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THE HUMAN SKELETON

From our head to our toes, our bones provide support for our bodies and help to form our shape. The skull protects the brain and forms the shape of our face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal col­umn.

The average human adult skeleton consists of 206 bones, attached to the muscles by ten­dons. Babies are born with 270 soft bones — about 64 more than an adult. These will fuse together by the age of twenty or twenty-five into the 206 hard, permanent bones. Bone building continues throughout life, as a body constantly renews and reshapes the bones' living tissue.

The skeleton has two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, the spine, the ribs and the sternum (breastbone) and includes 80 bones. The appendicular skeleton, consists of 126 bones, includes two limb girdles (the shoulders and pelvis) and their attached limb bones.

Axial skeleton (80 bones)

- skull — consisting of 1) the cranium (which encloses and protects the brain) and 2) the facial skeleton;

- jaw — the only freely movable bone of the skull;

- ribs, sternum (breastbone) — comprising the “thorax”/chest, protecting the heart and lungs;

- vertebral column — the "spine".

The vertebral column transmits the body weight from the head, thorax, and abdomen to the lower extremities and encloses and protects the spinal cord. The vertebral column has three groups of vertebrae and two sets of fused bones. These vertebrae include seven cervical (neck) vertebrae, twelve thoracic (upper back) vertebrae, and five lumbar (lower back) vertebrae. Five fused vertebrae form the sacrum and from three to five fused small vertebrae form the coccyx (tail bone). The vertebrae form a column of bone that protects the spinal cord. The thoracic vertebrae have facets (indentations) upon their surfaces that articulate (meet) with the ribs. The ribs form a cage that shelters the heart, lungs, liver, spleen, and pelvis helps to protect the bladder, intestines, and in women, the reproductive organs. Al­though they're very light, bones are strong enough to support our entire weight.

Appendicular skeleton consists of the pectoral girdle, the upper limbs, pelvic girdle, the lower limbs (126 bones, 64 in the shoulders and upper limbs and 62 in the pelvis and lower limbs):

- the pectoral girdle consists of the clavicle and scapula;

- upper extremity (the humerus, ulna, radius, the hand);

- pelvic girdle consists of two hip bones; each hip bone forms from fused bones. The two coxae, sacrum, and coccyx form the pelvis, a bowl-shaped cavity that supports and protects many abdominal organs;

- lower extremity (the femur, the tibia, the fibula, the foot).

Bones are fastened to other bones by long, fibrous straps called ligaments. The joints allow the bones to move. Cartilage, a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.

Bones are made up of calcium, phosphorus, sodium, and other minerals, as well as the protein collagen. Calcium is needed to make bones hard, which allows them to support body weight. Bones also store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of certain vitamins and minerals that you eat, especially vitamin D and calcium, directly af­fects how much calcium is stored in the bones. The soft bone marrow inside many of the bones is where most of the blood cells are made.