Video: Cranial fossae

What is something that provides our brains with support and protection, has more holes in it than a piece of Swiss cheese, and strikes absolute fear into your average Anatomy student? Eeehhh, how about the floor of the cranial cavity. Fortunately, learning about this intricate part of our anatomy doesn't have to be daunting and can be made easier by dividing it into three parts called the cranial fossae.

The base of the skull forms the inferior wall of the cranial cavity and comprises parts of the frontal, ethmoid, sphenoid, temporal, and occipital bones. These bones form the three cranial fossae – anterior, middle, and posterior. Let's waste no time and begin by looking at the anterior cranial fossa, which you can see highlighted here. It is the part of the skull base that supports the frontal lobe of the brain and is comprised anterolaterally by parts of the frontal bone, medially by parts of the ethmoid bone, and posteriorly by parts of the sphenoid bone.

Beginning with the frontal bone's contribution to the anterior cranial fossa, we see that a large part of the floor of the anterior cranial fossa is formed by the superior surface of the orbital plates of the frontal bone. Anteromedial to the orbital plates is a midline ridge known as the frontal crest, which provides an attachment for the falx cerebri. At the point where the frontal crest meets the anterior border of the ethmoid bone, a small foramen known as the foramen cecum may very occasionally be present. When patent, it gives passage to an emissary vein from the nasal cavity, which drains into the superior sagittal sinus.

Here we have highlighted the parts of the ethmoid bone that contribute to the anterior cranial fossa. First, let's identify the anterior ethmoidal foramen, which is this bilateral structure seen here. It transmits the anterior ethmoidal artery, veins and nerve from the orbit to the anterior cranial fossa. There's also the cribriform plate which has many small openings known as cribriform, or olfactory, foramina, which give it a sieve-like appearance. They provide passage for many of the olfactory fiber bundles which extend from the olfactory epithelium of the upper nasal mucosa to the olfactory bulb sitting on top of the cribriform plate. They carry signals related to the sense of smell from the nasal cavity.

Next up is the crista galli, which translates to rooster's crest. It's a thick, triangular-shaped, vertical bony projection from the midline of the cribriform plate that also acts as an attachment point for the falx cerebri. As mentioned earlier, the posteriormost part of the anterior cranial fossa is composed by parts of the sphenoid bone, which can be seen highlighted here.

In the context of the anterior cranial fossae, let's first identify the lesser wings, which are these small bony plates seen here. They unite medially at the sphenoidal yoke which is this flat anterior part of the surface of the body of the sphenoid bone.

Moving onto the middle cranial fossa, this butterfly-shaped 'bowl' houses the temporal lobes of the brain as you can see here. We can see that the boundary between the anterior and middle cranial fossae can be mostly drawn along the posterior margins of the lesser wings of the sphenoid bone which are known as the sphenoidal ridges. The middle cranial fossa itself consists mainly of parts of the sphenoid bone and temporal bones. Let's take a look.

So here you can see the parts of the sphenoid bone that contributes to the middle cranial fossa, and we're going to begin with this central part here which we know is the body of the sphenoid bone. The first landmark we'll identify is sella turcica, which literally means 'Turkish saddle', and if you look at it in sagittal section, you can certainly see why.

The middle, deepest portion of the sella turcica is known as the hypophyseal, or pituitary, fossa which, as the name suggests, houses the hypophysis, which you and I would more commonly refer to as the pituitary gland. The anterior and posterior walls of the sella turcica are termed the tuberculum sellae and dorsum sellae respectively.

Anterior to the tuberculum sellae is a small depression known as the chiasmatic sulcus which houses the optic chiasm. It's flanked on either side by the optic canals which gives passage to optic nerves.

The final landmarks which I'll point out to you in this region of the body of the sphenoid bone are three pairs of processes namely the anterior clinoid processes which are found at the medial ends of the sphenoidal ridges, the middle clinoid processes which are continuous with the tuberculum sellae, and finally, the posterior clinoid processes which extend from the dorsum sellae.

Moving laterally from the body of the sphenoid bone, we're now looking at the greater wings of the sphenoid bones which are much stronger and broader in form than their lesser counterparts and comprise the anterior third of the lateral portion of the middle cranial fossa.

Now let's move on to the parts of the temporal bone in the middle cranial fossa, specifically, its petrous part that contributes to the posterior two-thirds of the lateral portion of the middle cranial fossa, and you'll be glad to know that there are not many landmarks to identify here.

First, let's take a look at this depression here known as the trigeminal impression. This is the site where the ganglion of the trigeminal or fifth cranial nerve is lodged. Additionally, we have the arcuate eminence which is a rounded prominence on the superior aspect of the petrous portion of the temporal bone that marks the position of the anterior semicircular canal of the internal ear.

Now let's take a look at the several openings, or foramina, within the middle cranial fossa through which important neurovascular structures pass through. Starting in the greater wings, the smallest and most lateral of the bunch is known as the foramen spinosum. It transmits the middle meningeal artery and vein as well as the meningeal branch of the mandibular nerve. You can remember these by the mnemonic, 'SPINy MIDDLE MEN'.

Just medial to it is the foramen ovale, which transmits the mandibular nerve, the accessory meningeal artery, the lesser petrosal nerve, and an emissary vein. A useful mnemonic to remember these structures is 'O-MALE'.

If we continue anteromedially, we can next identify the foramen rotundum which transmits the maxillary nerve. To remember this pathway, use the mnemonic 'RO-MAX' – through the foramen ROtundum travels the MAXillary nerve.

Continuing in an anteromedial direction, we will then come across another opening called the optic canal, which allows for the passage of the optic nerve and the ophthalmic artery.

Now you may have noticed that these four openings are roughly oriented along a diagonal line from lateral to medial. An easy way to remember these openings is with the mnemonic, 'Stop Only Remembering One'.

Now, in between the greater and lesser wings, you will notice an elongated crack-like opening. This is the superior orbital fissure and it allows passage of several important structures to the eyes such as the oculomotor, trochlear, abducens nerves as well as branches of the ophthalmic division of the trigeminal nerve – so that's cranial nerves III, IV, V, and VI – and finally, we have the ophthalmic veins.

Moving posteriorly just medial to the foramen ovale, we find the foramen lacerum, located between the sphenoid, temporal, and occipital bones. It is actually not a true foramen in the sense that it is filled with cartilage in life and only gives passage for a small number of meningeal arteries and veins, however some structures like the deep and greater petrosal nerves do cross it horizontally.

Lying immediately posterolateral to the foramen lacerum is the internal opening of the carotid canal, the passageway for the internal carotid artery to enter the skull. Extending from the carotid canal is the carotid sulcus which marks the path of the internal carotid artery after it enters the cranial cavity.

Thankfully, that's just about everything there is to identify in the middle cranial fossa. So let's now take a look at the posterior cranial fossa, the section of the cranial base that houses the cerebellum and brainstem. It is composed mainly by the occipital and temporal bones and is bounded anteriorly on either side by the superior border of the petrous part of the temporal bone, which is known as the petrous ridge.

Looking at the contributions of the temporal bone to the posterior cranial fossa, we can see that it mainly involves the posterior surface of the petrous part and you'll be glad to know that there's only one landmark we need to identify this time around and that is the internal acoustic meatus which gives passage for both the vestibulocochlear and facial nerves as well as the labyrinthine artery and veins which supply the internal ear.

Now let's look at the contributions of the occipital bone which form the larger portion of the posterior cranial fossa. The first thing you may notice about it are these paired bowl-like impressions seen here. These are the cerebellar fossae which house the left and right cerebellar hemispheres. The ridge between the two fossae seen here is the internal occipital crest which provides attachment for the falx cerebelli which separates the cerebellar hemispheres.

At its posterior end, the internal occipital crest merges with the grooves for the transverse and superior sagittal sinuses to form a cross-shaped elevation known as the cruciform eminence. At its center is an additional bony projection known as the internal occipital protuberance.

Let's switch our focus for a moment to the anteriormost part of the occipital bone known as the basilar part. It joins with the body of the sphenoid bone and contributes to the cranial region known as the clivus that extends from the dorsum sellae to the foramen magnum upon which lies the brainstem.

There are four major openings within the posterior cranial fossa to note. The most obvious is, of course, the foramen magnum which translates to 'great hole'. You probably know already that it serves as the landmark and passageway of the brainstem as it transitions into the spinal cord. It is flanked on either side by two small openings known as the hypoglossal canals which, as the name suggests, transmits the hypoglossal nerves.

If we then look just a bit more laterally, we find another set of irregularly-shaped openings seen here along the border between the occipital and temporal bones. These are known as the jugular foramina. Due to their name, it's easy to remember that they each give passage for the internal jugular vein but it also transmits the 9th, 10th, and 11th cranial nerves.

And that brings us to the end of this tutorial on the cranial fossae. I hope you enjoyed this jam-packed tutorial. To solidify your knowledge and revise what we've learned today, check out our quizzes, atlas galleries, and other learning resources for this topic on our dedicated study unit page.

See you next time!