The human skeleton is composed of 300 bones at birth and by the time adulthood is reached, some bones have fused together to give a total of 206 bones in the body. The bone mass in the skeleton reaches maximum density around age 30. The human skeleton can be divided into the axial skeleton and the appendicular skeleton. The axial skeleton is formed by the vertebral column, the rib cage and the skull. The appendicular skeleton, which is attached to the axial skeleton, is formed by the pectoral girdles, the pelvic girdle and the bones of the upper and lower limbs.
The human skeleton serves six major functions; support, movement, protection, production of blood cells, storage of ions and endocrine regulation.
The human skeleton is not as sexually dimorphic as that of many other primate species, but subtle differences between sexes in the morphology of the skull, dentition, long bones, and pelves exist. In general, female skeletal elements tend to be smaller and less robust than corresponding male elements within a given population. The pelvis in female skeletons is also different from that of males in order to facilitate child birth.
- 1 Divisions
- 2 Function
- 3 Sexual dimorphism in the human skeleton
- 4 Disorders
- 5 References
The axial skeleton (80 bones) is formed by the vertebral column (32-34; the number of the vertebrae differs from human to human as the lower 2 parts, sacral and coccygeal bone may vary in length), the rib cage (12 pairs of ribs and the sternum), and the skull (22 bones and 7 associated bones). The upright posture of humans is maintained by the axial skeleton, which transmits the weight from the head, the trunk, and the upper extremities down to the lower extremities at the hip joints. The bones of the spine are supported by many ligaments. The erectors spinae muscles are also supporting and are useful for balance. The vertebral column consists of 5 parts. The most cranial (uppermost) part is made up by the cervical vertebras (7), followed by thoracic (12), lumbar (5), sacral (4-5) and coccygeal vertebras (3-4). Cervical vertebras make up the junction between the vertebral column and the cranium. Sacral and coccygeal vertebras are fused and thus often called "sacral bone" or "coccygeal bone" as unit. The sacral bone makes up the junction between the vertebral column and the pelvic bones.
The appendicular skeleton (126 bones) is formed by the pectoral girdles, the upper limbs, the pelvic girdle or pelvis, and the lower limbs. Their functions are to make locomotion possible and to protect the major organs of digestion, excretion and reproduction.
The skeleton serves six major functions; support, movement, protection, production of blood cells, storage of ions and endocrine regulation.
The skeleton provides the framework which supports the body and maintains its shape. The pelvis, associated ligaments and muscles provide a floor for the pelvic structures. Without the rib cages, costal cartilages, and intercostal muscles, the lungs would collapse.
The joints between bones allow 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.
The skeleton protects many vital organs:
- The skull protects the brain, the eyes, and the middle and inner ears.
- The vertebrae protect 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
Bone matrix can store calcium and is involved in calcium metabolism, and bone marrow can store iron in ferrotin 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. Hydroxyapatite is in turn composed of 39.8% of calcium, 41.4% of oxygen, 18.5% of phosphorus, and 0.2% of hydrogen.
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.
Sexual dimorphism in the human skeleton
Anatomical differences between human males and females are highly pronounced in some soft tissue areas, but tend to be limited in the skeleton. The human skeleton is not as sexually dimorphic as that of many other primate species, but subtle differences between sexes in the morphology of the skull, dentition, long bones, and pelves (sing. pelvis) are exhibited across human populations. In general, female skeletal elements tend to be smaller and less robust than corresponding male elements within a given population.
Long bones are generally larger in males than in females within a given population. Muscle attachment sites on long bones are often more robust in males than in females, reflecting a difference in overall muscle mass and development between sexes. Sexual dimorphism in the long bones is commonly characterized by morphometric or gross morphological analyses.
Human pelves exhibit greater sexual dimorphism than other bones, specifically in the size and shape of the pelvic cavity, ilia, greater sciatic notches, and the sub-pubic angle. The Phenice method is commonly used to determine the sex of an unidentified human skeleton by anthropologists with 96% to 100% accuracy in some populations.
There are many classified skeletal disorders. One of the most common is osteoporosis. Also common is scoliosis, a side-to-side curve in the back or spine, often creating a pronounced "C" or "S" shape when viewed on an x-ray of the spine. This condition is most apparent during adolescence, and is most common with females.
Osteoporosis is a disease of bone, which leads to an increased risk of fracture. In osteoporosis, the bone mineral density (BMD) is reduced, bone microarchitecture is disrupted, and the amount and variety of non-collagenous proteins in bone is altered. Osteoporosis is defined by the World Health Organization (WHO) in women as a bone mineral density 2.5 standard deviations below peak bone mass (20-year-old sex-matched healthy person average) as measured by DXA; the term "established osteoporosis" includes the presence of a fragility fracture. Osteoporosis is most common in women after the menopause, when it is called postmenopausal osteoporosis, but may develop in men and premenopausal women in the presence of particular hormonal disorders and other chronic diseases or as a result of smoking and medications, specifically glucocorticoids, when the disease is craned steroid- or glucocorticoid-induced osteoporosis (SIOP or GIOP).
Osteoporosis can be prevented with lifestyle advice and medication, and preventing falls in people with known or suspected osteoporosis is an established way to prevent fractures. Osteoporosis can also be prevented with having a good source of calcium and vitamin D. Osteoporosis can be treated with bisphosphonates and various other medical treatments.
- Miller, Larry (2007-12-09). "We're Born With 300 Bones. As Adults We Have 206". Ground Report.
- "How many bones does the human body contain?". Ask.yahoo.com. 2001-08-08. Retrieved 2010-03-04.
- Exploring our human bodies. San Diego Supercomputer Center Education
- Lee, Na Kyung; Sowa, Hideaki; Hinoi, Eiichi; Ferron, Mathieu; Ahn, Jong Deok; Confavreux, Cyrille; Dacquin, Romain; Mee, Patrick J.; McKee, Marc D.; Jung, Dae Young; Zhang, Zhiyou; Kim, Jason K.; Mauvais-Jarvis, Franck; Ducy, Patricia; Karsenty, Gerard (2007). "Endocrine Regulation of Energy Metabolism by the Skeleton". Cell 130 (3): 456–69. doi:10.1016/j.cell.2007.05.047. PMC 2013746. PMID 17693256.
- Buikstra, J.E.; D.H. Ubelaker (1994). Standards for data collection from human skeletal remains. Arkansas Archaeological Survey. p. 208.
- Phenice, T. W. (1969). "A newly developed visual method of sexing the os pubis". American Journal of Physical Anthropology 30 (2): 297–301. doi:10.1002/ajpa.1330300214. PMID 5772048.
- Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group. Technical Report Series 843. World Health Organization. 1994. ISBN 92-4-120843-0. PMID 7941614.[page needed]
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