The Probert Encyclopaedia of Medicine

SKULL

The skull is one of the principle groups of bones in the human anatomy. The skull consists of twenty-six bones: eight bones form the skull, which houses the brain and ear ossicles, plus fourteen facial bones, which form the front of the face, jaw, nose, orbits, and the roof of the mouth, three more bones make up the inner ear ossicles, and one more, the hyoid bone, is in the neck and is attached to the temporal bone by ligaments and anchors the tongue. The skull also contains a dental arcade of teeth, which are technically not bones, though they do share some of the compositional characteristics of bone tissue. Children may grow twenty deciduous teeth, which will eventually fall out and be replaced by the permanent teeth. The bones of the skull include the frontal bone, the occipital bone, two parietal bones, and two temporal bones. The lower rearmost part of each temporal bone is called the mastoid process, but because it is separated from the temporal bone, proper, by a suture, it is often considered a separate bone. The sphenoid bone forms the central base of the skull and spans the skull from side to side, the greater wings forming side plates of the skull. The sections of the ethmoid bone are positioned between the orbits, forming the walls and roof of the nasal cavity, while the three middle ear ossicles (stapes, malleus, and incus) are located within the temporal bones on each side of the skull. The U-shaped hyoid bone is found in the neck, and is attached by ligaments to the temporal bones. In the face, the two maxillary bones form much of the orbits, nose, upper jaw and roof of the mouth, while the zygomatic bones form the cheeks. The lachrymal bones are located on the inner sides of the orbits and are attached to the ethmoid bone and maxillary bones. Within the nasal cavity, the vomer is located in the low center and forms the thin flat bone of the nasal septum, while two inferior urbinates form the lower sides of the cavity and two palate bones form
e floor of the nasal cavity as well as the roof of the mouth.
The mandible is the only movable part of the skull, forming the lower jaw and mounting the teeth. The bones of the skull, with the exception of the mandible, are held together by very thin sutures, or seams, in which the periosteum of the individual bones interweave with each other, and are cemented by a fibrous, connective tissue. In the newborn, these sutures are not yet developed, with the bones being attached by cartilage which ossifies over time as the bones of the skull fuse together. The most evident external sutures of the skull include the coronal suture, joining the frontal bone and parietal bone, the sagittal suture, joining the parietal bones to each other, the lambdoid suture, joining the occipital and parietal bones, the squamous suture, joining the temporal and sphenoid bones to the parietal bone on each side of the skull.
The pterion is the short segment of the suture joining the squamous and parietal bones. The bones of the skull also feature a number of sinuses and foramina. Four pairs of sinuses flank the nasal cavity. Two are found in the maxillary bone, and are called maxillary sinuses. The sphenoid bone forms two paranasal sinuses called the sphenoids, and the ethmoid bone forms the two paranasal sinuses called ethmoids. Additionally, the frontal sinuses are located in the frontal bone just behind the roof of each orbit. The foramen magnum is a large, round opening in the base of the skull which admits the spinal cord, while at the base of each temporal bone is the external auditory meatus, which serve as the auditory canals. Just above each orbit in the frontal bone is a small notch or hole, called a supraorbital foramen, and just below each orbit, in the maxillary bone, is an infraorbital foramen. Two more openings, one on each side of the skull, can be found in the frontal processes of the zygomatic bones, and are called zygomatofacial foramina. On each side of the mandible, just below the lower canines, are the mental foramina. These facial foramina serve to admit blood vessels and nerves through and into the bone. The teeth are mounted in the maxillary bone and the mandible, and are brought together for chewing by the hinge-like motion of the mandible. An average adult will have thirty-two teeth, evenly arrayed on the maxilla and mandible.
Research Skull