The Cranium

The portion of the skull which encloses the brain is the cranium. There are eight cranial bones: frontal, two parietals, occipital, two temporals, ethmoid, and sphenoid. They are all held together with sutures.

The frontal bone forms the anterior portion of the cranium, including the ridges above the eyebrows, and the superior portion of the orbit or eye socket. The supraorbital foramina can be seen above the orbits.

The frontal is attached to the two parietals at the coronal suture (corona = L. crown). Examine the parietals carefully and you will see the two faint temporal lines which form an arch on each side of the skull. They serve as a point of attachment for the temporal muscles.

Note the depressions for blood vessels on the interior surface of the parietals. These depressions are sulci (sing. - sulcus). The two parietal bones are held together by the sagittal suture.

The posterior, inferior portion of the cranium is the occipital bone. The occipital is joined to both of the parietals by the lambdoidal suture. The large hole in the occipital is the foramen magnum. The spinal cord exits the brain there. On either side of the foramen magnum, on the inferior surface, are the two occipital condyles which form a joint with the vertebral column. "Under" (actually superior to) the occipital condyles are the hypoglossal canals.

Zygomatic Arch For Singing
Figure 9.1 Superior View of the Skull

Coronal suture Frontal Lambdoidal suture Nasal Occipital Parietal Sagittal suture Sutural bone

Figure 9.2 Skull, Lateral View

Coronal suture Ethmoid External auditory meatus Frontal Lacrimal Lambdoidal suture Mandible Mandibular condyle Mandibular fossa Mastoid process Maxilla Nasal Occipital Parietal Sphenoid Styloid process Squamosal suture Temporal Temporal lines Temporal process of zygomatic Zygomatic Zygomatic arch Zygomatic process of temporal

On each side of the skull there is a temporal bone. The temporal bones are joined to the parietals at the squamosal sutures. The flattened portions of the temporal bones are called the squamosal portions.

The temporal bone has many important bone markings. Projecting inferiorly is the slender, pointed styloid process. Unfortunately, this feature has been broken off most of the laboratory skulls. Find the styloid process on the drawing and where it was on the skull. The larger, blunt mastoid process is located lateral and posterior to the styloid process.

Superior to these two processes is the external auditory meatus. As the name suggests, this canal contains the internal organs of hearing. The external ear is attached at this point on the temporal bone.

On the interior sloping surface of the temporal bone, you can see the internal auditory meatus which contains the nerves of hearing and equilibrium. The petrous portion of the temporal bone is the thickened, triangular portion through which the auditory canals run. Within the petrous process are the organs of hearing and equilibrium including the three smallest bones of the body, the malleus, incus, and stapes. These bones play a vital role in hearing. Details of their function, as well as of the anatomy of the rest of the inner ear, will be dealt with in chapter twenty-five.

Between the petrous portion of the temporal bone and the occipital bone, inferior and posterior to the internal auditory meatus, you can see the large jugular foramen which holds the jugular vein. Find and label both internal and external openings of the jugular foramen. DO NOT PUT THE POINT OF A PENCIL OR PEN NEAR THE SKULL! DO NOT ATTEMPT TO PUT ANYTHING THROUGH THE FORAMINA OF THE SKULL!

Once you have located the external openings of the jugular foramina, you can easily see the external openings of the carotid canals immediately anterior to the exterior openings of the jugular foramina. The carotid canals are directed medially to their internal openings. These internal openings are not easily visible on the floor of the cranium. They are located along with some other foramina at the medial point of the petrous portions. We will learn the other foramina in this group later.

Figure 9.3 Base of the Skull

Carotid canal External auditory meatus Foramen lacerum Foramen magnum Foramen ovale Hypoglossal canal Incisive foramen Jugular foramen Maxilla Occipital Occipital condyle Palatine Sphenoid Styloid process Temporal Zygomatic

Once again, examine the external surface of the temporal bone. Anterior to the external auditory meatus is a depression, the mandibular fossa, which articulates with the mandible. The long anteriorly projected process which articulates with the cheekbone (zygomatic bone) is the zygomatic process of the temporal bone. This process is part of the zygomatic arch. Can you locate the suture between the zygomatic process of the temporal bone and the temporal process of the zygomatic bone?

Medial to the zygomatic arch is the sphenoid bone. This portion of the sphenoid bone is called the greater wing of the sphenoid. The rest of the sphenoid bone can be seen on the floor of the cranium.

The depression in the center of the superior surface of the sphenoid is the hypophyseal fossa. The pituitary gland (hypophysis) is located here. Note that the hypophyseal fossa is part of a ridge of bone that looks somewhat like a saddle with the fossa being the seat. This ridge of bone is the sella turcica (L. Turkish saddle). The two points of bone on the back of the saddle are the posterior clinoid processes. Anterior to the sella turcica are the two optic foramina which contain the optic nerves from the eyes. Lateral and posterior to the optic foramina are the anterior clinoid processes.

The sphenoid contains a number of other important markings. Lateral to each posterior clinoid process is the foramen lacerum. The internal opening of the carotid canal joins the internal opening of the foramen lacerum (see discussion of temporal bone). Lateral to the foramen lacerum are the foramen ovale and the foramen spinosum. The foramen ovale is larger, oval-shaped, and anterior.

The final bone of the cranium is the ethmoid. It can be seen projecting into the floor of the cranium through the frontal bone. The ridge of bone in the center is the crista galli (L. cock's comb). On either side of the crista galli is the cribriform plate which has numerous holes for the olfactory nerves from the nasal cavity. Projecting from the inferior surface of the ethmoid is the perpendicular plate which forms the upper part of the nasal septum.

Figure 9.4 Floor of the Cranium

Anterior clinoid process Cribriform plate Crista galli Ethmoid Foramen lacerum Foramen magnum Foramen ovale Foramen spinosum Frontal Hypoglossal canal Hypophyseal fossa Internal auditory meatus Jugular foramen Occipital Optic foramen Petrous portion of temporal Posterior clinoid process Sella turcica Sphenoid

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