Inside Your Skull

Dr. Stephen Flanagan

Welcome to the Flanatomy podcast. I’m your host, Dr. Stephen Flanagan. We’re diving into the wild world of anatomy with a side of crazy stories from my bizarre life. Let’s explore what makes you, you!

Keshia Rayna

And I’m Keshia Rayna, your co-host, keeping it one hundred and making sure Doc doesn’t get lost in his wild tales. I’m locked and loaded with my phone to fact-check and break down the nerdy stuff so y’all can vibe with the science. What’s on deck today, Doc?

Dr. Stephen Flanagan

We’re cracking open the skull today—the bony fortress guarding your brain and face, and the star of every anatomy lab horror story. This one’s for you college anatomy students, so grab your calipers, pipe cleaners, and a strong stomach, ‘cause we’re mapping the skull’s geography, bones, cavities, markings, foramen, and everything in between. Let me apologize ahead of time, AI is not always the best at pronunciations especially since we are bumping between a few linguistic roots.

Keshia Rayna

We should keep it to the cranium and facial bones, weaving in how those holes let nerves and vessels sneak through. For this episode.

Dr. Stephen Flanagan

That is a good idea. As always, I’ve got a couple of personal tales that’ll make you cringe—one involving a dog named Brutus and another with a cracked tooth from weightlifting.

Keshia Rayna

Let’s start with the big picture: skull geography. The skull’s divided into the cranium, which houses the brain, and the facial bones.

Dr. Stephen Flanagan

Exactly, and the cranium has two main parts: the vault (or calvaria), the rounded top like a dome, and the base, the floor with all the ridges and holes. The cranial vault is made of multiple obvious bones stitched together by sutures—those zigzag lines like the sagittal or coronal suture. Those sutures are fibrous joints, from Latin sutura, meaning seam, locking the bones tight but allowing slight flex in babies. In adults, they’re fused, making the vault a solid shield. when you trace those sutures with your finger it feels like reading Braille on bone.

Keshia Rayna

Skull Braille? That’s a vibe, Doc. So, vault’s the dome, base’s the floor. What about those fossae you always mention?

Dr. Stephen Flanagan

Spot on—the cranial base is divided into three fossae by internal bony ridges, like steps in a theater. This is a good spot to remind every one to say "fah-sa... like Mufasa from the lion king.

Keshia Rayna

Mufasa, that is hilarious! Did you see the 2024 live action Mufasa movie Flan?

Dr. Stephen Flanagan

No, how was it?

Keshia Rayna

It was good! But, it was a little strange watching photorealistic lions sing songs by Lin-Manuel Miranda...

Dr. Stephen Flanagan

Yeah, that seems odd. I'll have to watch it and get back to you. Let's see... we were on the fossa... if you take the top of the skull off and look down into the skull from above, The anterior cranial fossa is the front step, housing the frontal lobes of the brain, bounded by the frontal bone and lesser wings of the sphenoid. It’s shallow, like a ledge for your thinking cap. The middle cranial fossa is the middle step, deeper, holding the temporal lobes, pituitary gland, and more, framed by the sphenoid’s greater wings and petrous temporal bones. The posterior cranial fossa is the back step, deepest, cradling the cerebellum and brainstem, walled by the occipital bone and mastoid temporal parts. Fossae from Latin fossa, ditch—perfect for those brain depressions. In cadavers, you see these ridges clearly when you lift the brain—looks like a bony staircase.

Keshia Rayna

Bony staircase? Makes the skull sound like a haunted house. So, anterior for frontal lobes, middle for temporal and pituitary, posterior for cerebellum. What about the skull’s smaller cavities?

Dr. Stephen Flanagan

The skull’s packed with smaller cavities—like hidden rooms. The middle and inner ear cavities are in the lateral cranial base, in the temporal bone: middle ear’s the tympanic cavity with ossicles, inner ear’s the bony labyrinth with cochlea and semicircular canals. The nasal cavity lies in and posterior to the nose, divided by the septum, lined with conchae for air warming. The orbits are the eye sockets, conical holes formed by seven bones (frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, ethmoid), with fissures for optic nerves. Air-filled sinuses are in bones around the nasal cavity—frontal, ethmoid, sphenoid, maxillary—like hollow pockets lightening the skull.

Keshia Rayna

So, ears, nose, eyes, sinuses—all tucked in. Lets get into the cranial bones?

Dr. Stephen Flanagan

Sure! The cranial bones are eight large ones forming the cranium which houses the brain. there are 2 sets of Paired bones: the temporal and parietal. then 4 Unpaired bones: the frontal, occipital, sphenoid , and ethmoid. Let’s break ‘em down.

Dr. Stephen Flanagan

the Frontal bone, forms forehead and orbital roofs. as far as interesting markings go there are supraorbital ridges or brow ridges, supraorbital foramen which are little holes above the orbits (for supraorbital nerve/vessels). I have actually seen my own frontal bone! When I was a kid, in 4th or 5th grade, a dog bit me right in the eye—or at least it seemed he went for my eye. But my eyebrow ridges are so big the dog's bottom jaw grazed my cheek and latched onto my eyebrow. It tore a big enough chunk off the lateral portion of my eyebrow that I saw my skull when I looked in the mirror after the attack. The dog’s name was Brutus, a very close friend until that moment. So I like to think that if I could be in the same situation again, I would have said, "Et tu, Brute?" while clutching my bloody brow. I got a bunch of Stitches, and a pretty good scar, but the brow ridge, and reflexes saved my eye.

Keshia Rayna

"Et tu, Brute?" to a dog? Doc, you’re dramatic! I don't see a scar... there... where is it?

Dr. Stephen Flanagan

Luckily it is hidden underneath my bushy eyebrow. But look right here... just at the end of the brow....

Keshia Rayna

Oh, I see it! .... Gnarly!

Keshia Rayna

What are the other cranial bones?

Dr. Stephen Flanagan

Oh, right. We digressed.... the Occipital bone is a single bone, it forms the skull base and back. There are a lot of markings on this bad boy... the most obvious is the foramen magnum, literally, "the big hole" for the spinal cord passage. There are also the occipital condyles which articulate with atlas vertebra below. These are about the size a shape of little almonds below the skull. Then, I want you to image you are taking an exam. and the guy in front of you has a fresh hair cut. Can you picture that bulbous knob on the back of his head? reach around the back side of your own head.... Do you feel that little knob? That is the external occipital protuberance. It is for nuchal ligament attachment.

Keshia Rayna

Every time we talk about the occipital bone, I think of my son’s birth. He came out OP — occiput posterior, face-up instead of face-down. The back of his head was pressed right against my spine for most of labor. It made everything last longer and hurt more — the doctors called it “back labor” because the hardest part of the baby’s skull was grinding against my sacrum instead of flexing with the pelvis. They kept monitoring his heart rate closely because OP positions can put extra pressure on the umbilical cord and make delivery trickier. He ended up needing an assisted delivery with forceps. It was intense — not the smooth “sunny-side-up” story people joke about.

Dr. Stephen Flanagan

That sounds really intense, Keshia. I’m glad you both came through it okay. The occipital bone is strong, but labor is no joke — especially when the head’s position adds extra pressure.

Keshia Rayna

Yeah. It was a lot. But it also made me appreciate how perfectly the skull is shaped to protect the brain, even when it’s the reason things get complicated. Anyway… next bone is the sphenoid, right?

Dr. Stephen Flanagan

Yeah — the butterfly-shaped one in the middle base. Let’s move on…

Dr. Stephen Flanagan

Most people say "butterfly" but i think it looks like a bat. The Sphenoid bone is a single bone that runs behind your eyes. It makes up the middle cranial base. Sphenoid from Greek sphenoeides, which means wedge-like—fits between bones. It’s the “key bone”—holds everything together. It has a lot of notable Markings. The first of these marking has a fun name: the sella turcica, which is also known as the pituitary fossa.

Keshia Rayna

Sella turcica — That name’s actually a nod to military history, Doc. In the 16th century, European anatomists saw that bony depression and immediately thought of the high-arched, deep-seated saddles used by Ottoman Turkish cavalry. Those saddles were the Cadillac of saddles — high pommel in front, tall cantle in back, locked-in seat that let riders stay stable while firing bows or swinging sabers at full gallop. It was a total game-changer! The Turkish saddle gave horsemen stability and mobility over the older flat saddles that couldn’t match. This allowed the Turks to dominate battlefields from Vienna to Baghdad. So every time we say “sella turcica,” we’re quietly saluting the Ottoman Turks and their cavalry tech that helped them build one of the longest-lasting empires ever.

Dr. Stephen Flanagan

Yeah! I love the history that is intertwined through anatomy! If you look at a skull from above or in a sagittal section, you’ll see the hypophyseal fossa (the deepest part of the sella) sitting between two bony bumps: the tuberculum sellae in front (like the pommel) and the dorsum sellae in back (like the cantle of the saddle). The whole thing cradles the pituitary gland — the “master gland” — like a little bony seat for it. I guess you might think of the pituitary like the rump of a Sipahi in the 16th century.

Keshia Rayna

Nice! What other markings are there on that little bat-like sphenoid bone?

Dr. Stephen Flanagan

Well.. there are a lot of little holes. The optic canal which allows the optic nerve or CN 2 and the ophthalmic artery to pass from and to the eye. The superior orbital fissure where Cranial nerves 3, 4, 6 and a portion of the trigeminal nerve pass through as well as the ophthalmic vein. The foramen rotundum where a different portion of the trigeminal nerve passes through. The foramen ovale where the last bit of the trigeminal nerve and the mandibular nerve pass through the skull. And the foramen spinosum which allows passage of the middle meningeal artery into the skull. To help with that a bit.... rotundum means round, ovale means oval, spinosum means spiny and each hole has those features and it goes R, O, S from rostral, or the front of the skull, working to caudal, or the back of the skull.

Keshia Rayna

That sphenoid bone is a big favorite for the students and the instructors! Such a cool bone! And it definitely looks like a bat! What's next Flan?

Dr. Stephen Flanagan

I always like to go to the Ethmoid bone next. It is a single, sieve-like, bone between orbits. It is about the size and shape of a ping-pong ball. There are a few notable markings here like the crista galli. This comes from the Latin crista which means crest, and galli which means rooster, I suppose it does look like a rooster’s comb! The Crista Galli separates the frontal lobes of the brain, it is a little more complex than this but lets just leave it there. The cribriform plate or olfactory foramina, allows the axons projecting off the nasal epithelia to cross into the skull and form the olfactory bulbs and Cranial nerve 1. So that is the superior part, on the inferior side of the ethmoid bone you'll see these little wings called the middle nasal conchae. I should be more decriptive... the inferior side has 3 processes and the two lateral processes are called the left and right middle nasal conchae, they create air turbulence for inhaled air through the nose. I am going to do a lot more on this in another podcast, but air turbulence during inspiration is crazy important, you should breath through your nose when you can! Then the middle projection, between the middle nasal conchae, is the perpendicular plate of the ethmoid bone. It is the top of the line that you see looking into the nose hole... again "nose hole" is not the name anatomists use. The nose hole, or the main, large, pear-shaped opening for the nose on the front of a human skull is called the piriform aperture or anterior nasal aperture but I digress....again. Ethmoid itself is from Greek ethmos, meaning a sieve which it is! the bone is perforated for nerves. This references the cribiform plate that the olfactory nerves project through from the nasal epithelium... in other words this is how you can smell.

Keshia Rayna

That was quite the anatomical rant! Yo, Flan, did you lift today?

Dr. Stephen Flanagan

Yeah, why?

Keshia Rayna

Cause my olfactory nerves are firing action potentials through my cribiform plate right now telling me... you didn't shower!

Dr. Stephen Flanagan

burn!

Keshia Rayna

lets talk Parietal bones

Dr. Stephen Flanagan

The parietal bones are paired, so we have two, they form the skull sides/roof. not a lot of notable markings here so lets give the students a break.

Keshia Rayna

No rant?

Dr. Stephen Flanagan

Not on parietal bones. nice and easy.

Dr. Stephen Flanagan

Especially since the next bones on my list are the the Temporal bones. Now... here comes a long description! These bones are complex so bare with me. They are paired, and make up the lower sides of the skull. The temporal bones, house ears. The main depression in this bone is the external acoustic meatus or the ear canal. Then, the styloid process is also a very prominent structure. It is this little pokey projection for muscle attachment to the skull on the inferior side of the temporal bone. The mastoid process is also prominent but it looks more like a little stone and is the attachment point for the sternocleidomastoid muscle. You'll also see the jugular foramen when the temporal bone is articulated with the occipital bone which allows the passage of Cranial Nerves 9, 10, and 11, as well as allowing the internal jugular vein to carry blood out of the skull. The carotid canal is entirely within the petrous part of the temporal bone at the base of the skull and allows the passage of the internal carotid artery into the skull. It is a super round foramen that is long, like a canal rather than a hole. The word Temporal is from the Latin word tempus which means time. The reason for this is that his is the location on the body that first shows the passage of time as grey hair shows up on the temples.

Keshia Rayna

That was a lot! But i did see what you did there referring to the mastoid as looking like a stone. The petrous part of the temporal bone come from Latin petrosus, meaning "stone-like" or "rocky". But the Grey temples for time bit? Poetic. I didn't know that! Facial bones now?

Dr. Stephen Flanagan

OK. this is the time to transition from the 8 bones that make up the cranium to the 14 bones that shaping the face. Remember... 8 in the crate, 14 in the face. The Unpaired bones are the mandible which makes up the lower jaw, and the vomer which along with the perpendicular plate of the ethmoid bone makes up the nasal septum... the vertical line in the nose hole. The Paired bones are the maxillae which makes up the upper jaw and area below the eyes, the zygomatic bones which are the proper cheekbones, the nasal bones which are these curious little bones that makeup the nose bridge, the lacrimal bones that are just on the medial sides of the orbits that house the tear ducts, the palatine bones that make up the hard palate in the rear roof of the mouth, and the inferior nasal conchae, these are a separate bone that make up another, lower set of nasal conchae. I try to remind the students to use the mnemonic device for facial bones: My Mouth’s Palate Never Liked Zucchini in Vinegar. It works but there are other mnemonic devices, but I will try to just use the PC versions here.

Keshia Rayna

PC versions? too funny! I am sure there are markings you'd like to cover for each of those bones?

Dr. Stephen Flanagan

Always!

Dr. Stephen Flanagan

Let's do them in order starting with the Mandible. It is a U-shaped bone that makes up the lower jaw.

Keshia Rayna

The mandible is one of my favorite bones. Always thought I might go into dentistry. Do you know much about how it evolved.

Dr. Stephen Flanagan

Not really. I know a few details but I'd like to hear more.

Keshia Rayna

OK. Jaws themselves evolved around 420–390 million years ago in early vertebrates. Before that, jawless fish like lampreys just had a mouth opening without a true hinge. The jaw came from gill arches—those structures that supported breathing and feeding in ancient fish. The lower jaw part, basically the precursor to the mandible, started as cartilage from the first branchial arch, called Meckel's cartilage. In early fish and tetrapods, this cartilage supported a bunch of small bones that made up the lower jaw.

Dr. Stephen Flanagan

So reptiles still have multiple bones in their lower jaw?

Keshia Rayna

Exactly. In reptiles, the lower jaw is made of several bones like the dentary (the main tooth-bearing one), plus others like the articular, angular, and surangular. The jaw joint is between the articular bone on the lower jaw and the quadrate bone on the upper jaw. It's a simple hinge, good for wide gapes to swallow big prey, but not super precise for chewing.

Dr. Stephen Flanagan

And then mammals changed all that? How did we go from multiple bones to just one?

Keshia Rayna

That's the big dramatic shift! It happened in the synapsid lineage—the "mammal-like reptiles"—starting around 250–300 million years ago. Over time, the dentary bone got bigger and stronger, expanding to take over more of the jaw function. Muscles for chewing got more powerful, so the dentary needed better attachment sites—like the coronoid process at the top of each vertical ramus, for the temporalis muscle. Meanwhile, the other postdentary bones (articular, angular, etc.) started shrinking. A new joint formed between the dentary and the squamosal bone of the skull—this is our temporomandibular joint, or TMJ. The old articular-quadrate joint got freed up and gradually migrated into the middle ear, becoming the malleus and incus (two of our three ear ossicles). The angular became the tympanic bone that holds the eardrum. This happened gradually through transitional fossils like in cynodonts and early mammaliforms around 200–225 million years ago. It allowed for precise chewing and better hearing at the same time.

Dr. Stephen Flanagan

Wait, hold up—Kesh, you said two of the ear bones came from the old jaw joint, but what about the third one? The stapes, right? Where did that one come from?

Keshia Rayna

Good catch, Flan! Yeah, the stapes is the odd one out. While the malleus and incus were basically promoted from being part of the reptilian jaw hinge, the stapes has a completely different backstory. It didn’t come from the jaw at all—it was already an ear bone long before mammals showed up.

Dr. Stephen Flanagan

Already an ear bone? Like, in fish or something?

Keshia Rayna

Exactly. Go way back to early fish, around 400 million years ago or so. There was this bone called the hyomandibula—it was part of the second gill arch, the hyoid arch. In those ancient fish, the hyomandibula helped support the jaw and connected it to the skull. It was basically a strut that kept everything in place while the fish was sucking in water or grabbing food.

Dr. Stephen Flanagan

Whoa, that's a wild story! I knew parts of that but the whole story is amazing! Why is the mandible just one bone now? And what about the two halves fusing?

Keshia Rayna

Yep, classic exaptation—repurposing old structures for new jobs. In mammals, the entire lower jaw became just the dentary (what we call the mandible). Having a single, rigid bone per side gave better leverage for complex chewing and precise occlusion of teeth. As for the fusion: in most mammals, the two halves (left and right dentaries) stay separate at the midline symphysis, but in some groups it fuses into one solid bone. In humans specifically, the symphysis fuses postnatally, usually between 6 months and 2 years old, as we start chewing more. That fusion likely helps resist twisting forces during powerful bites.

Dr. Stephen Flanagan

Okay, another thing I have wondered about, since we are on it, humans have a chin but none of our ape relatives do. When did that show up?

Keshia Rayna

Great question—the chin is basically unique to Homo sapiens. Early hominins like Australopithecus (around 4–2 million years ago) had robust, projecting jaws with big teeth for tough plant foods. No real chin; the symphysis sloped backward. As we moved to Homo species—especially Homo erectus around 2 million years ago—jaws started shrinking. Tools, fire, cooking, and softer foods meant less need for massive chewing muscles and big teeth. By around 300,000 years ago in early Homo sapiens, the mandible got even more gracile: shorter, less forward-projecting, with smaller teeth in a parabolic arcade. The chin emerged as a bony buttress at the symphysis—probably to reinforce it against bending stresses or from changes in tongue and speech muscles pulling on the area. It's one of the last big facial changes that makes us look "modern."

Dr. Stephen Flanagan

So the chin isn't just for looks—it's tied to diet, speech, maybe both?

Keshia Rayna

Pretty much. It's likely a byproduct of overall facial reduction plus biomechanical needs, but it does help stabilize the jaw during talking and chewing. No other primate has it—our closest relatives like chimps have sloping jaws. Over the last 10,000 years with agriculture and processed food, jaws have shrunk even more in some populations, sometimes causing crowding issues.

Dr. Stephen Flanagan

Mind blown. Evolution turned a bunch of jaw bones into ear parts and gave us a chin. Thanks, Kesh—that actually makes sense now. You must have had an amazing Anatomy professor!

Keshia Rayna

Anytime! The mandible's story is one of the coolest examples of evolutionary tinkering. Unfortunately, I learned most of that doom scrolling!

Dr. Stephen Flanagan

Too funny! Lets see if we can tie that all together with the landmarks of the mandible. There is the mandibular foramen which allows the inferior alveolar nerve/vessels to travel to the teeth. This is actually more like a long tunnel that starts with the mandibular foramen and ends with the mental foramen which is a small opening on the front of the mandible. When you get a filling on the bottom jaw the dentist numbs the inferior alveolar nerve right at the mandibular foramen. That deadens the whole nerve which innervates the front of the face and makes your whole face feel numb.

Dr. Stephen Flanagan

Permission for one more tangent?

Keshia Rayna

permission granted!

Dr. Stephen Flanagan

I fractured a molar once lifting weights! I was clenching my jaw during a heavy deadlift and suddenly my tooth started hurting. The dentist confirmed and then began the root canal/crown procedure. When he numbed my inferior alveolar nerve right near the mental foramen, the shot felt like electrocution! I think he got the needle a little too close to the nerve. That mechanical pressure can cause a major influx of ions into the nerve and give you the zap sensation... but that is a topic for a different episode. anyway, now I have a fake tooth which is a little weird. Did you know that mandible is Latin for chewer?

Keshia Rayna

"Chewer" that is funny! You'd think I would have learned that on the Doom Scroll!

Dr. Stephen Flanagan

maybe, Doom scrolling is touch and go, good information but not always a complete story. Ok. Lets move on from the chewer. The Vomer is weird little bone. It would be hard to tell what it is if you saw it disarticulated. It is a single, plow-shaped bone that makes up the inferior nasal septum. I hate to say it again but... the vertical line in the nose hole.

Keshia Rayna

You and that anterior nasal aperture!

Dr. Stephen Flanagan

I know, I know, it isn't the nose hole! Ok, around the nasal aperture are the Maxillae. They are two, fused upper jaw bones that make up the front of the face. They form hard palate, and the orbital floors. There are a few landmarks to mention here like the alveolar process which are the tooth sockets, the infraorbital foramen which is a little hole below the eyes that the infraorbital nerve and blood vessels travel through. But, they are not super complex outside of that. Nice and easy!

Keshia Rayna

Very good. Ok. What's left? Zygomatic bones, Nasal bones, Palatine bones, and the Inferior nasal conchae. Any of those have specific landmarks to point out?

Dr. Stephen Flanagan

You know what.... none of those really have landmarks that we need to point out to intro anatomy students. In fact, if those are disarticulated from the skull they are very hard to identify. Let's see... Zygomatic bones have the temporal processes that form a portion of the zygomatic arch. Nasal bones... nothing really. Lacrimal bones have the lacrimal fossa that forms the tear duct. Palatine bones have the horizontal plate and the orbital processes. And the Inferior nasal conchae get their name because they are conch shell-shaped, kinda.

Keshia Rayna

Cool beans as long as everyone can ID the bones than they are basically good? Spend extra time on foramen?

Dr. Stephen Flanagan

I think that is probably right for most anatomy classes. And, yes, it is always a good idea to revisit the foramen of the cranial base! This is always hard for students and for the instructors it is easy to just run some pipe cleaners into the holes and ask for identifications on practicals. So it is something everyone should work extra hard on! The whole skull is perforated holes for nerves and vessels to pass into or out of the cranial case. The whole thing is a bit of a puzzle so in that way it can be fun. Let’s map ‘em by fossa. The Anterior cranial fossa has the cribriform foramina. The Middle cranial fossa has a lot: the optic canal, the superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum. The Posterior cranial fossa has a few: the foramen magnum is the most obvious, the jugular foramen between the temporal/occipital bones, hypoglossal canal one the occipital condyles, and the internal acoustic meatus. One good thing to take note of is that the Carotid canal for the internal carotid artery, shares a divot with foramen lacerum which lies between the sphenoid/temporal and occipital bones. Lacerum comes from the Latin lacer which means torn/ragged shape. In that tricky divot there is a smooth hole, the carotid canal and a jagged edge hole, foramen lacerum!

Keshia Rayna

Pipe cleaner quizzes are never comfortable! The tip about the Carotid and lacerum sharing a divot is a good point! Like anatomical roommates. What do you think doc, Wrap it up? We have been going for a while with all those tangents.

Dr. Stephen Flanagan

The skull’s a bony puzzle—vault dome, base steps, cavities hiding ears/nose/eyes, bones paired/unpaired, foramen like highways for nerves/vessels. It is always a long, but fun, discussion when talking about the dome! From Brutus’s bite to a cracked tooth—it’s your head’s headquarters!

Dr. Stephen Flanagan

Good bye everyone!

Dr. Stephen Flanagan

Catch ya next time on Flanatomy! Don't forget to like and subscribe!