Skeletal System: Facts, Function & Diseases

The adult human skeletal system consists of 206 bones, as well as a network of tendons, ligaments and cartilage that connects them. The skeletal system performs vital functions — support, movement, protection, blood cell production, calcium storage and endocrine regulation — that enable us to survive.

Animals with internal skeletons made of bone, called vertebrates, are actually the minority on Earth. As much as 98 percent of all animals are invertebrates, meaning they do not have internal skeletons or backbones.

The amount of bones a person is born with isn’t the final tally later on. Human infants are born with about 300 bones, some of which fuse together as the body develops. By the time humans reach adulthood, they have 206 bones, according to Arizona State University’s School of Life Sciences. Human males grow until their late teens and females grow until two years after the beginning of their menstrual cycle, typically. This is when the growth plates on bones usually close, halting bone expansion.

The skeletons of adult males and females have some variation, primarily to accommodate childbirth. The female pelvis is flatter, more rounded and proportionally larger, for example. A male’s pelvis is about 90 degrees or less of angle, whereas a female’s is 100 degrees or more. [Image Gallery: The BioDigital Human]

While they become brittle when outside of the body, bones are very much alive inside the body, being fed by a network of blood vessels from the circulatory system and nerves from the nervous system, according to Healthline.

A typical bone has a dense and tough outer layer. Next is a layer of spongy bone, which is lighter and slightly flexible. In the middle of some bones is jelly-like bone marrow, where new cells are constantly being produced for blood, according to the Merck Manuals.

Teeth are considered part of the skeletal system but they are not counted as bones. Teeth are made of dentin and enamel, which is strongest substance in your body. Teeth also play a key role in the digestive system.

The skeletal system has two distinctive parts: the axial skeleton and the appendicular skeleton, according to the U.S National Library of Medicine(NLM).

The axial skeleton, with a total of 80 bones, consists of the vertebral column, the rib cage and the skull. The axial skeleton transmits the weight from the head, the trunk and the upper extremities down to the lower extremities at the hip joints, which help humans maintain our upright posture, the NLM noted.

The appendicular skeleton has a total of 126 bones, and is formed by the pectoral girdles, the upper limbs, the pelvic girdle and the lower limbs, according to the NLM. Their functions are to make walking, running and other movement possible and to protect the major organs responsible for digestion, excretion and reproduction.

X-rays, MRIs, bone density tests and arthroscopy are some of the primary diagnostic tools used to detect diseases and deformities of the skeletal system. Bone scans and bone marrow biopsies are used to diagnose cancer, according to the Merck Manuals.

The primary skeletal conditions are metabolic bone diseases such as osteoporosis, osteomalacia, and a few other rarer conditions, said Dr. Nathan Wei of the Arthritis Treatment Center.