Here’s my face.

I went to the dentists’ office recently for a regular checkup and cleaning, and when my dentist learned that I taught human anatomy, he volunteered to send me a high-res copy of my panoramic x-ray. I couldn’t think of any plausible scenario wherein someone could use it for evil, and it has lots of cool stuff in it besides teeth, so decided to post it so I could yakk about it.

First things first: my teeth are in pretty good shape. I had to have my wisdom teeth (3rd molars) pulled back in 2009, and my upper 1st molar on the left has a root canal and a porcelain crown, which stands out bright white on the radiograph. Everyone else is present and looking good. If it’s been a while since you’ve covered this, the full human dentition consists of 2 incisors, 1 canine, 2 premolars, and 3 molars on each side, top and bottom, for a total of 32 teeth. Because I’ve had all four 3rd molars removed, I’m down to 28.

I could go on and on about the cool stuff in this image. Here are 12 things that stand out:

  1. The mandibular condyle, which is the articular end of the mandible that fits into the mandibular fossa, a shallow socket on the inferior surface of the temporal bone, to form the temporomandibular joint (TMJ). There’s an articular disk made of fibrocartilage inside the joint, which separates it into two fluid-filled spaces, one against the condyle and one against the fossa. This allows us to do all kinds of wacky stuff with our lower jaws besides simply opening and closing them, such as slide the jaw fore and aft or side to side. This is a strong contrast to most carnivores, which bite down hard and therefore need a jaw joint that works as a pure hinge. See this post for pictures and discussion of the jaw joint in a bear skull.
  2. The coronoid process of the mandible, which is a muscle attachment site. A few fibers of the masseter and buccinator muscles can encroach onto the coronoid process, but mostly it is buried in the temporalis, one of the primary jaw-closing muscles. Put your fingers on the side of your head a little above and in front of your ear and bite down. That muscle you feel bulging outward is the temporalis. Back in the 1960s, Melvin Moss (1968) discovered that if he removed the temporalis muscles from newborn rats, the coronoid processes would fail to develop. Moss’s ambition was to discover the quanta of anatomy, which in his view were “functional matrices” – finite sets of soft tissues related by development and function, which might contain “skeletal units” that grew because of the morphogenetic demands of the functional matrices. His tagline was, “Functional matrices evolve, skeletal units respond”. Not all of Moss’s ideas have aged well in light of what we now know about the genetic underpinnings of skeletal development, but he wasn’t completely wrong, either, and functional matrix theory is still an interesting and frequently productive way to think about the interrelationships of bones and soft tissues. For more horrifying/enlightening Moss experiments on baby rats, see this post.
  3. The mandibular angle, which is another muscle attachment. The medial pterygoid muscle attaches to the medial surface, and the masseter attaches laterally. You can feel this, too, by putting your fingers over your mandibular angle and biting down – that’s the masseter you feel bulging outward. Note that the angle flares downward and outward on either side of my jaw. This flaring of the angle tends to be more pronounced in males than in females, and it is one of many features that forensic anthropologists (like the one I belong to) take into account when attempting to determine biological sex from human skeletal remains. Like most sexually dimorphic features of the skeleton, this is a tendency along a spectrum of variation rather than a binary yes/no thing. There are women with flared jaw angles (Courtney Thorne-Smith, probably) and men with slender mandibles, so you wouldn’t want to sex a skeleton by that feature alone.
  4. The mandibular canal, a tubular channel through the mandible that houses the inferior alveolar artery, vein, and nerve. This neurovascular bundle provides innervation and blood supply to the tooth-bearing part of the mandible and to the teeth themselves, and emerges through the mental foramen to provide sensory innervation and blood supply to the chin.
  5. The upper surface of the hard palate, formed by the palatine process of the maxilla anteriorly and by the palatine bones posteriorly. The palate is the roof of the mouth and the floor of the nasal airways.
  6. The median septum of the nasal cavity, formed by cartilage anteriorly, the perpendicular plate of the ethmoid bone superiorly, and the vomer posteriorly and inferiorly.
  7. The blue lines are the inferior margins of my maxillary sinuses – air-filled spaces created when pneumatic diverticula of the nasal cavity hollow out the maxillae. You have these, too, as well as air spaces in your frontal, ethmoid, sphenoid, and temporal bones. It looks like many of the roots of my upper molars stick up into my maxillary sinuses. This is not an illusion, as shown below.
  8. When I had the root canal on my left upper 2nd molar, the endodontist filled the pulp cavities of the tooth roots with gutta-percha, a rigid natural latex made from the sap of the tree Palaquium gutta. Gutta-percha is bioinert, so it makes a good filling material (it was also used to insulate transoceanic telegraph cables), and it’s radiopaque, which allows endodontists to confirm that the cavities have been filled completely. The other teeth show the typical structure of a dense enamel crown, less dense dentine forming the bulk of the tooth, and radiolucent pulp cavities containing blood vessels and nerves.
  9. This is the rubber bit I gripped with my incisors to keep my teeth apart and my head motionless while the CT machine rotated around me to make the scan. Not that cool in a science sense, but I figured it deserved a label.
  10. Note that the roots of the canines go farther into the jaws than those of the other teeth. This is true for all four canines, it’s just easiest to see with this one. This is a pretty standard mammalian thing, for taxa that still have canines – they tend to be big and mechanically important, so they have deep roots. Even though our canines are absolutely and proportionally much smaller than those in the other great apes, we can still see traces of their earlier importance, like these deep roots.
  11. In places you can see the trabecular internal structure of my mandible clearly. As someone who geeks out pretty much anytime I get a look inside a bone, this tickled me.
  12. The remains of an alveolus or tooth socket. I had my 3rd molars out almost a decade ago, and by now the sockets will have mostly filled in with new trabecular bone. But you can still see the ghostly outline of at least this one – a sort of morphogenetic trace fossil buried inside my mandible. I assume that in another decade or two this will have disappeared through regular bone remodeling.

Here’s a closeup of my left upper 2nd premolar and first two (and only remaining) molars. The gutta-percha filling the pulp cavities of the three roots of the 1st molar is obvious. The disparity in root length is mostly illusory – this was an oblique shot and the two ‘short’ roots are foreshortened.

Here’s the same image with the roots of the 2nd molar traced in pink, and the inferior margin of the maxillary sinus traced in blue. It’s not that uncommon for upper molar roots to stick up into the maxillary sinuses. That was true of my 3rd molars as well, and when I had them taken out, the endodontist had to put stitches into my gums to close the holes. Otherwise I would have had open connections between my oral cavity and maxillary sinuses, which would have sucked and been dangerous. Nasal mucus in the maxillary sinuses could have drained into my mouth, and food I was chewing could have been forced up into the sinuses, where it would have decomposed and caused a truly vile sinus infection.

In a developmental sense, it’s not that the roots of the teeth grow upward into the sinuses, it’s that the sinuses grow downward, eroding the bone around the roots of the teeth. This happens well after the teeth are done forming – the sinuses continue to expand as long as the skull is growing, and they retain the potential to remodel the surrounding bone for as long as we live. Even in cases like mine where the roots of the molars stick up into the sinuses, the tooth roots are still covered by soft tissue, including branches of the superior alveolar artery, vein, and nerve that enter the pulp cavities of the tooth roots through foramina at their tips.

If you ask your dentist for copies of your own dental x-rays, you’ll probably get them. If you do, have fun exploring the weird territory inside your head.

Reference

  • Moss, M. L. (1968). A theoretical analysis of the functional matrix. Acta Biotheoretica, 18(1), 195-202.

 

Internal Iliac Arteries - MJW 2011

Here’s a thing I put together to help my students understand the many branches of the internal iliac artery in humans. In the image above, we’re looking in superomedial view into the right half of the sacrum and pelvis. Bones are white, ligaments blue, the piriformis muscle sort of meat-colored, and arteries red (for a tour of the pelvis identifying all of this stuff, see my pelvic foramina slideshow). At the top is a big inverted Y-shape: the common iliac arteries branching from the abdominal aorta, which continues on, much reduced, as the median sacral artery. The right common iliac artery is shown bifurcating into the external iliac artery, which continues on out of the pelvis to become the femoral artery, and the internal iliac artery, source of much fear and doubt.

The first thing to understand is that any particular branching pattern of the internal iliac arteries, whether in an anatomical altas, a lecture, revealed in a dream, or even in your own body, will probably have no bearing whatsoever on the branching pattern in the next person you encounter, alive or dead. Furthermore, the variation between right and left in a single person can be as great as that among different people. The branches to pelvic viscera are particularly fiendish; they sometimes travel far into the pelvis as a common trunk and then “starburst” near their target organs, making identification almost impossible. Do not waste your time trying to memorize any particular branching sequence. Instead, concentrate on matching the arteries to their targets; you will discover the identities of the branches by seeing where they are going, not the order in which they branch.

There are typically 10 named branches of the internal iliac artery. Authorities quibble on the details, as we’ll see in a moment, but if you know these 10, you’ll be fine for almost any conceivable purpose. A simple scheme of my own devising for remembering them is 2-4-4:

TWO to the back body wall:

  1. iliolumbar A—may arise from external or common iliac AA; sometimes double
  2. lateral sacral A—note branches to anterior sacral foramina and anastomoses with median sacral A

FOUR leaving the pelvis entirely:

  1. obturator A—often arises from the external iliac A instead, exits pelvis through obturator canal
  2. superior gluteal A—exits pelvis through suprapiriform foramen
  3. inferior gluteal A—exits pelvis through infrapiriform foramen, with internal pudendal A
  4. internal pudendal A—exits pelvis through infrapiriform foramen, with inferior gluteal A

FOUR to pelvic viscera:

  1. superior vesical A—usually the dominant artery of the anterior trunk, this is the patent part of the obliterated umbilical artery, which survives as the medial umbilical ligament
  2. inferior vesical A (males) / vaginal A (females)—may branch off uterine A (females) or superior vesical A (both)
  3. uterine A (females)—major artery to uterus, approaches laterally within the broad ligament
    A to ductus deferens (males)—extremely small and difficult to trace
  4. middle rectal A—usually the most inferior branch of the entire internal iliac tree (at least inside the pelvis)

My way to explain those last four is to extend my index finger and say, “Everybody has to pee, so up front we have superior vesical.” Then extend my pinky and say, “And everyone has to poop, so in back we have middle rectal.” Then extend digits three and four and explain that the identity of the middle two arteries varies between the sexes (but that the inferior vesical artery of males and the vaginal artery of females are basically the same vessel).

There is a LOT of variation in the descriptions of the internal iliac artery branches among different sources — almost as much variation as there is in the arteries themselves.

  • ​The Thieme Atlas of Anatomy, 2nd Ed (Gilroy et al. 2009), Table 19.1 on p. 254, includes the inferior vesical artery for both sexes. The artery to ductus deferens is listed as a branch of the superior vesical artery, and the uterine and vaginal arteries are listed separately, bringing the total for females to 11.
  • Clinically Oriented Anatomy, 7th Ed (Moore et al. 2013), Table 3.4 and pp. 350-355, lists the 10 branches I went through above. Moore et al. explicitly say that the vaginal artery is the female homolog of the inferior vesical artery (p. 351).
  • Gray’s Anatomy, 40th Ed (Standring et al. 2008), pp. 1085-1089, splits the difference. The artery to ductus deferens is not listed; instead, the ductus deferens is said to be supplied by the inferior vesical A (in contrast to Thieme, which has it is supplied by the superior vesical A). Both the vaginal and inferior vesical arteries are listed, but the vaginal artery is said to frequently replace the inferior vesical artery.

The upshot is that pretty much all of these sources agree on how the blood is getting distributed, there are just some minor differences over what we call certain vessels. I have never personally seen a dissection detailed enough to allow an interior vesical artery to be recognized separately from the vaginal artery — the vagina lies so close behind the bladder that whatever you call the artery that runs lateral to them, it could easily be supplying both structures, and probably does. As far as I’m concerned, the inferior vesical artery in males and the vaginal artery in females are the same artery, in that they both supply the inferior portion of the bladder. I think it’s just a historical hiccup that we call them by different names, possibly perpetrated by smelly, lonely, vagina-obsessed men of centuries past.

A final note, added in revision: some sources refer to two trunks or divisions of the internal iliac artery: a posterior trunk that gives rise to the iliolumbar, lateral sacral, and superior gluteal arteries, and an anterior trunk that gives rise to everything else. If that’s what your professor tells you, smile and nod and keep your heretical thoughts to yourself. Personally, I regard the notion of trunks of the internal iliac artery alongside phlogiston, luminiferous aether, and snorkeling sauropods, as romantic nonsense at best. I have seen an obturator artery arise from a superior gluteal artery and a pudendal artery arise from a superior vesical artery. In a world where variants like those can and do turn up frequently, the stability and reason implied by regular trunks is illusory.

References

  • Gilroy, A., MacPherson, B., and Ross, L. (eds.) 2009. Atlas of Anatomy, 2nd ed. Thieme, Stuttgart.
  • Moore, K.L., Dalley, A.F., and Agur, A.M. 2013. Clincially Oriented Anatomy, 7th ed. Lippincott Williams & Wilkins, Philadelphia.
  • Standring, S. 2008. Gray’s Anatomy, 40 ed. Churchill Livingstone, London.