1-8.
BONE STRUCTURE
a. General. Bone is a tissue composed of living cells (osteocytes) distributed in
an intercellular matrix that contains organic and inorganic substances. The organic
component, largely collagenous fibers, is responsible for the strength and resilience of
bone while the inorganic salts, mostly calcium phosphate, contribute to its hardness,
and rigidity. The inorganic constituents make up approximately 67% of bony matter in
the adult. The radiopacity of bone depends largely upon the amount of minerals
present. Lack of mineral content in the young and aged alters the radiopacity and
requires compensation.
b. Forms of Bone Tissue. There are two forms of bone tissue, cancellous and
compact. Cancellous or spongy bone consists of irregular strands of tissue, which
branch and join one another, forming a loose network in which the intercommunicating
spaces are filled with marrow. Compact or dense bone has a more solid, regular
appearance and its intercommunicating canals are microscopic in size. The basic
structure of these two types of bone is essentially the same. They differ mainly in the
relative amount of solid substance and the number, size, and arrangement of the
intercommunicating spaces they contain. Both cancellous and compact forms are
present in most bones of the body, but the extent and distribution of each varies
considerably. In adults, the exterior of all bones is compact bone while the interior is
usually cancellous.
c. A Typical Long Bone. In a typical long bone (figure 1-11), each end
(epiphysis) is largely cancellous and is covered by a thin layer of compact bone. The
reverse is true in the shaft (diaphysis), which is mostly compact bone tissue. The
central medullary canal, or cavity in the shaft of a long bone, is continuous with the
intercommunicating spaces in the cancellous bone located at the ends. Depending on
the age of the individual and the type of bone considered, either red or yellow marrow
fills these cavities. Red marrow, active in the production of blood cells, is present in all
bones at birth and blood cells are produced in all locations. With advancing age, the
production of blood cells decreases and red marrow is replaced by yellow marrow,
which consists mostly of fat cells. In the adult, red marrow is found mainly in the skull,
vertebrae, ribs, sternum, and the articular ends of some long bones.
d. Long Bone Structure. Each long bone, except for its articular surface, is
enclosed by a thick, fibrous sheet of membranous tissue, called the periosteum, which
develops when the perichondrium, the outer covering of the embryonic skeleton,
becomes permeated with blood vessels. The marrow cavity, and also the canal system,
are lined by a delicate layer of reticular (netlike) tissue, called the endosteum.
e. Some Other Bones. In flat bones, such as the ribs, one or more plates of
compact bone surround the cancellous bone. In many irregular bones, such as the
vertebrae, spongy bone is enclosed by a thin shell of compact bone.
MD0956
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