c. 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.
d. At their margins, flat bones are articulated with other flat bones and held
together by FCT. These fibrous connections are usually called sutures.
4-11. ORIGIN AND DEVELOPMENT
Flat bones generally begin as membranous, FCT models within the fetus. Again,
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
4-12. SPECIAL CONDITIONS OF THE FLAT BONES OF THE CRANIUM
The flat bones of the skull are somewhat special.
a. Curved Shape. They are generally curved. Together, they form a sphere
which surrounds and protects the brain.
b. Healing of Fractures. When the growth of the cranial flat bones is complete,
the osteogenic layer of the periosteum disappears.
(1) Cracks and/or line fractures of cranial flat bones will usually heal by the
activity of the osteoblasts within the bone.
(2) However, when bone substance is lost and a spatial defect ("hole")
remains, the missing portions of the table(s) will not be replaced. Osteoblastic activity
will repair only the margins of the spatial defect ("hole").
c. Variations in Brain Injury.
(1) In a young individual, the flat bones of the skull are not yet fully
developed. The cranium as a whole is relatively flexible. An injury to the brain,
resulting from a force applied to the cranium, will usually be located immediately below
the location of the applied force.
(2) In an older adult, the flat bones of the skull have fully developed and are
more or less fused to each other. The cranium is a relatively solid sphere. An injury to
the brain, resulting from a force applied to the cranium, will usually be found on the
opposite side from the applied force. Often, the applied force will be diverted around
the sphere to the base of the cranium. There, the diverted force may cause fractures of
the cranium at the apertures (openings) in its base.