Although a skeleton sometimes symbolizes death and creepy,
scary things, it is one of the body’s most life-giving systems. Unlike other
living organs, bones are firm and strong, but they have their own blood, lymphatic
vessels, and nerves.
There are two types of tissue inside bones:
- Compact
bone: This hard and dense tissue makes up the outer layer of most bones and
the main shaft of long bones, such as those in the arms and legs. Nerves and
blood vessels live inside this tissue.
- Spongy
bone: This tissue is made up of smaller plates filled with red bone marrow.
It is found at the ends of long bones, like the head of the femur, and at the
center of other bones.
Red bone marrow
forms most of the blood cells in the body and helps destroy old blood cells.
Another type of marrow, yellow bone
marrow, resides in the central cavities of long bones. It is mostly made up
of fat. However, if the body suffers large amounts of blood loss, it can
convert yellow marrow to red to make more blood cells.
The skull consists
of 22 separate bones that make up the cranium, the housing for the brain. Twenty-one
of those bones are fused together by sutures, nearly rigid fibrous joints. The
lower-most bone of the skull is the mandible,
or jawbone.
The spine, or vertebral
column, is a series of irregularly shaped bones in the back that connects
to the skull. At birth, humans have 33 or 34 of these bones. But bones fuse as
we age, and the result is 26 separate bones in the spines of adults.
The rib cage is
made up of 12 pairs of bones that encase vital organs in the chest. The bones
curve from the back at the vertebral column to the front of the body. The upper
seven pairs meet with the sternum, or chest bone. The remaining five pairs are attached
to each other via cartilage or do not connect.
The muscles of the shoulders and arms include the clavicle (collarbone), scapula (shoulder blade), humerus, radius, ulna, and the
bones of the wrist and hand.
The hip bones are
three sets of bones—ilia, ischia, and pubes—that fuse together as we grow older. These form the majority
of the pelvis at the base of the spine as well as the socket of the hip joint. The
sacrum—five fused bones and at the bottom of the spine—and the coccyx, or
tailbone, make the rest of the bones in the pelvic region.
The head of the femur,
the largest and longest bone in the body, creates the other half of the hip
joint and extends down to form part of the knee. It begins the bones of the leg.
The other bones of the leg include the tibia,
fibula, and the bones of the ankle
and foot.
The most common condition that affects bones is fracture, or
when a bone endures such a great impact that it breaks.
Other common conditions that affect the skeletal system
include:
- Osteoporosis:
This is a disease in which the bones become fragile and prone to fracture.
- Leukemia:
This is a cancer of the white blood cells.
- Osteopenia,
osteitis deformans, and osteomalacia: Similar to osteoporosis,
these are other types of bone loss.
- Scoliosis,
kyphosis, and lordosis: These are abnormalities of the spinal curve.
