Bones are rigid organs that are made up of two main types of bone tissue:
Compact bone makes up the dense outer layer of bones.
Spongy bone is lighter and less dense than compact bone, and is found toward the center of the bone. The tough, shiny, white membrane that covers all surfaces of bones is called the periosteum
Many bones also contain in their innermost part a soft connective tissue called bone marrow. There are two types of bone marrow: red marrow and yellow marrow. Red marrow makes blood cells. Yellow marrow consists of fat cells although it will have a different role to that of normal fat cells
The bones of newborn babies contain only red marrow. As children get older, thei red marrow is replaced by yellow marrow. In adults, red marrow is found mostly in the bones of the skull, the ribs, and pelvic bones.
Classification of bones
There are four main types of bones in the human body. They can be long, short, flat, or irregular. Identifying a bone as long, short, flat, or irregular is based on the shape of the bone not the size of the bone.
Long bone are longer than wide . They consist of a long shaft (diaphysis) with two bulky ends or extremities(epiphyses). The structure of a long bone is shown in Figure. They are primarily compact bone but may have a large amount of spongy bone at the ends or extremities. Long bones include femur, tibia, and fibula of the leg, the humerus, radius, and ulna of the arm and the phalanges of the fingers and toes.
Short bones are roughly cube shaped with vertical and horizontal dimensions approximately equal. They consist primarily of spongy bone, which is covered by a thin layer of compact bone. Short bones include the bones of the wrist and ankle.
Flat bones are thin, flattened, and usually curved. Most of the bones of the cranium are flat bones.
Irregular bones are not in any of the above three categories. They are primarily spongy bone that is covered with a thin layer of compact bone. The vertebrae and some of the bones in the skull are irregular bones.
In this video you can watch the different types of bones: