The framework of the body is built upon a series of bones, supplemented in certain regions by cartilage; the bony part of the framework constitutes the skeleton.
The skeletal system serves several functions, among them are:
- Protection and support: The ribs protect the organs of the thorax and the skull protects the brain. The legs support the weight of the entire body. The vertebrae also support the upper body.
- Movement: Most skeletal muscles attach to the bones of the skeletal system and use them as leverage points for movement of the body.
- Production of blood cells: The bone marrow produces blood cells in a process known as hematopoiesis.
- Storage: "Yellow bone marrow" or adipose tissue stores fat in the medullary cavity of long bones. Bones can also be broken down to release inorganic calcium and phosphorus stored in the non cellular matrix of the bone.
In the skeleton of the adult there are generally 206 distinct bones:
- Vertebral column 26
- Skull 22
- Hyoid bone 1
- Ribs and sternum 25
- Auditory ossicles 6
- Total 80
- Upper extremities 64
- Lower extremities 62
- Total 126
Types of Bones
Bones are divisible into four classes: Long, Short, Flat, and Irregular. Long Bones are found in the limbs and function as levers, they are longer than they are wide. Short Bones transfer forces of movement and are cube shaped as in the carpus and tarsus. Flat Bones are used for either extensive protection or the provision of broad surfaces for muscular attachment. The bones expand into broad, flat plates, as in the skull and the scapula. Irregular Bones have peculiar forms, cannot be grouped under the preceding heads, and are used for muscle attachement and articulation. Some irregular bones include the vertebræ, sacrum and coccyx.
Long bones consists of a body or shaft and two extremities. The body, or diaphysis is cylindrical, with a central cavity termed the medullary cavity. The wall consists of dense, compact tissue of considerable thickness in the middle part of the body, but becoming thinner toward the extremities. Within the medullary cavity is adipose tissue or "yellow bone marrow".
The extremities are refered to as the epiphysis. Within the epiphysis is the "spongy bone" also known as "red bone marrow". It is within this marrow that red blood cells are produced at an average rate of 2.5 million per second. Running horizontally across the spongy bone of this region is the Epiphyseal line which is a region of cell growth responsible for lateral bone growth during youth, when growth is complete this line calcifies and becomes known as the epiphyseal plate.
Nutrient foramen run through the compact bone and allow the passage of nutrients in and out of the bone. There is a thin outer layer of connective tissue called the PERIOSTEUM which is highly vascular and allows for muscle and tendon attachment, it is bound to the bone itself by PERFORATING FIBERS which are composed of collagen. This layer does not cover the articulating regions of the bone. The bones belonging to this class include: the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges.
Short bones are generally equal in length, width, and thickness. They are found in the wrists and ankles. Aside from points of insertion and vascular areas, short bones are almost completely covered by articular surfaces.
These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of spongy bone. They generally offer protection, as is the case with the bones of the cranium and with the ribs and sternum.