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Chapter 8: Skeletal System

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INTRODUCTION

Skeletal tissues form bones—the organs of the skeletal system

The relations of bones to each other and to other body structures provide a basis for understanding the function of other organ systems

The adult skeleton is composed of 206 separate bones

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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

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AXIAL SKELETON

Skull: composed of 28 bones in two major divisions: cranial bones and facial bones (Figures 8-2 to 8-7; Table 8-3) Cranial bones (Figure 8-8)

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

Parietal bones Form the bulging top of the cranium Form several sutures: lambdoid suture with the occipital

bone, squamous suture with the temporal bone and part of the sphenoid, and coronal suture with the frontal bone

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AXIAL SKELETON (cont.)

Cranial bones (cont.) Temporal bones

Form the lower sides of the cranium and part of the cranial floor Contain the inner and middle ears

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 vertebras

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AXIAL SKELETON (cont.)

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 (Figure 8-7)

Contains the sphenoid sinusesEthmoid bone

A complex, irregular bone that lies anterior to the sphenoid and posterior to the nasal bones

Forms anterior cranial floor, medial orbit walls, upper parts of the nasal septum, and side walls of the nasal cavity

The cribriform plate is located in the ethmoid

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AXIAL SKELETON (cont.)

Skull (cont.) Facial bones (Table 8-4)

Maxilla (upper jaw) Two maxillae form the keystone of the face Maxillae articulate with each other and with the nasal, zygomatic,

inferior concha, and palatine bones Forms parts of the orbital floors, roof of the mouth, and floor and

side walls of the nose Contains maxillary sinuses

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AXIAL SKELETON (cont.)

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 Nasal bones

Both nasal bones form the upper part of the bridge of the nose; cartilage forms the lower part

Articulate with the ethmoid, nasal septum, frontal bone, maxillae, and each other

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AXIAL SKELETON (cont.)

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 a groove for the nasolacrimal (tear) duct Articulates with the maxilla, frontal, and ethmoid bones

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

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AXIAL SKELETON (cont.)

Facial bones (cont.) Inferior nasal conchae (turbinates)

Form the lower edge projecting into the nasal cavity and form the nasal meatuses

Articulate with ethmoid, lacrimal, maxillary, and palatine bones Vomer bone

Forms the posterior portion of the nasal septum Articulates with the sphenoid, ethmoid, palatine, and maxillae

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AXIAL SKELETON (cont.)

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 the cranial and nasal cavities and paranasal sinuses

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AXIAL SKELETON (cont.) Fetal skull (Figure 8-11)

Characterized by unique anatomical features not seen in adult skull

Four fontanels, or “soft spots,” allow the skull to mold during the birth process and allow rapid growth of the brain (Table 8-5)

Permits differential growth or appearance of skull components over time Face: smaller proportion of total cranium at birth (one

eighth) than in adult (one half) Head at birth is one fourth the total height; at maturity

about one eighth 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

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AXIAL SKELETON (cont.) 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 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:

Seven cervical vertebrae 12 thoracic vertebrae Five lumbar vertebrae Sacrum: in adults, results from the fusion of five separate

vertebrae Coccyx: in adults, results from the fusion of four or five

separate vertebrae

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AXIAL SKELETON (cont.) Vertebral column (cont.)

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 a vertebral foramen Second cervical vertebra has an upward projection, the

dens, to allow rotation of the head Seventh cervical vertebra has a long, blunt spinous

process Each thoracic vertebra has articular facets for the ribs

Vertebral column as a whole articulated with the head, ribs, and iliac bones

Individual vertebrae articulate with each other in joints between their bodies and between their articular processes

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AXIAL SKELETON (cont.)

Sternum (Figure 8-15) Dagger-shaped bone in the middle of the anterior chest

wall made of three parts: Manubrium: the upper handle part Body: middle blade part Xiphoid process: blunt cartilaginous lower tip that 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 the costal cartilages

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AXIAL SKELETON (cont.)

Ribs (Figures 8-15 and 8-16) 12 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 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 and

lower parts of the arm, wrist, and hand Shoulder girdle (Figure 8-17)

Composed of the scapula and clavicle Clavicle forms the only bony joint with the trunk, the

sternoclavicular joint At its distal end, the clavicle articulates with the acromion

process of the scapula Humerus (Figures 8-18 and 8-19)

The long bone of the upper part of the arm Articulates proximally with the glenoid fossa of the scapula

and distally with the radius and ulna

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APPENDICULAR SKELETON (cont.)

Upper extremity (cont.) Ulna

The long bone found on the little finger side of the forearm

Articulates proximally with the humerus and radius and distally with a fibrocartilaginous disk

Radius The long bone found on the thumb side of the forearm Articulates proximally with the capitulum of the

humerus and the radial notch of the ulna; articulates distally with the scaphoid and lunate carpals and with the head of the ulna

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APPENDICULAR SKELETON (cont.)

Upper extremity (cont.) Carpal bones (Figure 8-20)

Eight small bones that form the wrist Carpals are bound closely and firmly by ligaments and

form two rows of four carpals each Proximal row is composed of the pisiform, triquetrum, lunate, and

scaphoid Distal row is composed of the hamate, capitate, trapezoid, and

trapezium The joints between the radius and carpals allow wrist

and hand movements

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APPENDICULAR SKELETON (cont.)

Upper extremity (cont.) Metacarpal bones

Form the framework of the hand The thumb metacarpal forms the most freely movable

joint with the carpals Heads of the metacarpals (the knuckles) articulate

with the phalanges

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APPENDICULAR SKELETON (cont.)

Lower extremity Consists of the bones of the hip, thigh, lower part of the

leg, ankle, and foot (Table 8-8) Pelvic girdle is composed 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 composed of three bones that fuse

together (Figure 8-22): Ilium: largest and uppermost Ischium: strongest and lowermost Pubis: anteriormost

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APPENDICULAR SKELETON (cont.)

Lower extremity (cont.) 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 talus

Fibula The smaller, more laterally and deeply placed of the two leg

bones Articulates with the tibia

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APPENDICULAR SKELETON (cont.)

Lower extremity (cont.) 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 composed of the calcaneus, talus,

navicular, cuneiforms, and medial three metatarsals Lateral longitudinal arch is composed of the calcaneus, cuboid,

and fourth and fifth metatarsals

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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

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THE BIG PICTURE: SKELETAL SYSTEM

Skeletal system is a good example of increasing structural hierarchy in the body Skeletal tissues 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 more than an assemblage of individual bones; it represents a complex and interdependent functional unit of the body