The human skeleton is made of bones, some of them joined together, supported and supplemented by a structure of ligaments, tendons, muscles, and cartilage.
The skeleton changes composition over a lifespan. Early in gestation, a fetus has no hard skeleton - bones form gradually during nine months in the womb. When a baby is born it has more bones than it will as an adult On average, an adult human has 206 bones in their skeleton (the number can vary slightly from individual to individual), but a baby is born with approximately 270. The difference comes from a number of small bones that fuse together during growth. These include the bones in the skull and the spine. The sacrum (the bone at the base of the spine) consists of six bones which are separated at birth but fuse together into a solid structure in later years.
There are 6 bones (three on each side) in the middle ear that articulate only with themselves, and one bone, the hyoid bone, which does not touch any other bones in the body.
The longest bone in the body is the femur and the smallest is the stapes bone in the middle ear.
The skeleton functions not only as the support for the body but also in haematopoiesis, the manufacture of blood cells that takes place in bone marrow. It is also necessary for protection of vital organs and is needed by the muscles for movement.
There are many differences between the male and female human skeletons, some more noticeable than others. Men tend to have slightly thicker and longer limb and digit bones while women tend to have larger pelvic bones in relation to body size. Women also tend to have narrower rib cages, smaller teeth, less angular mandibles, and less pronounced cranial features such as the brow ridges and occipital condyle (the small bump in the cranium's posterior). There are also a number of smaller differences between human male and female skeletons.
There is a myth that men have one less rib than women. This stems from a passage found in the Bible which states that Eve was created from one of Adam's ribs. However, both men and women have the same number of ribs: 12 pairs or 24 total.
See also: sexual dimorphism
One way to group the bones of the human skeleton is to divide them into two groups, namely the axial skeleton and the appendicular skeleton. The axial skeleton consists of bones in the midline and includes all the bones of the head and neck, the vertebrae, ribs and sternum. The appendicular skeleton consists of the clavicles, scapulae, bones of the upper limb, bones of the pelvis and bones of the lower limb.
The bones of the human skeleton are structurally and in many taxonomies organized as those of the:
The skeleton can be affected by many diseases that compromise physical mobility and strength. Skeletal diseases range from minor to extremely debilitating. Bone cancer and bone tumors are extremely serious and are sometimes treated by radical surgery such as amputation of the affected limb. Various forms of arthritis attack the skeleton resulting in severe pain and debility. Osteoporosis can increase the likelihood of fractures and broken bones, especially among post-menopausal women and the elderly.