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Other Terms: Body, Skeleton (in vivo), Set of all bones, Set of bones of body
This is not a static framework, but a highly dynamic internal scaffolding. It is dynamic in many ways. On one hand, it shows extreme flexibility of movement when acted upon by muscles. At another extreme, the cells of skeletal tissue are constantly monitoring and changing the micro-structure of this amazing tissue called bone, providing it with maximal strength, toughness, and resilience. In addition to its dynamic role of support it also provides a protective and stabilizing function. The skull and vertebral column surround the delicate central nervous structures, the brain and spinal cord, providing a strong, protective shell. This protective case, called the cranium, also fixes in space important nervous structures, such as the internal ear, that would not be able to function properly in an unstable environment. This dynamic framework also exhibits a tremendous capacity for growth and repair. It is a dynamic storehouse of calcium ions, ions that play a significant role in many of the body's functions. The skeleton consists of 206 separate bones, ignoring various sesamoid bones and the fact that some bones represent the fusion of multiple bones. These bones range in size from the small ear ossicles measuring a few millimeters in length, to the large femur measuring up to fifty centimeters. The skeleton is divisible into two portions, the axial skeleton and the appendicular skeleton. The axial skeleton includes the skull, vertebral column, ribs, and sternum. The appendicular skeleton consists of the bones of the limbs and their girdles. The individual bones of the skeleton come in a variety of shapes. Some are long and tubular, while others have the spread-winged appearance of a butterfly. Bones can be grouped into four shape categories. Although not that meaningful, the four categories descriptively group the bones. The four shape categories are: long bones, short bones, flat bones, and irregular bones. Long bones are unique in having a diaphysis or shaft with a medullary cavity. The other bone types lack this hollow tubular region. The short, flat, and irregular bones are similar in having outer plates of compact bone surrounding centers of cancellous bone. In general, long bones and short bones are found in the appendicular skeleton, while flat bones and irregular bones occur in the axial skeleton. In the right hands, the skeleton can be a library of information. Its markings, foramina, landmarks, and canals each tell a story about the soft tissues of the body. A strong foundation of skeletal anatomy goes a long way.
The word skeleton comes from the Greek term skello meaning to make dry. Early anatomists used the word to describe a dried body or mummy. Later, anatomists applied it to the dried bones of an animal in their natural position. In modern parlance it applies to any supporting or protecting framework.
The individual bones of the skeleton articulate in a variety of ways. These meeting places of the bones range from very stable junctions of limited movement, to highly mobile contiguous surfaces. Three basic joint types are defined. Each type is characterized by the type of tissue that unites the bones. Fibrous joints are bones joined by dense collagenous connective tissue. These joints typically have small ranges of motion. They are found throughout the skeleton, but are most common in the skull. Examples include cranial sutures, the gomphoses of the teeth, and the syndesmotic interosseous membrane uniting the tibia and fibula. Cartilaginous joints are the least numerous joints in the body. They are of two types. The first consists of tough pads of fibrocartilage interposed between bones, such as the intervertebral discs. Symphysis is the name given this type of joint. The second category of cartilage joint, a synchondrosis, is a temporary joint. These are the growth plates of bones. They are cartilaginous junctions of bone that allow the bone to grow in length. Eventually these cartilage plates are replaced by bone, resulting in bone fusion and the cessation of growth in length. The final category of joints is the synovial or capsular joint. This joint has an outer fibrous capsule and an inner loose connective tissue, or synovial, capsule surrounding the articular cartilage ends of contiguous bones. Within the capsule is a synovial fluid. This is a lubricant that allows the bones to move against one another in a frictionless manner. These are the most numerous type of skeletal union, providing a wide range of motion between neighboring bones. Examples include the highly mobile shoulder joint at one extreme to the intercarpal joints, showing very limited ranges of motion, at the other extreme.
Ossification of the skeleton occurs via two distinct modes. Endochondral ossification, the more widespread of the two modes, accounts for the majority of skeletal formation. The entire post-cranial skeleton and much of the cranial base forms endochondrally. This process involves the formation of cartilaginous anlagen, cartilage models of the bones, followed by the eventual replacement of these precursors by bone. The other form of ossification, intramembranous or dermal bone formation, is confined to the skull with the exception of a small intramembranous contribution to the clavicle's sternal end. Intramembranous ossification involves osteogenic centers forming in prenatal connective tissue. There is no precursor to the bone. Instead, the bone spreads as thin plates through the membranous connective tissue. Most of the bones of the cranial vault and face form via this mechanism.