Chapter 8



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 (Figure 8-1; Table 8-1)

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

Frontal bone

Forehead bone

Forms most of the roof of the orbits (eye sockets) and anterior part of the cranial floor

Axial Skeleton

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

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

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)

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)

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)

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

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 rocker-like 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.

Axial Skeleton

Vertebral column (cont.)

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

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

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

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

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 8-22):

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

Skeletal Differences in Men and Women

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

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)

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

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

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

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