Name:
Cortical/medullary bone; mineral storage    
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| Definition:
The cortical bone is the outer, compact or dense bone, and the medullary
bone is the inner, spongy, or cancellous bone. |
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 Structure:
A thick layer of cortical bone makes
up the shaft of the bone, and an inner spongy layer contains irregularly
shaped spaces lined with crystals of minerals and proteins. The medullary
shaft is the inner, tubular space, sometimes called the marrow cavity.
Both ends of long bones, the epiphyses, have epiphyseal lines, that indicate
where growth occurred during development.
The outside of the bone is covered by periosteum, and the inner spaces
are covered by a thin layer of endostium, both of which contain osteoblast
and osteoclast cells. The cortical bone is a lattice of tube-shaped osteons,
aka Haversian systems, tightly packed, but with blood and lymph channels
and nerves traversing them up and down, centrally (Haversian canals), and
perforating through them horizontally (VolkmannÕs canals) from the
periosteum into the medullary cavity.
 Function:
The strength of the femur comes from the thick
cortex, and the small flexibility of long bones
comes from the twisted bundles of collagen, along with mineral components
of the bone matrix, called osteons. Inside the cortical bone, remodeling
occurs rapidly (months)in the cancellous, or spongy bone, where the minerals
and proteins are stored in thin bridges, but it occurs slowly (years) in
the dense, cortical bone. Blood cell formation
(hematopoiesis) occurs in the cancellous bone, especially in the ends of
the bones. The medullary cavity is mainly filled with yellow fat.
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