Skeletal system

The human skeleton is made of individual or joined bones (such as the skull), supported and supplemented by a structure of ligaments, tendons, muscles, cartilage and other organs.

The skeleton is not unchanging; it changes composition over a lifespan. Early in gestation, a fetus has no hard skeleton — bones form gradually during nine months in the womb. At birth, all bones will have formed, but a newborn baby has more bones than an adult. On average, an adult human has 206 bones (according to Gray's Anatomy, but the number can vary slightly from individual to individual), but a baby is born with approximately 270 bones which fuse together to reach the number of 206 bones of the adult body. The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column. An infant is born with pockets of cartilage between particular bones to allow further growth. The sacrum (the bone at the base of the spine) consists of five bones which are separated at birth but fuse together into a solid structure in later years. Growing is usually completed between ages 12 and 14, at which point the bones have no pockets of cartilage left to allow more growth.

Not all bones are interconnected directly. There are 6 bones, auditory ossicles, (three on each side) in the middle ear that articulate only with each other. Another bone, the hyoid bone in the neck, does not touch any other bones in the body, and is supported by muscles and ligaments, it serves as the point of attachment to the tongue.

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 20% of the total body weight.

Functions
The most obvious function of bone is to: (1)support the body. It also the (2) site of haematopoiesis, the manufacture of blood cells, that takes place in bone marrow (which is why bone marrow cancer is very often a terminal disease). It is also necessary for (3) protection of vital organs. (4) Movement in vertebrates is dependent on the skeletal muscles, which are attached to the skeleton by tendons. Without the skeleton to give leverage, movement would be greatly restricted. Bone also serves as a (4) mineral storage deposit in which nutrients can be stored and retrieved.

Organization
One way to group the bones of the human skeleton is to divide them into 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, the arm bones, the bones of the pelvis and the leg bones.

A more systematic division of the bones of the human skeleton divides them into the:
 * skull
 * middle ear
 * shoulder girdle
 * ribcage
 * vertebra
 * arms
 * hands
 * pelvis
 * legs
 * feet

Gender differences


There are many differences between the male and female human skeletons. Men tend to have slightly thicker and longer limbs and digit bones, while women tend to have narrower rib cages, smaller teeth, less angular mandibles, and less pronounced cranial features such as the brow ridges and occipital protuberance (the small bump at the back of the skull). Most striking is the difference in hip bones, owing to differences related to the process of reproduction, and very likely also to the biological process of sexual selection. There are a number of smaller differences between human male and female skeletons as well.

The book Genesis of the Bible tells that Eve was created from one of Adam's ribs. This has lead some people to assume that men have one less rib than women, but upon inspection this is false: men and women both have 12 pairs of ribs. In addition, removed ribs usually regenerate within 2-3 months of sectional surgery, as in the surgical procedure rib thoracoplasty.

Diseases
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.

A fracture occurs when a bone is subjected to too much stress. Fractures are divided into "simple" and "compound" fractures, the meaning of which is not immediately apparent. A "simple" fracture means only that the damaged bone has not broken through the skin, and does not imply a single break. Likewise, a broken bone is called a "compound fracture" when it has broken through the skin, and again does not imply that the bone has broken in more than one place. Osteoporosis can increase the likelihood of fractures and broken bones, especially among post-menopausal women and the elderly.

Scoliosis is a condition where the spine is curved from side to side.