These include the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The parietal bone forms most of the upper lateral side of the skull see Figure 6. These are paired bones, with the right and left parietal bones joining together at the top of the skull.
Each parietal bone is also bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone. The temporal bone forms the lower lateral side of the skull see Figure 6. The temporal bone is subdivided into several regions Figure 6. The flattened, upper portion is the squamous portion of the temporal bone. Below this area and projecting anteriorly is the zygomatic process of the temporal bone, which forms the posterior portion of the zygomatic arch.
Posteriorly is the mastoid portion of the temporal bone. Projecting inferiorly from this region is a large prominence, the mastoid process , which serves as a muscle attachment site. The mastoid process can easily be felt on the side of the head just behind your earlobe.
Posterior to the mandibular fossa on the external base of the skull is an elongated, downward bony projection called the styloid process, so named because of its resemblance to a stylus a pen or writing tool.
This structure serves as an attachment site for several small muscles and for a ligament that supports the hyoid bone of the neck. See also Figure 6. The frontal bone is the single bone that forms the forehead.
At its anterior midline, between the eyebrows, there is a slight depression called the glabella see Figure 6. The frontal bone also forms the supraorbital margin of the orbit. Near the middle of this margin, is the supraorbital foramen, the opening that provides passage for a sensory nerve to the forehead. The frontal bone is thickened just above each supraorbital margin, forming rounded brow ridges.
These are located just behind your eyebrows and vary in size among individuals, although they are generally larger in males. Inside the cranial cavity, the frontal bone extends posteriorly. This flattened region forms both the roof of the orbit below and the floor of the anterior cranial cavity above see Figure 6. The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity Figure 6.
On the base of the skull, the occipital bone contains the large opening of the foramen magnum , which allows for passage of the spinal cord as it exits the skull. On either side of the foramen magnum is an oval-shaped occipital condyle. These condyles form joints with the first cervical vertebra and thus support the skull on top of the vertebral column. The sphenoid bone is a single, complex bone of the central skull Figure 6. The sphenoid forms much of the base of the central skull and also extends laterally to contribute to the sides of the skull see Figure 6.
Inside the cranial cavity, the right and left lesser wings of the sphenoid bone , which resemble the wings of a flying bird, form the lip of a prominent ridge that marks the boundary between the anterior and middle cranial fossae.
This bony region of the sphenoid bone is named for its resemblance to the horse saddles used by the Ottoman Turks, with a high back and a tall front. The rounded depression in the floor of the sella turcica is the hypophyseal pituitary fossa , which houses the pea-sized pituitary hypophyseal gland. The greater wings of the sphenoid bone extend laterally to either side away from the sella turcica, where they form the anterior floor of the middle cranial fossa.
The greater wing is best seen on the outside of the lateral skull, where it forms a rectangular area immediately anterior to the squamous portion of the temporal bone. On the inferior aspect of the skull, each half of the sphenoid bone forms two thin, vertically oriented bony plates.
The right and left medial pterygoid plates form the posterior, lateral walls of the nasal cavity. The somewhat larger lateral pterygoid plates serve as attachment sites for chewing muscles that fill the infratemporal space and act on the mandible. The ethmoid bone is a single, midline bone that forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit Figure 6.
On the interior of the skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity. Within the nasal cavity, the perpendicular plate of the ethmoid bone forms the upper portion of the nasal septum.
The ethmoid bone also forms the lateral walls of the upper nasal cavity. Extending from each lateral wall are the superior nasal concha and middle nasal concha, which are thin, curved projections that extend into the nasal cavity Figure 6.
In the cranial cavity, the ethmoid bone forms a small area at the midline in the floor of the anterior cranial fossa. This region also forms the narrow roof of the underlying nasal cavity. This portion of the ethmoid bone consists of two parts, the crista galli and cribriform plates. It functions as an anterior attachment point for one of the covering layers of the brain. Small nerve branches from the olfactory areas of the nasal cavity pass through these openings to enter the brain. A suture is an immobile joint between adjacent bones of the skull.
The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The long sutures located between the bones of the brain case are not straight, but instead follow irregular, tightly twisting paths.
These twisting lines serve to tightly interlock the adjacent bones, thus adding strength to the skull for brain protection.
The two suture lines seen on the top of the skull are the coronal and sagittal sutures. The coronal suture runs from side to side across the skull, within the coronal plane of section see Figure 6. It joins the frontal bone to the right and left parietal bones. The sagittal suture extends posteriorly from the coronal suture, running along the midline at the top of the skull in the sagittal plane of section see Figure 6.
It unites the right and left parietal bones. On the posterior skull, the sagittal suture terminates by joining the lambdoid suture. The cranium the skull minus the lower jaw bone, or mandible consists of 28 bones. Some bones are paired, which means there is a left and right, and some bones are unpaired, meaning there is just one.
Here is a list of those bones with a brief description:. Frontal bone Unpaired — this is the forehead, from the eyebrows to the top of the skull. Parietal bone Paired — the left and right parietal bones connect at the top of the skull. Occipital bone Unpaired — this is the back of the skull. The spinal cord goes through a hole in this area called the foramen magnum connecting with the brain.
Temporal bone Paired — the left and right temporal bones are found on either side of the skull. Sphenoid bone Unpaired — helps to form the floor and the sides of the cranium. Maxilla Paired — this is the upper jaw, where the upper teeth are located. The paranasal sinuses are named for the skull bone that each occupies. The frontal sinus is located just above the eyebrows, within the frontal bone. This irregular space may be divided at the midline into bilateral spaces, or these may be fused into a single sinus space.
The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is the maxillary sinus. These are paired and located within the right and left maxillary bones, where they occupy the area just below the orbits. The maxillary sinuses are most commonly involved during sinus infections.
Because their connection to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline sinus. It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the most posterior of the paranasal sinuses.
The lateral aspects of the ethmoid bone contain multiple small spaces separated by very thin bony walls. Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae. The hyoid bone is an independent bone that does not contact any other bone and thus is not part of the skull. The hyoid serves as the base for the tongue above, and is attached to the larynx below and the pharynx posteriorly.
The hyoid is held in position by a series of small muscles that attach to it either from above or below. Movements of the hyoid are coordinated with movements of the tongue, larynx, and pharynx during swallowing and speaking.
Skip to main content. Chapter 7- The Skeletal System. Search for:. Cranial Bones of the Skull: Cranial bones form the cranial cavity that surrounds and protects the brain and brainstem. Figure 2: Cranial Bones of Human Skull. Figure 3: Immovable Suture Joints of Skull. Figure 4 a Superior view of infant skull. Figure 4 b Lateral view of infant skull:. Figure 5: Facial Bones of Human Skull. Figure 6: Paranasal Sinuses.
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