The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs and heart.

The longest and heaviest bone in the body is the femur and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 20% of the total body weight, 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 six bones in the middle ear called the ossicles (three on each side) 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, and ossification of the mandible occurs in the fibrous membrane covering the outer surfaces of Meckel's cartilages. At birth a newborn baby has approximately 270 bones, whereas on average an adult human has 206 bones (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. The sacrum (the bone at the base of the spine) consists of five bones which are separate at birth but fuse together into a solid structure in later years. An infant is born with zones of cartilage, called epiphyseal plates, between segments of bone to allow further growth. Growing is usually completed between ages 13 and 19, at which time the epiphyseal plates of long bones close allowing no further growth.

Organization

Much of the human skeleton maintains the ancient segmental pattern present in all vertebrates (mammals, birds, fish, reptiles and amphibians) with basic units being repeated. This segmental pattern is particularly evident in the vertebral column and in the ribcage.fact: date=September 2008

There are 206 bones in the adult human skeleton, a number which varies between individuals and with age - newborn babies have 270 bones some of which fuse together. These bones are organized into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.

Axial skeleton

The axial skeleton (80 bones) is formed by the vertebral column (26), the thoracic cage (12 pairs of ribs and the sternum), and the skull (22 bones and 7 associated bones). The axial skeleton transmits the weight from the head, the trunk, and the upper extremities down to the lower extremities at the hip joints, and is therefore responsible for the upright position of the human body. Most of the body weight is located in front of the spinal column which therefore have the erector spinae muscles and a large amount of ligaments attached to it resulting in the curved shape of the spine. The 240 skeletal muscles acting on the axial skeleton position the spine, allowing for small movements in the thoracic cage for breathing, and the head, where they control the minute and complex facial movements.