The type of most bones of the human skull is flat bone. Flat bones are bones whose principle function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flat plates, as in the cranium (skull), the ilium (pelvis), sternum and the rib cage.
In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough; the inner is thin, dense, and brittle,
and hence is termed the vitreous table. The flat bones in the skull are firmly connected to each other by means of seams.
Flat bones are composed of two layers of compact bone enclosing between them a variable quantity of cancellous bone, which is the location of red bone marrow. In an adult, most red blood cells are formed in the marrow of flat bones. This intervening cancellous tissue is called the diploë, a spongy bone structure.
Geographically, the bones of the skull can be divided into two parts. The part that protects the brains is called the cranium. Its primary purpose is to protect the brain. The part that forms the face is called the facial skull.