Openings and what passes through them

An opening in the skull is never just a hole. It is the place where a named structure crosses from one space to another, and its shape tells you something about what it carries. Four words cover almost every opening you will meet.

A notch is an opening that the bone did not close; soft tissue completes it in life. The supraorbital notch is the usual example.

For every opening, answer the same four questions.

1. Which bone, or between which bones?
2. Which two spaces does it join?
3. What passes through? Learn the principal structure first; add the others once that is secure.
4. Which neighbor could I mistake it for, and what difference in position separates them?

The fourth question is the one students skip and examiners ask. On an unlabeled skull nothing tells you what runs through a hole; only its position does. So in this section position always comes first and contents second.

The anterior fossa has one set of openings to learn, and it is a set, not a single hole. On each side of the crista galli the cribriform plate of the ethmoid forms a narrow gutter pierced by many small cribriform foramina (cribrum is Latin for a sieve). Treat the plate as a perforated region: you are not asked to find an individual hole.

The direction of travel is upward. Receptor cells in the lining of the nasal roof send their fine axons up through the plate, and these end at once in the olfactory bulb, which rests on the upper surface of the plate. CN I is therefore not one trunk but a spray of filaments.

Most of the openings of the cranial base are in the middle fossa, and five of them are in a single bone. Go to the internal cranial base view and find the sella turcica. On each side of it, the openings of the sphenoid form a curve that runs from the midline outward and backward: optic canal → superior orbital fissure → foramen rotundum → foramen ovale → foramen spinosum. Learn that curve as a sequence and each opening has an address.

Specimen still

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.

Optic canal. The optic canal is a short round tunnel through the root of the lesser wing, at the front outer corner of the sella turcica and just medial to the anterior clinoid process. It runs forward and a little outward into the apex of the orbit. It carries the optic nerve (CN II) and, with it, the ophthalmic artery.

Superior orbital fissure. Immediately lateral to the optic canal, and separated from it only by a thin bar of bone, is a long slit: the gap between the lesser wing above and the greater wing below. The superior orbital fissure is widest at its medial end and narrows as it runs upward and outward. It also opens into the orbit. Through it pass the three nerves that move the eye, the oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves; the three branches of the ophthalmic nerve (V1), named lacrimal, frontal and nasociliary; and the superior ophthalmic vein, which drains the orbit backward.

Foramen rotundum. Just below and behind the medial end of the fissure, in the greater wing, is a small round opening that faces forward more than downward. The foramen rotundum is really a short canal that runs forward into the pterygopalatine fossa. It carries the maxillary nerve (V2). Because it points forward, it has no mouth on the under-surface of the skull: from below you will not find it.

Foramen ovale. Further back and further out, still in the greater wing, is a larger oval hole that passes straight down through the floor. The foramen ovale opens into the infratemporal fossa and carries the mandibular nerve (V3). Other structures are listed for it but are variable: an accessory meningeal artery may enter here, and the lesser petrosal nerve occasionally leaves through it. Learn V3 as the answer and the others as possible companions.

Foramen spinosum. Behind and lateral to the foramen ovale, at the back corner of the greater wing, is the smallest opening of the set. The foramen spinosum is named for the spine of the sphenoid, a small downward point of bone right beside it on the external surface. It also opens into the infratemporal fossa, but its traffic runs the other way: the middle meningeal artery enters the skull here. From the foramen, branching grooves for the artery run up the inner surface of the side wall toward the pterion. A middle meningeal vein and a small meningeal branch of the mandibular nerve accompany the artery.

Foramen lacerum. Medial to the foramen ovale, at the side of the body of the sphenoid, the tip of the petrous part of the temporal bone stops short. The jagged gap left between it, the sphenoid and the basilar part of the occipital bone is the foramen lacerum (lacerum means torn). On a dry skull it can look like the largest opening in the region. In life it is filled with fibrocartilage, and no major structure passes down through it.

Carotid canal. The important structure here is the internal carotid artery, and it uses a different opening. The carotid canal begins as a round hole on the under-surface of the petrous part of the temporal bone, runs upward for a short distance, then bends forward and medially inside the bone. It ends at the petrous apex, in the upper part of the foramen lacerum. The artery emerges there, crosses the top of the cartilage, and turns upward beside the body of the sphenoid into the cavernous sinus next to the sella turcica. A plexus of sympathetic nerve fibers, the internal carotid plexus, travels on the artery through the canal.

Behind the petrous ridge the openings are fewer and larger. Three are paired and one is the largest opening of the skull. Work from the petrous slope down to the midline.

Specimen still

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.

Internal acoustic meatus. About halfway along the back slope of the petrous part, facing the brainstem, is a round opening that leads sideways into the bone: the internal acoustic meatus. It is a short passage that ends blindly inside the bone. Two nerves enter it together: the facial nerve (CN VII) and the vestibulocochlear nerve (CN VIII), with the small labyrinthine artery to the inner ear. At the bottom of the meatus they separate. CN VIII ends in the inner ear, inside the petrous part, and never leaves the skull. CN VII continues into its own tunnel, the facial canal.

Jugular foramen. Directly below the internal acoustic meatus, in the seam between the petrous part of the temporal bone and the occipital bone, is a large, irregular opening: the jugular foramen. A broad curved groove on the inner wall of the fossa leads into it; this groove holds the sigmoid sinus, the last of the venous channels inside the skull. At the foramen the sigmoid sinus becomes the internal jugular vein. Three cranial nerves leave through the same opening: the glossopharyngeal (CN IX), vagus (CN X) and accessory (CN XI) nerves. A smaller venous channel, the inferior petrosal sinus, also drains through the foramen.

Hypoglossal canal. Lower and more medial still, just above the rim of the foramen magnum at its front outer edge, a short tunnel passes forward and outward through the occipital bone, above the occipital condyle. The hypoglossal canal carries the hypoglossal nerve (CN XII), the motor nerve of the tongue. From outside, its exit is hidden above the front of the occipital condyle.

Foramen magnum. The foramen magnum (“great opening”) is the oval hole in the floor of the fossa, entirely within the occipital bone, joining the cranial cavity to the vertebral canal. Here the medulla oblongata becomes the spinal cord, wrapped in the meninges. The two vertebral arteries come up through it to supply the brain. The spinal roots of the accessory nerve (CN XI) also ascend through it: they arise from the upper spinal cord, enter the skull through the foramen magnum, and then leave again through the jugular foramen.

Turn to the inferior view. Most of what you find are the outer mouths of openings you already know, but one opening exists only on this surface.

Stylomastoid foramen. On the under-surface of the temporal bone, in the angle between the styloid process in front and the mastoid process behind, is a small round hole: the stylomastoid foramen. Its name is its address. It is the lower end of the facial canal, and the facial nerve (CN VII) leaves the skull here on its way to the muscles of facial expression.

This route explains a clinical rule. A lesion at the internal acoustic meatus affects the facial nerve and hearing and balance, because CN VIII lies beside it there. A lesion at or beyond the stylomastoid foramen paralyzes the face alone.

The outer mouths. Medial to the styloid process find two openings, one in front of the other. The round, smooth-walled one in front is the lower opening of the carotid canal, where the internal carotid artery enters the bone. The larger, irregular one behind it is the jugular foramen, where the internal jugular vein begins. Artery in front, vein behind, with a ridge of bone between them. Further forward and medially, at the tip of the petrous part, is the ragged outer face of the foramen lacerum. On the under-surface of the greater wing, just behind the root of the lateral pterygoid plate, are the foramen ovale and, behind and lateral to it, the foramen spinosum next to the spine of the sphenoid.

The remaining openings are on the facial skeleton. Three of them are the points where the skin nerves of the face come to the surface, one for each division of the trigeminal nerve, and they line up.

In the anterior view these three openings lie close to one vertical line on each side of the face. From top to bottom they belong to V1, V2 and V3, the same order in which those divisions left the middle cranial fossa (superior orbital fissure, foramen rotundum, foramen ovale).

Mandibular foramen. Select the mandible and turn it to see the inner surface of the ramus. Near its center is the mandibular foramen, partly covered in front by a small tongue of bone, the lingula. It is the entrance to the mandibular canal, a tunnel that runs forward inside the body of the mandible beneath the roots of the teeth. The inferior alveolar nerve (a branch of V3) and its vessels enter here and supply the lower teeth. The nerve ends as the mental nerve, which comes back out to the surface at the mental foramen.

Openings of the palate. In the inferior view, three sets of openings mark the hard palate.

Everything in this section on one page. Read each row as a sentence: this opening, in this bone, joins this space to that space and carries this. Cover the last column and test yourself, then cover the first.

Three groups of openings are asked about together because they lie close together or carry the same nerve. In each, the contents are easy to memorize and easy to swap. What keeps them apart is where each opening is.

A memory aid for the three divisions of the trigeminal nerve is “Standing Room Only”: Superior orbital fissure for V1, foramen Rotundum for V2, foramen Ovale for V3. The spinosum is not on that list because its principal content is an artery.

Core activity · Almost Twins

Almost twins: openings

Two look-alikes side by side. Decide what they share, then pick the clue that separates them and say why it works.

Foramen rotundum vs foramen ovaleForamen ovale vs foramen spinosumInternal acoustic meatus vs stylomastoid foramenOptic canal vs superior orbital fissureJugular foramen vs hypoglossal canal

A · Foramen rotundumB · Foramen ovale

Look-alike A

Foramen rotundum

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.

• Greater wing of the sphenoid, middle cranial fossa
• At the front, just below the superior orbital fissure; small, and it faces forward
• Leads to the pterygopalatine fossa; no mouth on the under-surface of the skull
• Maxillary nerve (V2)

Look-alike A

Foramen rotundum

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.

• Greater wing of the sphenoid, middle cranial fossa
• At the front, just below the superior orbital fissure; small, and it faces forward
• Leads to the pterygopalatine fossa; no mouth on the under-surface of the skull
• Maxillary nerve (V2)

Look-alike B

Foramen ovale

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.

• Greater wing of the sphenoid, middle cranial fossa
• Behind and lateral to the rotundum; the largest of the wing's openings, and it passes straight down
• Leads to the infratemporal fossa; found again in the inferior view
• Mandibular nerve (V3)

Step 1 · What they share

What do these two share?

Select every feature that both show.

ABoth pierce the greater wing of the sphenoidBBoth open from the middle cranial fossaCBoth carry a division of the trigeminal nerveDBoth can be found in the inferior viewEBoth lead to the infratemporal fossa

Check

Reset this pair

Openings make most sense when you follow one structure from where it starts to where it ends. The routes below are fixed by the anatomy, and each one ties two or more openings together.