Skeletal System Anatomy and Physiology

Bone is the body's framework, its mineral bank, and its blood cell factory. Know the names, the landmarks, and the joints, because fracture care, traction, IM injection sites, and obstetric pelvic measurements all depend on this anatomy.

Functions of the Skeletal System

Beyond giving the body shape, bones do several jobs.

1. Support. Bones are the steel girders and reinforced concrete of the body, the internal framework that holds us up and cradles soft organs. The leg bones act as pillars under the trunk; the rib cage supports the thoracic wall.
2. Protection. Bones shield soft organs. The fused skull encloses the brain, the vertebrae surround the spinal cord, and the rib cage protects the thoracic organs.
3. Movement. Skeletal muscles, attached to bones by tendons, use bones as levers to move the body.
4. Storage. Fat is stored in internal bone cavities, and bone stores minerals, chiefly calcium and phosphorus. Most of the body's calcium sits in bone as calcium salts, a ready reserve of calcium ions for the blood.
5. Blood cell formation. Hematopoiesis occurs in the marrow cavities of certain bones.

Anatomy of the Skeletal System

The skeleton has two divisions: the axial skeleton (the bones forming the longitudinal axis of the body) and the appendicular skeleton (the bones of the limbs and girdles).

Classification of Bones

The adult skeleton has 206 bones. There are two types of osseous tissue, compact bone and spongy bone, and bones fall into four shape groups: long, short, flat, and irregular.

• Compact bone. Dense, smooth, and homogeneous.
• Spongy bone. Long, needle-like pieces of bone with lots of open space.
• Long bones. Longer than they are wide, with a shaft and a head at each end, mostly compact bone.
• Short bones. Generally cube-shaped and mostly spongy bone. Sesamoid bones, which form within tendons, are a special type of short bone.
• Flat bones. Thin, flattened, usually curved, with two layers of compact bone sandwiching a layer of spongy bone.
• Irregular bones. Bones that fit none of the other categories.

Long Bone

A long bone is described by both gross and microscopic anatomy.

Gross Anatomy

• Diaphysis. The shaft. Makes up most of the bone's length, is compact bone, and is covered by a fibrous connective tissue membrane, the periosteum.
• Sharpey's fibers. Hundreds of connective tissue (perforating or Sharpey's) fibers that secure the periosteum to the underlying bone.
• Epiphyses. The ends of the long bone. Each is a thin layer of compact bone enclosing spongy bone.
• Articular cartilage. Glassy hyaline cartilage covering the external surface in place of periosteum, giving a smooth, slippery, low-friction joint surface.
• Epiphyseal line. In adult bone, a thin line of bony tissue spanning the epiphysis.
• Epiphyseal plate. The epiphyseal line is a remnant of the epiphyseal plate, a flat plate of hyaline cartilage in young, growing bone that drives lengthwise growth. By the end of puberty, when hormones stop long-bone growth, the plates are fully replaced by bone, leaving only the epiphyseal lines.
• Yellow marrow. In adults, the shaft cavity (the medullary cavity) stores adipose (fat) tissue as yellow marrow.
• Red marrow. In infants, this area makes blood cells. In adults, red marrow is confined to the spongy bone of flat bones and the epiphyses of some long bones.
• Bone markings. Bone surfaces are scarred with bumps, holes, and ridges that mark where muscles, tendons, and ligaments attached and where vessels and nerves passed.
• Categories of bone markings. Two kinds: projections (processes) that grow out from the surface, and depressions (cavities) that indent it. Memory trick: terms beginning with T are projections, those beginning with F (except facet) are depressions.

Microscopic Anatomy

To the naked eye, spongy bone looks spiky and open while compact bone looks very dense.

• Osteocytes. Mature bone cells, found in tiny cavities called lacunae.
• Lamellae. Lacunae arranged in concentric circles around central (Haversian) canals.
• Osteon (Haversian system). Each unit of a central canal plus its matrix rings.
• Canaliculi. Tiny canals radiating from the central canals to all lacunae, forming a transport system that connects every bone cell to its nutrient supply through the hard matrix.
• Perforating (Volkmann's) canals. Complete the pathway from the bone surface to the central canals, running into compact bone at right angles to the shaft.

Axial Skeleton

The axial skeleton forms the longitudinal axis of the body and has three parts: the skull, the vertebral column, and the bony thorax.

Skull

The skull is built from two sets of bones: the cranium and the facial bones.

Cranium

The cranium encloses and protects the brain and is built from eight large flat bones.

• Frontal bone. Forms the forehead, the brow ridges, and the superior part of each orbit.
• Parietal bones. Paired bones forming most of the superior and lateral cranial walls. They meet at the midline sagittal suture and form the coronal suture where they meet the frontal bone.
• Temporal bones. Lie inferior to the parietal bones, joining them at the squamous sutures.

Bone markings on the temporal bone:

1. External acoustic meatus. A canal leading to the eardrum and middle ear, the route sound takes into the ear.
2. Styloid process. A sharp, needlelike projection just inferior to the external acoustic meatus.
3. Zygomatic process. A thin bridge of bone joining the cheekbone (zygomatic bone) anteriorly.
4. Mastoid process. A rough projection full of air cavities (mastoid sinuses), posterior and inferior to the external acoustic meatus, providing an attachment site for some neck muscles.
5. Jugular foramen. At the junction of the occipital and temporal bones, it passes the jugular vein, the largest vein of the head, which drains the brain. Just anterior to it, the internal acoustic meatus transmits cranial nerves VII and VIII.

• Occipital bone. Joins the parietal bones at the lambdoid suture. Its base holds the foramen magnum, the large opening that lets the spinal cord connect with the brain.
• Sphenoid bone. The butterfly-shaped bone spanning the skull's width and forming part of the cranial floor. Its midline depression, the sella turcica (Turk's saddle), cradles the pituitary gland.
• Foramen ovale. A large oval opening in line with the posterior sella turcica, passing fibers of cranial nerve V to the chewing muscles of the lower jaw.
• Optic canal. Passes the optic nerve to the eye.
• Superior orbital fissure. A slitlike opening passing the cranial nerves that control eye movement.
• Sphenoid sinuses. Air cavities riddling the central sphenoid.
• Ethmoid bone. Irregularly shaped, anterior to the sphenoid. Forms the roof of the nasal cavity and part of the medial orbital walls.
• Crista galli. A projection from its superior surface; the outermost brain covering attaches here.
• Cribriform plates. Holey areas that pass nerve fibers from the olfactory receptors of the nose to the brain.
• Superior and middle nasal conchae. Ethmoid extensions that form part of the lateral nasal walls and increase turbulence of air flowing through the nasal passages.

Facial Bones

Fourteen bones make up the face. Twelve are paired; only the mandible and vomer are single.

• Maxillae. The two maxillae (maxillary bones) fuse to form the upper jaw. All facial bones except the mandible join the maxillae, making them the keystone bones of the face. They carry the upper teeth in the alveolar margin.
• Palatine bones. Paired bones posterior to the palatine processes of the maxillae, forming the posterior hard palate.
• Zygomatic bones. The cheekbones, also forming much of the lateral orbital walls.
• Lacrimal bones. Finger-sized bones forming part of each medial orbital wall, each grooved as a tear passageway.
• Nasal bones. Small rectangular bones forming the bridge of the nose.
• Vomer bone. The single midline bone of the nasal cavity, forming most of the bony nasal septum.
• Inferior nasal conchae. Thin, curved bones projecting medially from the lateral nasal walls.
• Mandible. The lower jaw, the largest and strongest facial bone. It joins the temporal bones, forming the only freely movable joints in the skull. Its horizontal part (the body) forms the chin; two upright bars (the rami) connect it to the temporal bones.

The Hyoid Bone

Though not part of the skull, the hyoid bone is closely tied to the mandible and temporal bones.

• Location. Suspended in the midneck about 2 cm (1 inch) above the larynx, anchored by ligaments to the styloid processes of the temporal bones.
• Parts. Horseshoe-shaped with a body and two pairs of horns (cornua), it is a movable base for the tongue and an attachment point for neck muscles that raise and lower the larynx during swallowing and speech.

Fetal Skull

The skull of a fetus or newborn differs from the adult skull in several ways.

• Size. The adult skull is about one-eighth of total body length; the newborn skull is one-fourth of its body length.
• Fontanels. The newborn skull has fibrous regions not yet converted to bone. These fibrous membranes connecting the cranial bones are the fontanels.
• Anterior fontanel. The largest, diamond-shaped, allowing the fetal skull to compress slightly during birth.

Vertebral Column (Spine)

The vertebral column is the axial support of the body, running from the skull it supports to the pelvis, where it transmits body weight to the lower limbs.

• Composition. Formed from 26 irregular bones connected and reinforced by ligaments into a flexible, curved structure.
• Spinal cord. Runs through the central cavity, surrounded and protected by the column.
• Vertebrae. Before birth the spine has 33 separate bones (vertebrae), but 9 fuse into two composite bones, the sacrum and the coccyx, at the inferior column.
• Cervical vertebrae. Of the 24 single bones, the 7 in the neck are cervical.
• Thoracic vertebrae. The next 12 are thoracic.
• Lumbar vertebrae. The remaining 5, supporting the lower back, are lumbar.
• Intervertebral discs. Pads of flexible fibrocartilage that cushion the vertebrae, absorb shock, and allow spinal flexibility.
• Primary curvatures. The thoracic and sacral curves, present at birth.
• Secondary curvatures. The cervical and lumbar curves, which develop after birth.
• Body (centrum). The disc-like, weight-bearing part of the vertebra, facing anteriorly.
• Vertebral arch. Formed by the joined posterior extensions (the laminae and pedicles) of the vertebral body.
• Vertebral foramen. The canal the spinal cord passes through.
• Transverse processes. Two lateral projections from the vertebral arch.
• Spinous process. A single projection from the posterior vertebral arch (the fused laminae).
• Superior and inferior articular processes. Paired projections lateral to the vertebral foramen that let a vertebra form joints with its neighbors.

Cervical Vertebrae

The seven cervical vertebrae (C1 to C7) form the neck region.

• Atlas (C1). Has no body. The superior surfaces of its transverse processes hold large depressions that receive the occipital condyles of the skull.
• Axis (C2). Acts as the pivot for rotation of the atlas and skull. Its large upright process, the dens, is the pivot point.
• Foramina. The transverse processes of the cervical vertebrae hold foramina that pass the vertebral arteries up to the brain.

Thoracic Vertebrae

The twelve thoracic vertebrae (T1 to T12) are all typical.

• Size. Larger than cervical vertebrae and the only vertebrae that articulate with the ribs.
• Shape. The body is heart-shaped with two costal facets on each side that receive the rib heads.
• Transverse processes. Each articulates with the knoblike tubercles of the nearby ribs.
• Spinous process. Long and hooked sharply downward, giving a side-view profile like a giraffe's head.

Lumbar Vertebrae

The five lumbar vertebrae (L1 to L5) have massive, blocklike bodies.

• Spinous processes. Short and hatchet-shaped.
• Strength. Because most spinal stress falls on the lumbar region, these are the sturdiest vertebrae.

Sacrum

The sacrum is formed by the fusion of five vertebrae.

• Alae. The winglike alae articulate laterally with the hip bones at the sacroiliac joints.
• Median sacral crest. The roughened posterior midline ridge, formed by the fused spinous processes.
• Posterior sacral foramina. Flank the median sacral crest laterally.
• Sacral canal. The vertebral canal continues through the sacrum and ends in a large inferior opening, the sacral hiatus.

Coccyx

The coccyx is formed by the fusion of three to five tiny, irregularly shaped vertebrae. It is the human tailbone, a remnant of the tail other vertebrates have.

Thoracic Cage

The sternum, ribs, and thoracic vertebrae make up the bony thorax, routinely called the thoracic cage because it forms a protective, cone-shaped cage of slender bones around the thoracic organs.

Sternum

The sternum (breastbone) is a typical flat bone, formed by the fusion of three bones: the manubrium, body, and xiphoid process.

• Landmarks. Three important bony landmarks: the jugular notch, the sternal angle, and the xiphisternal joint.
• Jugular notch. The concave upper border of the manubrium, easily palpated, generally at the level of the third thoracic vertebra.
• Sternal angle. The transverse ridge where the manubrium and body meet at a slight angle, at the level of the second ribs.
• Xiphisternal joint. Where the body and xiphoid process fuse, at the level of the ninth thoracic vertebra.

Ribs

Twelve pairs of ribs form the walls of the bony thorax.

• True ribs. The first seven pairs, attaching directly to the sternum by costal cartilages.
• False ribs. The next five pairs, attaching indirectly to the sternum or not at all.
• Floating ribs. The last two pairs of false ribs, with no sternal attachment.

Appendicular Skeleton

The appendicular skeleton has 126 bones: the limbs plus the pectoral and pelvic girdles that attach the limbs to the axial skeleton.

Bones of the Shoulder Girdle

Each shoulder (pectoral) girdle has two bones: a clavicle and a scapula.

• Clavicle. The collarbone, a slender, doubly curved bone. It attaches to the manubrium medially and the scapula laterally, helping form the shoulder joint, bracing the arm away from the thorax, and helping prevent shoulder dislocation.
• Scapulae. The shoulder blades, triangular bones that flare like wings when we move the arms posteriorly.
• Parts of the scapula. A flattened body and two important processes, the acromion and the coracoid.
• Acromion. The enlarged end of the scapular spine, connecting with the clavicle at the acromioclavicular joint.
• Coracoid. The beaklike process pointing over the top of the shoulder, anchoring some arm muscles. Just medial to it, the suprascapular notch serves as a nerve passageway.
• Borders of the scapula. Three: superior, medial (vertebral), and lateral (axillary).
• Angles of the scapula. Three: superior, inferior, and lateral. The glenoid cavity, a shallow socket that receives the head of the arm bone, sits in the lateral angle.
• Free movement of the shoulder girdle. The girdle attaches to the axial skeleton at only one point, the sternoclavicular joint. The scapula's loose attachment lets it slide against the thorax, and the shallow glenoid cavity leaves the shoulder joint poorly reinforced by ligaments.

Bones of the Upper Limb

Thirty separate bones form each upper limb, building the arm, forearm, and hand.

Arm

The arm is a single bone, the humerus, a typical long bone.

• Anatomical neck. A slight constriction just inferior to the head.
• Tubercles. The greater and lesser tubercles, two bony projections separated by the intertubercular sulcus, are sites of muscle attachment.
• Surgical neck. Just distal to the tubercles, named because it is the most frequently fractured part of the humerus.
• Deltoid tuberosity. A roughened area at the midshaft where the deltoid muscle attaches.
• Radial groove. Runs obliquely down the posterior shaft, marking the course of the radial nerve.
• Trochlea and capitulum. At the distal end, the medial spool-like trochlea and the lateral ball-like capitulum both articulate with the forearm bones.
• Fossae. The anterior coronoid fossa and posterior olecranon fossa, flanked by the medial and lateral epicondyles, let the ulnar processes move freely as the elbow flexes and extends.

Forearm

Two bones, the radius and the ulna, form the forearm.

• Radius. In anatomical position, the radius is the lateral (thumb-side) bone. When the palm faces backward, the distal radius crosses over and ends up medial to the ulna.
• Radioulnar joints. The radius and ulna articulate proximally and distally at small radioulnar joints and are connected along their length by the flexible interosseous membrane.
• Styloid process. Both the ulna and radius have a styloid process at the distal end.
• Radial tuberosity. The disc-shaped radial head joins the capitulum of the humerus. Just below the head, the radial tuberosity anchors the biceps tendon.
• Ulna. In anatomical position, the ulna is the medial (little-finger-side) bone.
• Trochlear notch. Formed by the coronoid and olecranon processes at the proximal ulna. Together these grip the trochlea of the humerus in a pliers-like joint.

Hand

The hand skeleton has carpals, metacarpals, and phalanges.

• Carpal bones. Eight carpal bones in two irregular rows of four form the carpus (wrist), bound by ligaments that restrict movement.
• Metacarpals. Numbered 1 to 5 from the thumb to the little finger. With a clenched fist, the metacarpal heads stand out as the knuckles.
• Phalanges. The finger bones. Each hand has 14 phalanges, three in each finger (proximal, middle, distal) except the thumb, which has two (proximal and distal).

Bones of the Pelvic Girdle

The pelvic girdle is formed by two coxal bones (ossa coxae), the hip bones.

• Pelvic girdle. Large, heavy, and securely attached to the axial skeleton. Bearing weight is its key function, since the whole upper body rests on the bony pelvis.
• Sockets. Deep and heavily reinforced by ligaments, receiving the thigh bones and attaching the limbs firmly to the girdle.
• Bony pelvis. Encloses and protects the reproductive organs, urinary bladder, and part of the large intestine.
• Ilium. A large, flaring bone forming most of the hip bone, connecting posteriorly with the sacrum at the sacroiliac joint. Hands on the hips rest over the alae (winglike portions) of the ilia.
• Iliac crest. The upper edge of an ala, a key landmark for intramuscular injections. It ends anteriorly in the anterior superior iliac spine and posteriorly in the posterior superior iliac spine.
• Ischium. The "sit-down" bone, forming the most inferior part of the coxal bone.
• Ischial tuberosity. A roughened area that bears weight while sitting.
• Ischial spine. Superior to the tuberosity, an important landmark in pregnancy because it narrows the pelvic outlet the baby must pass through during birth.
• Greater sciatic notch. Passes blood vessels and the large sciatic nerve from the pelvis into the thigh.
• Pubis. The pubic bone, the most anterior part of the coxal bone.
• Obturator foramen. An opening that passes blood vessels and nerves into the anterior thigh.
• Pubic symphysis. The cartilaginous joint where the pubic bones fuse anteriorly.
• Acetabulum. The deep socket where the ilium, ischium, and pubis fuse (the name means "vinegar cup"). It receives the head of the thigh bone.
• False pelvis. Superior to the true pelvis, medial to the flaring portions of the ilia.
• True pelvis. Surrounded by bone, inferior to the flaring ilia and the pelvic brim. In women, its dimensions matter because they must be large enough to let the infant's head pass during childbirth.
• Outlet and inlet. The outlet (inferior pelvic opening, measured between the ischial spines) and the inlet (superior opening between the right and left pelvic brim) are critical dimensions, carefully measured by the obstetrician.

Bones of the Lower Limbs

The lower limbs carry the total body weight when upright, so the bones of their three segments (thigh, leg, and foot) are thicker and stronger than the comparable upper-limb bones.

Thigh

The femur is the only thigh bone and the heaviest, strongest bone in the body.

• Parts. Its proximal end has a ball-like head, a neck, and greater and lesser trochanters (separated anteriorly by the intertrochanteric line and posteriorly by the intertrochanteric crest).
• Gluteal tuberosity. On the shaft, a site for muscle attachment along with the trochanters.
• Head. Articulates with the acetabulum in a deep, secure socket.
• Neck. A common fracture site, especially in old age.
• Lateral and medial condyles. At the distal femur, articulating with the tibia and separated posteriorly by the deep intercondylar fossa.
• Patellar surface. The smooth anterior distal surface that joins the patella (kneecap).

Leg

Two bones, the tibia and fibula, form the leg, connected along their length by an interosseous membrane.

• Tibia. The shinbone, larger and more medial. Its proximal medial and lateral condyles articulate with the distal femur to form the knee joint.
• Tibial tuberosity. A roughened area on the anterior tibia where the patellar (kneecap) ligament attaches.
• Medial malleolus. A distal process forming the inner bulge of the ankle.
• Anterior border. The sharp anterior tibial ridge, unprotected by muscle and easily felt under the skin.
• Fibula. Thin and sticklike, lying along the tibia and forming joints with it proximally and distally. It takes no part in the knee joint.
• Lateral malleolus. The distal fibula, forming the outer part of the ankle.

Foot

The foot (tarsals, metatarsals, and phalanges) has two jobs: it supports body weight and works as a lever to propel us forward when we walk and run.

• Tarsus. The posterior half of the foot, made of seven tarsal bones.
• Calcaneus and talus. The two largest tarsals carry most of the weight: the calcaneus (heel bone) and the talus (ankle), which sits between the tibia and the calcaneus.
• Metatarsals. Five metatarsals form the sole.
• Phalanges. 14 phalanges form the toes, three in each toe except the great toe, which has two.
• Arches. Three strong arches: two longitudinal (medial and lateral) and one transverse.

Joints

Joints (articulations) do two things: they hold bones together securely and give the rigid skeleton mobility.

• Classification. Joints are classified functionally and structurally.
• Functional classification. Based on how much movement the joint allows.
• Types of functional joints. Synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).
• Diarthroses. Freely movable joints predominate in the limbs, where mobility matters.
• Synarthroses and amphiarthroses. Immovable and slightly movable joints are mostly in the axial skeleton, where firm attachment and organ protection matter.
• Structural classification. Fibrous, cartilaginous, and synovial joints, based on whether fibrous tissue, cartilage, or a joint cavity separates the bones.

Fibrous Joints

In fibrous joints, bones are united by fibrous tissue.

• Examples. The skull sutures are the best example. The irregular bone edges interlock and are bound tightly by connective tissue fibers, allowing essentially no movement.
• Syndesmoses. Here the connecting fibers are longer than in sutures, so the joint has more give. The joint connecting the distal tibia and fibula is a syndesmosis.

Cartilaginous Joints

In cartilaginous joints, bone ends are connected by cartilage.

• Slightly movable examples. The pubic symphysis of the pelvis and the intervertebral joints of the spine, where bone surfaces are connected by fibrocartilage discs.
• Immovable examples. The hyaline cartilage epiphyseal plates of growing long bones and the cartilaginous joints between the first ribs and the sternum.

Synovial Joints

In synovial joints, the articulating bone ends are separated by a joint cavity holding synovial fluid. They account for all joints of the limbs.

• Articular cartilage. Covers the ends of the articulating bones.
• Fibrous articular capsule. A sleeve of fibrous connective tissue enclosing the joint, lined by a smooth synovial membrane (the source of the name).
• Joint cavity. Enclosed by the capsule and filled with lubricating synovial fluid.
• Reinforcing ligaments. Usually strengthen the fibrous capsule.
• Bursae. Flattened fibrous sacs lined with synovial membrane and holding a thin film of synovial fluid, common where ligaments, muscles, skin, tendons, or bones rub together.
• Tendon sheath. An elongated bursa that wraps completely around a tendon subjected to friction, like a bun around a hotdog.

Types of Synovial Joints by Shape

The shape of the articulating surfaces sets the movements allowed. Synovial joints are classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket.

• Plane joint. Flat articular surfaces allowing only short gliding movements. These are nonaxial (gliding without rotation around an axis). The intercarpal joints of the wrist are the best example.
• Hinge joint. The cylindrical end of one bone fits a trough on another, allowing angular movement in one plane like a mechanical hinge. These uniaxial joints include the elbow, the ankle, and the joints between finger phalanges.
• Pivot joint. The rounded end of one bone fits a ring of bone, rotating only around its long axis, so these are also uniaxial. Examples are the proximal radioulnar joint and the joint between the atlas and the dens of the axis.
• Condyloid joint. An egg-shaped surface fits an oval concavity, allowing the bone to move side to side and back and forth but not rotate around its long axis. Movement occurs around two axes, so these are biaxial.
• Saddle joints. Each surface has both convex and concave areas, like a saddle. These biaxial joints allow about the same movements as condyloid joints. The carpometacarpal joints of the thumb are the best example.
• Ball-and-socket joint. A spherical head fits a round socket, allowing movement in all axes, including rotation. These multiaxial joints are the most freely moving synovial joints. The shoulder and hip are examples.