The skull protects the brain. More specifically, a set of eight interlocking bones called the neurocranium (or braincase) forms a continuous shell around the entire brain. These bones range from about 6 mm thick at the temples to over 10 mm at the back of the head, and together they create one of the strongest natural structures in the human body.
The Eight Bones of the Braincase
The neurocranium isn’t a single piece of bone. It’s made up of eight separate bones that fit together like puzzle pieces, each covering a different region of the brain.
- Frontal bone (1): Forms the forehead and protects the frontal lobe, the part of the brain responsible for decision-making, personality, and voluntary movement.
- Parietal bones (2): Sit at the crown of the skull and act as a roof over much of the brain, covering parts of the front, middle, and rear brain cavities.
- Temporal bones (2): Located on the sides of the head, near the ears. These are the thinnest cranial bones at roughly 6 mm, and they house the structures for hearing and balance.
- Occipital bone (1): Covers the back and base of the skull, forming the rounded bulge at the nape of the neck. It protects the cerebellum and brainstem, the regions that control coordination, breathing, and heart rate.
- Sphenoid bone (1): A butterfly-shaped bone that sits deep in the middle of the skull, connecting to nearly every other cranial bone. It cradles the pituitary gland, which regulates hormones throughout the body.
- Ethmoid bone (1): A small bone nestled between the eye sockets that helps form the roof of the nasal cavity and separates it into two passages.
How Thick the Skull Actually Is
Skull thickness varies by location and, to a lesser extent, by sex. The thickest region is the back of the head: the occipital bone averages about 10 mm in both men and women. The parietal bones at the crown come in close behind at roughly 9.6 to 10.1 mm. The frontal bone averages 7.8 mm in men and 8.6 mm in women. The thinnest spots are the temporal bones on the sides, averaging just 6 mm.
That variation matters. The temporal region is the most vulnerable part of the skull, and it sits right over the middle meningeal artery. A blow to the temple can be especially dangerous not because the brain underneath is more critical, but because the bone there is thinnest and a major blood vessel runs just beneath it. Research on blunt impact tolerance found that the temporo-parietal region can begin to fracture at forces around 2,300 Newtons, while peak fracture forces in that area averaged roughly 5,600 Newtons.
Protection Beyond Bone
The skull is only the outermost layer of defense. Between the bone and the brain itself, three membrane layers called meninges add additional protection.
The outermost membrane, the dura mater, is a tough, thick layer that lines the inside of the skull. One side attaches directly to the bone, while the other connects to the middle layer beneath it. The dura also folds inward to create partitions between different sections of the brain, keeping them anchored in place.
The middle layer, the arachnoid mater, has a spiderweb-like structure with thin projections that connect it to the innermost layer. Between these two layers sits a fluid-filled space that plays a critical cushioning role.
The innermost layer, the pia mater, wraps tightly around the brain’s surface like shrink wrap, following every fold and groove. Together, the three layers form a suspension system that holds the brain in place while absorbing shock.
How Cerebrospinal Fluid Cushions the Brain
Filling the space between the middle and inner membranes is cerebrospinal fluid, a clear liquid that essentially lets the brain float inside the skull. This buoyancy effect dramatically reduces the brain’s effective weight, preventing it from pressing against the base of the skull under its own mass. The fluid also acts as a shock absorber during movement. Without it, even normal activities like walking or turning your head quickly would cause the brain to bounce against the inner surface of the skull.
How Infant Skulls Differ
Babies aren’t born with a fully fused skull. Their cranial bones are connected by flexible seams called sutures, and at certain intersections, soft spots called fontanelles remain open. This flexibility serves two purposes: it allows the skull bones to overlap slightly during birth so the head can pass through the birth canal without compressing the brain, and it gives the brain room to grow rapidly during the first years of life.
The smaller soft spot at the back of the head (the posterior fontanelle) typically closes by about two months of age. The larger one at the top of the head (the anterior fontanelle) takes longer. It closes by 12 months in about 38 percent of infants, and by 24 months in 96 percent, with a median closure age of roughly 14 months. During infancy and childhood, the sutures remain somewhat flexible, which also helps absorb minor bumps as babies learn to hold their heads up, roll over, and sit. If the sutures fused too early, the growing brain would have nowhere to expand, leading to serious developmental problems.
Why Some Skull Regions Are More Vulnerable
The skull’s protective ability isn’t uniform. The frontal bone is reinforced by air-filled cavities called sinuses that can help absorb frontal impacts. The occipital bone at the back is the thickest and sits over a naturally curved surface, which distributes force more evenly. The temporal region, being both thin and relatively flat, offers the least resistance to direct blows.
The base of the skull is also a structural weak point. It’s riddled with openings for blood vessels and nerves, including the large hole (foramen magnum) where the spinal cord connects to the brainstem. Forces transmitted upward through the spine, such as during a fall from height, can cause fractures along the skull base even without a direct hit to the head.
Taken together, the system works in layers: bone absorbs and distributes impact force, the dura mater provides a tough internal lining, the arachnoid and pia mater suspend the brain in place, and cerebrospinal fluid cushions it from all directions. No single element does the job alone.