![]() ![]() This “rule” applies to all images except direct lateral projections. If the deviation involves more than a 5-degree change, it must be compensated for by a change in either part rotation or central ray angulation. Any deviation from this relationship indicates a comparable change in the position and relationship of the internal structures. The length and width of the normally shaped head vary by 1 inch (2.5 cm). Internal deviations from the norm are usually indicated by external deviations and can be estimated with a reasonable degree of accuracy. Crania vary in size and shape, with resultant variation in the position and relationship of internal parts. The average cranium measures approximately 6 inches (15 cm) at its widest point from side to side, 7 inches (17.8 cm) at its longest point from front to back, and 9 inches (22 cm) at its deepest point from the vertex to the submental region. The average or so-called normal cranium is more or less oval in shape, wider in back than in front. The deep depression posterior to the petrous ridges is the posterior cranial fossa, which protects the cerebellum, pons, and medulla oblongata (see Fig. The middle cranial fossa accommodates the temporal lobes and associated neurovascular structures and extends from the lesser wings of the sphenoid bone to the apices of the petrous portions of the temporal bones. It is associated mainly with the frontal lobes of the cerebrum. The anterior cranial fossa extends from the anterior frontal bone to the lesser wings of the sphenoid. Internally, the cranial floor is divided into three regions: the anterior, middle, and posterior cranial fossae (see Fig. The thickness and degree of mineralization in normal adult crania show comparatively little difference in radiopacity from person to person, and the atrophy of old age is less marked than in other regions of the body. The cranium develops rapidly in size and density during the first 5 or 6 years, after which a gradual increase occurs until adult size and density are achieved, usually by the age of 12 years. The posterior and sphenoidal fontanels normally close in the 1st and 3rd months after birth, and the anterior and mastoid fontanels close during the 2nd year of life. Each sphenoidal fontanel is found at the site of the pterion the mastoid fontanels are found at the asteria. Two fontanels are also on each side at the inferior angles of the parietal bones. Posteriorly and in the midsagittal plane is the posterior fontanel, located at the point labeled lambda in Fig. The anterior fontanel is located at the junction of the two parietal bones and the one frontal bone at the bregma. Two of the fontanels are situated in the midsagittal plane at the superior and posterior angles of the parietal bones. They contain a small amount of calcium, are indistinctly marked, and present six areas of incomplete ossification called fontanels ( Fig. In a newborn infant, the bones of the cranium are thin and not fully developed. At the junction of the occipital bone, parietal bone, and mastoid portion of the temporal bone is the asterion. On the lateral aspect of the skull, the junction of the parietal bone, squamosal suture, and greater wing of the sphenoid is the pterion, which overlies the middle meningeal artery. The lambda is the junction of the lambdoidal and sagittal sutures. Between the occipital bone and the parietal bones is the lambdoidal suture. Between the temporal bones and the parietal bones are the squamosal sutures. The junction of the coronal and sagittal sutures is the bregma. The sagittal suture is located on the top of the head between the two parietal bones and just behind the coronal suture line (not visible in Figs. The coronal suture is found between the frontal and parietal bones. The sutures are named coronal, sagittal, squamosal, and lambdoidal (see Figs. The outer plate, or table, is thicker than the inner table over most of the vault, and the thickness of the layer of spongy tissue varies considerably.Įxcept for the mandible, the bones of the cranium and face are joined by fibrous joints called sutures. The bones of the cranial vault are composed of two plates of compact tissue separated by an inner layer of spongy tissue called diploë. ![]() B, Sagittal MRI of cranium showing contents and position of brain. 20-3 A, Lateral aspect of interior of cranium. ![]()
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