The Skull: A Box Built From Twenty-Two Bones

Twenty-two bones, eight of them around the brain

Count them once and the architecture stops being a list and becomes a building. The skull is assembled from TWENTY-TWO bones: EIGHT CRANIAL bones that enclose the brain, and FOURTEEN FACIAL bones that build the face beneath it. The cranial eight are the FRONTAL bone in front, the two PARIETAL bones forming the roof and sides, the OCCIPITAL bone behind and below, the two TEMPORAL bones low on each side, the single ETHMOID deep between the orbits, and the butterfly-shaped SPHENOID spanning the width of the base like a keystone. Almost all of them are FLAT bones of the kind set out in bones and their structure — formed by intramembranous ossification, built of two hard plates with spongy bone between — and they meet one another not at movable joints but at SUTURES, immovable fibrous joints where interlocking bony teeth are laced together, rigid in the adult and unmistakable to the eye as jagged running lines. The whole assembly divides naturally into two: the domed CALVARIA or vault that caps the brain, and the CRANIAL BASE on which the brain sits. Take a skull apart along its sutures and it is a three-dimensional jigsaw; leave it whole and it is a single rigid shell, which is precisely the point.

The vault, its seams, and two named crossings

Three great sutures divide the roof, and where they meet, the bones are named after the gap that once lay there. Look down on the top of the vault and three sutures organise the whole roof. The CORONAL suture runs across the crown from side to side, separating the frontal bone in front from the two parietals behind. The SAGITTAL suture runs front-to-back along the midline between the two parietal bones. The LAMBDOID suture runs across the back, separating the parietals from the occipital bone, and named for its resemblance to the Greek letter lambda. Lower on each side the SQUAMOUS suture arcs above the ear where the flat squamous part of the temporal bone overlaps the parietal like the edge of a scale. Two crossings have their own names and are landmarks a surgeon and a radiologist both use. The BREGMA is the point where the coronal and sagittal sutures meet — the junction of frontal and parietals — a soft gap in the newborn, a hard cross in the adult. The LAMBDA is the point where the sagittal and lambdoid sutures meet — the junction of parietals and occipital. In life these are not decoration: they are the reference points from which the surgeon measures, and the lines along which the infant skull can still move.

The pterion — a coin of thin bone over an artery

One H-shaped meeting of four bones is the single most dangerous point on the vault. On the side of the head, a couple of finger-breadths above the midpoint of the zygomatic arch, four bones come together in a small H-shaped suture pattern: the FRONTAL, the PARIETAL, the SQUAMOUS part of the TEMPORAL, and the GREATER WING of the SPHENOID. This meeting-place is the PTERION, and it is the THINNEST part of the skull — a place where four bones taper to their edges over a single small area. That would be merely a curiosity except for what lies on the inside surface directly beneath it: the ANTERIOR DIVISION of the MIDDLE MENINGEAL ARTERY, grooving the bone as it climbs. A blow to the temple that fractures the pterion tears that artery, and because it is an artery under systemic pressure, blood is driven into the space between the bone and the tough outer covering of the brain — stripping the DURA off the skull and forming an EXTRADURAL (epidural) HAEMATOMA, described with the coverings of the brain in the meninges and dural venous sinuses. On CT it is a BICONVEX (lens-shaped) collection that does not cross the sutures, because the dura is firmly anchored at the suture lines and the expanding blood cannot pass them. This is the lesion behind the boxer felled by a temple shot, the child struck by a cricket ball, and the young man of our opening scene — and the lucid interval is its signature, the deceptive quiet while the clot silently grows.

The diploë, and the newborn's soft spots

Slice through a vault bone and you find it is not solid: two dense plates — an OUTER TABLE and an INNER TABLE of compact bone — sandwich a layer of spongy bone called the DIPLOË, its marrow spaces drained by wide, thin-walled DIPLOIC VEINS that connect the veins of the scalp outside to the dural venous sinuses inside. This tripled construction gives strength for little weight, but the inner table is thinner and more brittle than the outer, which is why a blow can fracture the inner table while the outer looks intact. In the NEWBORN the story is different again, because the sutures have not yet closed. Where several bones approach at the vault's corners there remain wide membranous gaps — the FONTANELLES. The ANTERIOR fontanelle, a diamond at the bregma between the two frontal and two parietal bones, is the largest and the last to close, at around EIGHTEEN MONTHS. The POSTERIOR fontanelle at the lambda closes far earlier, by two to three months, and two smaller LATERAL fontanelles — the sphenoidal and mastoid — sit low on each side. These gaps are not a weakness of design but a requirement of birth: they let the bones of the vault overlap and slide during delivery, MOULDING the head to pass through the pelvis, and afterwards they allow the vault to keep pace with a brain that doubles in size in the first year.

The base from inside — three terraced fossae

Lift off the vault and the floor of the box drops in three broad steps, each holding a different part of the brain. Seen from inside, the cranial base is not flat but TERRACED, stepping downwards from front to back in three FOSSAE. The ANTERIOR CRANIAL FOSSA is the highest and holds the FRONTAL LOBES. Its floor is built from the orbital plates of the FRONTAL bone, the CRIBRIFORM PLATE of the ETHMOID in the midline, and the LESSER WINGS of the SPHENOID behind. Rising from the cribriform plate is a small vertical blade, the CRISTA GALLI — literally the cock's comb — to which the falx cerebri anchors, and on either side of it lie the OLFACTORY BULBS, receiving the threads of the first cranial nerve as they pass up through the sieve-like holes of the cribriform plate from the nose, the nasal roof being explored in the nose and paranasal sinuses. The MIDDLE CRANIAL FOSSA steps down behind, deeper on each side and holding the TEMPORAL LOBES, its central part carrying the PITUITARY gland. Its floor is the butterfly of the SPHENOID — the central body with the GREATER WINGS spreading laterally — together with the TEMPORAL bones. And behind and below everything is the deep POSTERIOR CRANIAL FOSSA, the largest and lowest, cradling the BRAINSTEM and the CEREBELLUM, floored by the OCCIPITAL bone and the PETROUS parts of the temporal bones. Each step down matches the part of the brain it carries: the light frontal lobes ride highest, the great mass of the cerebellum sits deepest.

The sella turcica, and the holes in the floor

At the very centre of the base, on the body of the sphenoid, is a saddle-shaped hollow — the SELLA TURCICA, the "Turkish saddle" — and seated in its deepest part, the hypophyseal fossa, is the PITUITARY GLAND, the master endocrine gland, hanging on its stalk from the base of the brain and roofed by a fold of dura. Its position matters clinically because it sits directly below the OPTIC CHIASM: a pituitary tumour enlarging upward presses on the crossing fibres and classically robs a person of their outer fields of vision, a bitemporal hemianopia. The middle fossa is also the most heavily perforated part of the base. Through the SPHENOID pass, in order, the OPTIC CANAL (carrying the optic nerve into the orbit), the SUPERIOR ORBITAL FISSURE (the great slit into the orbit for the nerves of eye movement), the FORAMEN ROTUNDUM, the FORAMEN OVALE and the FORAMEN SPINOSUM. Each transmits a specific nerve or vessel, and their exact contents — which nerve leaves by which hole, and what a fracture through each one takes with it — are set out on their own in the foramina of the skull base; here it is enough to know that the routes into the ORBIT run forward from the middle fossa, a doorway explored in the orbit and its contents. The posterior fossa carries the largest opening of all, the FORAMEN MAGNUM, through which the brainstem becomes the spinal cord, flanked by the JUGULAR FORAMEN, the INTERNAL ACOUSTIC MEATUS and the HYPOGLOSSAL CANAL.

The petrous temporal — the pyramid that holds the ear

The most extraordinary single piece of the base is the PETROUS part of the temporal bone — petrous meaning "rocky", and it is the DENSEST bone in the body. It juts up and forwards as a three-sided PYRAMID, its ridge dividing the middle cranial fossa in front from the posterior fossa behind, and it earns its density by what it protects: hollowed inside it, encased in solid bone, are the MIDDLE and INNER EAR — the ossicles that carry sound, the cochlea that hears, and the semicircular canals that sense balance — the whole apparatus laid out in the ear. Through the posterior face of the pyramid opens the INTERNAL ACOUSTIC MEATUS, admitting the facial (VII) and vestibulocochlear (VIII) nerves to the depths of the bone. Because it is so hard, the petrous bone resists ordinary fracture; but when a great force does crack it, the fracture runs into the ear itself, and the consequences declare themselves at the surface — blood behind the eardrum, cerebrospinal fluid leaking from the ear, and damage to the seventh and eighth nerves running through it. The petrous temporal is where the skull is at its most solid and, when it fails, at its most revealing.