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Summary
THE PLANNER <
I
he Structure of Bone Controls
Function and Growth 114
• The skeletal system consists of all bones attached at joints and
the cartilage between the joints. The functions of the skeletal
system include support, protection, movement, mineral ho-
meostasis, blood cell production, and triglyceride storage.
• Bones are classified according to shape and include long
bones, short bones, flat bones, and irregular bones. A typi-
cal long bone has a number of features, including diaphysis
(shaft), epiphyses (ends), metaphysis, articular cartilage,
medullary (marrow) cavity, and periosteum (covering).
• As shown, bone tissue consists of various cells (osteogenic
cells, osteocytes, osteoblasts, and osteoclasts) surrounded
by a protein and mineral extracellular matrix. Compact bone
tissue has arrays of osteons with little space between them,
while spongy bone tissue consists of trabeculae with red
marrow-filled spaces.
Microscopic structure of bone • Figure 5.2
Canaliculi
Central
(haversian)
canal
Lacuna with
osteocyte
Concentric
lamellae
• Bone formation (ossification) occurs via two processes:
intramembranous ossification (in which mesenchyme is
replaced by bone tissue) and endochondral ossification (in
which cartilage is replaced by bone tissue).
• Bone grows near the ends (epiphyseal plates) and increases
in length. Bone is constantly destroyed by osteoclasts and
remade by osteoblasts (in a process called bone remodel-
ing). Fractures (broken bones) are healed by bone remodel-
ing and ossification. •
• Bone growth depends on the presence of minerals, vita-
mins, and hormones. Bones store and release calcium, mag-
nesium, and phosphate under the control of parathyroid
hormone and calcitonin and thereby regulate the calcium
concentration in the blood.
550x
Sectional view of an osteon
2
The Axial Skeleton Is Composed
of 80 Bones 122
• The skull consists of 8 cranial bones and 14 facial bones,
which protect the brain, provide attachment points for
muscles, and have openings for blood vessels and nerves.
• Bones of the skull have air-filled cavities (paranasal sinuses)
that are lined with mucous membranes and open into the
nasal cavity.
• As shown, the bones of the vertebral column include 7 cervi-
cal vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, the
sacrum (5 fused vertebrae), and the coccyx (4 fused verte-
brae).
The vertebral column • Figure 5.10a
Cervical
vertebrae (7)
Thoracic
vertebrae (12)
Lumbar
vertebrae (5)
Sacrum (1)
Coccyx (1)
• The vertebral column is curved and provides strength, sup-
port, and balance. It protects the spinal cord and enables
movement.
• Each vertebra consists of a body, a vertebral arch, and seven
processes. The vertebrae vary in size and shape, depending
on where in the spinal column they are located.
146 CHAPTER 5
The Skeletal System
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