The cranium is the upper bony structure of the skull, including everything except for the mandible, or jawbone. The cranium protects the brain and head, and supports facial structures such as the eyes and ears, holding them in the proper place to receive sensory information most efficiently. Animals that have a skull are collectively known as craniates.
The cranium is not a single bone, but many. They are joined by sutures, rather than joints, which allow for very little movement. The human cranium consists of 21 bones and is subdivided into the neurocranium or brain case, which surrounds and protects the brain, and the splanchnocranium, which supports the structures of the face.
The bones in the neuocranium are the frontal bone, the occipital bone, the sphenoid bone, the ethmoid bone, two parietal bones, and two temporal bones. In addition to the mandible, the splanchnocranium includes two maxilla, two palatine bones, two zygomatic bones, two nasal bones, two lachrymal bones, two inferior nasal conchae, and the vomer bone. The cranium also contains the paranasal sinuses, which are air filled passages communicating with the nasal passages inside the nostrils.
Amniotes, a group of animals including mammals, birds, and reptiles, can be classified according to type of skull. Some amniotes have bilaterally symmetrical holes, called temporal fenestrae, in the temporal bone of the cranium. Animals with no temporal fenestrae are classified as anapsids. Turtles are the only living anapsids.
Synapsids have one small low opening on each side of the skull behind the eye, while diapsids have two openings on each side. Both groups are very diverse. Euryapsids, which are now extinct, had a high opening on each side of the skull behind the eye. They evolved from diapsids, but lost the lower opening. Mammals are considered synapsids, though they have evolved to no longer have temporal fenestrae.
The craniums of fish are less well developed than those of amniotes. Most fish skulls have a reduced structure, and consist mostly of cartilage rather than bone. Lamprey eels have the simplest cranium of all living craniates, consisting only of a loose structure of cartilanginous fibers that does not fully enclose the brain. Ray-finned fishes or actinopterygii have a more well-formed and bony cranium than other varieties of fish.