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Chapter 8
Skeletal System
Introduction
• Skeletal tissues form bones—the organs of the skeletal
system
• The relationship of bones to each other and to other
body structures provides a basis for understanding the
function of other organ systems
• The adult skeleton is composed of 206 separate bones
Divisions of Skeleton
• The adult skeleton is composed of 206 separate bones
• Axial skeleton
– 80 bones of the head, neck, and torso
• 74 bones that form the upright axis of the body
• 6 tiny middle ear bones
• Appendicular skeleton
– 126 bones that form the appendages to the axial skeleton
• The upper and lower extremities
Axial Skeleton
• Skull
– 28 bones in two major divisions
• Cranial bones
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–
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–
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Frontal
Parietal (2)
Temporal (2)
Occipital
Sphenoid
Ethmoid
• Facial bones
– Maxilla (2)
– Zygomatic (2)
– Nasal (2)
– Mandible
– Vomer
– Lacrimal (2)
– Palatine (2)
– Inferior nasal concha (2)
Axial Skeleton
• Cranial Bones
– Frontal bone
• Forms the forehead and anterior part of the top
of the cranium
• Contains the frontal sinuses
• Forms the upper portion of the orbits
• Forms the coronal suture with the two parietal bones
Axial Skeleton
• Cranial bones (cont.)
– Parietal bones
• Form the bulging top of the cranium
• Form several sutures
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–
–
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Lambdoidal suture with occipital bone
Squamous suture with temporal bone
Coronal suture with frontal bone
Part of sphenoid
Axial Skeleton
• Cranial bones (cont.)
– Temporal bones
• Form the lower sides of the cranium and part of the cranial floor
• Contain the inner and middle ears
Axial Skeleton
• Cranial bones (cont.)
– Occipital bone
• Forms the lower, posterior part of the skull
• Forms immovable joints with three other cranial bones
and a movable joint with the first cervical vertebra
Axial Skeleton
• Cranial bones (cont.)
– Sphenoid bone
• A bat-shaped bone located in the central portion of the cranial floor
• Anchors the frontal, parietal, occipital, and ethmoid bones and forms
part of the lateral wall of the cranium and part of the floor of each
orbit
• Contains the sphenoid sinuses
Axial Skeleton
• Cranial bones (cont.)
– Ethmoid bone
• A complicated, irregular bone that lies anterior to the sphenoid and
posterior to the nasal bones
• Forms the anterior cranial floor, medial orbit walls, upper parts of the
nasal septum, and sidewalls of the nasal cavity
• The cribriform plate is located in the ethmoid
Axial Skeleton
• Facial bones
– Maxilla (upper jaw)
• Two maxillae form the keystone of the face
• Maxillae articulate with each other and with nasal, zygomatic,
inferior concha, and palatine bones
• Forms parts of the orbital floors, roof of the mouth, and floor and
sidewalls of the nose
• Contains maxillary sinuses
Axial Skeleton
• Facial bones (cont.)
– Mandible (lower jaw)
• Largest, strongest bone of the face
• Forms the only movable joint of the skull with the temporal bone
– Zygomatic bone
• Shapes the cheek and forms the outer margin of the orbit
• Forms the zygomatic arch with the zygomatic process of the
temporal bones
Axial Skeleton
• Facial bones (cont.)
– Nasal bone
• Both nasal bones form the upper part of the bridge of the nose,
whereas cartilage forms the lower part
• Articulates with the ethmoid bone, nasal septum, frontal bone,
maxillae, and the other nasal bone
Axial Skeleton
• Facial bones (cont.)
– Lacrimal bone
• Paper-thin bone that lies just posterior and lateral to each nasal
bone
• Forms the nasal cavity and medial wall of the orbit
• Contains groove for the nasolacrimal (tear) duct
• Articulates with the maxilla and the frontal
and ethmoid bones
Axial Skeleton
• Facial bones (cont.)
– Palatine bone
• Two bones form the posterior part of the hard palate
• Vertical portion forms the lateral wall of the posterior part
of each nasal cavity
• Articulates with the maxillae and the sphenoid bone
Axial Skeleton
• Facial bones (cont.)
– Inferior nasal conchae (turbinates)
• Form lower edge projecting into the nasal cavity and form the nasal
meati
• Articulate with ethmoid, lacrimal, maxillary, and palatine bones
– Vomer bone
• Forms posterior portion of the nasal septum
• Articulates with the sphenoid, ethmoid, and palatine
bones and maxillae
Axial Skeleton
• Fetal skull
– Characterized by unique anatomic features not seen in adult
skull
– Fontanels or “soft spots” (4) allow skull to “mold” during birth
process and permit rapid growth of brain (Table 8-5)
Axial Skeleton
• Hyoid bone
– U-shaped bone located just above the larynx and below the
mandible
– Suspended from the styloid processes of the temporal bone
– Only bone in the body that articulates with no other bones
Axial Skeleton
• Vertebral column
– Forms the flexible longitudinal axis of the skeleton
– Consists of 24 vertebrae plus the sacrum and coccyx
– Segments of the vertebral column:
•
•
•
•
•
Cervical vertebrae, 7
Thoracic vertebrae, 12
Lumbar vertebrae, 5
Sacrum—in adult, results from fusion of five separate vertebrae
Coccyx—in adult, results from fusion of four or five separate vertebrae
Axial Skeleton
• Vertebral column (cont.)
– Characteristics of the vertebrae
• All vertebrae, except the first, have a flat, rounded body anteriorly and
centrally, a spinous process posteriorly, and two transverse processes
laterally
• All but the sacrum and coccyx have vertebral foramen
• Second cervical vertebra has upward projection, the dens, to allow rotation
of the head
• Seventh cervical vertebra has long, blunt spinous process
• Each thoracic vertebra has articular facets for the ribs
Axial Skeleton
• Vertebral column (cont.)
– Vertebral column as a whole articulates with the head, ribs, and
iliac bones
– Individual vertebrae articulate with each other in joints between
their bodies and between their articular processes
Axial Skeleton
• Sternum
– Dagger-shaped bone in the middle of the anterior chest wall
made up of three parts:
• Manubrium—the upper, handle part
• Body—the middle, blade part
• Xiphoid process—the blunt cartilaginous lower tip, which ossifies
during adult life
Axial Skeleton
• Sternum (cont.)
– Manubrium articulates with the clavicle and first rib
– Next nine ribs join the body of the sternum,
either directly or indirectly, by means of the
costal cartilage
Axial Skeleton
• Ribs (Figures 8-15 and 8-16)
– Twelve pairs of ribs, with the vertebral column and sternum, form
the thorax
– Each rib articulates with the body and transverse process of its
corresponding thoracic vertebra
– Ribs 2 through 9 articulate with the body of the vertebra above
Axial Skeleton
• Ribs (cont.)
– From its vertebral attachment, each rib curves outward, then
forward and downward
– Rib attachment to the sternum:
• Ribs 1 through 8 join a costal cartilage that attaches it to the
sternum
• Costal cartilage of ribs 8 through 10 joins the cartilage of the rib
above to be indirectly attached to the sternum
• Ribs 11 and 12 are floating ribs, because they do not attach even
indirectly to the sternum
Appendicular Skeleton
• Upper extremity
– Consists of the bones of the shoulder girdle, upper arm, lower
arm, wrist, and hand
– Shoulder girdle
• Made up of scapula and clavicle
• Clavicle forms only bony joint with trunk, the sternoclavicular joint
• At its distal end, clavicle articulates with the acromion process of the
scapula
Appendicular Skeleton
• Upper extremity (cont.)
– Humerus
• The long bone of the upper arm
• Articulates proximally with the glenoid fossa of the scapula and
distally with the radius and ulna
Appendicular Skeleton
• Upper extremity (cont.)
– Ulna
• Long bone found on little finger side of forearm
• Articulates proximally with humerus and radius and distally with a
fibrocartilaginous disk
Appendicular Skeleton
• Upper extremity (cont.)
– Radius
• Long bone found on thumb side of forearm
• Articulates proximally with capitulum of humerus and radial notch of
ulna; articulates distally with scaphoid
and lunate carpals and with head of ulna
Appendicular Skeleton
• Upper extremity (cont.)
– Carpal bones
• Eight small bones that form wrist
• Carpals are bound closely and firmly by ligaments and form two
rows of four carpals each
– Proximal row is made up of pisiform, triquetrum, lunate, and scaphoid
– Distal row is made up of hamate, capitate, trapezoid,
and trapezium
• The joints between radius and carpals allow wrist and hand
movements
Appendicular Skeleton
• Upper extremity (cont.)
– Metacarpal bones
• Form framework of hand
• Thumb metacarpal forms the most freely movable joint with the
carpals
• Heads of metacarpals (knuckles) articulate with phalanges
Appendicular Skeleton
• Lower extremity
– Consists of the bones of hip, thigh, lower leg, ankle, and foot
Appendicular Skeleton
• Lower extremity (cont.)
– Pelvic girdle is made up of the sacrum and the two coxal
bones, bound tightly by strong ligaments
• A stable circular base that supports the trunk and attaches the
lower extremities to it
• Each coxal bone is made up of three bones that fuse together:
– Ilium—largest and uppermost
– Ischium—strongest and lowermost
– Pubis—anteriormost
Appendicular Skeleton
• Lower extremity (cont.)
– Femur—longest and heaviest bone in the body
– Patella—largest sesamoid bone in the body
– Tibia
• The larger, stronger, and more medially and superficially located of the
two leg bones
• Articulates proximally with the femur to form the
knee joint
• Articulates distally with the fibula and the talus
– Fibula
• The smaller, more laterally and deeply placed
of two leg bones
• Articulates with tibia
Appendicular Skeleton
• Lower extremity (cont.)
– Foot
• Structure is similar to that of the hand, with adaptations for
supporting weight
• Foot bones are held together to form spring arches
– Medial longitudinal arch is made up of calcaneus, talus, navicular,
cuneiforms, and medial three metatarsals
– Lateral longitudinal arch is made up of calcaneus, cuboid, and fourth
and fifth metatarsals
Skeletal Differences
in Men and Women
• Male skeleton is larger and heavier than female skeleton
• Pelvic differences
– Male pelvis—deep and funnel-shaped with a narrow pubic arch
– Female pelvis—shallow, broad, and flaring with
a wider pubic arch
Cycle of Life: The Aging Skeleton
• Aging changes begin at fertilization and continue
over a lifetime
– Changes can be positive or negative
• Normal bone development is a skeletal aging process
– Intramembranous ossification
– Endochondral ossification
– Appearance of ossification centers and closure of epiphyseal plates can
be used to estimate potential growth and height
Cycle of Life: The Aging Skeleton
• Characteristics of bone during age
• Bone produced early in life is properly calcified but not
brittle
• Osteoblastic activity during early periods of bone remodeling results
in deposition of more bone than is resorbed
– Prior to puberty results in growth of bones
– After puberty and until early thirties, replaced bone
is stronger
Cycle of Life: The Aging Skeleton
– Negative outcomes of skeletal aging begin between 30 and 40
years of age
• Decrease in osteoblast numbers with production of lower quality
matrix
• Increase in osteoclast numbers and activity with increased bone
loss
• Mature osteocytes coalesce and shrink, producing a honeycomb of
tiny holes in the compact bone
Cycle of Life: The Aging Skeleton
– Negative outcomes (cont.)
• Skeleton as a whole loses strength, and fracture risk increases
• Decrease in number of trabeculae in spongy bone in vertebral
bodies and other bones results in “spontaneous” as well as
compression fractures
• Overall height decreases beginning at about age 35
• Osteoporosis is a common and very serious bone disease in old
age
The Big Picture
• Skeletal system is a good example of increasing structural hierarchy
in the body
– Skeletal tissues are grouped into discrete organs—bones
– Skeletal system consists of bones, blood vessels, nerves, and other
tissues grouped to form a complex operational unit
– Integration of skeletal system with other body organ systems permits
homeostasis to occur
– Skeletal system is more than an assemblage of individual bones—it
represents a complex and interdependent functional unit of the body
Mechanisms of Disease—
Bone Fractures
• Fracture defined as partial or complete break in continuity of a bone
– Mechanical stress and traumatic injury are most common causes
– Pathological or spontaneous fractures occur in absence of trauma
– Stress fractures may not be apparent in clinical examination or standard
x-ray images but can be seen in bone scans
• Bone damage is microscopic
• Caused by repetitive trauma (e.g., marathon runners)
Mechanisms of Disease—
Bone Fractures
•
Fracture defined (cont.)
– Displaced, open or compound fractures—do not produce a break in the skin and
pose less danger of infection
– Nondisplaced, closed or simple fractures—do not produce a break in the skin
and pose less danger of infection
– Fracture types:
•
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Impacted—one end of fracture driven into diaphysis of other fragment
Complete—break extends across entire section of bone
Incomplete—some fracture components still partially joined
Dentate—fracture components jagged and fit together like teeth on a gear
Comminuted—crushed, small, crumbled bone fragments near fracture
Mechanisms of Disease—
Bone Fractures
– Fracture types (cont.)
• Avulsion—bone fragments pulled away from underlying bone surface
or bone totally torn from body part
• Linear—fracture line parallel to the bone’s long axis
• Transverse—fracture line at right angle to long axis of bone
• Oblique—fracture line slanted or diagonal to longitudinal axis
• Spiral—fracture line spirals around long axis
• Hairline—common in skull—fracture components small and aligned;
if fracture is pushed downward, called a depressed fracture
Mechanisms of Disease—
Bone Fractures
– Fracture types (cont.)
• Greenstick—bone bent but broken only on one side (common in
children)
• Pott’s—fracture of lower tibia
• Colles’—fracture of distal radius
• LeFort—fracture of face and/or base of skull
• Hangman’s—fracture of posterior elements in upper cervical spine,
especially the axis
• Blowout—fracture of the eye orbit
Mechanisms of Disease—
Bone Fractures
– Osgood-Schlatter disease
– Avulsion fracture of tibial tuberosity fragments the surface
• Caused by powerful contraction of quadriceps muscle group pulling
on patellar ligament attached to tibial tuberosity
• Common in adolescent athletes in whom patellar ligament is
stronger than underlying bone
Mechanisms of Disease—
Treatment of Fractures
• Clinical signs of fracture include pain, loss of function, false
motion, soft tissue edema, deformity, and crepitus
• Initial treatment is realignment and immobilization of bone
fragments
– Closed reduction—alignment completed without surgery
– Open reduction—surgery required to align and internally immobilize
bone fragments with screws, wires, plates, or other orthopedic devices
– After reduction, immobilization generally accomplished by casts, splints,
and bandages
– Traction sometimes used—especially in children
– Restoration of function is treatment priority following healing
Mechanisms of Disease—Mastoiditis
• Inflammation of air spaces within mastoid portion of temporal
bone
– Pus may enter mastoid air spaces from middle ear infection or otitis
media
– Mastoid air cells do not drain into nose as do paranasal sinuses
– Infectious material may erode thin, bony partition separating air
cells from cranial cavity, causing intracranial infection
– Treatment is antibiotic therapy and surgical incision of eardrum to
drain pus from middle ear
– Surgical removal of part of mastoid process of temporal bone—
mastoidectomy—is rare
Mechanisms of Disease—
Abnormal Spinal Curvatures
• Normal curvature of spine is convex through the
cervical and lumbar regions
– Normal curves give spine strength for support of body and
balance required to stand and walk
• Abnormal curvatures
– Lordosis—abnormally accentuated lumbar curve
(“swayback”)
• Frequently seen during pregnancy
• May be secondary to traumatic injury
Mechanisms of Disease—
Abnormal Spinal Curvatures
• Abnormal curvatures (cont.)
– Kyphosis—abnormally accentuated thoracic curvature
(“hunchback”)
• Frequent consequence of vertebral compression fractures in
osteoporosis
• Sign of Scheuermann’s disease, which may develop
in children at puberty
Mechanisms of Disease—
Abnormal Spinal Curvatures
• Abnormal curvatures (cont.)
– Scoliosis—abnormal side-to-side spinal curvature
• Often appears before adolescence
• Treatments vary with severity of curvature
– Milwaukee brace
– Transcutaneous stimulation
– Surgical grafting to the deformed vertebrae of bone from elsewhere in
skeleton or of metal rods