BOLUS
B
Treatment sometimes involves
antibiotic
drugs.
However, a boil that is opened
surgically to release the pus usually
heals rapidly without the need for
drug treatment.
R a i s e d s u r f a c e
o f b o i l
P u s - fi lle d c a v it y
S w e a t g l a n d
B l o c k e d h a i r
f o l l i c l e
N e r v e
Cross-section of a boil
Following bacterial infection ofthe hair follicle,
there is a buildup of pus, and a raised tender
lump appears on the surface ofthe skin.
bolus
A soft mass of chewed food that is pro-
duced by the action of the tongue, the
teeth, and the saliva to facilitate swal-
lowing of the food. The term bolus is
also used to describe a single dose of a
drug that is rapidly injected into a vein.
bonding
The
reciprocal process by which a
strong tie, both psychological and emo-
tional, is established between a parent
and a newborn child. The process of
The bonding process
By the maintenance of close physical contact,
bonding gradually becomes established.
bonding may be delayed if a baby is
premature or ill and has to be separated
from his or her parents immediately
after birth (for example, by being placed
in an incubator).
bonding, dental
Techniques that use plastic resins and
acrylic or porcelain veneers to repair,
restore,
or cosmetically improve the
teeth.
Dental bonding may sometimes be
used as an alternative to crowning (see
crown, dental
) and can also be used to
protect the teeth.
bone
The structural material of the
skeleton
that provides a rigid framework for the
muscles and protects certain organs of
the
body.
In
combination with the
joints and the muscles, the bones form
the locomotor system.
STRUCTURE
Bone consists of several layers. The sur-
face has a thin covering known as the
periosteum, a membrane that contains a
network of blood vessels and nerves.
Beneath the periosteum is an inner shell
of hard (also called compact or cortical)
bone composed of columns of bone
cells (osteoclasts and osteoblasts). Each
column has a central hollow (haversian
canal) that is important for the nutri-
tion, growth, and repair of the bone.
The direction of the haversian canals
corresponds with the mechanical forces
acting on the bone.
Inside the hard shell, bone has a cen-
tral meshlike structure (which is known
as spongy,
cancellous,
or
trabecular
bone). The cavity in the centre of cer-
tain bones, and the spaces in spongy
bone, contain
bone marrow,
in which
the red blood cells, platelets, and most
of the white blood cells are formed.
GROWTH
Bone is continuously reabsorbed by
osteoclasts and replaced by osteoblasts.
The osteoblasts encourage deposition of
calcium
phosphate
on
the
protein
framework of the bone; the osteoclasts
remove it. The actions of these cells are
controlled by growth hormone, secreted
by the pituitary gland, the sex hormones
oestrogen and testosterone, the adrenal
hormones, the thyroid hormone thyro-
calcitonin, and parathyroid hormone.
These hormones also work to maintain
the level of calcium in the blood.
Most bones begin to develop in the
embryo during the fifth or sixth week
of pregnancy, at which time they take
the form
of
cartilage.
This
cartilage
begins to be replaced by hard bone, in a
process known as
ossification,
at around
the seventh or eighth week of pregnancy;
this process is not complete until early
adult life. At birth, many bones consist
mainly of cartilage, which ossifies later
in life. The
epiphyses
(the growing ends
of the long bones) are separated from
the bone shaft
(diaphysis)
by the epiphy-
seal plate. Some bones in the body, such
as certain skull bones, do not develop
from cartilage, and these are known as
membranous bones.
bone abscess
A localized collection of pus in a bone
(see
osteomyelitis).
bone age
A measure of skeletal maturity used to
assess physical development in children.
X-rays,
which show how much bones
have grown in a particular body area,
are used to determine bone age. (See
also
age
.)
bone cancer
Malignant
growth in bone. Bone cancer
may originate in the bone itself (primary
bone cancer) or, more commonly, may
occur as a result of cancer spreading
from elsewhere in the body (secondary,
or metastatic, bone cancer).
PRIMARY BONE CANCER
Cancers that originate in the bone are
rare; the type of primary bone cancer
that occurs most often is
osteosarcoma
.
Other types include
chondrosarcoma
and
fibrosarcoma.
Cancers can also start in the
bone marrow, but these are not usually
considered to be bone cancers (see
mul-
tiple
myeloma
and
leukaemia).
The
treatment of primary bone cancer will
depend on the extent to which the dis-
ease has spread. If it remains confined
to bone, it may be possible to remove
the cancer and fill the defect with a
bone
graft.
In other cases, amputation may be
recommended.
Radiotherapy
or
chemo-
therapy,
or both, may also be needed.
SECONDARY BONE CANCERS
The cancers that spread most readily to
form secondary bone cancer are those
of the breast, lung, prostate, thyroid,
and kidney. These bone metastases occur
commonly in the spine, pelvis, ribs, and
skull. Pain is usually the main symptom.
Affected bones are abnormally fragile
and may fracture easily. Bone cancer that
affects the spine may cause collapse or
crushing of vertebrae, damaging the
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