Structure and growth of bones
Bone is a connective tissue made of long-extended cells and hard intercellular substance; nerves and blood vessels supply it, and the periosteum covers it. By shape bones are tubular (femur, humerus), flat (scapula, skull bones), spongy (vertebrae, ribs) and mixed. A tubular bone has a compact middle and spongy ends; yellow marrow fills its cavity and red marrow lies between the spongy plates – blood cells are made in red marrow. Growth in length happens through the cartilage at the bone ends, thickening through the periosteum; growth is regulated by the pituitary growth hormone, and a person grows and forms until about 22–25 years. Mineral salts (calcium phosphate, calcium carbonate) make bone hard and organic substances make it flexible; children have more organic matter, so their bones bend, while in the elderly minerals dominate and bones are brittle.
Observation with chicken bones (with adults): an adult soaks one of two cleaned bones in vinegar for several days; compare their flexibility. Safety: only adults handle acid and fire; wash hands.