A human skull is a smooth, rounded structure made up of 22 interlocking bones, roughly the size of a volleyball but slightly elongated from front to back. It appears off-white to pale yellow when dried, with a surface that ranges from glassy smooth across the forehead to rough and ridged where muscles once attached at the back and sides. What strikes most people first are the obvious openings: two large, roughly rectangular eye sockets, a heart-shaped nasal opening in the center, and rows of teeth set into the upper and lower jaws.
The Overall Shape and Structure
The skull has two main sections. The upper dome, called the cranium, is made of 8 bones that fit together like a three-dimensional jigsaw puzzle to form a protective shell around the brain. Below that, 14 bones make up the face, giving the skull its recognizable features: cheekbones, a nasal bridge, an upper jaw, and eye sockets.
The cranium is surprisingly thin in places, sometimes just a few millimeters thick, but it gets its strength from a sandwich-like structure. Two layers of dense bone surround a spongy middle layer, similar to corrugated cardboard. This design keeps the skull light enough that your neck muscles can support it comfortably. Hollow air pockets called sinuses, located behind the forehead, cheeks, and nasal area, reduce the skull’s weight even further.
From the front, the skull looks somewhat oval, widest at the temples. From the side, you can see how the face projects forward from the rounded braincase, with the brow ridge overhanging the eye sockets and the jaw extending below. From above, it resembles an egg, slightly wider toward the back than the front.
The Visible Lines Between Bones
One of the most distinctive features of a skull is the network of wavy, zigzag lines running across its surface. These are sutures, the joints where the skull’s individual bones meet and eventually fuse together. They look like irregular stitching or the interlocking teeth of a zipper, and they’re visible even on adult skulls long after the bones have fully joined.
The most prominent suture runs ear to ear across the top of the head, roughly where a crown would sit. This is where the large forehead bone meets the two bones forming the top and sides of the skull. Another major suture runs straight down the center of the skull from front to back, and a third crosses the back of the head in an inverted V shape. Above each ear, a curved suture marks where the side bones meet the temple bones. Together, these lines give the skull a patchwork quality that becomes obvious when you look closely.
Key Features of the Face
The eye sockets are deep, cone-shaped cavities that angle slightly inward. Each one is formed by parts of seven different bones coming together, which is why the rim of the socket can feel smooth in some spots and ridged in others. Just above the sockets, the brow ridge creates a subtle shelf of bone that varies significantly from person to person.
The nasal opening is a pear-shaped hole in the center of the face, wider at the bottom than the top. Two small, thin nasal bones form the bridge of the nose at the top of this opening, but the rest of what you think of as your nose is cartilage, which disappears after death. Inside the nasal cavity, you can see thin, scroll-shaped bones called conchae that project inward from the side walls.
The cheekbones sit just below and to the outside of the eye sockets, forming a prominent ridge. From each cheekbone, a bony arch extends backward toward the ear, creating the curved bar you can feel on the side of your head between your eye and your ear. This arch is one of the skull’s most recognizable profile features.
The Jaw and Teeth
The lower jaw is the skull’s only movable bone. It’s U-shaped, with a broad, curved body that holds the lower teeth and two vertical branches that angle upward on each side. At the top of each branch, a rounded knob fits into a socket just in front of the ear opening, forming the hinge joint that lets you open and close your mouth. On a bare skull, the lower jaw often falls away from the rest of the structure since it’s only held in place by muscles and ligaments in life.
A full adult skull contains sockets for 32 teeth, 16 in the upper jaw and 16 in the lower. The upper teeth are set into two bones that form the central part of the face, extending from below the nose to the roof of the mouth. The tooth sockets are clearly visible as a row of deep pits along the curved edge of both jaws. In older skulls or those from elderly individuals, some sockets may be partially or fully filled in with bone where teeth were lost during life.
The Back and Base of the Skull
The back of the skull is a smooth, rounded surface dominated by the occipital bone. Near the center, there’s often a small bump you can feel on your own head, where neck muscles attach. The bone surface here tends to be rougher and more ridged than the top or sides, showing the pull marks left by powerful muscles that held the head upright.
Flipping the skull over reveals the base, which is far more complex than the smooth dome above. The most prominent feature is the foramen magnum, a large oval hole near the center-back of the base. This opening, whose Latin name translates to “large hole,” is where the spinal cord connects to the brain. It’s roughly 3 centimeters wide, and its position on the underside of the skull (rather than the back) reflects our upright posture.
The interior base of the skull is divided into three step-like shelves, each sitting at a slightly different level. The shallowest shelf sits behind the forehead and cradles the front portions of the brain. The middle shelf drops down a step and contains a small saddle-shaped depression in its center that houses the pituitary gland. The deepest shelf, at the back, is a bowl-shaped depression that holds the brainstem and the cerebellum. Scattered across the base are dozens of small holes and channels where blood vessels and nerves pass through to reach the face, eyes, ears, and neck.
How Male and Female Skulls Differ
Male and female skulls are built from the same bones, but they develop differently during puberty. Males experience a longer and more intense growth spurt, which pushes the face further forward and downward relative to the braincase. The result is a set of consistent visual differences, though there’s overlap between individuals.
Male skulls tend to be larger overall, with a more rugged, angular appearance. The brow ridges are more prominent, the forehead slopes back at a steeper angle, and the back of the skull shows more pronounced ridges where neck muscles attached. The eye sockets on a male skull tend to be squarer and sit lower in the face, with blunter rims. The cheekbones are heavier and the cheek arches flare wider. The chin is typically squared off, sometimes with two slight points, compared to the more rounded, single-pointed chin common in female skulls.
Female skulls generally appear smoother, with a more rounded, vertical forehead, sharper eye socket rims, and less prominent brow ridges. The overall architecture is more gracile, with thinner bone and less pronounced muscle attachment sites. The bump behind the ear (the mastoid process) tends to be smaller in females, which is one of the most reliable single indicators forensic specialists use when examining skeletal remains.
How an Infant Skull Looks Different
An infant skull looks strikingly different from an adult’s. The bones are thinner, more flexible, and haven’t yet fused together. Instead of tight, zigzag sutures, an infant skull has wide, fibrous gaps between the bones, and at several points where multiple bones meet, there are soft, membrane-covered openings called fontanelles. The most noticeable one sits on top of the head toward the front, the familiar “soft spot” that parents are warned to protect.
These gaps exist for two reasons: they allow the skull bones to overlap slightly during birth, making it possible for the head to pass through the birth canal, and they give the brain room to grow rapidly in the first years of life. The skull reaches about 75% of its adult size by age two. The bones themselves start as single solid layers and gradually develop the three-layer sandwich structure of adult skull bone within the first six years. The fontanelles close during infancy, and the sutures between the major bones gradually fuse over the following years, eventually producing the interlocked, rigid structure of the adult skull.
Proportionally, an infant skull is also shaped differently. The face is much smaller relative to the braincase, giving babies their characteristic large-foreheaded, small-faced appearance. As the child grows, the face elongates and the jaw expands to accommodate adult teeth, shifting the proportions toward the more balanced look of an adult skull.