Skull
The skull is the ossified, bony structure that encloses and protects the brain, internal extensions of sensory organs, and some facial structures. The skull is usually considered to consist of a cranial section (the cranium) and a facial region.
The cranium is a large rounded, dome-shaped region of the skull that is composed of paired left and right frontal bones, parietal bones, temporal bones, and an unpaired occipital bone that forms the posterior base of the skull.
The bones of the cranium are fused by sutures—joints that run jaggedly along the interface between the bones. At birth, the sutures are soft, broad, and cartilaginous. This flexibility allows the skull to grow as the child matures. The sutures eventually fuse and become rigid and ossified near the end of puberty or early in adulthood. The coronal suture unites the frontal bone with the parietal bones. In anatomical nomenclature, the primary coronal plane is, of course, the plane that runs through the length of the coronal suture. At right angles to the coronal suture, the metopic suture separates the frontal bones in the midline region. The area formed by the fusion of the four bones near the top of the skull is termed the anterior fontanel or bregmatic fontanel (also commonly known as the topmost "soft spot" in a baby's skull). As with the sutures, the fontanels are soft at birth to permit growth. The fontanels shirk and close during childhood and are usually fully closed and hardened by young adulthood.
The sagittal suture unties the two large domed-shaped parietal bones along the midline of the body. The suture is used as an anatomical landmark in anatomical nomenclature to establish what are termed sagittal planes of the body. The primary sagittal plane is the sagittal plane that runs through the length of the sagittal suture. Sagittal planes run anteriorly and posteriorly, are always at right angles to the coronal planes. The lambdoidal suture unites the left and right parietal bones with occipital bone. The area where the two parietals and the unpaired occipital bone meet is termed the posterior fontanel, lamdoidal fontanel, or lambda point (also commonly called the rear "soft spot" on a baby's skull). Like the anterior fontanel, the posterior fontanel closes and hardens with age, but is an important feature that allows growth of the skull during embryological and childhood development.
Along the sides of the cranium, the squamosal suture unties the temporal bone lying above (superior to) the ear and ear canal with the parietal bone. The anterior region of the temporal bones is united with the great wing of the sphenoid bone by continuation of the squamosal suture. The junction of the temporal, parietal, frontal and great wing of the sphenoid takes place at the sphenoid fontanel. The posterior border of the temporal bone on each side unties with the corresponding mastoid bone
A mastoid fontanel lies at the posterior region of the side of the skull where the parietal, occipital, and mastoid bones unite. A mastoid process extends anteriorly toward the ear canal. A bony finger-like styloid process protrudes the area interior to the external auditory opening (external auditory meatus).
The facial area of the skull is composed of the left and right zygomatic arches that extend from the lowest, most anterior margins of the temporal bone where the temporal bones articulate with the mandible (the temporomandibular joint) into the zygomatic bone itself. The zygomatic arches and zygomatic bones thicken to become prominent facial landmarks forming the lower and side orbits of the eyes. The orbits are separated by a number of smaller bones in the nasal region including the ethmoid, lacrimal, and nasal bones. The maxilla and upper teeth form the most inferior region of the facial portion of the skull and are fused to the zygomatic bones.
The mandible is not considered a formal portion of the skull. In decayed bodies, the mandible becomes detached from the skull as the temporomandibular joint and supporting ligaments deteriorate.
A number of small openings allow nerves and blood vessels to penetrate the skull. These openings are termed foramen and are generally named for the bone they penetrate. For example, opening in the parietal bones are termed parietal foramen. A large foramen magnum at the rear and base of the skull allows the spinal cord to exit the skull into the vertebral column. Rounded, smooth, bony protuberances termed the occipital condyles lie on the anterior sides of the foramen magnum and help articulate the skull with the vertebral column.
The external occipital crest marks the posterior midline of the occipital bone. The crest runs from the foramen magnum upward (superiorly) to a bony knot-like external occipital protuberance.
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