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The skull has two basic divisions based on whether the bone is involved in encasing the brain or not. The bones that make up the brain enclosure are termed cranial bones or the neurocranium, and the rest of the bones are considered facial or viscerocranial bones. Some cranial bones contribute to what the layperson might consider parts of the face but that is not the way they are divided by anatomists.
The bones that make up the cranial vault are known as flat bones. This type of bone consists of three layers with inner and outer tables of compact bone sandwiching the diploe which is a thin layer of spongy bone. These lack the marrow cavities found in long bones.
As bone is heavy and the head must be supported by the neck the bones are thickened where necessary for strength but in areas where there is no stress on them, they are less robust. To further lighten the skull there are also bones with membrane lined hollow areas within them. These areas are known as sinuses. All but one of these sinuses drain into the nasal cavity and will be discussed when examining the specific bones that contain them.
In addition, there are areas of the skull that have multi-bone depressions. That are referred to often in anatomic descriptions. These depressions are known under the general term of fossa. One set is found at the base of the skull and can be only visualized with the brain removed. They can be seen in Appendix D though they are not labeled as such the three hollows are clear. The anterior one is simply called the anterior cranial fossa, the middle one is the middle cranial fossa and predicably the posterior one is called the posterior cranial fossa.
The other set, while they can be seen on a lateral view of the skull is best understood with a diagram showing the inferior view of the skull (Appendix C). There is an indentation of the side of the skull that extends from the temporal lines completely down to the inferior border of the skull. It is divided into three spaces. The space above the zygomatic arch (This is unlabeled but is where area where the pink and aqua bones meet in Appendix B) is the temporal fossa and below that is found the infratemporal fossa or sometimes referred to as the infratemporal space. Deep to the infratemporal space is another depression known as the pterygopalatine fossa or sphenopalatine fossa. This cone shaped area is difficult to see in two-dimensional diagrams but is found between the medial and lateral pterygoid plates and the maxilla. It is a passageway that connects various parts of the face and skull so will be mentioned often in discussions of nerve and blood vessel pathways.Diagram Reference Guide