Structure and Development of a Typical Flat Bone
The flat bones have two layers of dense bony tissue, called tables.
There is an inner table and an outer table.
Generally, between the two tables is a layer of cancellous bony tissue.
The spaces of this cancellous bony tissue are filled with red marrow.
In adults, the red marrow of the flat bones is the primary blood-cell forming area
of the body.
As with the cancellous tissue of the long bone, the cancellous tissue of
the flat bone is organized into trabeculae.
The trabeculae are oriented in the same directions as the lines of applied forces,
much like the struts of a building.
Adjacent to the nasal cavities, many flat bones are hollowed to form the
paranasal sinuses. These hollow spaces take the place of cancellous bony tissue. The
development of the mastoid bone is likewise formed by the extension of the air-filled
cavity of the middle ear into the mastoid bone.
The outer surface of the outer table and the under surface of the inner table
are covered with periosteum. The periosteum is similar to that described for the
"typical" long bone.
At their margins, flat bones are articulated with other flat bones and held
together by FCT. These fibrous connections are usually called sutures.
Origin and Development
Flat bones generally begin as membranous, FCT models within the fetus.
Then an invasion of material forms an ossification center.
This center tears down and replaces the FCT with bone tissue. The ossification
center continues to grow outward.
In time, a full plate of bone has been formed. Then, the flat bone grows at its margins
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