The skeleton, labeled: sternum, ribs, skull, hip and limb bones

Axial skeleton

The vertical, central axis of the body: the bones of the head, neck, chest and back. In an adult it has 80 bones: the skull, the vertebral column (including the sacrum and coccyx) and the thoracic cage (the ribs and sternum).

Appendicular skeleton

The bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton. In an adult it has 126 bones.

What makes up the axial skeleton

• Skull

  Axial skeleton

  Formed by 22 bones: 8 in the brain case and 14 facial bones. The hyoid bone and the three ear ossicles in each middle ear are associated with the head but are counted separately.

• Vertebral column

  Axial skeleton

  24 vertebrae, plus the sacrum and the coccyx.

• Thoracic cage

  Axial skeleton

  12 pairs of ribs and the sternum.

An adult skeleton has 206 bones. The skeleton supports the body, gives muscles something to act on, protects organs such as the brain, spinal cord, heart and lungs, and stores minerals such as calcium and phosphate. The marrow inside bones stores fat and houses the tissue that makes blood cells.

Brain case

• Frontal bone

  Unpaired

  Joins the right and left parietal bones at the coronal suture. It forms most of the floor of the anterior cranial fossa.

• Parietal bones

  Paired

  Form most of the upper lateral side of the skull. The right and left bones join at the top of the skull.

• Temporal bones

  Paired

  Form the lower lateral side of the skull. Landmarks include the mastoid process (the bump behind the earlobe), the styloid process and the mandibular fossa.

• Occipital bone

  Unpaired

  The single bone of the posterior skull and the posterior base of the cranial cavity. The foramen magnum, the opening for the spinal cord, is in it.

• Sphenoid bone

  Unpaired

  A single, complex bone of the central skull, called a keystone bone because it joins almost every other skull bone. The sella turcica is part of it.

• Ethmoid bone

  Unpaired

  Lies at the midline within the central skull. Its crista galli and cribriform plates sit in the floor of the anterior cranial fossa, and it contributes to the nasal septum and the wall of the orbit.

The brain case has eight bones: the paired parietal and temporal bones plus the unpaired frontal, occipital, sphenoid and ethmoid bones.

Facial bones

• Maxilla

  Paired

  Forms the upper jaw. The maxillary sinuses, the largest paranasal sinuses, lie within the maxillary bones below the orbits.

• Palatine

  Paired

  One of the six paired facial bones.

• Zygomatic

  Paired

  The cheekbone. Its temporal process and the zygomatic process of the temporal bone form the zygomatic arch.

• Nasal

  Paired

  One of the six paired facial bones.

• Lacrimal

  Paired

  One of the six paired facial bones.

• Inferior nasal concha

  Paired

  One of the six paired facial bones.

• Vomer

  Unpaired

  One of the two unpaired facial bones.

• Mandible

  Unpaired

  The lower jaw and the only moveable bone of the skull. Its condyle joins the temporal bone at the temporomandibular joint.

The facial bones are 14 bones: six paired (maxilla, palatine, zygomatic, nasal, lacrimal and inferior nasal concha) and two unpaired (vomer and mandible). The facial bones form the upper and lower jaws, the nose and nasal cavity and the orbits.

Skull sutures

• Coronal suture

  Top of the skull

  Runs side to side and joins the frontal bone to the right and left parietal bones.

• Sagittal suture

  Top of the skull

  Runs along the midline from the coronal suture and unites the right and left parietal bones. At the back it joins the lambdoid suture.

• Lambdoid suture

  Back of the skull

  Joins the occipital bone to the right and left parietal and temporal bones. It is named for its upside-down V shape, like the Greek letter lambda.

• Squamous suture

  Side of the skull

  Unites the squamous portion of the temporal bone with the parietal bone.

• Pterion

  Side of the skull

  Where four bones meet: the frontal bone, the parietal bone, the squamous part of the temporal bone and the greater wing of the sphenoid. The textbook calls it the weakest part of the skull.

A suture is an immobile joint between adjacent skull bones, with the narrow gap between them filled by dense, fibrous connective tissue. The twisting paths of the sutures between brain-case bones lock the bones together tightly.

Other skull landmarks

• Hyoid bone

  Upper neck

  A small U-shaped bone that does not join any other bone, so it is not part of the skull. It is the base for the tongue and is attached to the larynx and the pharynx.

• Paranasal sinuses

  Inside skull bones

  Air-filled spaces named for the bone they sit in: frontal, maxillary, sphenoid and the ethmoid air cells. They lighten the skull and add resonance to the voice.

• Foramen magnum

  Occipital bone

  The “great aperture” in the posterior cranial fossa through which the spinal cord passes.

Regions of the vertebral column

• Cervical vertebrae

  C1 to C7, the neck

  Small bodies, because they carry the least body weight. Cervical vertebrae usually have a bifid (Y-shaped) spinous process.

• Thoracic vertebrae

  T1 to T12, the chest

  Have facets for rib attachment, and a long spinous process that angles downward.

• Lumbar vertebrae

  L1 to L5, the lower back

  Carry the greatest amount of body weight, so they have a large, thick body and a short, blunt spinous process.

• Sacrum

  Fused

  A triangular bone formed by the fusion of five sacral vertebrae. It joins the ilium of each hip bone at the sacroiliac joints.

• Coccyx

  Fused

  The tailbone, formed by the fusion of four very small coccygeal vertebrae. It is not weight bearing when standing.

Atlas and axis

• Atlas (C1)

  Cervical

  Supports the skull. It has no body or spinous process: it is a ring made of an anterior and a posterior arch.

• Axis (C2)

  Cervical

  The axis for rotation when you turn the head right or left. It is recognized by the dens (odontoid process), which projects upward from the body and joins the anterior arch of the atlas.

The four curves

Thoracic curve

A primary curve, kept from the single curve of the fetal spine.

Sacrococcygeal curve

A primary curve, formed by the sacrum and coccyx.

Cervical curve

A secondary curve that develops after birth as an infant begins to hold the head upright when sitting.

Lumbar curve

A secondary curve that develops as a child begins to stand and walk.

Parts of a typical vertebra

Body

The anterior part of the vertebra, which supports body weight.

Vertebral arch

The posterior part, made of the right and left pedicles and laminae.

Vertebral foramen

The large opening between the arch and the body, which holds the spinal cord.

Spinous process

The single process that projects posteriorly at the midline of the back.

Transverse processes

A pair of processes that project laterally. A typical vertebra has seven processes in all.

Intervertebral disc

Separates and unites the bodies of adjacent vertebrae. It has a fibrous outer layer, the anulus fibrosus, and a gel-like center, the nucleus pulposus.

Sternum anatomy

• Manubrium

  Sternum, upper part

  The wider, superior portion. Its top border has the U-shaped jugular (suprasternal) notch, and the clavicular notch on each side is where the clavicle joins at the sternoclavicular joint. The first ribs attach here.

• Sternal angle

  Where manubrium meets body

  The junction between the manubrium and body is not flat but forms a slight bend. The second rib attaches here, which makes it a landmark for counting ribs.

• Body of the sternum

  Sternum, central part

  The elongated central portion. Ribs 3 to 7 attach to it.

• Xiphoid process

  Sternum, lower tip

  The small inferior tip. It is cartilaginous early in life and gradually ossifies starting in middle age.

Parts of a rib

Head of the rib

The posterior end, which articulates with the costal facet on the body of the vertebra.

Neck of the rib

The narrowed region lateral to the head.

Tubercle of the rib

A small bump on the posterior surface that articulates with the transverse process of the vertebra.

Angle of the rib

The point of greatest curvature, just lateral to the tubercle.

Costal groove

A shallow groove along the inferior margin of each rib, for the passage of blood vessels and a nerve.

Costal cartilage

The hyaline cartilage in which each rib ends toward the front. Most ribs attach to the sternum through it.

Rib cage anatomy: true, false and floating ribs

True ribs, 1 to 7

Also called vertebrosternal ribs. The costal cartilage of each attaches directly to the sternum.

False ribs, 8 to 12

Also called vertebrochondral ribs. Their cartilages do not attach directly to the sternum. For ribs 8 to 10 the cartilage attaches to the cartilage of the next higher rib.

Floating ribs, 11 and 12

The last two false ribs. They are short and do not attach to the sternum at all; their small cartilages end in the muscles of the lateral abdominal wall.

The thoracic cage is the 12 pairs of ribs with their costal cartilages, and the sternum. The ribs are anchored at the back to the 12 thoracic vertebrae (T1 to T12), and the cage protects the heart and lungs.

Pectoral girdle

• Clavicle

  Pectoral girdle

  The collarbone, the only long bone that lies horizontally. Its sternal end joins the manubrium (the only bony connection between the pectoral girdle and the axial skeleton) and its acromial end joins the acromion of the scapula.

• Scapula

  Pectoral girdle

  The shoulder blade, a flat triangular bone on the posterior shoulder. It does not articulate with the ribs. The glenoid cavity joins the humerus to form the shoulder joint.

• Scapular spine and acromion

  Scapula

  The spine is a long ridge across the posterior surface; it extends laterally as the acromion, the bony tip of the shoulder.

• Coracoid process

  Scapula

  A hook-like projection (coracoid = “crow’s beak”) that points anteriorly, below the lateral end of the clavicle.

Arm, forearm and hand

• Humerus

  Arm

  The single bone of the upper arm. The head joins the glenoid cavity of the scapula. The greater and lesser tubercles are separated by the intertubercular groove, and the deltoid tuberosity is on the shaft.

• Distal humerus

  Arm

  The trochlea joins the ulna and the capitulum joins the radius at the elbow. The medial epicondyle is larger than the lateral one.

• Ulna

  Forearm, medial

  The medial bone of the forearm. Its olecranon process is the bony tip of the elbow, and the trochlear notch fits around the trochlea of the humerus.

• Radius

  Forearm, lateral

  The bone on the thumb side of the forearm. Its disc-shaped head joins the capitulum of the humerus.

• Carpal bones

  Wrist

  Eight small bones in two rows of four, at the base of the hand.

• Metacarpal bones

  Palm

  Five bones numbered 1 to 5 starting at the thumb. They form the knuckles at their distal ends.

• Phalanges

  Fingers and thumb

  14 bones: the thumb has two (proximal and distal) and each of digits 2 to 5 has three (proximal, middle and distal).

Bones of the wrist (carpals)

Proximal row, thumb side to little-finger side

Scaphoid (“boat-shaped”), lunate (“moon-shaped”), triquetrum (“three-cornered”), pisiform (“pea-shaped”).

Distal row, little-finger side to thumb side

Hamate (“hooked bone”), capitate (“head-shaped”), trapezoid, trapezium (“table”).

Memory phrase from the textbook

“So Long To Pinky, Here Comes The Thumb” gives the carpals in that order: scaphoid, lunate, triquetrum, pisiform, then hamate, capitate, trapezoid, trapezium.

The upper limb has 30 bones on each side: the humerus, ulna and radius, eight carpals, five metacarpals and 14 phalanges. Only the scaphoid, lunate and triquetrum take part in the radiocarpal joint; the scaphoid and lunate meet the radius.

Hip bone anatomy

• Hip bone (coxal bone)

  Pelvic girdle

  Each adult hip bone is formed by three bones that fuse in the late teenage years: the ilium, ischium and pubis. Each joins the axial skeleton at the sacrum.

• Ilium

  Hip bone, upper part

  The fan-like superior region and the largest part of the hip bone. Its curved upper margin is the iliac crest. It joins the sacrum at the sacroiliac joint.

• Ischium

  Hip bone, lower back part

  The posteroinferior region. The ischial tuberosity supports the body when sitting.

• Pubis

  Hip bone, front part

  The anterior portion. The two pubic bones meet at the pubic symphysis.

• Acetabulum

  Hip bone

  The deep cup-shaped cavity where the ilium, ischium and pubis converge. It receives the head of the femur at the hip joint.

• Bony pelvis

  Four bones

  The right and left hip bones, the sacrum and the coccyx. It supports the weight of the upper body when sitting and transfers it to the lower limbs when standing.

Thigh, leg and foot

• Femur

  Thigh

  The thigh bone, the longest and strongest bone of the body. Its head joins the acetabulum, and the greater and lesser trochanters are below the neck. The condyles at the distal end join the tibia at the knee.

• Patella

  Knee

  The kneecap, the largest sesamoid bone of the body. It lies in the tendon of the quadriceps femoris and articulates with the patellar surface of the femur.

• Tibia

  Leg, medial

  The shin bone: the larger, weight-bearing bone of the leg and the second longest bone of the body. Its distal end forms the medial malleolus.

• Fibula

  Leg, lateral

  The slender bone on the lateral side of the leg. It does not bear weight. Its distal end forms the lateral malleolus.

• Tarsal bones

  Posterior foot

  Seven bones: talus, calcaneus, cuboid, navicular and the medial, intermediate and lateral cuneiforms.

• Metatarsal bones

  Mid-foot

  Five bones numbered 1 to 5 starting on the medial side. The first is shorter and thicker than the others, and the second is the longest.

• Phalanges

  Toes

  14 bones: the big toe (hallux) has two and each of the other four toes has three.

Tarsal bones

Talus

The most superior tarsal bone. Its upper surface joins the tibia and the fibula to form the ankle joint.

Calcaneus

The heel bone, the largest bone of the foot. Body weight passes from the tibia to the talus to the calcaneus.

Cuboid

Joins the front end of the calcaneus.

Navicular

Joins the front of the talus and, in turn, the three cuneiform bones.

Cuneiforms

Three wedge-shaped bones: medial, intermediate and lateral.

The lower limb has 30 bones on each side: the femur, patella, tibia, fibula, seven tarsals, five metatarsals and 14 phalanges.

Five bone shapes

• Long bones

  Longer than wide

  Examples: femur, tibia, fibula, humerus, ulna, radius, metacarpals, metatarsals and phalanges. They work as levers.

• Short bones

  Cube-like

  Only the carpals of the wrists and the tarsals of the ankles. They give stability and support with some limited motion.

• Flat bones

  Thin and often curved

  Cranial bones, scapulae, sternum and ribs. They are attachment points for muscles and often protect internal organs.

• Irregular bones

  Complex shapes

  Vertebrae and many facial bones.

• Sesamoid bones

  Small and round, in tendons

  They protect tendons from compressive forces. The patellae are the only sesamoid bones found in everyone.

Parts of a long bone

Diaphysis

The tubular shaft between the two ends. Its wall is dense compact bone.

Medullary cavity

The hollow region inside the diaphysis, filled with yellow marrow.

Epiphysis

The wider section at each end, filled with spongy bone. Red marrow fills the spaces in the spongy bone.

Metaphysis

The narrow area where each epiphysis meets the diaphysis. It contains the epiphyseal plate (growth plate).

Epiphyseal plate

A layer of hyaline cartilage in a growing bone. When growth in length stops, in early adulthood (about 18 to 21 years), bone replaces the cartilage and the plate becomes the epiphyseal line.

Periosteum

The fibrous membrane covering the outer surface of the bone, except at joints. Tendons and ligaments attach at it.

Endosteum

The delicate membrane lining the medullary cavity, where bone growth, repair and remodeling occur.

Articular cartilage

A thin layer of cartilage covering the epiphyses where bones meet at a joint. It reduces friction and acts as a shock absorber.

Epiphyseal plate zones

Reserve zone

Closest to the epiphyseal end. Small chondrocytes here do not take part in growth but secure the plate to the bone of the epiphysis.

Proliferative zone

Stacks of slightly larger chondrocytes that divide by mitosis to replace those that die at the diaphyseal end of the plate.

Zone of maturation and hypertrophy

Older, larger chondrocytes. Growth in length results from cell division in the proliferative zone and the maturation of cells in this zone.

Zone of calcified matrix

Closest to the diaphysis. Most chondrocytes are dead because the matrix around them has calcified, and osteoblasts lay bone onto the remaining calcified cartilage, which connects the plate to the diaphysis.

On the epiphyseal side of the plate cartilage is formed, and on the diaphyseal side it is ossified, so the diaphysis grows in length. Bone also grows in diameter, a process called appositional growth, which can continue after growth in length stops.

Bone cells

Osteogenic cell

Undifferentiated, with high mitotic activity. The only bone cell that divides, and it develops into osteoblasts.

Osteoblast

Forms new bone: it secretes the collagen matrix and calcium salts. When it is trapped in the calcified matrix it becomes an osteocyte.

Osteocyte

The primary cell of mature bone, housed in spaces called lacunae.

Osteoclast

Breaks down (resorbs) old bone. It is multinucleated and comes from monocytes and macrophages.