Chapter 6
The Skeletal System
FUNCTIONS OF THE SKELETAL
SYSTEM
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Supports and gives shape to the body
Protects internal organs
Helps make movements possible
Stores calcium
Hemopoiesis, or blood cell formation
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TYPES OF BONES
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Four major types, according to overall
shape of the bone
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Long—Example: humerus (upper arm)
Short—Example: carpals (wrist)
Flat—Example: frontal (skull)
Irregular—Example: vertebrae (spinal cord)
Some also recognize a sesamoid (round)
bone category
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Example: patella (kneecap)
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STRUCTURE OF LONG BONES
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Structural components (Figure 6-1)
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Diaphysis or shaft—hollow tube of hard
compact bone
Medullary cavity—hollow area inside diaphysis
bone that contains yellow marrow
Epiphyses, or ends of the bone—spongy bone
that contains red bone marrow
Articular cartilage—covers epiphyses and
functions as a cushion
Periosteum—strong membrane covering bone
everywhere except at joint surfaces
Endosteum—lining of medullary cavity
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MICROSCOPIC STRUCTURE OF BONE
AND CARTILAGE
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Bone types (Figure 6-2)
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Spongy
• Texture results from needlelike threads of bone called
trabeculae surrounded by a network of open spaces
• Found in epiphyses of bones
• Spaces contain red bone marrow
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Compact
• Structural unit is an osteon—calcified matrix arranged in
multiple layers or rings called concentric lamella
• Composed of osteocytes, lacunae, and canaliculi
• Covered by periosteum
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MICROSCOPIC STRUCTURE OF BONE
AND CARTILAGE
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Cartilage (Figure 6-4)
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Cell type called chondrocyte
Matrix is gel-like and lacks blood vessels
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BONE FORMATION AND GROWTH
(Figures 6-5 and 6-6)
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Early bone development (before birth)
consists of cartilage and fibrous structures
Cartilage models gradually replaced by
calcified bone matrix—process called
endochondral ossification
Osteoblasts form new bone, and
osteoclasts resorb bone
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DIVISIONS OF SKELETON
Skeleton composed of the following divisions
and their subdivisions:
 Axial skeleton
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Skull
Spine (vertebral column)
Thorax
Appendicular skeleton
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Upper extremities, including shoulder
(pectoral) girdle
 Lower extremities, including hip (pelvic) girdle
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Location and description of bones—see
Figures 6-7 to 6-17 and Tables 6-2 to 6-6
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DIFFERENCES BETWEEN A MAN’S AND A
WOMAN’S SKELETON
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Size—male skeleton generally larger
Shape of pelvis—male pelvis deep and
narrow; female pelvis broad and shallow
Size of pelvic inlet—female pelvic inlet
generally wider, normally large enough for
baby’s head to pass through it (Figure 618)
Pubic angle—angle between pubic bones
of female generally wider
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JOINT (ARTICULATIONS)
Every bone except the hyoid (which anchors the tongue) connects to
at least one other bone
 Kinds of joints (Figures 6-19 to 6-21)
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Synarthroses (no movement)—fibrous connective tissue grows
between articulating bones; example: sutures of skull
 Amphiarthroses (slight movement)—cartilage connects articulating
bones; example: symphysis pubis
 Diarthroses (free movement)—most joints belong to this class
• Structures of freely movable joints—joint capsule and ligaments hold
adjoining bones together but permit movement at joint
• Articular cartilage—covers joint ends of bones and absorbs joints
• Synovial membrane—lines joint capsule and secretes lubricating fluid
• Joint cavity—space between joint ends of bones
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Types of freely movable joints—ball-and-socket, hinge, pivot,
saddle, gliding, and condyloid
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