The human skeleton is a tough, flexible structure that supports weight and protects internal organs. It 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. The difference comes from a number of small bones that fuse together during growth. They leave pockets of cartilage to allow more growth. 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. Growing is usually completed between ages 12 and 14. This means that the bones have no pockets of cartilage left, so there is no material left to allow more growth.
Not all bones are interconnected directly. There are 6 bones (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.
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 support the body. It also the 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 protection of vital organs. Movement 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 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:
 A prepared human skeleton, as used in biology education |
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 larger pelvic bones(so when they have kids they will come out without crushing their pelvises) 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 reproduction, and very likely also to the biological process of sexual selection. There are also a number of smaller differences between human male and female skeletons. Removed ribs can regenerate within 2-3 months of sectional surgery, as in the surgical procedure rib thoracoplasty. |
