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The Skeletal System

A major overall function of the skeletal system is support and protection of the body’s internal organs. Bones make movement possible, store substances such as lipids and calcium, and are the site for hemopoiesis (red cell formation) in red bone marrow.

Functions of the Skeletal System

The following functions are performed by the skeletal system:
a. Support—The skeletal system is a bony framework that supports tissues and organs.
b. Protection—It encases delicate organs.
c. Movement—Muscles are anchored to bones and, as they contract, they pull on and move bones.
d. Storage—Bones maintain homeostasis of blood calcium. When the amount of calcium in blood increases to above normal, calcium moves out of blood and into bones for storage. When blood calcium decreases to below normal, it comes out of storage and enters the blood.
e. Hemopoiesis—Red bone marrow makes blood cells.

TYPES OF BONES

A. Long—example: humerus (upper arm)
B. Short—example: carpals (wrist)
C. Flat—example: frontal (skull)
D. Irregular—example: vertebrae (spinal cord)

STRUCTURE OF LONG BONES

A. Structural components
1. Diaphysis or shaft; a hollow tube made of compact bone
2. Medullary cavity; the hollow area inside the diaphysis that contains soft yellow bone marrow
3. Epiphyses or ends of the bone; spongy bone contains red bone marrow
4. Articular cartilage; covers epiphyses as a cushion; thin layer of cartilage covering each epiphysis; acts as a cushion between joint surfaces
5. Periosteum; strong membrane covering bone except at joint surfaces; a strong, fibrous membrane that covers a long bone everywhere except at its joint surfaces
6. Endosteum; lines medullary cavity

MICROSCOPIC STRUCTURE OF BONE AND CARTILAGE

A. Bone types
1. Spongy
a. Texture results from needlelike threads of bone called trabeculae surrounded by a network of open spaces
b. Found in epiphyses of bones
c. Spaces contain red bone marrow
2. Compact
a. Structural unit is haversian system—composed of concentric lamella, lacunae containing osteocytes, and canaliculi, all covered by periosteum
B. Cartilage
1. Cell type called chondrocyte
2. Matrix is gel-like and lacks blood vessels

BONE FORMATION AND GROWTH

A. Sequence of development early—cartilage models replaced by calcified bone matrix
B. Osteoblasts form new bone, and osteoclasts resorb bone

DIVISIONS OF THE SKELETON

Skeleton composed of the following divisions and their subdivisions:
A. Axial skeleton
1. Skull
2. Spine
3. Thorax
4. Hyoid bone
B. Appendicular skeleton
1. Upper extremities, including shoulder girdle
2. Lower extremities, including hip girdle

DIFFERENCES BETWEEN A MAN’S AND A WOMAN’S SKELETON

A. Size—male skeleton generally larger
B. Shape of pelvis—male pelvis deep and narrow; female pelvis broad and shallow
C. Size of pelvic inlet—female pelvic inlet generally wider; normally large enough for baby’s head to pass through it
D. Pubic angle—angle between pubic bones of female generally wider

JOINTS (ARTICULATIONS)

A. Kinds of joints
1. Synarthroses (no movement)—fibrous connective tissue grows between articulating bones; for example, sutures of skull
2. Amphiarthroses (slight movement)—cartilage connects articulating bones; for example, symphysis pubis
3. Diarthroses (free movement)—most joints belong to this class
a. Structures of freely movable joints—joint capsule and ligaments hold adjoining bones together but permit movement at joint
b. Articular cartilage—covers joint ends of bones and absorbs jolts
c. Synovial membrane—lines joint capsule and secretes lubricating fluid
d. Joint cavity—space between joint ends of bones
B. Types of freely movable joints—ball-and-socket, hinge, pivot, saddle, gliding, and condyloid
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