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.

Osteoporosis is a prevalent disease, particularly among the elderly, resulting in the loss of bone tissue. In osteoporosis, bone loses calcium, becomes thinner and may disappear completely, according to Wei. Osteomalacia is a softening of the bones, according to the Mayo Clinic. It is often caused by a vitamin D deficiency and results from a defect in the bone-building process. Osteoporosis, on the other hand, develops in previously constructed bones.

Arthritis is a group of more than 100 inflammatory diseases that damage joints and their surrounding structures. Arthritis can attack joints, joint capsules, the surrounding tissue or parts throughout the body. It usually affects the joints of the neck, shoulders, hands, lower back, hips or knees. "The diagnosis is suspected by a careful history and physical exam and confirmed through laboratory and imaging studies. Treatment depends on the type of arthritis," Wei said.

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 typically becomes evident during adolescence, the Merck Manuals noted. Two to 3 percent of the population — an estimated 6 to 9 million people in the United States — suffers from scoliosis, according to the American Association of Neurological Surgeons.

About 90 percent of people will experience lower back pain at some point in their lives, according to Dr. James Nace of LifeBridge Health. "Patients can often be helped with things such as anti-inflammatory medications, but in some cases may need treatments such as topical medications, patches or electrical stimulation." 

One of the much rarer diseases of the skeletal system is bone cancer. It may originate in the bones or spread there from another part of the body. The American Cancer Society estimates around 3,450 new cases will be diagnosed for bones and joint cancer and around 1,590 people will die from it in 2018 in the United States. Bone cancer accounts for less than 0.2 percent of all cancers, according to the American Cancer Society. Cancers that metastasize — originate from other parts of the body and then spread to the bones — are much more common than primary bone cancer.  

Bone cancer is a malignancy arising in the bones and supporting structures such as cartilage, according to Dr. Robert Christie, medical oncologist and hematologist at Virginia Cancer Specialists, a practice in The U.S. Oncology Network. "Unfortunately, these bone cancers are often seen in younger patients in their 20s and 30s versus lung cancer and breast cancer which are typically diagnosed later in life."

While leukemia is a cancer that primarily affects the blood, the skeletal system is involved as the cancer starts in the marrow of the bone. With this type of cancer, abnormal white blood cells multiply uncontrollably, affecting the production of normal white blood cells and red blood cells, according to the American Cancer Society.

Bursitis is a disorder that most commonly affects the shoulder and hip joints, Nace said. It is caused by an inflammation of the bursa, small fluid-filled bags that act as lubricating surfaces for muscles to move over bones.

The skeletal system is also susceptible to breaks, strains and fractures. While bones are meant to protect the body's vital organs, it takes about 10 to 16 pounds of pressure to break an average bone. Bones such as the skull and femur are much tougher to break.

Orthopedics is the medical specialty responsible for treating entire skeletal system. In the United States, orthopedic surgeons have typically completed four years of undergraduate education and four years of medical school. They then undergo residency training in orthopedic surgery. The American Board of Orthopaedic Surgery oversees the certification process for this specialty. Many go on to further specialize in specific areas, such as the spine, hand or sports injuries.

Humans have been dealing with injuries and disease from the beginning of time. Some important milestones in the history of orthopedics include:

  • In the Paleolithic period, early man engraved human bones after eating their owners. "Engraving is usually considered part of a modern behavior kit, a new way of expression typical of our species," said study lead author Silvia Bello, a paleoanthropologist at the Natural History Museum in London. 
  • Hippocrates, the ancient Greek father of medicine, develops splints for fractures of the tibia.
  • During the Roman era, Galen (199-129 B.C.) describes the skeletal system and the surrounding muscles. Medical experts of the time also develop the first artificial prostheses.
  • Ambroise Pare (1510-1590), the father of French surgery, develops techniques for amputations and artificial limbs.
  • Antonius Mathysen (1805-1878), a Dutch military surgeon, in 1851 invents the plaster of Paris (POP) bandage. A POP cast remains the primary method of fracture immobilization today.
  • In 1895, Wilhelm Conrad Roentgen accidentally discovers an image cast from his cathode ray generator, projected far beyond the possible range of the cathode rays. He wins the Nobel Prize for Physics in 1901 for his discovery of X-rays.
  • Sir Reginald Watson-Jones (1902-1972) publishes "Fractures and Joint Injuries" in 1940, which remains a standard reference for several decades.
  • In 1949, H. Lowry Rush (1879-1965) uses stainless steel pins to treat long bone fractures.
  • Marijuana is found to possibly heal bone fractures, according to research by the Tel Aviv University's Sackler Faculty of Medicine in 2015.
  • 2016 sees the creation of living bone grown from the cells of patients for the first time.
All about your body's skeleton, the framework of bones that keeps you together.
All about your body's skeleton, the framework of bones that keeps you together.
Credit: Ross Toro, Livescience contributor

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Additional reporting by Alina Bradford, Live Science contributor.