The bones in our body are made up of four distinct layers. Each of these layers has important responsibilities and are vital to our day-to-day life. The outer layer of bone is called compact bone. Compact bone is solid throughout and enables our body to be supported. Compact bone is so dense that surgeon’s need to use a saw to cut through it. The next layer of bone is called spongy bone and it is lightweight and porous, which prevents our bones from getting too heavy, but still be strong. It makes up about 20% of our skeleton, and it’s found mostly in long bones, joints, and in our spine. The center of many bones are filled with a gelatinous substance called bone marrow. Bone marrow is responsible for creating red blood cells and some of our white blood cells. Bone marrow also makes platelets for our circulatory system. In addition, bone marrow stores iron for when our body needs it. We have two different types of bone marrow: yellow and red. We are born with red marrow, but it slowly changes to yellow over time. Finally, the outermost part of bone is covered in periosteum, a thin, fibrous membrane that covers all bones. The membrane is responsible for helping to create new bone in children and does the same when adults sustain an injury. The periosteum is the site at which ligaments, tendons, and muscles attach to bone! It also contains the blood vessels that allow nutrients and oxygen to the get to the bones! Knowing the four layers of bones can help us to understand just how complex our human body really is!