The
Skeleton: The
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The average human adult skeleton has 206 bones joined to ligaments
and tendons to form a protective and supportive framework.
The skeleton has two main parts: the axial skeleton and the appendicular
skeleton.
The axial skeleton consists of the skull, the spine, the ribs and
the sternum (breastbone) and includes 80 bones.
The appendicular skeleton
includes two limb girdles (the shoulders and pelvis) and their attached limb
bones. This part of the skeletal system contains 126 bones, 64 in the shoulders
and upper limbs and 62 in the pelvis and lower limbs.
There are only minor differences between the skeletons of the male and the female: the men's bones tend to be larger and heavier than corresponding women's bones and the women's pelvic cavity is wider to accommodate childbirth.
The skeleton plays an important part in movement by providing a series of independently
movable levers, which the muscles can pull to move different parts of the body.
It also supports and protects the internal body organs. 
The skeleton is not just a movable frame, however; it is an efficient factory
which produces red blood cells from the bone marrow of certain bones and white
cells from the marrow of other bones to destroy harmful bacteria. The bones
are also a storehouse for minerals - calcium, for example - which can be supplied
to other parts of the body.
Babies are born with 270 soft bones - about 64 more than an adult; and many of these will fuse together by the age of twenty or twenty-five into the 206 hard, permanent bones.
Bones:
A living bone consists of three layers: the periosteum, or outside skin of
the bone; the hard compact bone; and the bone marrow.
If we were to cut a
living bone in half, we would see that it contains various layers.
First is
a layer of thin, whitish skin which is packed with nerves and blood vessels and
supplies the cells of which the hard bone below is built.
Next is a dense,
rigid bone called the compact bone. It is shaped like a cylinder and is so hard
that surgeons must use a saw to cut through it. It is honeycombed with thousands
of tiny holes and passageways, through which run nerves and blood vessels that
supply oxygen and nutrients to the bone. This dense layer supports the weight of
the body and is made up of mostly calcium and minerals, so that it feels no
pain.
The "skin," however, is very sensitive, so that when a bone is broken,
injured nerve fibers run through the compact bone and send messages which relay
the pain signals to the brain.
If we cut though the compact bone, we find
that its cylinder surrounds and protects the spongy bone marrow which contains a
material much like gelatin. This marrow produces either red blood cells (which
carry oxygen), white blood cells (which fight infection), or platelets (that
help stop bleeding).
These three bone layers work together with nerve signals which speed back and forth and blood streams which move between the layers. Thighbones are usually stronger, pound for pound, than reinforced concrete.
Joints: 
A ligament is a tough band of white, fibrous, slightly elastic tissue. This
is an essential part of the skeletal joints; binding the bone ends together to
prevent dislocation and excessive movement that might cause breakage.
Ligaments also support many internal organs; including the uterus, the
bladder, the liver, and the diaphragm and helps in shaping and supporting the
breasts. Ligaments, especially those in the ankle joint and knee, are sometimes
damaged by injury. A "torn" ligament usually results from twisting stress when
the knee is turned while weight is on that particular leg. Minor sprains are
treated with ice, bandages and sometimes physical therapy, but if the ligament
is torn, the joint may be placed in a plaster cast to allow time to heal or it
may require surgical repairs. If a ligament is made up of several thick bands of
fibrous branches, it is called a "collateral ligament."
The word "ligament" comes from the Latin word, "ligamentum," meaning a band or tie.
Tendons attach muscles to bones. Tendons are tough and do not stretch - they are inelastic - this ensures that all the movement of the muscle is passed on to the bone.