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 fibres, 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.
B.FORMS OF B ONE TISSU. There are two forms of bone tissue, cancellous and compact.Cancellous or
spongy bone consists of irregular strands of tissue, which branch and joins one another, forming a loose
networkin 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 arepresent 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 whilethe
interior is usually cancellous.
C. ATYPICALLONG B ONE. 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, either red or yellow marrow fills these cavities. Red marrow,
active in the production of blood cells, ispresent 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. ONG 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, is 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. 1-11. DESCRIPTIVE TERMINOLOGY
By reviewing the following terms, it will help you understand the discussion of bones more easily.
a. Extremity – The distal or terminal portion of a bone. An arm or leg is also referred to as