Skull
The
skull or
cranium is a
bony structure found in many animals which serves as the general framework for the
head. Those animals having skulls are called
Craniates. The skull supports the structures of the
face and protects the
brain against injury.
Protection of the brain is only one part of the function of a bony skull. For example, a fixed distance between the eyes is essential for
stereoscopic vision, and a fixed position for the ears helps the brain to use auditory cues to judge direction and distance of sounds. The skull also serves as a mount for the
teeth and
jaws, to permit eating, and in some animals (e.g. horned
ungulates) it has a defensive function: the
frontal bone is where horns are mounted.
The skulls of animals show shocking variety.
In humans, the adult skull is normally made up of 29 bones. Except for the
mandible, all of the bones of the skull are joined together by
sutures, rigid articulations permitting very little movement.
Eight bones form the
neurocranium (braincase), a protective vault of bone surrounding the brain and
medulla oblongata. Fourteen bones form the
splanchnocranium, the bones supporting the face. Encased within the
temporal bones are the six
ear ossicles of the
middle ear. The
hyoid bone, supporting the
larynx, is usually not considered as part of the skull, as it does not articulate with any other bones.
The skull also contains the
sinus cavities, which are air-filled cavities lined with
respiratory epithelium, which also lines the large airways. The exact functions of the sinuses are unclear; they may contribute to lessening the weight of the skull with a minimal reduction in strength, or they may be important in improving the resonance of the voice. In some animals, such as the
elephant, the sinuses are extensive. The elephant skull needs to be very large, to form an attachment for muscles of the neck and trunk, but is also unexpectedly light; the comparatively small brain-case is surrounded by large sinuses which reduce the weight.
The
meninges are the three layers, or membranes, which surround the structures of the
central nervous system. They are known as the
dura mater, the
arachnoid mater and the
pia mater. Other than being classified together, they have little in common with each other.
In humans, the
anatomical position for the skull is the
Frankfurt plane, where the lower margins of the
orbits and the upper borders of the
ear canals are all in a horizontal plane. This is the position where the subject is standing and looking directly forward. For comparison, the skulls of other species, notably
primates and
hominids, may sometimes be studied in the Frankfurt plane. However, this does not always equate to a natural posture in life.
*
Bone terminology*
Anatomical terms of location*
Great muscles of the head (cat)*
Phrenology, the pseudoscientific process of determining personality from the shape of the head.
*White, T.D. 1991. Human osteology. Academic Press, Inc. San Diego, CA.
*
Animal Skull Collection (A U.S. high-school teacher's labour of love: more than 300 animal skull images.)
*
Site with pictures of various animal skulls (commercial supplier)
*
Skull terminology site by Texas A&M*
Anatomy of cranial cavity.
