A typical bone (A) is actually made of two types of bony tissue. On the outside is a type of ‘skin’ called the periosteum. Below this is a thin layer of thick, dense, ‘solid’ bone. It is known as hard or compact bony tissue. Inside this, and forming the bulk of the middle of the bone, is a different bony tissue, more like a sponge or honeycomb. It has gaps and spaces, and it is called spongy, or alveolar, bony tissue. It is much lighter than the outer compact bone, and the spaces are filled with blood vessels, and jelly – like bone marrow for making new blood cells.