dle protect the heart and lungs, whereas bones of the pelvic
girdle protect the lower abdominal and internal reproductive
organs. Whenever limbs or other body parts move, bones
and muscles interact.
Blood Cell Formation
The process of blood cell formation, called
sis), or hemopoiesis, begins in the yolk
sac, which lies outside the embryo (see chapter 23, p. 891).
Later in development, blood cells are manufactured in the
liver and spleen, and still later, they form in bone marrow.
Bones shape, support, and protect body structures, as well as
aid body movements. They house tissues that produce blood
cells and store various inorganic salts.
Support, Protection, and Movement
Bones give shape to structures such as the head, face, tho-
rax, and limbs. They also support and protect. For example,
the bones of the lower limbs, pelvis, and vertebral column
support the body’s weight. The bones of the skull protect the
eyes, ears, and brain. Bones of the rib cage and shoulder gir-
close to the new blood vessels, building spongy
bone nearby. Granulation tissue develops, and in
regions farther from a blood supply, fibroblasts
hen seven-year-old Jacob fell from
the tree limb, he had been hanging
about eight feet from the ground. He
landed in a crumpled heap, crying, with his right
leg at an abnormal angle. Emergency medical
technicians immobilized the leg and took Jacob
to the emergency department at the nearest hos-
pital, where an X ray indicated a broken tibia. He
spent the next six weeks in a cast, and the bone
continued to heal over several months. By the
next summer, Jacob was again climbing trees—
but more carefully.
Many of us have experienced fractured, or
broken, bones. A fracture is classiF
ed by its cause
and the nature of the break. ±or example, a break
due to injury is a traumatic fracture, whereas
one resulting from disease is a
A broken bone exposed to
the outside by an opening in the skin is termed
compound (open) fracture.
It has the added dan-
ger of infection, because microorganisms enter
through the broken skin. A break protected by
uninjured skin is a
. ±igure 7A shows
several types of traumatic fractures.
Repair of a Fracture
When a bone breaks, blood vessels in it rupture,
and the periosteum is likely to tear. Blood from
the broken vessels spreads through the damaged
area and soon forms a blood clot, or
Vessels in surrounding tissues dilate, swelling and
Within days or weeks, developing blood ves-
sels and large numbers of osteoblasts originat-
ing from the periosteum invade the hematoma.
The osteoblasts rapidly divide in the regions
produce masses of fibrocartilage. Meanwhile,
phagocytic cells begin to remove the blood
clot as well as any dead or damaged cells in the
incomplete, and the
break occurs on the
convex surface of the
bend in the bone.
complete, and the break
occurs at a right angle to
the axis of the bone.
caused by twisting a
occurs at an angle
other than a right angle
to the axis of the bone.
involves an incomplete
is complete and
fragments the bone.
Various types of fractures.