Skeletal System — revision sheet

The skeletal system comprises the 206 bones of the adult skeleton together with the cartilages, joints and ligaments that connect them. It supports and protects the body, provides levers for movement, houses haematopoietic marrow and acts as the main reservoir of calcium and phosphate.

Bones in the adult skeleton: 206 (80 axial, 126 appendicular)Ossification elements at birth: ≈270–300Skeleton as % of body mass: ≈15% (≈10–12 kg in an adult)Body calcium stored in bone: ≈99% (≈1–1.2 kg)Serum total calcium: 2.2–2.6 mmol/L (8.5–10.5 mg/dL)Serum ionised calcium: 1.1–1.3 mmol/L (≈45–50% of total)Compact : spongy bone (by mass): ≈80 : 20Bone matrix composition (wet weight): ≈60–70% mineral, ≈20–30% organic (≈90% type I collagen), ≈10% waterSkeletal turnover (adult): ≈5–10% per yearLongest / strongest bone: Femur (≈26% of stature)Smallest bone: Stapes (≈3 mm)Largest sesamoid bone: Patella

Bone tissue & cells

Bone tissue (osseous tissue) Tissue: A mineralised connective tissue of cells embedded in a matrix of type I collagen and hydroxyapatite, combining tensile and compressive strength with low weight. It forms every bone of the skeleton in two architectures — compact and spongy.
Bone matrix Tissue: The extracellular material of bone: an organic osteoid of type I collagen and non-collagenous proteins, mineralised by crystals of calcium hydroxyapatite.
Compact bone Tissue: Dense bone that forms the outer shell of every bone and the thick wall of long-bone shafts. It is built from closely packed cylindrical osteons aligned with the long axis, resisting bending and compression.
Spongy bone Tissue: A three-dimensional lattice of thin bony plates and struts (trabeculae) whose spaces contain bone marrow. It lightens bones and transmits load along stress lines while providing a large surface for remodelling.
Osteon Tissue: The cylindrical structural and functional unit of compact bone: concentric lamellae of mineralised matrix arranged around a central (Haversian) canal carrying vessels and nerves.
Periosteum Membrane: The vascular, richly innervated connective-tissue membrane covering the external surface of bones except at articular surfaces. Its inner layer contains osteoprogenitor cells responsible for appositional growth and fracture repair.
Endosteum Membrane: A thin cellular layer lining the medullary cavity, the trabeculae of spongy bone and the central and perforating canals. It contains osteoprogenitor cells, osteoblasts, bone-lining cells and osteoclasts.
Osteoprogenitor cell Cell: Mitotically active stem/progenitor cells of mesenchymal origin, found in the periosteum, endosteum and canal linings, that differentiate into osteoblasts.
Osteoblast Cell: Bone-forming cell that secretes osteoid (type I collagen and non-collagenous proteins) and initiates its mineralisation. Osteoblasts also control osteoclast formation through RANKL and osteoprotegerin.
Osteocyte Cell: A mature bone cell — a former osteoblast entombed within mineralised matrix in a lacuna — that maintains the matrix and acts as the mechanosensor and endocrine regulator of bone.
Osteoclast Cell: A large, multinucleated bone-resorbing cell derived from the monocyte–macrophage lineage. It dissolves mineral with acid and digests collagen with proteases in a sealed resorption compartment.

Long bone anatomy

Diaphysis Bone feature: The tubular shaft of a long bone: a thick wall of compact bone surrounding the medullary cavity. It forms from the primary ossification centre.
Epiphysis Bone feature: The expanded end of a long bone, made of spongy bone with a thin compact shell and capped by articular cartilage. It ossifies from a secondary ossification centre separated from the shaft by the growth plate until maturity.
Metaphysis Bone feature: The flared region of a long bone between the diaphysis and the epiphysis, adjoining the growth plate, where newly formed bone is added during growth.
Medullary cavity Space: The central cavity of a long-bone shaft, lined by endosteum and filled with bone marrow — red in infants, mainly yellow (fatty) in adults.
Epiphyseal plate Cartilage: A disc of hyaline cartilage between epiphysis and metaphysis that produces growth in length of long bones; after puberty it ossifies, leaving the epiphyseal line.
Articular cartilage Cartilage: A layer of hyaline cartilage, 2–4 mm thick in large joints, covering the articular surfaces of synovial joints. It provides a smooth, almost frictionless, load-distributing bearing surface.

Skull

Skull Bone group: The skeleton of the head — 22 bones (8 cranial, 14 facial) joined mostly by immovable sutures, with the mandible as the only freely movable bone. It encloses the brain, special sense organs and the openings of the respiratory and digestive tracts.

Skull — cranial bones

Frontal bone Bone: The single bone of the forehead that forms the anterior cranial vault, the roofs of the orbits and most of the floor of the anterior cranial fossa. It contains the frontal sinuses.
Parietal bone Bone: A paired, quadrilateral, curved plate forming most of the roof and upper sides of the cranial vault.
Occipital bone Bone: The trapezoid-shaped bone forming the back and much of the base of the cranium. It surrounds the foramen magnum and articulates with the atlas through its occipital condyles.
Temporal bone Bone: A paired, complex bone of the side and base of the skull that houses the middle and inner ear, forms the mandibular fossa of the temporomandibular joint and contributes the posterior zygomatic arch.
Sphenoid bone Bone: A single, bat- or butterfly-shaped bone in the centre of the cranial base that articulates with every other cranial bone. It carries the sella turcica for the pituitary and many of the key foramina of the middle cranial fossa.
Ethmoid bone Bone: A light, cuboidal, spongy bone between the orbits that forms the roof of the nasal cavity (cribriform plate), the upper nasal septum, the superior and middle nasal conchae and the ethmoidal air cells.

Skull — facial bones

Nasal bone Bone: Paired small oblong bones that meet in the midline to form the bony bridge of the nose.
Maxilla Bone: The paired bones of the upper jaw. Together they form most of the face, the upper dental arch, the anterior hard palate, the floor of the orbits and the lateral walls of the nasal cavity, and contain the maxillary sinuses.
Zygomatic bone Bone: The paired cheekbone forming the prominence of the cheek, the lateral wall and floor of the orbit and the anterior part of the zygomatic arch.
Lacrimal bone Bone: The smallest and most fragile facial bone, a thin plate in the anterior medial wall of the orbit that helps form the fossa for the lacrimal sac.
Palatine bone Bone: A paired L-shaped bone at the back of the nasal cavity forming the posterior third of the hard palate, part of the lateral nasal wall and a small part of the orbital floor.
Inferior nasal concha Bone: A paired, independent scroll-shaped bone projecting from the lateral wall of the nasal cavity — the largest of the three conchae and, unlike the superior and middle conchae, a separate bone.
Vomer Bone: A thin, unpaired, plough-shaped bone forming the posteroinferior part of the bony nasal septum.
Mandible Bone: The largest and strongest facial bone and the only movable bone of the skull (apart from the ossicles). It carries the lower teeth and articulates with both temporal bones at the temporomandibular joints.

Skull — sutures & landmarks

Cranial sutures Joint: Fibrous joints uniting the bones of the skull by a thin layer of dense connective tissue. They allow moulding at birth and growth of the vault in childhood, then progressively fuse in adulthood.
Fontanelles Region: Membrane-covered gaps between the incompletely ossified cranial bones of the fetus and infant, found where several sutures meet. They permit moulding during birth and rapid brain growth, and are a clinical window on intracranial pressure and hydration.
Pterion Bone feature: An H-shaped junction on the side of the skull where the frontal, parietal, temporal (squamous part) and sphenoid (greater wing) bones meet. It is the thinnest part of the lateral skull and overlies the anterior branch of the middle meningeal artery.
Cranial fossae and foramina of the skull base Region: The floor of the cranial cavity is stepped into anterior, middle and posterior cranial fossae that cradle the brain. Their foramina transmit the cranial nerves and the major vessels of the head.
Bony orbit Region: A pyramidal bony socket formed by seven bones that houses and protects the eyeball, extraocular muscles, nerves, vessels and lacrimal gland.

Hyoid

Hyoid bone Bone: A U-shaped bone in the anterior neck at the level of C3, between the mandible and the thyroid cartilage. It is the only bone that does not articulate with any other bone, being suspended by muscles and ligaments.

Vertebral column

Vertebral column Bone group: The flexible, curved column of 33 vertebrae (26 separate bones in the adult) linked by intervertebral discs, facet joints and ligaments. It supports the head and trunk, transmits weight to the pelvis and protects the spinal cord.
Typical vertebra Bone: The basic unit of the vertebral column: an anterior weight-bearing body and a posterior vertebral arch that encloses the vertebral foramen and carries seven processes.
Cervical vertebrae Bone group: The seven vertebrae of the neck (C1–C7) — the smallest and most mobile, identified by the foramen in each transverse process. C1 (atlas) and C2 (axis) are specialised for movements of the head.
Atlas (C1) Bone: The first cervical vertebra: a ring of bone without a body or spinous process that supports the skull on its lateral masses and rotates around the dens of the axis.
Axis (C2) Bone: The second cervical vertebra, distinguished by the tooth-like dens (odontoid process) that projects upward as a pivot around which the atlas and head rotate.
Thoracic vertebrae Bone group: The twelve vertebrae of the chest (T1–T12), characterised by costal facets on their bodies and transverse processes for articulation with the ribs.
Lumbar vertebrae Bone group: The five largest vertebrae (L1–L5), in the lower back, built to bear the weight of the upper body. They have massive kidney-shaped bodies and lack transverse foramina and costal facets.
Sacrum Bone: A large triangular bone formed by fusion of five sacral vertebrae, wedged between the hip bones. It forms the posterior wall of the pelvis and transmits body weight to the pelvic girdle through the sacroiliac joints.
Coccyx Bone: The small triangular "tailbone" at the end of the vertebral column, formed by 3–5 (usually 4) rudimentary fused coccygeal vertebrae.
Intervertebral disc Cartilage: A fibrocartilaginous pad between adjacent vertebral bodies, made of a gelatinous nucleus pulposus surrounded by a laminated annulus fibrosus. It absorbs shock, distributes load and allows movement between vertebrae.

Thoracic cage

Thoracic cage Bone group: The osteocartilaginous cage formed by the 12 thoracic vertebrae, 12 pairs of ribs with their costal cartilages, and the sternum. It protects the thoracic and upper abdominal organs and is the moving framework of breathing.
Ribs Bone group: Twelve pairs of curved flat bones forming most of the thoracic wall. Ribs 1–7 are true (vertebrosternal), 8–10 false (vertebrochondral) and 11–12 floating (vertebral).
Costal cartilages Cartilage: Bars of hyaline cartilage that prolong the ribs anteriorly and connect them to the sternum or to one another, giving the thoracic wall its elasticity.
Sternum Bone: The flat midline bone of the anterior chest wall (≈17 cm long), composed of the manubrium, body and xiphoid process. It anchors the clavicles and the costal cartilages of the true ribs.

Upper limb — pectoral girdle

Clavicle Bone: An S-shaped long bone lying horizontally at the root of the neck that acts as a strut holding the upper limb away from the trunk. It is the only bony link between the upper limb and the axial skeleton.
Scapula Bone: A flat, triangular bone on the posterolateral chest wall (ribs 2–7) that forms the posterior pectoral girdle, the socket of the shoulder joint and the attachment for 17 muscles.

Upper limb — arm

Humerus Bone: The long bone of the arm, articulating with the scapula at the shoulder and with the radius and ulna at the elbow. Three major nerves are in direct contact with it — axillary, radial and ulnar.

Upper limb — forearm

Radius Bone: The lateral bone of the forearm (on the thumb side), narrow above and broad below. It rotates around the ulna in pronation and supination and forms the main forearm component of the wrist joint.
Ulna Bone: The medial bone of the forearm (on the little-finger side), large proximally where it forms the main hinge of the elbow with the humerus, and small distally where its head is separated from the carpus by an articular disc.

Upper limb — hand

Carpal bones Bone group: Eight short bones of the wrist arranged in proximal and distal rows of four. Together they form an arch whose palmar concavity, roofed by the flexor retinaculum, is the carpal tunnel.
Metacarpals Bone group: Five miniature long bones forming the skeleton of the palm, numbered I (thumb) to V (little finger). Their heads form the knuckles.
Phalanges of the hand Bone group: Fourteen small long bones forming the skeleton of the digits: three (proximal, middle and distal) in each finger and two (proximal and distal) in the thumb.

Upper limb — carpal bones

Scaphoid Bone: The boat-shaped, largest bone of the proximal carpal row on the thumb side, which bridges both carpal rows. It is the most commonly fractured carpal bone and is notorious for non-union and avascular necrosis.
Lunate Bone: A crescent (moon)-shaped bone in the centre of the proximal carpal row that articulates with the radius. It is the most commonly dislocated carpal bone and the site of Kienböck disease.
Triquetrum Bone: A pyramid-shaped bone on the ulnar side of the proximal carpal row, carrying the pisiform on its palmar surface.
Pisiform Bone: A small pea-shaped sesamoid bone in the tendon of flexor carpi ulnaris, lying on the palmar surface of the triquetrum. It is the last carpal bone to ossify.
Trapezium Bone: The lateral bone of the distal carpal row, at the base of the thumb, whose saddle-shaped distal surface forms the highly mobile carpometacarpal joint of the thumb.
Trapezoid Bone: The smallest bone of the distal carpal row, wedge-shaped, lying between the trapezium and capitate and articulating with the base of the 2nd metacarpal.
Capitate Bone: The largest carpal bone, in the centre of the distal row, whose rounded head articulates with the scaphoid and lunate. It is the first carpal to ossify.
Hamate Bone: A wedge-shaped bone on the ulnar side of the distal carpal row with a prominent hook (hamulus) projecting into the palm, which bounds both the carpal tunnel and Guyon’s canal.

Lower limb — pelvic girdle

Bony pelvis Bone group: The basin-shaped ring formed by the two hip bones, the sacrum and the coccyx. It transfers body weight to the lower limbs, protects the pelvic viscera and, in females, forms the bony birth canal — the source of the classic sex differences.
Hip bone Bone: A large, irregular bone formed by fusion of the ilium, ischium and pubis at the acetabulum. The two hip bones form the pelvic girdle, articulating with the sacrum posteriorly, each other anteriorly and the femur laterally.
Ilium Bone: The largest, uppermost part of the hip bone, consisting of a body (contributing to the acetabulum) and a broad wing (ala) whose upper margin is the iliac crest.
Ischium Bone: The posteroinferior part of the hip bone, comprising a body and a ramus. Its ischial tuberosity bears the body’s weight when sitting.
Pubis Bone: The anteroinferior part of the hip bone, consisting of a body and superior and inferior rami. The two pubic bones meet at the pubic symphysis.

Lower limb — thigh & knee

Femur Bone: The longest, strongest and heaviest bone of the body (≈26% of stature), forming the skeleton of the thigh between the hip and the knee.
Patella Bone: The largest sesamoid bone in the body, a flat triangular bone in the tendon of quadriceps femoris in front of the knee. It increases the leverage of the quadriceps and protects the knee joint.

Lower limb — leg

Tibia Bone: The large, medial, weight-bearing bone of the leg (the second longest bone), articulating with the femur at the knee and the talus at the ankle. Its anteromedial surface is subcutaneous along its whole length.
Fibula Bone: The slender lateral bone of the leg. It bears little weight but forms the lateral malleolus of the ankle mortise and gives attachment to many muscles.

Lower limb — foot

Tarsal bones Bone group: Seven short bones forming the posterior half of the foot: talus, calcaneus, navicular, cuboid and three cuneiforms. They transmit body weight from the leg to the ground and form the posterior arches of the foot.
Metatarsals Bone group: Five long bones of the forefoot, numbered I (hallux) to V, connecting the tarsus to the toes. Their heads form the weight-bearing "ball" of the foot.
Phalanges of the foot Bone group: Fourteen short long bones forming the toes: two in the great toe (hallux) and three in each of the lesser toes.
Arches of the foot Region: The tarsal and metatarsal bones are arranged in longitudinal and transverse arches, maintained by bone shape, ligaments, the plantar aponeurosis and muscle tendons, that make the foot both a shock-absorbing spring and a rigid lever.

Lower limb — tarsal bones

Talus Bone: The second largest tarsal bone, which transmits the whole weight of the body from the tibia to the foot. It has no muscle attachments and is about 60% covered by articular cartilage.
Calcaneus Bone: The largest and strongest tarsal bone, forming the heel. It transmits body weight from the talus to the ground and is the lever for the calf muscles via the calcaneal (Achilles) tendon.
Navicular Bone: A boat-shaped tarsal bone on the medial side of the foot, between the head of the talus and the three cuneiforms. Its tuberosity is the main insertion of tibialis posterior.
Cuboid Bone: A cube-shaped tarsal bone on the lateral side of the foot, between the calcaneus and the 4th and 5th metatarsals. It is the keystone of the lateral longitudinal arch.
Cuneiform bones Bone: Three wedge-shaped tarsal bones — medial, intermediate and lateral — between the navicular and the bases of the first three metatarsals. Their wedge shape forms the transverse arch of the foot.

Joints — classification

Fibrous joints Joint: Joints in which bones are united by dense fibrous connective tissue, with no joint cavity. They allow little or no movement and comprise sutures, syndesmoses and gomphoses.
Cartilaginous joints Joint: Joints in which bones are united by cartilage, without a joint cavity. Primary joints (synchondroses) use hyaline cartilage; secondary joints (symphyses) use fibrocartilage and lie in the midline.
Synovial joints Joint: The freely movable joints of the body (diarthroses), characterised by a joint cavity containing synovial fluid, articular cartilage and a fibrous capsule lined by synovial membrane. They are classified into six types by the shape of their articular surfaces.

Joints — synovial components

Joint capsule Membrane: The sleeve-like envelope that encloses a synovial joint, consisting of an outer fibrous layer of dense connective tissue and an inner synovial membrane.
Synovial membrane Membrane: A thin, vascular connective-tissue membrane lining the inner surface of the joint capsule and all non-articular surfaces inside a synovial joint. It secretes and resorbs synovial fluid.
Synovial fluid Fluid: A small volume of clear, viscous fluid in synovial joint cavities that lubricates the articular surfaces and nourishes the avascular articular cartilage.
Bursa Membrane: A flattened, closed sac lined by synovial membrane and containing a capillary film of synovial fluid, located where tendons, muscles or skin glide over bone or other structures.

Joints — axial skeleton

Temporomandibular joint Joint: The synovial joint between the head of the mandible and the mandibular fossa and articular tubercle of the temporal bone, divided by an articular disc into upper (gliding) and lower (hinge) compartments.
Atlanto-occipital joint Joint: A pair of condyloid synovial joints between the occipital condyles and the superior articular facets of the atlas, permitting nodding (flexion–extension) of the head.
Atlanto-axial joint Joint: Three synovial joints between the atlas and axis — a median pivot joint around the dens and two lateral plane joints — that allow rotation of the head ("no" movement).
Intervertebral joints Joint: Adjacent vertebrae are joined by a secondary cartilaginous joint between their bodies (the intervertebral disc) and by paired plane synovial zygapophysial (facet) joints between their articular processes, reinforced by a series of ligaments.
Costovertebral joints Joint: Plane synovial joints linking each rib to the thoracic vertebral column: the joint of the head of the rib (with the vertebral bodies and disc) and the costotransverse joint (rib tubercle with the transverse process).

Joints — upper limb

Sternoclavicular joint Joint: A saddle-type synovial joint, divided by an articular disc, between the medial end of the clavicle and the manubrium and first costal cartilage. It is the only joint connecting the upper limb to the axial skeleton.
Acromioclavicular joint Joint: A plane synovial joint between the lateral end of the clavicle and the medial margin of the acromion. Its stability depends chiefly on the coracoclavicular ligament.
Shoulder joint Joint: The ball-and-socket synovial joint between the head of the humerus and the shallow glenoid cavity of the scapula — the most mobile and the most frequently dislocated major joint in the body.
Elbow joint Joint: A compound hinge-type synovial joint formed by the humero-ulnar and humeroradial articulations, sharing a capsule with the proximal radioulnar joint. It permits flexion and extension of the forearm.
Radioulnar joints Joint: The radius and ulna are united by proximal and distal pivot-type synovial joints and by the interosseous membrane (a syndesmosis), which together permit pronation and supination of the forearm.
Wrist joint Joint: The condyloid (ellipsoid) synovial joint between the distal radius and articular disc above and the scaphoid, lunate and triquetrum below. With the midcarpal joint it allows flexion, extension, abduction, adduction and circumduction of the hand.
Joints of the hand Joint: The carpometacarpal, metacarpophalangeal and interphalangeal joints of the hand — including the saddle joint of the thumb — which together give the hand its range of grips and fine movements.

Joints — pelvis & lower limb

Sacroiliac joint Joint: The strong, weight-bearing joint between the auricular surfaces of the sacrum and ilium. It is a compound joint — synovial anteriorly and a syndesmosis posteriorly — permitting only slight movement.
Pubic symphysis Joint: A secondary cartilaginous joint (symphysis) in the anterior midline uniting the bodies of the two pubic bones through a fibrocartilaginous interpubic disc.
Hip joint Joint: The ball-and-socket synovial joint between the head of the femur and the acetabulum of the hip bone. It is built for stability and weight bearing, with a deep socket and the strongest ligaments in the body.
Knee joint Joint: The largest and most complex joint of the body — a modified (bicondylar) hinge synovial joint between the femoral condyles, the tibial plateau and the patella. Its stability depends on menisci, cruciate and collateral ligaments and the quadriceps.
Ankle joint Joint: The hinge-type synovial joint between the trochlea of the talus and the mortise formed by the distal tibia and the medial and lateral malleoli. It permits dorsiflexion and plantarflexion of the foot.
Subtalar joint Joint: The synovial joint between the talus and the calcaneus where, together with the transverse tarsal joint, inversion and eversion of the foot take place.

Joints — knee ligaments & menisci

Menisci of the knee Cartilage: Two crescent-shaped wedges of fibrocartilage on the tibial plateau — the C-shaped medial and the more circular lateral meniscus — that deepen the articular surfaces and transmit and distribute load across the knee.
Anterior cruciate ligament Ligament: An intracapsular, extrasynovial ligament running from the anterior intercondylar area of the tibia upward, backward and laterally to the medial side of the lateral femoral condyle. It prevents anterior displacement of the tibia on the femur and is the most commonly completely torn knee ligament.
Posterior cruciate ligament Ligament: The stronger of the two cruciate ligaments, running from the posterior intercondylar area of the tibia upward, forward and medially to the lateral side of the medial femoral condyle. It prevents posterior displacement of the tibia on the femur.
Tibial collateral ligament Ligament: A broad, flat band on the medial side of the knee from the medial femoral epicondyle to the medial tibia. It resists valgus and external rotation forces and is firmly attached to the medial meniscus.
Fibular collateral ligament Ligament: A strong, rounded cord on the lateral side of the knee from the lateral femoral epicondyle to the head of the fibula. It resists varus stress and, unlike the tibial collateral ligament, is separate from the meniscus.

Mnemonics

Carpal bones (lateral → medial; proximal row then distal row): "Some Lovers Try Positions That They Can't Handle" — Scaphoid, Lunate, Triquetrum, Pisiform | Trapezium, Trapezoid, Capitate, Hamate. "Trapezium by the thumb".
Tarsal bones: "Tiger Cubs Need MILC" — Talus, Calcaneus, Navicular, Medial, Intermediate and Lateral cuneiforms, Cuboid.
Cranial bones (8): "PEST OF 6" — Parietal (2), Ethmoid, Sphenoid, Temporal (2), Occipital, Frontal; the "6" reminds you there are six types.
Facial bones (14): "Virgil Can Not Make My Pet Zebra Laugh" — Vomer, (inferior nasal) Conchae, Nasal, Maxillae, Mandible, Palatine, Zygomatic, Lacrimal.
Vertebrae by region: "Breakfast at 7, lunch at 12, dinner at 5" — 7 cervical, 12 thoracic, 5 lumbar (then 5 fused sacral and ≈4 coccygeal).
Contents of the foramen ovale: "OVALE" — Otic ganglion (lies just below), V3 (mandibular nerve), Accessory meningeal artery, Lesser petrosal nerve, Emissary vein.
Salter–Harris growth-plate fractures: "SALTR" — I Straight across (Slipped), II Above (metaphysis — most common), III Lower (epiphysis), IV Through (metaphysis + plate + epiphysis), V Rammed (crush, worst prognosis).
Nerves at risk with humeral fractures (proximal → distal): "ARM": Axillary — surgical neck; Radial — mid-shaft (radial groove); Median — supracondylar. Ulnar — medial epicondyle.

From the Corpus anatomy atlas — for education only.