the femoral nerve (L2–L4).

The four vasti + rectus femoris = quadriceps femoris. Their tendons merge to form the quadriceps tendon, which contains the patella (sesamoid) and continues below as the patellar ligament (tendon) to the tibial tuberosity.

Pes anserinus (“goose’s foot”) is the conjoined tendinous attachment of sartorius, gracilis, and semitendinosus on the medial tibia just below the tibial condyle. One muscle from each thigh compartment.

Muscles of the Medial Thigh

The medial compartment contains adductors of the hip, innervated by the obturator nerve (L2–L4).

Adductor magnus is the largest muscle in the body. Its gap between the adductor and hamstring parts forms the adductor hiatus, which allows the femoral vessels to pass from the anterior thigh to the posterior popliteal fossa.

Femoral Triangle and Subsartorial Canal

The femoral triangle is a triangular depression at the top of the anteromedial thigh. Analogous to the cubital fossa.

Boundaries:

• Superior: inguinal ligament
• Lateral: sartorius
• Medial: adductor longus
• Floor: iliopsoas (lateral) + pectineus (medial)

Contents (lateral to medial) — mnemonic: NAVEL (Nerve, Artery, Vein, Empty space, Lymphatics):

1. Femoral nerve — fans out immediately into multiple branches
2. Femoral artery — gives off profunda femoris artery
3. Femoral vein — receives great saphenous vein

The subsartorial (adductor) canal is the tunnel deep to sartorius from the apex of the femoral triangle to the adductor hiatus. It contains the femoral artery, femoral vein, saphenous nerve (branch of femoral nerve), and nerve to vastus medialis. The femoral artery becomes the popliteal artery as it passes through the adductor hiatus.

Muscles of the Gluteal Region

The gluteal region contains three large gluteal muscles and a group of small lateral rotators.

Large Gluteal Muscles

Gluteus medius and minimus and TFL are the critical muscles for single-limb stance — they contract contralaterally to prevent the pelvis from dropping when one foot leaves the ground.

Gluteal Neurovascular Landmarks

Piriformis is the key landmark. All structures exiting the greater sciatic foramen are named relative to it:

• Superior gluteal nerve and artery: exit above piriformis → supply gluteus medius, minimus, and TFL
• Inferior gluteal nerve and artery: exit below piriformis → supply gluteus maximus
• Sciatic nerve: exits below piriformis (usually)

Small Lateral Rotators (Deep Gluteal Muscles)

All perform hip lateral rotation. From superior to inferior:

The superior and inferior gemellus muscles sandwich obturator internus — “the gemellus sandwich.” Obturator internus is unique: it originates inside the pelvis and exits through the lesser sciatic foramen, making a near-right-angle turn.

Superior gemellus receives nerve to obturator internus; inferior gemellus receives nerve to quadratus femoris — each gemellus is innervated by the nerve of the muscle just inferior to it.

Muscles of the Posterior Thigh

The posterior compartment contains the hamstring muscles, innervated by the tibial division of the sciatic nerve (with one exception).

True hamstrings must: (1) cross both the hip and knee, (2) originate on the ischial tuberosity, and (3) be innervated by the tibial nerve. This defines semitendinosus, semimembranosus, and the long head of biceps femoris.

The short head of biceps femoris is the exception: it does not cross the hip joint, does not originate on the ischial tuberosity, and is innervated by the common fibular nerve, not the tibial nerve.

Popliteal Fossa

The popliteal fossa is the diamond-shaped space at the back of the knee — the “knee pit.”

Boundaries:

• Superior lateral: biceps femoris
• Superior medial: semimembranosus and semitendinosus
• Inferior lateral: lateral head of gastrocnemius
• Inferior medial: medial head of gastrocnemius
• Floor: femur, capsule of knee joint, popliteus

Contents (lateral to medial, then deep):

• Tibial nerve (central, most superficial nerve)
• Common fibular nerve (lateral, follows biceps femoris)
• Popliteal artery (deepest structure, continuation of femoral artery)
• Popliteal vein (between nerve and artery)
• Small saphenous vein drains into popliteal vein here

Knee Joint

The knee is the largest and most superficial joint in the body. It has three articulations sharing one joint capsule:

1. Medial femorotibial articulation
2. Lateral femorotibial articulation
3. Patellofemoral articulation (patella glides on patellar surface of femur)

The fibula does not participate in the knee joint. The joint type is a modified hinge (also called bicondylar), allowing flexion/extension plus a small amount of rotation.

Ligaments of the Knee

Intrinsic ligaments (part of the capsule):

• Patellar ligament (tendon): quadriceps tendon → patella → tibial tuberosity; replaces the anterior fibrous capsule
• Tibial (medial) collateral ligament (MCL/TCL): flat sheet; medial epicondyle of femur to medial tibial condyle; blends with medial meniscus — if MCL tears, medial meniscus is often torn too
• Oblique popliteal ligament: extension of semimembranosus tendon; reinforces posterior capsule obliquely
• Arcuate popliteal ligament: arches over the popliteus muscle entering the capsule

Extrinsic ligaments (not part of capsule):

• Fibular (lateral) collateral ligament (LCL/FCL): rope-like; lateral epicondyle of femur to head of fibula; completely separate from capsule and lateral meniscus
• Anterior cruciate ligament (ACL): tibial attachment anterior to intercondylar eminence → lateral femoral condyle. Prevents anterior displacement of tibia. Tightens on extension. Tested by the anterior drawer test. Common injury in hyperextension.
• Posterior cruciate ligament (PCL): tibial attachment posterior to intercondylar eminence → medial femoral condyle. Prevents posterior displacement of tibia. Tightens on flexion. Stronger than ACL; rarely torn.

The cruciate ligaments are inside the joint capsule but outside the synovial membrane (the synovial membrane folds back to exclude them).

Menisci

The medial and lateral menisci are C-shaped (medial) and oval-shaped (lateral) fibrocartilaginous plates on the tibial plateau. They deepen the tibial articular surface and absorb compressive loads.

• Medial meniscus: C-shaped; anterior and posterior attachments are far apart; firmly attached to the MCL (hence torn with MCL injury)
• Lateral meniscus: oval, almost circular; attachments are close together; not attached to the LCL

Proximal Tibiofibular Joint

A plane joint between the head of the fibula and the inferior surface of the lateral tibial condyle. Supported by anterior and posterior ligaments of the fibular head.

Bones and Joints of the Ankle and Foot

Tarsal Bones (7)

From proximal to distal:

• Talus: articulates with tibia and fibula superiorly; head articulates with navicular anteriorly; body transmits weight to calcaneus. The trochlea is the superior articular surface. Unique: no muscular attachments.
• Calcaneus: largest, strongest tarsal; forms the heel. Calcaneal tuberosity = heel. Sustentaculum tali (talar shelf) projects medially to support the talus.
• Navicular: anterior to talus; boat-shaped.
• Cuboid: anterior to calcaneus; on the lateral side.
• Medial (1st) cuneiform, Intermediate (2nd) cuneiform, Lateral (3rd) cuneiform: anterior to navicular; three wedge-shaped bones.

Digits are numbered 1 (hallux/big toe, medial) to 5 (pinky toe, lateral) — opposite to the hand.

Talocrural Joint (Ankle Joint)

The ankle joint is a hinge joint between the trochlea of the talus and the malleolar mortise (arch formed by the distal tibia + medial malleolus + lateral malleolus). Movements: dorsiflexion (bringing the dorsum of the foot toward the shin) and plantar flexion (pointing the foot down). The trochlea is wider anteriorly → ankle is more stable in dorsiflexion.

Medial (deltoid) ligament: 4 parts (anterior tibiotalar, tibionavicular, tibiocalcaneal, posterior tibiotalar); very strong; resists eversion.

Lateral collateral ligament: 3 parts (anterior talofibular, calcaneofibular, posterior talofibular); weaker than medial; inversion sprains usually tear the anterior talofibular ligament first.

Tibiofibular Syndesmosis

A syndesmosis between the distal tibia (fibular notch) and fibula; held by anterior/posterior tibiofibular ligaments and interosseous ligament. Essential for maintaining the integrity of the malleolar mortise.

Subtalar Joint

The subtalar joint (talocalcaneal joint) is a plane joint between the body of the talus and the calcaneus. It allows inversion and eversion of the foot. Dorsiflexion/plantar flexion = talocrural; inversion/eversion = subtalar.

Plantar Ligaments

• Plantar calcaneonavicular ligament (spring ligament): from sustentaculum tali to navicular; supports the head of the talus; essential for maintaining the medial longitudinal arch
• Short plantar ligament (plantar calcaneocuboid): calcaneus to cuboid
• Long plantar ligament: calcaneus to bases of metatarsals 2–5 (most superficial)

The plantar aponeurosis runs from the calcaneal tuberosity to the bases of the proximal phalanges, supporting the longitudinal arch.

Muscles of the Anterior Compartment of the Leg

The anterior compartment contains dorsiflexors and toe extensors, innervated by the deep fibular nerve (a branch of the common fibular nerve). Arterial supply: anterior tibial artery → dorsalis pedis at the ankle.

Superior and inferior extensor retinacula hold tendons in place anterior to the ankle, preventing bowstringing.

Muscles of the Lateral Compartment of the Leg

Two muscles only, both performing plantar flexion and eversion, innervated by the superficial fibular nerve.

Both muscles pass posterior to the lateral malleolus, held by superior and inferior fibular retinacula. Blood supply: fibular (peroneal) artery (branch of posterior tibial artery, runs in the posterior compartment but perforates into lateral compartment).

Muscles of the Posterior Compartment of the Leg

The largest compartment, with two groups separated by a deep transverse fascia. All innervated by the tibial nerve; blood supply from the posterior tibial artery (with fibular artery for lateral compartment).

Superficial Group

Gastrocnemius + soleus = triceps surae. The calcaneal (Achilles) tendon is the common tendon; the thickest and strongest tendon in the body. Rupture typically occurs near the calcaneal insertion and produces an audible snap.

Deep Group

Medial Malleolus Mnemonic: Tom, Dick, and Not Harry

Structures crossing behind the medial malleolus (deep to flexor retinaculum) from anterior to posterior:

1. T — Tibialis posterior tendon
2. D — Flexor Digitorum longus tendon
3. aN — posterior tibial Artery + tibial Nerve (accompanied by posterior tibial veins)
4. H — Flexor Hallucis longus tendon

The Foot

Plantar Aponeurosis

The plantar aponeurosis is a thick, fibrous sheet just deep to the plantar skin. It anchors the skin and supports the longitudinal arch.

Muscles of the Foot (Intrinsic)

The foot muscles are studied in four layers from superficial to deep.

Layer 1 (most superficial, just deep to plantar aponeurosis):

Layer 2:

Layer 3:

Layer 4 (deepest):

Key difference from the hand: in the foot, the second digit (not the third) is the midline for abduction/adduction. Both directions of movement of the second toe are abduction.

Dorsal intrinsic muscles (on dorsal foot surface):

• Extensor hallucis brevis and Extensor digitorum brevis: innervated by the deep fibular nerve; extend the toes they supply.

Neurovascular Supply of the Foot

The tibial nerve divides just below the medial malleolus into:

• Medial plantar nerve: innervates muscles on the medial foot; sensory to medial 3½ toes (plantar surface)
• Lateral plantar nerve: the “obturator nerve of the foot” — innervates most foot muscles (the rule); sensory to lateral 1½ toes

The lateral plantar nerve is the dominant nerve. Exceptions innervated by the medial plantar nerve: abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, and the first lumbrical.

The posterior tibial artery divides into medial and lateral plantar arteries alongside their corresponding nerves.

Unit 6: Neuroanatomy

The Skull

The skull is formed by two sets of bones: the neurocranium (protecting the brain) and the viscerocranium (facial skeleton).

Neurocranium: frontal bone, two parietal bones, occipital bone (with foramen magnum — the large opening for the spinal cord), two temporal bones (complex; contain the structures of hearing and balance; site of the external acoustic meatus and the TMJ).

Viscerocranium: two zygomatic bones (cheekbones), two maxillae (upper jaw, teeth, hard palate), mandible (lower jaw), nasal bones (bridge of nose), sphenoid (butterfly-shaped; visible from lateral and anterior views; extends into the orbit).

Meninges

Three layers of connective tissue surround the brain and spinal cord:

1. Dura mater (“tough mother”): outermost; thick, fibrous, inextensible. Has two layers: periosteal (adheres to inner skull) and meningeal (invaginates to form dural folds).
2. Arachnoid mater (“spider mother”): middle layer; web-like delicate membrane. Not closely applied to the brain surface.
3. Pia mater (“soft/tender mother”): innermost; one cell thick; intimately adherent to every fold of the brain; cannot be separated from nervous tissue.

The subarachnoid space lies between the arachnoid and pia mater. It contains cerebrospinal fluid (CSF).

Dural Folds

The meningeal layer of the dura mater invaginates to form partitions:

• Falx cerebri: sickle-shaped; separates the left and right hemispheres of the cerebrum
• Tentorium cerebelli: tent-shaped horizontal partition; separates the cerebellum from the occipital lobe of the cerebrum
• Falx cerebelli: smaller sickle beneath the tentorium; separates the two cerebellar hemispheres

Dural Venous Sinuses

Where the two dural layers separate, venous blood collects in dural venous sinuses:

• Superior sagittal sinus: along the superior edge of the falx cerebri
• Inferior sagittal sinus: along the inferior edge of the falx cerebri
• Straight sinus: where inferior sagittal sinus meets the tentorium cerebelli
• Transverse sinuses: along the posterior edges of the tentorium cerebelli
• Sigmoid sinuses: S-shaped continuation of the transverse sinuses → exit the skull as the internal jugular veins

Cerebrospinal Fluid

CSF is produced by the choroid plexus in the ventricles (cavities within the brain). It flows through the ventricular system, exits into the subarachnoid space, and is reabsorbed into the venous system via arachnoid villi projecting into the superior sagittal sinus.

CSF functions: buoyancy (reduces effective brain weight from ~1400g to ~25g), shock absorption, and waste clearance.

Blood Supply to the Brain: Circle of Willis

The brain is supplied by two pairs of arteries:

1. Internal carotid arteries (from common carotid → from brachiocephalic trunk on right, from aorta on left): give rise to the middle cerebral artery (lateral brain) and anterior cerebral artery (medial/superior brain)
2. Vertebral arteries (from subclavian arteries, passing through transverse foramina of cervical vertebrae): join to form the basilar artery → posterior cerebral arteries (posterior brain)

These are connected by communicating arteries:

• Anterior communicating artery: bridges the two anterior cerebral arteries
• Posterior communicating arteries (2): connect internal carotid to posterior cerebral arteries

Together they form the circle of Willis, an arterial anastomosis at the base of the brain providing collateral flow if one artery is occluded.

The Brain

The brain comprises:

• Cerebrum: two hemispheres (left and right), each with four lobes. Connected by the corpus callosum.
• Cerebellum: “little brain”; coordinates movement and balance; two hemispheres separated by the falx cerebelli
• Diencephalon: thalamus (relay station) and hypothalamus (autonomic control, connects to pituitary)
• Brainstem: midbrain (superior), pons (middle), medulla oblongata (inferior) → continuous with spinal cord

Cerebral Lobes

Each cerebral hemisphere has four lobes:

• Frontal lobe: anterior; motor cortex, prefrontal cortex
• Parietal lobe: posterior to central sulcus; somatosensory cortex
• Temporal lobe: lateral, below lateral sulcus; auditory cortex, memory
• Occipital lobe: posterior; visual cortex

Key sulci: central sulcus (separates frontal from parietal lobe), lateral sulcus (separates frontal/parietal from temporal), parieto-occipital sulcus (visible on medial surface, separates parietal from occipital).

Gyri are the raised folds; sulci are the grooves between them. The folding dramatically increases cortical surface area.

Spinal Cord

The spinal cord begins at the foramen magnum (where the medulla oblongata continues downward) and ends at approximately the L1–L2 vertebral level as the conus medullaris. The vertebral column is considerably longer than the spinal cord.

There are 31 pairs of spinal nerves. Below the conus medullaris, the lumbar and sacral spinal nerve roots descend within the vertebral canal to reach their intervertebral foramina, forming the cauda equina (“horse’s tail”).

Cranial Nerves

There are 12 pairs of cranial nerves, arising from the inferior/ventral surface of the brain (mostly brainstem). They may carry sensory information, motor commands, or both. They are numbered I–XII in Roman numerals.

Mnemonic for names: “Oh Oh Oh To Touch And Feel Very Good Velvet Ah Heaven” (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal).

Mnemonic for sensory (S), motor (M), or both (B): “Some Say Marry Money But My Brother Says Big Brains Matter Most” — S, S, M, M, B, M, B, S, B, B, M, M.

Anatomical note: Cranial nerve numbers roughly correspond to their superior-to-inferior position on the brainstem. At the pontomedullary junction, CN VI (abducens) is near the midline, with CN VII (facial) and CN VIII (vestibulocochlear) just lateral — a cluster of three nerves at “6 o’clock” on the pons. The vagus nerve (CN X) is a useful checkpoint — if it is CN X, the count is on track.

Key clinical distinctions:

• Trigeminal (V) vs. Facial (VII): Trigeminal carries sensation from the face; Facial carries motor to facial expression. A common source of confusion.
• Facial (VII) vs. Glossopharyngeal (IX) for taste: Facial serves the anterior 2/3 of the tongue; Glossopharyngeal serves the posterior 1/3.
• Accessory (XI): the only cranial nerve that also has a spinal root (from the upper cervical spinal cord), which ascends through the foramen magnum to join the cranial root before exiting.