The parietal bone forms most of the upper and lateral side of the skull (see Figure 7.3.3). These are paired bones, with the right and left parietal bones joining together at the top of the skull forming the sagittal suture. Each parietal bone is also bounded anteriorly by the frontal bone at the coronal suture, inferiorly by the temporal bone at the squamous suture, and posteriorly by the occipital bone at the lambdoid suture. Figure 7.3.5. Superior view of the skull. Temporal Bone The ramus on each side of the mandible has two upward-going bony projections. The more anterior projection is the flattened coronoid process of the mandible, which provides attachment for one of the biting muscles. The posterior projection is the mandibular condyles, which is topped by the oval-shaped condyle. The condyle of the mandible articulates (joins) with the mandibular fossa and articular tubercle of the temporal bone. Together these articulations form the temporomandibular joint, which allows for opening and closing of the mouth (see Figure 7.3.3). The broad U-shaped curve located between the coronoid and condylar processes is the mandibular notch. Internal acoustic meatus—This opening is located inside the cranial cavity, on the medial side of the petrous ridge. It connects to the middle and inner ear cavities of the temporal bone.

Each lacrimal bone is a small, rectangular bone that forms the anterioromedial wall of the orbit (see Figure 7.3.2 and Figure 7.3.3). The anterior portion of the lacrimal bone forms a shallow depression called the lacrimal fossa, and extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid (tears of the eye), which serves to maintain the moist surface of the eye, drains at the medial corner of the eye into the nasolacrimal canal. This duct then extends downward to open into the nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, thus causing a runny nose. Inferior Nasal Conchae Jugular foramen—The opening in the temporal bone directly posterior to the carotid canal. The is the point of exit for the internal jugular vein. Inside the nasal area of the skull, the nasal cavity is divided into halves by the nasal septum. The upper portion of the nasal septum is formed by the perpendicular plate of the ethmoid bone and the lower portion is the vomer bone. When looking into the nasal cavity from the front of the skull, two bony plates are seen projecting from each lateral wall. The larger of these is the inferior nasal concha, an independent bone of the skull. Located just above the inferior concha is the middle nasal concha, which is part of the ethmoid bone. A third bony plate, also part of the ethmoid bone, is the superior nasal concha. It is much smaller and out of sight, above the middle concha. The superior nasal concha is located just lateral to the perpendicular plate, in the upper nasal cavity. Lateral View of Skull The cranium (also known as the neurocranium) is formed by the superior aspect of the skull. It encloses and protects the brain, meninges, and cerebral vasculature. The cranium consists of eight bones. These include the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. Parietal BoneCleft lip is a common developmental defect that affects approximately 1:1000 births, most of which are male. This defect involves a partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft (gap).

The cranium contains and protects the brain. The interior space that is almost completely occupied by the brain is called the cranial cavity. This cavity is bounded superiorly by the rounded top of the skull, which is called the calvaria (skullcap), and the lateral and posterior sides of the skull. The bones that form the top and sides of the cranium are usually referred to as the “flat” bones of the skull. Articular tubercle—The smooth ridge located immediately anterior to the mandibular fossa. Both the articular tubercle and mandibular fossa contribute to the temporomandibular joint, the joint that provides for movements between the temporal bone of the skull and the mandible. Maxillary fracture – associated with high-energy trauma. Fractures affecting of maxillary bones are classified using the Le Fort classification, ranging from 1 to 3. The anterior cranial fossa is the shallowest of the three cranial fossae. It extends from the frontal bone anteriorly to the lesser wing of the sphenoid bone posteriorly. It is divided at the midline by the crista galli and cribriform plates of the ethmoid bone. The middle cranial fossa is located in the central skull, and is deeper than the anterior fossa. The middle fossa extends from the lesser wing of the sphenoid bone anteriorly to the petrous ridge posteriorly. It is divided at the midline by the sella turcica. The posterior cranial fossa is the deepest fossa. It extends from the petrous ridge anteriorly to the occipital bone posteriorly. The large foramen magnum is located at the midline of the posterior fossa. The bones of the skull can be considered as two groups: those of the cranium (which consist of the cranial roof and cranial base) and those of the face.On the lateral side of the cranium, above the level of the zygomatic arch, is a shallow space called the temporal fossa. Arising from the temporal fossa and passing deep to the zygomatic arch is another muscle that acts on the mandible during chewing, the temporalis. Figure 7.3.3 – Lateral View and Sagittal Section of Skull: (a) Lateral View of Skull. The lateral skull shows the large rounded brain case, zygomatic arch, and the upper and lower jaws. The zygomatic arch is formed jointly by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone. The shallow space above the zygomatic arch is the temporal fossa. (b) Sagittal Section of Skull. This midline view of the sagittally sectioned skull shows the nasal septum. Bones of the Cranium The brain case is that portion of the skull that surrounds and protects the brain. It is subdivided into the rounded top of the skull, called the calvaria, and the base of the skull. There are eight bones that form the brain case. These are the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The facial bones support the facial structures, and form the upper and lower jaws, nasal cavity, nasal septum, and orbit. There are 14 facial bones. These are the paired maxillary, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones, and the unpaired vomer and mandible bones. The floor of the brain case is referred to as the base of the skull or cranial floor. This is a complex area that varies in depth and has numerous openings for the passage of cranial nerves, blood vessels, and the spinal cord. Inside the skull, the base is subdivided into three large spaces, called the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa (fossa = “trench or ditch”) ( Figure 7.3.4). From anterior to posterior, the fossae increase in depth. The shape and depth of each fossa correspond to the shape and size of the brain region that each houses. Figure 7.3.4 – Cranial Fossae: The bones of the brain case surround and protect the brain, which occupies the cranial cavity. The base of the brain case, which forms the floor of cranial cavity, is subdivided into the shallow anterior cranial fossa, the middle cranial fossa, and the deep posterior cranial fossa.

Content: trochlear, abducens, oculomotor, ophthalmic, maxillary, mandibular nerves, pituitary gland, internal carotid artery, temporal lobes of the brain

Base of the skull (inferior view)

Content: brainstem, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, hypoglossal nerves, internal jugular vein The walls of each orbit include contributions from seven skull bones ( Figure 7.3.15). The frontal bone forms the roof and the zygomatic bone forms the lateral wall and lateral floor. The medial floor is primarily formed by the maxilla, with a small contribution from the palatine bone. The ethmoid bone and lacrimal bone make up much of the medial wall and the sphenoid bone forms the posterior orbit. The right and left inferior nasal conchae form a curved bony plate (turbinate) that projects into the nasal cavity space from the lower lateral wall (see Figure 7.3.12). The inferior concha is the largest of the nasal conchae and can easily be seen when looking into the anterior opening of the nasal cavity. Vomer Bone Content: frontal lobe of the cerebral cortex, olfactory bulb, olfactory tract, optic nerve, orbital gyri The skull is a bony structure that supports the face and forms a protective cavity for the brain. It is comprised of many bones, which are formed by intramembranous ossification, and joined by sutures (fibrous joints).