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Skeletal System

Learning Outline

Skeletal System

The specific structures of the gross anatomy of the skeleton, such as names of specific bones and bone features are not covered in this outline.

If you want an optional overview of the bones of the skeleton click here.

Functions of the skeletal system


Framework of body, holding other organs in place


Attachment sites for skeletal muscles

Movable joints; leverage for movement


Hard covering of thoracic organs, brain, spinal cord, other soft structures

Protection as in bodily defense against injury

Mineral and fat storage

Calcium & phosphorus salts stored in bone tissue

Yellow fat (yellow bone marrow) stored in bone cavities

Blood cell production

Hematopoiesis (hemato = “blood” poiesis = “making”)

Red bone marrow is blood-forming tissue inside some bones

Female human skeletonMale Human Skeleton
Human skeletal system (female left, male right)
click either image to see expanded views
from Bernhard Ungerer used by permission

Bone organs (gross structure)

Bone number

206 bones is standard / typical (but nearly everyone has more/fewer)

Regions of skeleton

Axial skeleton—forms “axis” of body

  • Includes bones of skull, vertebral column, thorax

Appendicular skeleton—forms appendages (arms, legs)

  • Includes bones of shoulder/arm/hand and hip/leg/foot

Bone categories

Long bones

  • Examples: radius, ulna, tibia, fibula, humerus, femur, metacarpal bones, metatarsal bones

Short bones

  • Examples: carpal bones, tarsal bones

Flat bones

  • Examples: sternum, ribs, scapula

Irregular bones

  • Examples: vertebrae, pelvic bones, skull bones of face

Sesamoid bones

  • Example: patella

Long bone structure

Epiphyses (sing. epiphysis) are end regions

  • Usually have spongy bone on inside, compact bone on outside

Diaphysis is middle “shaft” region (pl. diaphyses)

  • Usually compact bone on outside, cavity on inside
  • Medullary cavity contains yellow marrow
    • Lined with thin membrane called endosteum

Bone covered with periosteum (dense fibrous sheet) and articular (joint) cartilage

long bone
Long bone structure

Flat bone structure

Short and irregular bones have a similar structure to flat bones

Internal and external table

  • Thick sheet of compact bone that forms the hard outer shell of bone
  • Covered with periosteum

Diploe (diploë)

  • Cancellous (spongy) bone filled with red marrow forming inner region of bone

Bone tissue (microscopic structure)

Compact bone

Hard bone forming outer shell of all bone organs

Bone matrix is collagen fibers with apatite mineral (calcium/phosphorus) crystals encrusted on the fibers

  • Always starts out as fibrous membrane or cartilage, then turns to bone
    • Endochondral ossification — cartilage becomes bone Gray's Anatomy
      • Epiphyseal plate is cartilage between epiphysis and diaphysis as they grow together
    • Intramembranous ossification — membrane becomes bone
      • Fontanels are “soft spots” in infant skull where ossification is not complete
        • Allow for deformation of skull during childbirth

Osteon (haversian system) is a tapered, cylindrical unit that makes up compact bone tissue Gray's Anatomy slide

  • Central canal (haversian or osteonal canal) surrounded by concentric lamellae (layers) of hard bone matrix Gray's Anatomy Gray's Anatomy
    • Transverse (Volkmann) canals connect central canals side to side, forming blood networks
  • Osteoblasts (“bone makers”) make matrix, then are trapped and now “retired” and called osteocytes (“bone cells”) Gray's Anatomy
    • Lacuna (pl. lacunae) are the spaces in which osteocytes are found
    • Osteocytes may “come out of retirement” during remodeling
  • Canaliculi (“tiny canals”) connect the lacunae (each housing one osteocyte) to each other and to the central canal
    • Processes of adjacent osteoblasts are joined by gap junctions, making them a syncytium of cells
  • Lamellae (sing. lamella)
    • Concentric lamellae form osteons
    • Intersititial lamellae, found between osteons, are left over from preliminary osteons and woven bone
    • Circumferential lamellae form continuous boundaries at the outer and inner surfaces of the compact bone (around the inner and outer circumference)

Around 21 million osteons in adult skeleton

Each osteon is 100-400 µm in diameter (1 inch = 25,400 µm)

A medium osteon has about 30 lamellae (each about 3 µm)

The central canal is around 50 µm in diameter

Cancellous (spongy) bone

Made up of irregular, fractal trabeculae of hard bone surrounded by red bone marrow (liquid)

  • Cancellous bone is also called trabecular bone

Red marrow is myeloid tissue (myelo = “marrow”)

  • Hematopoiesis

microscopic bone structure
Compact Bone (osteon highlighted in green)
Click image to enlarge it

Osteon photo
Osteon (closer view)
Click image to enlarge it


Bone is constantly being torn down and built up—this is remodeling slide

Role of bone cells pencast icon

  • Osteoblasts make new bone matrix (using Ca++ from blood)
  • Osteoclasts (“bone breakers”) dissolve bone matrix (releasing Ca++ to blood)

Role of hormones

  • Calcitonin (CT; from thyroid) increases Ca++ storage (out of blood)
  • Parathyroid hormone (PTH) gets Ca++ out of storage (into blood)

Age effects

  • More bone is made than is lost until age 25 (usually rapid until puberty, then slows)
  • About as much bone is made as is lost 25-50 (can vary)
  • More bone is lost than is made 50-120 (usually slight)
  • Yellow marrow replaces red marrow, reducing total RBC production

Stress effects

  • Mechanical stress can affect bones (fractures, pressure, exercise, etc.)
  • Stress increases bone density (therefore, gravity and exercise increase bone density)
  • Usually accounts for changes in bone density in old age

Skeletal variations


  • Male — heavier, larger; more defined markings; deep pelvis skull icon
  • Female — lighter, smaller; less defined markings; shallow/broad pelvis skull icon

Age (see section above)


  • Toxins
  • Stress (including fractures)—see above

lion trackRequired—please read the A&P Connect article Skeletal Variations online at the Evolve website

Kevin in the Paris catacombsA good place to study normal variations in human skeletons is in the underground ossuary (place for bones) in Paris—also known as the “Paris catacombs.” bones from the Paris catacombsSkeletal remains of thousands of people buried in 18th century cemeteries were moved to abandoned chalk quarries under the streets of Paris and can be visited today. Click each photo to enlarge it. Curious about the Paris catacombs? Click here.

One frontal bone or two?

Figure A is a photograph from the Paris catacomb showing a skull with a sagittal suture separating the frontal part of the skull into a left frontal bone and right frontal bone.

Figure B is a photograph of a nearby skull that is “standard” in that it has no sagittal suture dividing the frontal bone—it has a single frontal bone.

It is very likely that neither individual was aware of these facts while they were alive. This is an example of how the human skeleton can vary from one person to another.

Skull with frontal suturestandard frontal bone


See Tables 9-1, 9-2 and 9-3 in textbook


Joint is where two or more bones come together (join)

Arthro = joint

  • Arthritis = joint inflammation (many causes / types) slide


  • Fibrous structures that connect one bone to another Gray's Anatomy Gray's Anatomy

Structural categories slide

Fibrous — bones are joined by fibrous tissue

Cartilaginous — bones are joined by cartilage

Synovial — bones are joined at a fluid-filled space lined with synovial membrane

Functional categories slide

Immovable — bones don’t move relative to one another

  • Synarthroses

Slightly movable — bones can move, but not much

  • Amphiarthroses

Freely movable — bones have significant movement

  • Diarthroses


Fibrous joints are synarthrotic joints


  • Fibrous bands called ligaments connect the bones
  • Example: joints between the distal radius and ulna Gray's Anatomy


  • Fibrous tissue connects flat bones that fit together like jigsaw puzzle pieces
  • Example: joints between flat bones of skull Gray's Anatomy


  • Fibrous tissue connects root of tooth to socket of jaw bone Gray's Anatomy


Cartilaginous joints are amphiarthrotic)


  • Hyaline cartilage between bones
  • Example: first rib-sternum (costosternal) joint Gray's Anatomy


  • Fibrocartilage between bones
  • Examples:
    • between BODIES of vertebra Gray's Anatomy Gray's Anatomy
    • pubic symphysis Gray's Anatomy Gray's Anatomy


Synovial joints are diarthrotic Gray's Anatomy Gray's Anatomy

Uniaxial — single axis of movement

  • Hinge Gray's Anatomy
  • Pivot Gray's Anatomy Gray's Anatomy

Biaxial — two axes of movement

  • Saddle Gray's Anatomy
  • Condyloid Gray's Anatomy

Multiaxial — multiple axes of movement

  • Ball and socket Gray's Anatomy Gray's Anatomy
  • Gliding Gray's Anatomy

Bursa Gray's Anatomy Gray's Anatomy


Types of movements slide

Angular movements

Angular movements increase or decrease the angle of a joint

Flexion — decreases angle of joint

  • Ankle flexion — special case
    • Plantar flexion — moves toes inferiorly
    • Dorsiflexion — moves toes superiorly

Extension — increases angle of joint

  • Hyperextension — goes beyond anatomical position

Abduction — moves part away from midline of body or region

Adduction — moves toward from midline of body or region

Circular movements

Circular movements move body parts in a circle

Rotation — pivots part on its axis

Circumduction — moves distal end of part in a circle-like path

Supination — twisting of limb (e.g. arm or leg) away from median

Pronation — twisting of limb toward median

Gliding movement

Special movements

Ankle movements

  • Inversion — move sole toward median
  • Eversion — move sole away from median

Forward and back movements

  • Protraction — move part anteriorly
  • Retraction — move part posteriorly

Up and down movements

  • Elevation — move part superiorly
  • Depression — move part inferiorly

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