Bones of the Cranium

The human skull consists of eight cranial bones and facial bones: frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones. A skull contains two each of temporal and parietal bones. The frontal bone is large and located at the top front of the skull. It makes up the forehead and the roofs of the eye sockets, or orbits. The bottom forward edge of the frontal bone meets the facial bones. The paired parietal bones are located on each side of the skull and extend to the top and rear of the skull. They are located behind the frontal bone. Parietal bones form a large portion of the cranial cavity, the space within the skull that holds the brain and meninges and relies on cerebrospinal fluids to cushion the brain from impacts. Each temporal bone is located on either side of the head, just anterior the ear, below a parietal bone, and behind the facial bones. A thick region of each temporal bone, the petrous portion, contains the middle ear and bones, or ossicles, comprising the inner ear. The occipital bone lies at the lower rear of the cranium. The ethmoid bone is located at the front of the cranium, between eye orbits, which are the sockets that contain the eyes. Other bony structures in the ethmoid bone make up the breathing passages of the nose. The sphenoid bone makes contact with all other cranial bones. It is located at the skull's base, in the center, below the frontal and parietal bones, behind the ethmoid bone, and in front of the temporal and occipital bones.

Skull bones, including cranial bones, fuse together at sutures. A skull suture is a seam that forms a joint between skull bones. Sutures are not straight and may have many small jagged edges. These intricate edges give the sutures stability. Sutures occur only in the skull. As a human grows from infant to adult, the bones of the cranium also increase in size, and the sutures harden. These sutures are semirigid, in that they hold the bones of an immature skull together yet allow the bones to increase in size. If the skull were one solid bone, it would not yield and allow growth. The frontal bone and parietal bones meet at the coronal suture. The sagittal suture at the top of the cranium connects the two parietal bones. The occipital bone meets the paired parietal bones at the lambdoid suture. The squamous suture connects the parietal and temporal bones. The jagged junctures may also be called markings. As with bones throughout the body, cranial bones have several characteristic markings, each called a process. Processes are markings, which are growths of tissues, such as those found on the cranium. The frontal, sphenoid, and ethmoid bones contain sinuses, or cavities. A sinus is an air space within the skull and facial bones that is associated with the nose and throat and is found in the forehead, cheekbones, and behind the nose. These cavities reduce the weight of the skull and affect the voice and breathing. In addition to sinuses, the cranium has a number of foramina. A foramen (plural, foramina) is an opening or passageway that allows muscle tissues, nerves, and blood vessels to pass from one area of the body to another. The carotid artery passes through the carotid foramen, and the jugular vein and several nerves pass through the jugular foramen. The occipital bone includes the important passageway for the spine, which is called the foramen magnum. There are two projections, each called an occipital condyle, that sit on either side of the foramen magnum. An occipital condyle is one of two large, convex processes that allow attachment of the spine to the skull by articulating with the first cervical vertebra of the spine. The optic nerve and ophthalmic artery also pass through a foramen called the optic foramen. Several other nerves pass through the sphenoid's superior orbital fissure, which is still considered a foramen even though it is more of a cleft than a passageway.

Many other markings are found in cranial bones. For example, depressions in the parietal bones support blood vessels in the cranium. Depressions and projections in the temporal bones connect with the jawbone and allow the jaw to move up and down, and attachments for neck muscles allow the head to move.

Fourteen bones create the facial structures of the skull: two nasal bones, two maxillary bones (or maxillae), two zygomatic bones, the mandible (or lower jaw), a lacrimal bone under each orbit (eye socket), two palatine bones, two inferior nasal conchae, and the vomer. Each type of bone has a specific function. These bones shape the face, form portions of the orbits and nasal cavities, and hold the teeth. The mandible opens and closes the mouth, connecting to the skull with four muscles to facilitate movement. The human face includes 14 bones that link with the cranial bones and form facial structures. These structures create facial shape; hold the teeth, eyes, and ear ossicles in place; form the nasal cavity; and make up the palate.

The two nasal bones are small and elongated. They make up the bridge of the nose. The two maxillary bones together shape the upper jawbone. All other facial bones except the mandible (the lower jawbone) articulate with the maxillae. Each maxillary bone holds a sinus, and each features an alveolar process that holds the sockets (alveoli) for teeth of the upper jaw. Most of the hard palate that makes up the roof of the mouth is formed by the fused palatine processes, projections of each maxillary bone.

The two zygomatic bones are known as the cheekbones. They give cheeks their rounded shape and make up the bottom of the orbits, also called eye sockets, and form part of the orbits' outer wall. The term orbit may be either the bony socket or the contents of the socket. A zygomatic process, or marking, projects to the rear of each zygomatic bone, directly above the ear opening. Connected to a companion protrusion or process from each temporal bone, these processes form the zygomatic arch on each side of the face.

Mandibles are the largest facial bones and are the only moveable skull bones. The mandible includes a wide, curved portion, called the body, and a perpendicular section, called the mandibular ramus, on each side of the body. Each ramus features a bony projection (an outgrowth from the bone). that articulates with the temporal bone; together these form the temporomandibular joint. A jaw muscle, the temporalis, attaches to another process on the mandible, the coronoid process. An alveolar process along the mandible body holds sockets for the lower teeth.

The smallest facial bones are the two lacrimal bones. Thin and curved, the lacrimals are located behind and beside the nasal bones. They form part of the middle walls of the orbits. The two palatine bones form the rear of the hard palate, which separates the nasal and oral cavities. Palatine bones also contribute to parts of the nasal cavity and a small section of the orbits.

The inferior nasal conchae form part of the side walls of the nasal cavity. The two bones sit below the superior and middle conchae that are part of the ethmoid bone. Like those structures, the inferior nasal conchae increase surface area in the nose and help filter inhaled air. Finally, the vomer makes up the bottom rear portion of the nasal septum, the partition between the nostrils. The septum consists of the vomer articulated with a plate of the ethmoid bone, together with attached cartilage.

A few other bones are associated with the skull in addition to the facial and cranial bones. Three auditory ossicles attach to one another in each middle ear. An auditory ossicle is a tiny bone that fits with the other two auditory ossicles, which together transmit sound wave vibrations from the tympanum, or eardrum, to the inner ear. Human ears have three ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). The hyoid, also known as the hyoid apparatus, is a single U-shaped bone that supports and anchors the tongue and provides places for muscle attachment. Specifically, it provides a site for attaching the tongue's muscles to the hyoid, which is in the throat. This bone is unique in that it does not connect with other bones but is suspended from the temporal bone by ligaments and tendons.

The eight cranial bones that make up the human skull house many cavities or contained spaces. The largest is the cranial cavity, which contains the brain. Three depressions, or fossae (anterior, middle, and posterior), form the interior floor, providing room for the brain and surrounding fluid. A ridge called the crista galli is located in the center of the anterior fossa, in the ethmoid bone. The crista galli is the attachment spot for the dura mater, the outermost of three meninges, which are membranes that protect the brain.

The paired orbits, or eye sockets, are skull cavities formed by several bones. The frontal and sphenoid bones frame each orbital roof, and the sphenoid and zygomatic bones compose the lateral (outer) orbital walls. The two maxilla (plural maxillae) bones make up each orbital floor, and the lacrimal, ethmoid, and palatine bones together form each medial (inner) orbital wall. Orbits are not completely round but rather are cone-shaped, tapering toward the back. Much of the eye is contained within its orbit, where it moves or orbits freely, which is the source of the structure's name. The orbital bones provide protection as well as attachment points for muscles that move the eye. Lacrimal (tear) glands, blood vessels, fat, and nerves are also held within orbits.

Other skull cavities include the middle- and inner-ear cavities, buccal (or oral) cavity, nasal cavity, and paranasal sinuses. The middle-ear cavities contain the auditory ossicles. The buccal cavity forms the mouth. It is framed by the maxillae and the mandible—the jaws—and is separated from the nasal cavity by the palate. Like the orbits, the nasal cavity is composed of several bones. The frontal, nasal, sphenoid, and ethmoid bones frame the nasal cavity roof. The lateral walls are formed by the ethmoid, inferior nasal concha, lacrimal, and palatine bones. The maxilla and palatine combine to form the nasal cavity floor and side walls, as well as the hard palate at the roof of the mouth. The paired maxillae form the maxillary arch, which holds the upper row of teeth in place. The vomer and ethmoid bones make up the rigid part of the septum that divides the nasal cavity. Several bones that form the nasal cavity contain spaces, or cavities, within them. Each is referred to as a paranasal sinus, a space or cavity within a nasal cavity bone. Each paranasal sinus opens into the nasal cavity. Paranasal sinuses are labeled according to the bone in which they are located: frontal, ethmoid, sphenoid, and maxillary sinuses. Frontal paranasal sinuses are present just above the eyes, in the lower center of the forehead. The several small ethmoid sinuses occur just behind the eyes and the bridge of the nose. Sphenoid sinuses are found fairly deep within the upper region of the skull, behind the eyes and the ethmoid sinuses. Maxillary sinuses extend across a broad area on each side of the nose, reaching from under each eye to above the teeth in the upper jaw. All paranasal sinuses contain air and are lined with mucous membranes. Their presence lightens the front portion of the skull. The sinus chambers also resonate sounds generated by the vocal cords during speech. This effect is made particularly clear when sinuses are filled with mucus—as with a cold or sinus infection—and sounds have to travel through liquid rather than air.