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Chapter 8
Skeletal System
Introduction
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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 (Figure 8-1; Table 8-1)
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Axial skeleton—the 80 bones of the head, neck, and torso; composed of 74
bones that form the upright axis of the body and six tiny middle ear bones
Appendicular skeleton—the 126 bones that form the appendages to the axial
skeleton; the upper and lower extremities
Axial Skeleton
• Skull—made up of 28 bones in two major divisions: cranial bones and facial
bones (Figures 8-2 to 8-7;
Table 8-3)
 Cranial bones
• Frontal bone (Figure 8-8, C)
 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
• Parietal bones (Figure 8-8, A)
 Form the bulging top of the cranium
 Form several sutures: lambdoidal suture with occipital bone;
squamous suture with temporal bone and part of sphenoid; and
coronal suture with frontal bone
• Temporal bones (Figure 8-8, B)
 Form the lower sides of the cranium and part
of the cranial floor
 Contain the inner and middle ears
• Occipital bone (Figure 8-8, D)
 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
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Frontal bone
• Forehead bone
• Forms most of the roof of the orbits (eye sockets) and anterior part of the
cranial floor
Axial Skeleton
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Cranial bones (cont.)
• Sphenoid bone (Figure 8-8, E)
 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 (Figure 8-7)
 Contains the sphenoid sinuses
• Ethmoid bone (Figure 8-8, F)
 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
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Facial bones (Table 8-4)
• Maxilla (upper jaw) (Figure 8-8, H)
 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
• Mandible (lower jaw) (Figure 8-8, M)
 Largest, strongest bone of the face
 Forms the only movable joint of the skull with the temporal bone
• Zygomatic bone (Figure 8-8, I)
 Shapes the cheek and forms the outer margin of the orbit
 Forms the zygomatic arch with the zygomatic process of the temporal
bones
• Nasal bone (Figures 8-8, L, and 8-10)
 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
• Lacrimal bone (Figure 8-8, K)
 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
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Facial bones (cont.)
• Palatine bone (Figure 8-8, J)
 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
• 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 (Figure 8-8, G)
 Forms posterior portion of the nasal septum
 Articulates with the sphenoid, ethmoid, and palatine
bones and maxillae
• Eye orbits (Figure 8-7)
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Right and left eye orbits
• Contain eyes, associated eye muscles, lacrimal apparatus, blood
vessels, and nerves
• Thin and fragile orbital walls separate orbital structures from cranial and
nasal cavities and paranasal sinuses
• Traumatic injuries may result in “blowout fractures” (Figure 8-7, C)
• “Raccoon eyes”—clinical sign of blowout fracture
(Figure 8-7, D)
• Fetal skull (Figure 8-11)
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Characterized by unique anatomic features not seen in adult skull
Fontanels (unossified areas) or “soft spots” (4) allow skull to “mold” during
birth process and permit rapid growth of brain (Table 8-5)
Permits differential growth or appearance of skull components over time
• Face—smaller proportion of total cranium at birth (1/8) than in adult (1/2)
• Head at birth is ¼ total body height; at maturity is about 1/8 body height
• Sutures appear with skeletal maturity (Table 8-5)
• Paranasal sinuses—change in size and placement with skeletal maturity
(Figure 8-9)
• Appearance of deciduous and, later, permanent teeth
• Hyoid bone (Figure 8-12)
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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 (Figure 8-13)
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
 Characteristics of the vertebrae (Figure 8-14; Table 8-6)
• 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
 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
 Convexity persists in the thoracic and sacral regions
 Concavity persists in the cervical and lumbar regions
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C-Spine
• First or upper seven
• Foramen in each transverse process for transmission of vertebral artery,
vein and plexus of nerves.
• Short bifurcated spinous processes except on first and seventh vertebra.
• Small bodies with large, triangular spinal foramina.
 Atlas (C1)
• Lacks a body and spinous process
• Superior articulating processes are concave ovals that act like rockerlike cradle for the condyles of the occipital bone.
• Named atlas because it supports the head.
• Allows up and down movement of the head.
 Axis (C2)
• Second cervical vertebra
• Allows rotation movement of head
• Has dens, or odontoid process which is a peg or tooth-like projection
upward from the body of the axis that allows for side to side rotation of
the head.
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Axial Skeleton
• Vertebral column (cont.)
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T-Spine
• Next 12 vertebrae
• 12 pair of ribs attach to these vertebrae
• Stronger, with more massive bodies than C-spine
• Contain no transverse foramina
• Two sets of facets for articulation with ribs
• Elongated spinous processes
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Lumbar spine
• Five vertebrae
• Strong, massive, superior articulating processes directed medially
instead of upward
• Inferior articulating processes , laterally instead of upward
• Short blunt spinous process with large amount of muscle attachment
• Sternum (Figure 8-15)
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
 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
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• 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
 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
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Appendicular Skeleton
• Upper extremity (Table 8-7)
Consists of the bones of the shoulder girdle, upper arm, lower arm, wrist,
and hand
 Shoulder girdle (Figure 8-17)
• 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
 Humerus (Figures 8-18 and 8-19)
• The long bone of the upper arm
• Articulates proximally with the glenoid fossa of the scapula and distally
with the radius and ulna
 Ulna
• Long bone found on little finger side of forearm
• Articulates proximally with humerus and radius and distally with a
fibrocartilaginous disk
 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
 Carpal bones (Figure 8-20)
• 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
 Metacarpal bones
• Form framework of hand
• Thumb metacarpal forms the most freely movable joint with the carpals
• Heads of metacarpals (knuckles) articulate with phalanges
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Appendicular Skeleton
• Lower extremity
Consists of the bones of hip, thigh, lower leg, ankle, and foot (Table 8-8)
Pelvic girdle is made up of the sacrum and the two coxal bones, bound
tightly by strong ligaments (Figure 8-21)
• 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 (Figure 822):
 Ilium—largest and uppermost
 Ischium—strongest and lowermost
 Pubis—anteriormost
 Femur—longest and heaviest bone in the body (Figure 8-23)
 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
 Foot (Figures 8-24 and 8-25)
• 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
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Skeletal Differences in Men and Women
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Male skeleton is larger and heavier than female skeleton
Pelvic differences (Figure 8-26; Table 8-9)
 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
• 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
 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
• 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
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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 xray images but can be seen in bone scans
• Bone damage is microscopic
• Caused by repetitive trauma (e.g., marathon runners)
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:
• 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
• 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
• 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
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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—Abnormal Spinal Curvatures
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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
 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
 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