Aging of the Skull

Key topic in this section:



¦ Comparison of fetal, child, and adult skulls



Although many centers of ossification are involved in the formation of the skull, fusion of the centers produces a smaller number of composite bones as development proceeds. For example, the ethmoid bone forms from three separate ossification centers, and the occipital bone forms from four separate ossification centers. At birth, some of these ossification centers have not yet fused, so an infant initially has two frontal bone elements, four occipital bone elements, and a number of sphenoid and temporal elements.



The shape and structure of cranial elements differ in the skulls of infants and adults, causing variations in proportions and size. The most significant growth in the skull occurs before age 5, when the brain is still growing and exerting pressure against the internal surface of the developing skull bones. Brain growth is 90-95% complete by age 5, at which time cranial bone growth is also nearly complete, and the cranial sutures are almost fully developed. Note that the skull grows at a much faster rate than the rest of the body. Thus, the cranium of a young child is relatively larger than that of an adult. Figure 7.27 shows lateral and superior views of a neonatal (infant) cranium.



The infant’s cranial bones are connected by flexible areas of dense regular connective tissue, and in some regions the brain is covered only by this connective tissue sheet, since the bones aren’t yet big enough to fully surround the brain. The regions between the cranial bones are thickened, fibrous membrane remnants that are not yet ossified, called fontanelles (fon'ta-nel'; little spring; sometimes spelled fontanels). Fontanelles are sometimes referred to as



Table 7.4



Sex Differences in the Skull



View



Female Skull



Male Skull



Anterior View



Lateral View




Skull Feature



Female Skull Characteristic



Male Skull Characteristic



General Size and Appearance



More gracile and delicate



Nuchal Lines and External Occipital Protuberance



Mastoid Process



Squamous Part of Frontal Bone



Supraorbital Margin



Superciliary Arches



Mandible (general features)



Mental Protuberance (chin)



Mandibular Angle



External surface of occipital bone is relatively smooth, with no major bony projections



Relatively small



Usually more vertically oriented and rounded than in males



Thin, sharp border Little or no prominence Smaller and lighter



More pointed and triangular-shaped, less forward projection



Typically greater than 125 degrees



Sinuses



Teeth



Smaller in total volume Relatively smaller



More robust (big and bulky), more prominent muscle markings



Well-demarcated nuchal lines and prominent bump or “hook” for external occipital protuberance



Large, may project inferior to external acoustic meatus



Exhibits a sloping angle



Thick, rounded, blunt border More prominent and bulky Larger, heavier, more robust Squarish, more forward projection



Flared, less obtuse, less than 125 degrees (typically about 90 degrees)



Larger in total volume



Relatively larger




The “soft spots” on a baby’s head. When a baby passes through the birth canal, the cranial bones overlap at these fontanelles in order to ease the baby’s passage. Newborns frequently have a “cone-shaped” head due to this temporary deformation, but by a few days after birth, the cranial bones have returned to their normal position.



The fontanelles are present until many months after birth, when skull bone growth finally starts to keep pace with brain growth. The small mastoid and sphenoidal fontanelles close relatively quickly, compared to the larger posterior and anterior fonta-nelles. The posterior fontanelle normally closes around 9 months of age, while the larger anterior fontanelle doesn’t close until about 15 months of age. It is not uncommon to see rhythmic pulsations of the blood vessels internal to these fontanelles.



Although the skull may come close to its adult size by age 5, it still undergoes many more changes in subsequent years. The maxillary sinus becomes a bit more prominent beginning at age 5, and by age 10 the frontal sinus is becoming well formed. Later, the cranial sutures start to fuse and ossify. As a person ages, the teeth start to wear down from use, a process called dental attrition. Finally, if an individual loses some or all of his teeth, the alveolar processes of the maxillae and mandible regress, become less prominent, and eventually disappear.

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