The cranial base and the spaces of the skull

Switch the specimen to the internal cranial base view. The skull cap (calvaria) is removed and you are looking straight down at the floor the brain rests on. The face is toward the top of the screen and, because you are looking from above, the patient's right is on your right. This floor is the cranial base. What you see now is its upper, internal surface. Its lower, external surface is the underside of the skull, which you will turn to later in this section.

The floor is not flat. It drops in three steps from front to back, and each step is a cranial fossa (fossa means a trench or hollow). The anterior cranial fossa is the highest and shallowest. The middle cranial fossa is lower and shaped like a butterfly. The posterior cranial fossa is the lowest, deepest and largest. Each is molded to the part of the brain that lies in it, so once you can name the fossa you also know what rests there.

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Internal cranial base view. Skull cap removed, looking down into the cranial cavity: the face is toward the top, the patient's right is on your right.

Two ridges do the dividing, and both are easy to feel on a real skull. The front ridge is the sharp posterior edge of the lesser wings of the sphenoid; everything in front of it is anterior fossa. The back ridge is the thick upper border of the petrous part of the temporal bone on each side (the petrous ridge), which runs obliquely forward and toward the midline; everything behind it is posterior fossa. What lies between the two ridges is middle fossa. The next subtopic follows each boundary to the midline.

• If the surface you are looking at is directly above an eye socket, you are in the anterior fossa.
• If you can see the saddle of the sella turcica, or a deep hollow beside it, you are in the middle fossa.
• If you can see the foramen magnum, or the smooth bowl behind a petrous ridge, you are in the posterior fossa.

The three fossae are separated by parts of three bones. Learn the boundaries as lines you could trace with a fingertip from the side wall to the midline, because the same lines tell you which fossa an opening belongs to.

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Internal cranial base view. Skull cap removed, looking down into the cranial cavity: the face is toward the top, the patient's right is on your right.

Anterior to middle: the lesser wings. Each lesser wing of the sphenoid is a thin, pointed plate that ends behind in a sharp free edge. Follow that edge toward the midline and it thickens into a small knob, the anterior clinoid process, which overhangs the opening of the optic canal. Between the two optic canals a low ridge on the body of the sphenoid completes the line. In front of this line you are over the orbits; behind it the floor drops into the middle fossa.

The center of the middle fossa: the sella turcica. The body of the sphenoid rises in the midline as the sella turcica (Latin for “Turkish saddle”). From front to back it has a low front bump (the tuberculum sellae), a seat (the hypophyseal fossa, which holds the pituitary gland) and a tall back (the dorsum sellae), whose upper corners are the posterior clinoid processes. The air-filled sphenoidal sinus lies inside the body just below the seat, which you can confirm in the midsagittal view.

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Internal cranial base view. Skull cap removed, looking down into the cranial cavity: the face is toward the top, the patient's right is on your right.

Middle to posterior: the petrous ridges. The petrous part of the temporal bone (petrous means rock-like; it houses the middle and inner ear) lies on the floor like a pyramid on its side. Its base is at the side wall near the ear. Its tip, the petrous apex, points forward and medially toward the body of the sphenoid and stops just short of it, at the ragged gap called the foramen lacerum. The upper border of the pyramid, the petrous ridge, is the boundary. Its front slope faces the middle fossa. Its back slope faces the posterior fossa and carries the internal acoustic meatus. In the midline the dorsum sellae joins the right and left ridges.

The posterior fossa: the occipital bone. Behind the petrous ridges almost everything is occipital bone. Its basilar part climbs forward from the foramen magnum to meet the body of the sphenoid; the smooth slope the two make together is the clivus, on which the pons and medulla rest. Its lateral parts flank the foramen magnum and carry the occipital condyles on their under-surface. Its broad squamous part, the occipital squama, forms the bowl for the cerebellum. Where the petrous part of the temporal bone meets the occipital bone, a large irregular gap is left on each side: the jugular foramen.

Now turn the skull over: choose the inferior view. The mandible is hidden so that it does not cover the base. The face is again toward the top of the screen, but you are looking up from below, so the sides are mirrored: the patient's left is on your right, exactly as in the anterior view and the opposite of the internal view you have just left.

The underside looks crowded, but it sorts into three bands from front to back.

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Inferior (external base) view. From below, mandible removed: the face is toward the top, the patient's left is on your right.

The middle band holds the detail. Find its landmarks in order, from the midline outward.

Return to the anterior view and look into an eye socket. Each orbit is a pyramid lying on its side. Its base is the opening you are looking into, ringed by a thick margin. Its tip, the apex, points backward and medially, toward the center of the skull. Between base and apex are four walls: a roof, a floor, a medial wall and a lateral wall. The medial walls of the two orbits are nearly parallel, with only the nasal cavity between them. The lateral walls splay outward at about 45 degrees, which widens the field of vision to the side.

Seven bones line each orbit: frontal, sphenoid, zygomatic, maxilla, ethmoid, lacrimal and palatine. The useful way to learn them is wall by wall, together with what lies on the other side of each wall, because that tells you where a fracture or an infection will go.

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Anterior view. Facing the skull: the patient's left is on your right.

Notice which bones appear more than once. The sphenoid closes the back of the orbit and reaches three walls (roof, lateral, medial). The maxilla forms most of the floor and the front of the medial wall. The zygomatic bone turns the corner between the lateral wall and the floor. The frontal bone has the roof almost to itself. The ethmoid, lacrimal and palatine bones each appear once.

The rim is built from only three bones. The frontal bone forms the supraorbital margin, which carries the supraorbital notch. The zygomatic bone forms the lateral margin and the outer half of the infraorbital margin. The maxilla forms the inner half of the infraorbital margin and, with its frontal process, the medial margin.

How the orbit connects to its neighbors. At the apex, the optic canal and the superior orbital fissure both lead back into the middle cranial fossa. Along the junction of the lateral wall and the floor runs a second long gap, the inferior orbital fissure, which opens backward into the pterygopalatine fossa (a small space behind the maxilla) and, further forward, into the infratemporal fossa. At the front of the medial wall the lacrimal groove leads down into the nasolacrimal canal, which carries tears to the nasal cavity, below the inferior concha. The groove for the infraorbital nerve in the floor becomes a canal and opens on the face at the infraorbital foramen.

The nasal cavity is the tall, narrow space behind the nose, between the two orbits and above the palate. In the anterior view you look into it through the pear-shaped front opening of the bony nose (the piriform aperture). It runs back to the two choanae you found on the external base. A midline partition, the nasal septum, divides it into right and left halves. Each half is narrow at the top and wide at the bottom, and has a roof, a floor, a medial wall (the septum) and a lateral wall.

The bony septum. Open the midsagittal view: the left half of the skull is removed, the face points to your left, and the septum is the thin vertical plate filling the nasal region. It has two bony parts. The perpendicular plate of the ethmoid hangs down from the cribriform plate and forms the upper part. The vomer, a separate bone shaped like a plowshare, forms the lower and back part and reaches the body of the sphenoid behind; its free back edge is the blade you saw between the choanae. Along the floor, the vomer sits on a low crest raised by the maxillae and the palatine bones. In front, the two bony parts leave a wide angle that is filled in life by the septal cartilage, which continues forward into the external nose. A dry skull has lost that cartilage, so the front of the septum looks open.

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Midsagittal section view. Left half removed: you see the inside of the right half, and the face points to your left.

Conchae. The lateral wall is not flat. Three curved shelves of bone, the nasal conchae (singular concha, a shell), project from it and curl downward. The superior and middle nasal conchae are parts of the ethmoid. The inferior nasal concha is the largest and is a separate bone, one of the fourteen facial bones, fixed to the lateral wall. Covered with mucosa in life, the conchae (then called turbinates) enlarge the surface that warms, moistens and cleans the air. From the front only the inferior and middle conchae can be seen; the small superior concha is hidden high up and far back.

Meatuses. The air channel beneath each concha is a meatus (a passage), and it takes the name of the concha that roofs it. So the inferior meatus lies under the inferior concha, between it and the floor. The middle meatus lies under the middle concha. The superior meatus lies under the superior concha. The small pocket above and behind the superior concha, against the body of the sphenoid, is the sphenoethmoidal recess.

How the nasal cavity connects to its neighbors. Forward it opens on the face through the piriform aperture, and backward into the pharynx through the choanae. Upward, the many small holes of the cribriform plate link it to the anterior cranial fossa. From the orbit it receives the nasolacrimal canal. Through its lateral wall, high up and far back near the end of the middle concha, the sphenopalatine foramen leads to the pterygopalatine fossa. Through its floor, just behind the incisor teeth, the incisive canals lead down to the roof of the mouth. And every paranasal sinus opens into it.

The hard palate is one plate with two jobs: its upper surface is the floor of the nasal cavity and its lower surface is the roof of the mouth. In the inferior view it fills the horseshoe of the upper teeth.

Four pieces of bone make it. In front, the palatine process of each maxilla grows in from the tooth-bearing alveolar process and meets its partner in the midline; together the two form the larger front part of the palate, roughly two-thirds to three-quarters of it. Behind, the horizontal plate of each palatine bone completes it. The seams between the four pieces make a cross: a midline suture from front to back, and a transverse suture where the maxillae meet the palatine bones. The back edge of the hard palate is free; the soft palate hangs from it in life.

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Inferior (external base) view. From below, mandible removed: the face is toward the top, the patient's left is on your right.

• The incisive fossa: a single pit in the midline just behind the central incisor teeth. The incisive canals from the nasal cavity open into it.
• The greater palatine foramen: one on each side, at the back outer corner of the palate, close to the last molar tooth, at the edge of the horizontal plate.
• The lesser palatine foramina: one or more small holes just behind each greater palatine foramen.

These three sets of openings mark the palate. What each one carries is in Section 5: Openings of the face, jaw and palate.

You now have the main spaces of the skull: the cranial cavity with its three fossae, the two orbits, the nasal cavity and, below the palate, the mouth. Three smaller spaces on the outside of the base will keep appearing as destinations. The infratemporal fossa lies below the zygomatic arch, deep to the ramus of the mandible. The pterygopalatine fossa is a narrow cleft behind the maxilla and in front of the pterygoid process. The neck lies below the base. Every opening in the next section is a doorway between two of these.