Human skeleton
The biggest bone in the body is the femur in the thigh and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 30–40% of the total body weight,[1] and half of this weight is water.
Fused bones include those of the pelvis and the cranium. Not all bones are interconnected directly: there are three bones in each middle ear called theossicles that articulate only with each other. The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments.
Development
Early in gestation, a fetus has a cartilaginous skeleton from which the long bones and most other bones gradually form throughout the remaining gestation period and for years after birth in a process called endochondral ossification. The flat bones of the skull and the clavicles are formed from connective tissue in a process known as intramembranous ossification, andossification of the mandible occurs in the fibrous membrane covering the outer surfaces of Meckel's cartilages. At birth, a newborn baby has over 300 bones, whereas on average an adult human has 206 bones[2] (these numbers can vary slightly from individual to individual). The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column.
Organization
See also: List of bones of the human skeleton
There are over 206 bones in the adult human skeleton, a number which varies between individuals and with age – newborn babies have over 270 bones[3][4][5] some of which fuse together into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.
Function
The skeleton serves six major functions.
Support
The skeleton provides the framework which supports the body and maintains its shape. The pelvis and associated ligaments and muscles provide a floor for the pelvic structures. Without the ribs, costal cartilages, the intercostal muscles and the heart would collapse.
Movement
The joints between bones permit movement, some allowing a wider range of movement than others, e.g. the ball and socket joint allows a greater range of movement than the pivot joint at the neck. Movement is powered by skeletal muscles, which are attached to the skeleton at various sites on bones. Muscles, bones, and joints provide the principal mechanics for movement, all coordinated by the nervous system.
Protection
The skeleton protects many vital organs:
- The skull protects the brain, the eyes, and the middle and inner ears.
- The vertebrae protects the spinal cord.
- The rib cage, spine, and sternum protect the lungs, heart and major blood vessels.
- The clavicle and scapula protect the shoulder.
- The ilium and spine protect the digestive and urogenital systems and the hip.
- The patella and the ulna protect the knee and the elbow respectively.
- The carpals and tarsals protect the wrist and ankle respectively.
Blood cell production
The skeleton is the site of haematopoiesis, which takes place in red bone marrow.
Storage
Bone matrix can store calcium and is involved in calcium metabolism, and bone marrow can store iron in ferritin and is involved in iron metabolism. However, bones are not entirely made of calcium,but a mixture of chondroitin sulfate and hydroxyapatite, the latter making up 70% of a bone.
Endocrine regulation
Bone cells release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat.
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