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Chapter 5
The Skeletal System
Introduction
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The adult skeleton is composed of 206
bones
The skeletal system is subdivided into 2
divisions:
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Axial skeleton
Appendicular skeleton
The skeletal system includes joints,
cartilages, & ligaments
Bones: An Overview
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Functions of the bones
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Support—bones form the internal framework
that supports & anchors all soft organs
Protection—bones protect body organs
Movement—skeletal muscles, attached to
bones by tendons, use the bones as levers to
move the body & its parts
Bones: An Overview
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Functions of the bones
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Storage—fat is stored in the internal cavities
of bones, and bone serves as a storehouse for
minerals such as calcium
Blood cell formation—this process is also
called hematopoiesis & occurs within the
marrow of certain bones
Classification of Bones
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There are 2 basic types of bones
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Compact bone—dense & looks smooth
Spongy bone—composed of small needlelike
pieces of bone & lots of open space
Bones are classified into 4 groups
according to shape
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Long bones—longer than they are wide;
mostly compact bone; includes all the bones
of the limbs
Classification of Bones
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Bones are classified into 4 groups
according to shape
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Short bones—cube-shaped; mostly spongy
bone; includes all the bones of the wrist &
ankle & also the patella
Flat bones—thin, flat, & curved; 2 thin layers
of compact bone with a layer of spongy bone
in between; includes the bones of the skull,
ribs, & sternum
Irregular bones—bones that do not fit any of
the above categories; includes the bones of
the hip & vertebrae
Classification of Bones
Structure of a Long Bone
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Diaphysis
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Shaft of the bone
Makes up most of the bone’s length & is
composed of compact bone
Periosteum
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Fibrous connective tissue membrane
Covers & protects the diaphysis
Structure of a Long Bone
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Epiphyses
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Ends of the long bone—proximal & distal
Consists of a thin layer of compact bone that
encloses an area filled with spongy bone
Articular cartilage
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Covers the external surface of the epiphyses
It provides a smooth, slippery surface that
decreases friction at joint surfaces
Structure of a Long Bone
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Medullary cavity
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The cavity of the shaft that is primarily a
storage area for adipose tissue
Also known as the yellow marrow cavity
Structure of a Long Bone
Bone Formation, Growth, &
Remodeling
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The skeleton is formed from two of the
strongest & most supportive tissues in the
body—cartilage & bone
In embryos, the skeleton is primarily made
of hyaline cartilage, but in the young child
most of the cartilage has been replaced by
bone.
Bone Formation, Growth, &
Remodeling
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Ossification—process of bone formation &
involves 2 phases:
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The hyaline cartilage is completely covered
with bone matrix by bone-forming cells called
osteoblasts.
The hyaline cartilage is digested away,
opening a medullary cavity within the newly
formed bone.
Bone Formation, Growth, &
Remodeling
Bone Formation, Growth, &
Remodeling
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Bones are always changing & are
remodeled in response to changes in 2
factors:
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Calcium levels in the blood
If blood calcium levels are too low, osteoclasts
(bone-destroying cells) break down bone matrix &
release calcium into the blood.
 If blood calcium levels are too high, calcium is
deposited in bone matrix as hard calcium salts.
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The pull of gravity & muscles on the skeleton
Bone Formation, Growth, &
Remodeling
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Bone remodeling is essential if bones are
to retain normal proportions & strength
during long-bone growth as the body
increases in size & weight
Bone Fractures
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Bones are susceptible to fractures, or
breaks, all through life.
During youth, most fractures result from
exceptional trauma that twists or smashes
the bones. In old age, bones thin &
weaken, & fractures occur more often.
Bone Fractures
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2 common types of fractures: (p 123)
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Simple (closed) fractures = a fracture in
which the bones breaks cleanly but does not
penetrate the skin
Compound (open) fractures = when the
broken bone ends penetrate through the skin
Fractures are treated by reduction—
realignment of the broken bone ends
Bone Fractures
Axial Skeleton
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Forms the longitudinal axis of the body
Divided into 3 parts
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Skull—formed by 2 sets of bones
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Cranium—encloses & protects the brain tissue;
made up of several smaller bones (pp 126-127)
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Frontal bone—forms the forehead
Parietal bones—forms the superior & lateral walls
Temporal bones—inferior to the parietal bones
Occipital bone—forms the back of the cranium
Sphenoid bone—butterfly-shaped & forms part of the
floor of the cranial cavity
Ethmoid bone—lies anterior to the sphenoid & forms the
roof of the nasal cavity
Axial Skeleton
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Facial bones—hold the eyes in position & allow the
facial muscles to show our feelings (p 129)
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Maxillae—upper jaw
Palatine bones—form the posterior part of the hard
palate
Zygomatic bones—cheekbones
Lacrimal bones—medial walls of the eyes & has a groove
that serves as a passageway for tears
Nasal bones—forms the bridge of the nose
Vomer bone—single bone in the median line of the nasal
cavity (nasal septum)
Inferior conchae—curved bones projecting from the
lateral walls of the nasal cavity
Mandible—lower jaw; the largest & strongest bone of the
face!!!
Axial Skeleton
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Fetal skeleton—the skull of a fetus or newborn
infant is different from an adult skull. When a
baby is born, its skeleton is unfinished. The skull
also has regions that yet to be converted to bone
which are called fontanels. These are also known
as the “soft spots.”
Fetal Skull
Axial Skeleton
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Vertebral Column (p 131)
Extends from the skull to the pelvis
 Formed from 26 irregular bones
 Running through the central cavity of the vertebral
column is the spinal cord
 The vertebrae are separated by pads of cartilageintervertebral discs-cushion the vertebrae
 The spinal cord curves & forms an S-shaped
structure
 Abnormal spinal curvatures (p 132)
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Scoliosis
Kyphosis
lordosis
Axial Skeleton
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3 main types of vertebrae
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Cervical vertebrae
 Identified as C1 to C7
 Form the neck region to the spine
Thoracic vertebrae
 Identified as T1 to T12
 Larger than cervical vertebrae
Lumbar vertebrae
 Identified as L1 to L5
 Where most of the stress on the vertebral column
occurs
Sacrum—formed by the fusion of 5 vertebrae
 Coccyx—formed by the fusion of 3-5 tiny
vertebrae; also known as the human “tailbone”
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Axial Skeleton
Axial Skeleton
Axial Skeleton
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Bony thorax—aka the thoracic cage (p 135)
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Sternum
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Aka the breastbone
A flat bone & the result of the fusion of 3 bones:
 Manubrium
 Body
 Xiphoid process
Ribs
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12 pairs
Males DO NOT have 1 rib less than females!!!
3 types of ribs:
 True ribs—the 1st 7 pairs that are attached directly to the
sternum
 False ribs—the next 5 pairs that are attached to the sternum
 Floating ribs—the last 2 pairs of false ribs that lack sternal
attachment
Axial Skeleton
Appendicular Skeleton
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Composed of 126 bones of the limbs &
pectoral & pelvic girdles
Bones of the shoulder girdle (p 139)
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Clavicle—aka the collarbone
Scapula—aka the shoulder blades
Bones of the upper limbs (pp 140-141)
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Arm—formed by a single bone=humerus
Forearm—2 bones:
Radius—on the thumb side
 Ulna—on the pinky side
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Appendicular Skeleton
Appendicular Skeleton
Appendicular Skeleton
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Hand—3 sets of bones
Carpals—8 bones that make up the wrist
 Metacarpals—the palm of the hand
 Phalanges—bones of the fingers
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Bones of the pelvic girdle (p 142)
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Formed by 2 coxal bones called hip bones
Each hip bone is formed by the fusion of 3
bones:
Ilium
 Ischium
 pubis
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Appendicular Skeleton
Appendicular Skeleton
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Differences between the male & female
pelvis:
Female pelvis is shallower
 Female bones are lighter & thinner
 Female sacrum is shorter & less curved
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Bones of the lower limbs (pp 144-145)
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Thigh—formed by a single bone=femur
Leg—2 bones
Tibia—shinbone=larger & more medial
 Fibula—lies next to the tibia
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Appendicular Skeleton
Appendicular Skeleton
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Foot—3 sets of bones
Tarsals—7 bones that make up the posterior foot
 Metatarsals—the sole of the foot
 Phalanges—the bones of the toes
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Appendicular Skeleton
Joints
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Also called articulations
2 main functions
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Hold bones together securely
Give the skeleton mobility
Classified in 2 ways
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Functionally
Structurally
Joints
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Functional classification--focuses on the amount
of movement allowed by the joint
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Immovable joint
Slightly movable joints
Freely movable joints
Structural classification—focuses on the type of
tissue
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Fibrous joints—bones are united by fibrous tissue
Cartilaginous joints—bones are united by cartilage
Synovial joints—joint cavity contains synovial fluid
Joints
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Types of synovial joints based on shape
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Plane joint—short slipping or gliding; ie joints
of the wrist
Hinge joint—movement in one direction; ie
elbow, ankle, & the joints between the bones
of the fingers
Pivot joint—rotating movement; ie between
the radius & ulna
Condyloid joint—bones move from side to side
or back & forth; ie your knuckles
Joints
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Saddle joint—same movement as condyloid;
ie twiddling your thumbs
Ball-and-socket joint—head of one bone fits
into a round socket; ie shoulder & hip joint
Joints
Joints
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Inflammatory disorders of joints
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Arthritis
Inflammation of the joint
 Most widespread disease in the U.S.
 Symptoms=pain, stiffness, & swelling of the joint
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Osteoarthritis
Most common type of arthritis
 Chronic degenerative condition that affects the
aged
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Joints
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Rheumatoid arthritis
Chronic inflammatory disorder
 Affects more women than men
 The body’s immune system attempts to destroy
tissues
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Gout
Uric acid accumulates in the blood & may be
deposited as needle-shaped crystals in the soft
tissues of joints
 Usually affects the great toe
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