The term skull is usually understood to include all of the bones of the
head. The head consists of the cranium and the face, and it is therefore
convenient to divide the bones into cranial and facial groups.
The =cranial bones= (Ossa cranii) inclose the brain with its membranes
and vessels and the essential organs of hearing. They concur with the
facial bones in forming the orbital and nasal cavities, in which the
peripheral organs of sight and of smell are situated.
The =facial bones= (Ossa faciei) form the skeleton of the oral and nasal
cavities, and also support the larynx and the root of the tongue.
Most of the bones of the skull are flat bones, developed in membrane;
those of the cranial base may be classed as irregular, and are cartilage
bones. Only two form permanent movable joints with other parts of the
skull. The mandible or lower jaw-bone forms diarthrodial joints with the
temporal bones, and the hyoid bone is attached to the latter by bars of
cartilage. The other bones form immovable joints, most of which
disappear with age.
In order to study the separate bones, skulls of young subjects are
necessary, since later most of the lines of demarcation become
effaced. The relations of each bone to its surroundings should be
specially noted, since the final object is to understand the skull as
a whole. In the descriptions which follow the skull is considered with
its long axis horizontal, and that of the horse will serve as a type.