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Did you know..

..we have about 150 less bones now than we did when we were born? Babies are born with 350 bones, but as a child grows, some of these bones join together and an adult ends up with about 206 bones. The largest bone is the femur, or thigh bone, and the smallest is the stapes, or stirrup bone, in the middle ear, measuring about 2.6 to 3.4mm long.

Bones form the rigid framework of the human body. Each individual bone is named, however they are all part of the same skeletal system, and although they are all different in shape and size, each (long) bone has the same basic structure.

Diaphysis: This is the name given to the longest part of the bone, the main, middle region. Here we find the marrow cavity (medullary cavity), endosteum and yellow bone marrow.

Epiphysis: This is simply the end of bone. It is covered by hyaline cartilage to reduce friction between joints.

Bone marrow: This is the soft tissue which fills the cavities of the bones. It is a network of blood vessels and connective tissue fibres that produce new blood cells. There are two kinds of bone marrow:

  • Yellow bone marrow: mainly fatty tissue which is found generally in the long bones; and
  • Red bone marrow: which is found in the spongy bones, such as the vertebrae, sternum and ribs. New blood cells and platelets are formed here.

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Periosteum: The outer surface of bone is called the periosteum. It's a thin, dense membrane that contains nerves and blood vessels that nourish the bone. 

Compact bone: The next layer is made up of compact bone. This part is smooth and very hard. It's the part you see when you look at a skeleton. Within the compact bone are many layers of cancellous bone, which looks a bit like a sponge. Cancellous bone is not quite as hard as compact bone, but it is still very strong. In many bones, the cancellous bone protects the innermost part of the bone, the bone marrow.

Endosteum: Between the compact bone and cancellous bone is the endosteum.

Marrow cavity: As the name suggests, this is a hollow space at the centre of the diaphysis which contains yellow bone marrow.

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So, how is a long bone structured?

Lets work from the outside in. If we look at a long bone, the first thing we notice is its shape. Long and thin down the middle, and wide and oval shaped at the ends. We now know that this is the diaphysis and epiphysis of the bone. The epiphysis is covered with hyaline cartilage, and this serves to protect the surface of the epiphysis, as this is the point where joints occur (such as in your elbow or knee). Inside the epiphysis is spongy bone tissue and red bone marrow, which produces blood cells and platelets. 

The outer layer of compact bone is lined with periosteum, which is the connective tissue membrane that covers the entire bone. Under the periosteum is the compact bone. Compact bone is very strong and hard. Underneath is the endosteum, which covers the cancellous bone. Within the cancellous bone is a hollow space, the marrow cavity (medullary cavity), containing yellow bone marrow.

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