Main Model
Anterior : Frontal bone
Cranium
The cranium (skull) is the skeleton of the head. A series of bones form its two parts, the neurocranium and viscerocranium. The neurocranium is the bony case of the brain and its membranous coverings, the cranial meninges. It also contains proximal parts of the cranial nerves and the vasculature of the brain. The neurocranium in adults is formed by a series of eight bones: four singular bones centered on the midline (frontal, ethmoidal, sphenoidal, and occipital), and two sets of bones occurring as bilateral pairs (temporal and parietal).
The neurocranium has a dome-like roof, the calvaria (skullcap), and a floor or cranial base (basicranium). The bones forming the calvaria are primarily flat bones (frontal, parietal, and occipital) formed by intramembranous ossification of head mesenchyme from the neural crest. The bones contributing to the cranial base are primarily irregular bones with substantial flat portions (sphenoidal and temporal) formed by endochondral ossification of cartilage (chondrocranium) or from more than one type of ossification. The ethmoid bone is an irregular bone that makes a relatively minor midline contribution to the neurocranium but is primarily part of the viscerocranium. The so-called flat bones and flat portions of the bones forming the neurocranium are actually curved, with convex external and concave internal surfaces.
Most calvarial bones are united by fibrous interlocking sutures; however, during childhood, some bones (sphenoid and occipital) are united by hyaline cartilage (synchondroses). The spinal cord is continuous with the brain through the foramen magnum, a large opening in the cranial base.
The viscerocranium (facial skeleton) comprises the facial bones that mainly develop in the mesenchyme of the embryonic pharyngeal arches. The viscerocranium forms the anterior part of the cranium and consists of the bones surrounding the mouth (upper and lower jaws), nose/ nasal cavity, and most of the orbits (eye sockets or orbital cavities).
The viscerocranium consists of 15 irregular bones: 3 singular bones centered on or lying in the midline (mandible, ethmoid, and vomer), and 6 bones occurring as bilateral pairs (maxillae; inferior nasal conchae; and zygomatic, palatine, nasal, and lacrimal bones). The maxillae and mandible house the teeth - that is, they provide the sockets and supporting bone for the maxillary and mandibular teeth. The maxillae contribute the greatest part of the upper facial skeleton, forming the skeleton of the upper jaw, which is fixed to the cranial base. The mandible forms the skeleton of the lower jaw, which is movable because it articulates with the cranial base at the temporomandibular joints.
Several bones of the cranium (frontal, temporal, sphenoid, and ethmoid bones) are pneumatized bones, which contain air spaces (air cells or large sinuses), presumably to decrease their weight. The total volume of the air spaces in these bones increases with age.
In the anatomical position, the cranium is oriented so that the inferior margin of the orbit and the superior margin of the external acoustic opening of the external acoustic meatus of both sides lie in the same horizontal plane. This standard craniometric reference is the orbitomeatal plane (Frankfort horizontal plane).
Facial Aspect of Cranium
Features of the anterior or facial (frontal) aspect of the cranium are the frontal and zygomatic bones, orbits, nasal region, maxillae, and mandible.
The frontal bone, specifically its squamous (flat) part, forms the skeleton of the forehead, articulating inferiorly with the nasal and zygomatic bones. In some adults a frontal suture persists; this remnant is called a metopic suture. It is in the middle of the glabella, the smooth, slightly depressed area between the superciliary arches. The frontal suture divides the frontal bones of the fetal cranium.
The intersection of the frontal and nasal bones is the nasion (Latin nasus, nose), which in most people is related to a distinctly depressed area (bridge of nose). The nasion is one of many craniometric points that are used radiographically in medicine (or on dry crania in physical anthropology) to make cranial measurements, compare and describe the topography of the cranium, and document abnormal variations. The frontal bone also articulates with the lacrimal, ethmoid, and sphenoids; a horizontal portion of bone (orbital part) forms both the roof of the orbit and part of the floor of the anterior part of the cranial cavity.
The supra-orbital margin of the frontal bone, the angular boundary between the squamous and orbital parts, has a supra-orbital foramen (notch) in some crania for passage of the supra-orbital nerve and vessels. Just superior to the supra-orbital margin is a ridge, the superciliary arch, that extends laterally on each side from the glabella. The prominence of this ridge, deep to the eyebrows, is generally greater in males.
The zygomatic bones (cheek bones, malar bones), forming the prominences of the cheeks, lie on the inferolateral sides of the orbits and rest on the maxillae. The anterolateral rims, walls, floor, and much of the infra-orbital margins of the orbits are formed by these quadrilateral bones. A small zygomaticofacial foramen pierces the lateral aspect of each bone. The zygomatic bones articulate with the frontal, sphenoid, and temporal bones and the maxillae.
Inferior to the nasal bones is the pear-shaped piriform aperture, the anterior nasal opening in the cranium. The bony nasal septum can be observed through this aperture, dividing the nasal cavity into right and left parts. On the lateral wall of each nasal cavity are curved bony plates, the nasal conchae.
The maxillae form the upper jaw; their alveolar processes include the tooth sockets (alveoli) and constitute the supporting bone for the maxillary teeth. The two maxillae are united at the intermaxillary suture in the median plane. The maxillae surround most of the piriform aperture and form the infra orbital margins medially. They have a broad connection with the zygomatic bones laterally and an infra-orbital foramen inferior to each orbit for passage of the infra-orbital nerve and vessels.
The mandible is a U-shaped bone with an alveolar process that supports the mandibular teeth. It consists of a horizontal part, the body, and a vertical part, the ramus. Inferior to the second premolar teeth are the mental foramina for the mental nerves and vessels. The mental protuberance, forming the prominence of the chin, is a triangular bony elevation inferior to the mandibular symphysis (Latin symphysis menti), the osseous union where the halves of the infantile mandible fuse.
Lateral Aspect of Cranium
The lateral aspect of the cranium is formed by both neurocranium and viscerocranium. The main features of the neurocranial part are the temporal fossa, the external acoustic meatus opening, and the mastoid process of the temporal bone. The main features of the viscerocranial part are the infratemporal fossa, zygomatic arch, and lateral aspects of the maxilla and mandible.
The temporal fossa is bounded superiorly and posteriorly by the superior and inferior temporal lines, anteriorly by the frontal and zygomatic bones, and inferiorly by the zygomatic arch. The superior border of this arch corresponds to the inferior limit of the cerebral hemisphere of the brain. The zygomatic arch is formed by the union of the temporal process of the zygomatic bone and the zygomatic process of the temporal bone.
In the anterior part of the temporal fossa, 3-4 cm superior to the midpoint of the zygomatic arch, is a clinically important area of bone junctions: the pterion (Greek pteron, wing). It is usually indicated by an H-shaped formation of sutures that unite the frontal, parietal, sphenoid (greater wing), and temporal bones. Less commonly, the frontal and temporal bones articulate; sometimes all four bones meet at a point.
The external acoustic meatus opening (pore) is the entrance to the external acoustic meatus (canal), which leads to the tympanic membrane (eardrum). The mastoid process of the temporal bone is postero-inferior to the external acoustic meatus opening. Anteromedial to the mastoid process is the styloid process of the temporal bone, a slender needle-like, pointed projection. The infratemporal fossa is an irregular space inferior and deep to the zygomatic arch, and the mandible and posterior to the maxilla.
Occipital Aspect of Cranium
The posterior or occipital aspect of the cranium is composed of the occiput (Latin back of head, the convex posterior protuberance of the squamous part of the occipital bone), parts of the parietal bones, and mastoid parts of the temporal bones.
The external occipital protuberance, is usually easily palpable in the median plane; however, occasionally (especially in females) it may be inconspicuous. A craniometric point defined by the tip of the external protuberance is the inion (Greek nape of neck). The external occipital crest descends from the protuberance toward the foramen magnum, the large opening in the basal part of the occipital bone.
The superior nuchal line, marking the superior limit of the neck, extends laterally from each side of the protuberance; the inferior nuchal line is less distinct. In the center of the occiput, lambda indicates the junction of the sagittal and the lambdoid sutures. Lambda can sometimes be felt as a depression. One or more sutural bones (accessory bones) may be located at lambda or near the mastoid process.
Superior Aspect of Cranium
The superior (vertical) aspect of the cranium, usually somewhat oval in form, broadens posterolaterally at the parietal eminences. In some people, frontal eminences are also visible, giving the calvaria an almost square appearance.
The coronal suture separates the frontal and parietal bones, the sagittal suture separates the parietal bones, and the lambdoid suture separates the parietal and temporal bones from the occipital bone. Bregma is the craniometric landmark formed by the intersection of the sagittal and coronal sutures. Vertex, the most superior point of the calvaria, is near the midpoint of the sagittal suture.
The parietal foramen is a small, inconstant aperture located posteriorly in the parietal bone near the sagittal suture; paired parietal foramina may be present. Most irregular, highly variable foramina that occur in the neurocranium are emissary foramina that transmit emissary veins connecting scalp veins to the venous sinuses of the dura mater.