(The skeletal system) - What are bones? The word skeleton comes from the Greek for ‘dried Up,” but bones are not really as dry as rocks. In fact, bones are one-third water. They contain both living and nonliving parts. Minerals make the bones hard and strong, but they are not alive. Bone cells make the skeleton a lving frameswork. Bones have blood and nerves inside, and they grow and change. Bones are covered everywhere except at the joints by a thin white3 memebrane called the periosteum. (Periosteum is Latin for ‘surrounding the bone.”). the layer contains living cells that help in the growth and repair of bones. It also contains blood vessels and nervous. Beneath the periosteum is the hard part of bone, called compaact bone. Compact bone is so hard that surgeons have to cut it with a saw. Through tiny holes in thsi layer, nevrves and blood vessels pass inside the bone. insdie the layer of compact bone is a lacy network of bone with many small spaces inside. It looks rather like that cut edge of a sponage. This kind of bone is called spongy bone, or cancellous bone. (Cancellous comes from a Latin word that means “covered with bars.”) Cancellous bone is nearly as strong as compact bone, but it is much lighter becasue part of it isjsut emplty space. In the center of some bojnes there is a jellylike sustance called bone marrow. bone marrow prodcues red blood cells, which carry oxygen throught the bloodstrema, white blood cells that fight inflectioni, and blood platelets that help us to stop bleeding when we are cut. Bone sems nonliving, like a stone. And indeed, part of the bone, called the matrix, is made of nonliving substances. It contains a mineral part: a type of calcium phosphate, a salt containing the eleemnts calcium and phosphorus. bone matrix also contains protein fibers. These fibers are nonliving, like the protains in your hair and nails. When you cut your fingernails or hair, you do not bleed or feel anything. But if you could cut a very that slice of bone and look at it under a microscope, you would see numerous living bone cells, called osteocytes, embedded between layers of hard bony tissues . (The name osteocyte cames from words meaning “bone” and “cell”.). the bone substance itself is arranged in long cylinders, nested one inside another. These groups of cylinders are called Haversian systems. The ostrecytes are found in small hollows between the bony cylinders, and there is a hollow central channel inthemiddle of the smallest cylinder. Thsi central channel is called a Haversian cancal. Within the Havensisn canal is one or more blood vessels. bone cells, like all other living cells, need a constant supply of oxygen adn food materials, which are carried to them in teh bloodstream. Side branches of blood vessels cut through the covering of the bone. Though bones are all made of the same basic substance, theycame in many differnt sizes and shapes. Bones are usually divided into four grups: long, shorts, flat, and irregular. Long bones are found in the extiremities. They are usually rodlike with knobby ends. They may be very big, such as the femur, or small like the bones in the little toe. Short bones, such as those in the wrist and ankles are cube-shaped. Flat bones are 2 plates of compact bone that exclose a layer of spongy bone like a sandwich. The ribs, the shoulder blades, the breastbone, and msot of the bones of the skull are flat bones. Irregular bones are a catchall category that includes the vertebrae, the bones that do not fit into the other groups.