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105 after birth. The gradual replacement of cartilage and its intercellular sub‑ stance by immature bone cells (osteoblasts) and calcium deposits is called ossifcation (bone formation). The formation of bone is dependent to a great extent on a proper sup‑ ply of calcium and phosphorus to the bone tissue. These minerals must be taken into the body along with a suffcient amount of vitamin D. Vitamin D helps the passage of calcium through the lining of the small intestine and into the bloodstream. Once calcium and phosphorus are in the bones, os‑ teoblastic activity produces an enzyme which splits phosphorus substances and causes the formation of a calcium-phosphate compound giving bone its characteristic hard quality. Bones all over the body are of several different types. Long bones are found in the thigh, lower leg, upper and lower arm. These strong bones are broad at the ends where they join with other bones, and have large surface areas for muscle attachment. Short bones are found in the wrist and ankle and have small, irregular shapes. Flat bones are found covering soft body parts. These are the shoulder bone, ribs and pelvic bones. Sesamoid bones are small, rounded bones. They are found near joint; the kneecap is the largest example of this type of bone. The middle region of a long bone is called the diaphysis. Each end of a long bone is called an epiphysis. The periosteum is a strong, fbrous, vascular membrane that covers the surface of a long bone, except at the ends of the epiphyses. Bones other than long bones are completely covered by the periosteum as well. Beneath the periosteum is the layer of immature cells (osteoblasts). The ends of long bones are covered by a thin layer of cartilage called ar‑ ticular cartilage. This layer cushions the bones at the place where they meet with other bones (joints). Compact (cortical) bone is a layer of hard, dense tissue which lies under the periosteum in all bones and chiefly around the diaphysis of long bones. Within the compact bone is a system of small canals called haversian canals. They are flled with blood vessels which bring oxygen and nutrients to the bone and remove waste product such as carbon dioxide. Compact bone is tunneled out in the diaphysis of long bones by a central medullary cavity which contains yellow bone marrow. Cancellous bone, sometimes called spongy bone, is much more porous and less dense than compact bone. This bone tissue is found in the epiphy‑