It is shaped like a cylinder and is so hard that surgeons must use a saw to cut through it. It is honeycombed with thousands of tiny holes and passageways, through which run nerves and blood vessels that supply oxygen and nutrients to the bone. This dense layer supports the weight of the body and is made up of mostly calcium and minerals, so that it feels no pain. If we cut though the compact bone, we find that its cylinder surrounds and protects the spongy bone marrow which contains a material much like gelatin. These three bone layers work together with nerve signals which speed back and forth and blood streams which move between the layers.
Thighbones are usually stronger, pound for pound, than reinforced concrete.
A longitudinal row of the bodies supports the weight of the head and trunk. The intervertebral disks, which separate joining vertebrae, are fastened to the roughened upper and lower surfaces of the bodies. These disks cushion and soften the forces created by walking and jumping, which might otherwise fracture the vertebrae or jar the brain. The pedicles, laminae, and spinous process together complete a bony vertebral arch around the vertebral opening, through which the spinal cord passes. Various ligaments and muscles are attached to the spinal process and the transverse process.