Biomedical Engineering Reference
In-Depth Information
body, helps with movement, produces blood cells, and stores important minerals. It is
made up of strong, rigid bones that are composed of specialized connective tissue, bear
weight, and form the major supporting elements of the body. Some support also comes
from cartilage that is a smooth, firm, resilient, nonvascular type of connective tissue.
Since the bones of the skeleton are hard, they protect the organs, such as the brain and
abdominal organs, that they surround.
There are 8 cranial bones that support, surround, and protect the brain. Fourteen facial
bones form the face and serve as attachments for the facial muscles that primarily move
skin rather than bone. The facial bones, except for the lower jaw (mandible), are joined with
each other and with the cranial bones. There are 6 auditory ossicles, 3 in each ear, that trans-
mit sound waves from the external environment to the inner ear. The hyoid bone, which is
near the skull but is not part of it, is a small U-shaped bone that is located in the neck just
below the lower jaw. It is attached to the skull and larynx (voice box) by muscles and liga-
ments and serves as the attachment for several important neck and tongue muscles.
The vertebral column starts out with approximately 34 bones, but only 26 independent
ones are left in the average human adult. There are 7 cervical bones including the axis,
which acts as a pivot around which the head rotates, and the atlas, which sits on the axis
and supports the “globe” of the head. These are followed by 5 cervical, 12 thoracic, and
5 lumbar vertebrae and then the sacrum and the coccyx. The last two consist of 5 fused
vertebrae. The vertebral column supports the weight of and allows movement of the head
and trunk, protects the spinal cord, and provides places for the spinal nerves to exit from
the spinal cord. There are four major curves (cervical, thoracic, lumbar, and sacral/coccy-
geal) in the adult vertebral column that allow it to flex and absorb shock. While movement
between any two adjacent vertebrae is generally quite limited, the total amount of move-
ment provided by the vertebral column can be extensive. The thoracic cage consists of
12 thoracic vertebrae (which are counted as part of the vertebral column), 12 pairs of ribs
and their associated cartilage, and the sternum (breastbone). It protects vital organs and
prevents the collapse of the thorax during ventilation.
Bones are classified as long, short, flat, or irregular, according to their shape. Long bones,
such as the femur and humerus, are longer than they are wide. Short bones, such as those
found in the ankle and wrist, are as broad as they are long. Flat bones, such as the sternum
and the bones of the skull, have a relatively thin and flattened shape. Irregular bones do not
fit into the other categories and include the bones of the vertebral column and the pelvis.
Bones make up about 18 percent of the mass of the body and have a density of 1.9 g/cm
3
.
The two types of bone are spongy and compact (cortical). Spongy bone forms the ends
(epiphyses) of the long bones and the interior of other bones and is quite porous. Compact
bone forms the shaft (diaphysis) and outer covering of bones and has a tensile strength
of 120 N/mm
2
, compressive strength of 170 N/mm
2
, and Young's modulus of 1.8
10
4
N/mm
2
. The medullary cavity, a hollow space inside the diaphysis, is filled with fatty
yellow marrow or red marrow that contains blood-forming cells.
Bone is a living organ that is constantly being remodeled. Old bone is removed by special
cells, osteoclasts, and new bone is deposited by osteoblasts. Bone remodeling occurs during
bone growth and in order to regulate calcium availability. The average skeleton is totally
remodeled about three times during a person's lifetime. Osteoporosis is a disorder in which
old bone is broken down faster than new bone is produced so the resulting bones are weak
and brittle.
