Bones (or osseous tissues) are specialised connective tissues consisting of living cells and a matrix which consists of water (25%), extracellular fibres (25%) and inorganic salts (50%). The durability of the bone results from the deposition of calcium salts in the matrix.
Bones are the primary structure of the skeletal system. The human skeleton contains 206 bones, which lie buried within the muscles and other soft tissues to form the body's supporting framework.
Bones perform several functions:
- Stabilise and support the entire body
- Protect the internal tissues and organs (for example, the ribs protect the lungs and heart, the skull protects the brain and the vertebrae protect the spinal cord)
- Provide a surface for the attachment of muscles and ligaments
- Permit movement of the body by forming joints that are moved by muscles
- Contain the bone marrow (a tissue located within the internal cavities of the bones), which produces red blood cells
- Store and provide a reservoir of mineral salts
Bones in the human being are classified according to their shape: long, short, flat, irregular and sesamoid.
Long bones have
greater length than width. They act as levers that are pulled by contracting
Long bones include:
- The femur (in the thigh)
- Tibia and fibula (in the leg)
- Humerus (in the arm)
- Radius and ulna (in the forearm)
- Phalanges (in the fingers and toes).
Short bones are about equal length, width and thickness. They are found where only limited movement is required, such as the carpal bones in the wrists and the tarsal bones in the ankles.
Flat bones are thin and are actually more curved than flat. They provide protection of the soft tissues and afford extensive areas for muscle attachment. Flat bones include the cranial bones, which protect the brain; the sternum (or breast-bone) and the ribs, which protect the thoracic organs; and the scapula (the shoulder blade).
Irregular bones have complex shapes that can not fit neatly into any other category. They include vertebrae, mandible and hip bones, and function to protect the deeper organs or tissues.
Sesamoid bones are small in size and embedded within certain tendons where there is considerable friction, tension and physical stress, such as the palms and the soles. Their function is to protect tendons from excessive wear and tear. They vary in number from person to person. The patella or kneecap is the largest sesamoid bone and is present in every person.