[This post received first place in the 2024 Blog Extravaganza at Adam Mastroianni’s Experimental History. Many thanks, Adam!]

I first had this thought in 2019, and I started this draft in early 2020, but…you know how that particular story turned out.

I’m picking it back up again now because I’ve had the titular point reinforced on several trips and projects over the past couple of years. And because I think it’s ultimately a hopeful message. If you are interested in making anatomical discoveries, good! Because relative to a single human life, the work to be done is effectively infinite.

But wait, you might say, how could that possibly be true? Have we not been plumbing the depths of the human body literally for thousands of years? Have we not imaged people down to micron resolution with every available scanning modality?

We’ve been at this a while, how are we not done yet? Left: Da Vinci. Right: Hua Shou, 1340s, Ming Dynasty.

And what about other extant critters? Chickens are one of the commonly-used model organisms in laboratory studies, and the basis for a multi-billion-dollar food industry. Surely we must know everything there is to know about their anatomy? (Spoiler alert: we do not.)

What about fossils? Are we not even now engaged in a massive, civilization-wide, distributed project to scan museum collections? Can we not publish entire dinosaur skeletons as 3D files in the supplementary information to our papers (Lacovara et al. 2014)? There will always be new fossils to discover, but can’t we at least say that the ones we’ve digitized are completely known?

Where is all this new anatomy hiding?

I’ll tell you.

(Warning: dissection images inbound. Nothing too gory, but still.)

I’m going to draw a lot from human anatomy, because it’s one of the areas where I have the most hands-on experience, and because humans are one of the best-studied organisms on the planet. So if there are macroscopic structures awaiting discovery in humans, imagine how much more true that will be of every other species that we haven’t been studying with extreme diligence and self-interest for millennia.

The Human Factors

Part of the reason why we are still making new discoveries in human anatomy is because we’ve made the process of finding, recognizing, and publishing new structures surprisingly difficult. None of these barriers were put in place deliberately (we could quibble about barriers to publication), but they’re slowing the advance of anatomical knowledge nonetheless.

1. Not everyone gets to look, and everyone who does is on the clock

I’d originally put this point farther down, but for human anatomy it is the subtext and background radiation for everything else I have to say, so I’m giving it pride of place.

When we described the long cutaneous branch of the obturator nerve a few years ago (Staples et al. 2019, this post), I wondered why it hadn’t been discovered sooner. I hypothesized that it fell into a perceptual blind spot: the people with the best chance to discover it were medical students and surgeons, and each group faced a significant barrier. Surgeons had the expertise to recognize and preserve this tiny, delicate nerve, but they didn’t have the time or operative freedom to flay their patients open from ankle to groin to trace its path. Med students had the opportunity to chase the variant nerve all the way down the lower extremity, but only if they managed to preserve it while skinning the limbs, and if they recognized it as anomalous – neither of which was likely on Day 1 when they did the skinning.

Preserving that very long, very skinny nerve in dissection is not easy. Modified from Staples et al. (2019: figure 5).

Later I realized that these same factors apply to all kinds of anatomical discoveries. No shadowy Illuminati group deliberately made this decision, but as a civilization we have collectively ‘decided’ that three groups of people would get to peer inside the human body, and they’d all be hobbled. Surgeons are under immense pressure to make smaller incisions, do less invasive surgeries, and keep their patients on the table for as little time as possible, because small holes and short surgeries generally correlate with better outcomes. I’m not saying this is wrong – it is undoubtedly the right decision in the vast majority of cases – but it does mean that our most experienced anatomists have very little opportunity to investigate possibly new anatomical features, unless they happen to impede a surgery.

The second group that gets the privilege of hands-on exploration of the human body is medical students, and they’re also on the clock. Med school is legitimately challenging – we use the metaphor “drinking from the firehose” a lot – and med students usually have a long list of structures to find in a 3-4 hour dissection. I don’t think anyone could reasonably blame med students for not being “discovery oriented”; the fact is that when you’re going to spend between 100 and 200 hours dissecting an entire human body, at some point it becomes a job, and with all the other subjects med students are expected to master (biochemistry, cell biology, physiology, microbiology, pharmacology, etc.), it’s not their only job, and not always the top priority in a given day or week.

That leaves the third group: anatomy teachers, like me. With dozens or hundreds of med students to do the dissecting for us, shouldn’t we be in a perfect position to recognize interesting things in the anatomy lab? To some extent, yes, but the clue is in the question. I’m in the anatomy lab to teach, and teaching a big room full of very smart, very motivated folks who have Wikipedia and Radiopaedia and their textbooks and the campus library on their phones and tablets is a bit of a high-wire act, requiring dedication and focus – on teaching, not on discovery. So I keep my antennae out, probably more than most, but I’m still relying on the med students to make the discoveries, and I suspect that is true of most anatomy teachers.

2. Anatomical knowledge is oddly siloed

If you crack open the 40th edition of Gray’s Anatomy, published in 2008, and turn to page 1419, the very first sentence about the fibularis tertius muscle reads, “Fibularis tertius (peroneus tertius) is a muscle unique to humans.”

That bold assertion would probably come as a surprise to Dudley Morton, who published a paper titled “The peroneus tertius muscle in gorillas” in The Anatomical Record…in 1924. And to William Straus, Jr, who described and illustrated the peroneus tertius muscle in chimps and gorillas in a 1930 paper in The Quarterly Review of Biology.

Morton (1924), first page and figure 1.

How did this happen? The Anatomical Record and The Quarterly Review of Biology are not obscure sources, they’re highly-regarded journals with global readership. There’s a long story here, involving the prominent (not to say tyrannical) Victorian anatomist Richard Owen, the Gilded Age quest to tally anatomical features separating humans from apes, and some extremely dubious evolutionary hypotheses, but the short version is that comparative anatomy, physical anthropology, and clinical anatomy are three distinct fields. Each field has its own preferred publication venues, citation classics, and bodies of “common knowledge”. Ideas that were sunk long ago in one field may still be viable in another, because the debunking happened in a paper that few people outside of its home field have ever read or cited. And not just hypotheses, but even basic facts, like whether the peroneus tertius muscle is actually unique to humans (for avoidance of doubt, it most certainly is not).

This weird balkanization of science doesn’t make it harder to spot anomalous and potentially new anatomical structures in the dissection lab, but it can impair people’s efforts to understand the evolutionary history and clinical importance of a given body part, especially if they happen to fall into one literature silo and never learn that the other, parallel ones exist.

3. There are many barriers to publication

Crucially, both surgeons and med students live notoriously busy lives. Even if they notice and preserve something interesting, plowing through the literature, getting publication-quality photos, and actually writing and formatting a paper all take time. Hardly anyone has the time to do all the work by themselves, but collaboration means coordinating the efforts of multiple busy people. Then there’s the task of finding an appropriate journal – loads of otherwise promising OA outlets don’t take anatomical case studies, for example. And finally there is the gauntlet of peer review, about which we’ve already spilled many words.

A slide from my 2019 SVPCA talk, “How to make new discoveries in (human) anatomy.”

Now, in point of fact, surgeons, med students, and anatomy professors do find and publish new anatomical discoveries. But there are enough hurdles just on our side to explain why we’re not done yet, and may never be.

Nature Doesn’t Make It Easy

Beyond the speed bumps we humans have accidentally erected there lurks the unending, phenomenal complexity of nature, which throws up its own barriers to discovery.

1. Humans and other animals are hideously complex

Dissection-based human anatomy courses run between 100 and 200 hours not because that’s an administratively or pedagogically convenient number – I and everyone else in medical education, and especially the bean counters, can assure you it is not – but because that’s simply how long it takes to find all the bits. Minimally – we expect that students will take advantage of open lab hours on evenings and weekends to tidy up their dissections. And that’s relatively hasty, on-the-clock dissecting for teaching purposes. The professionally prepared plastinated cadavers for exhibits like Body Worlds can require 500 to 1000 hours of dissection.

That might sound ridiculous. After all, professional butchers, and hunters and farmers who dress their own kills and livestock, all get very good at taking apart large mammals much faster than that. But there’s a world of difference between taking apart a carcass as efficiently as possible – for which I give all those folks full props – and trying to dissect and put a name to all the parts.

Esophageal plexus and other neck viscera in left lateral view. For more about that variant nerve, see this post. Altounian et al. (2015: figure 4).

I was confronted with the frankly appalling complexity of the human body about a decade ago, when as part of a student project (Altounian et al. 2015) I did a deep dissection of the esophageal plexus. I went in after hours to do the extra dissecting work, just like we encourage the med students to do, and it took me something like four hours. It was rewarding work, but it’s probably telling that in ten years I’ve never done it again.

Incidentally, I don’t think this gets much easier as animals get smaller. A chicken or a cat has about the same number of body parts as a human, they’re just smaller and harder to see. Frogs seem to be a little simpler than shrews or hummingbirds, but it may also be that we know them less well, and dissect them less patiently and completely. At some point gross anatomy has to give way to histology as body parts become microscopic, but that doesn’t mean that the animals in question aren’t still pretty darned complex.

In sum, humans and other animals have lots and lots of parts. But it gets worse.

2. Anatomical variation is extremely common

It took me a long time to realize that there’s a needle-and-haystack problem with recognizing genuinely new anatomical structures from the common variations that turn up all the time. This is one of those things that might seem hard to believe unless you’ve experienced it, but we humans are crazy variable under the hood. In my program we encourage the students to log interesting variations on the whiteboard in the lab, not least so that everyone can beware of the variant anatomy while studying for their practical exams. If the students are really diligent about the logging, something like a third of the donor bodies end up written on the board. And those are the variations the students found and worried might distract their studying, not all the variations that exist. Oh, and we reset the log between each of our five curriculum blocks through the year. So essentially every cadaver has a chance to end up on the ‘variation board’ at least once, and some might be up there three or four times.

Here’s why this is relevant: numerous times I’ve seen some variation in lab, in a body system or region in which I was not familiar with the primary literature, and I’ve thought “cool variation” and moved on. Then later I’ll get curious and look it up, or I’ll be researching something completely different and stumble over a mention of that same variation. A couple of times that variation has turned out to be so phenomenally rare that if I’d only gotten good photos at the time, I could have had a nice little paper.

So to a first approximation, almost every human being has at least one anatomical variation notable enough that a med student would write it on a whiteboard. And this is actually a problem, because those of us who work in anatomy labs see so many of those common variations that sometimes it keeps us from recognizing the truly novel and important stuff.

3. Some body parts have distractors

What we now call the anterolateral ligament (ALL) of the knee was first discovered by a French surgeon 145 years ago (Segond 1879), and independently rediscovered sporadically throughout the twentieth century, but it wasn’t widely recognized as a body part normally present in most people until a pair of papers in 2012 and 2013 brought it to global prominence (Vincent et al. 2012, Claes et al. 2013).

A diagram from my 2019 SVPCA talk, showing the ALL (red) sandwiched between the patellar retinaculum and the iliotibial tract.

Given the vast amount of time, money, and effort that humankind has put into understanding, rehabbing, rebuilding, and replacing our knee joints, the absurdly long period during which the ALL escaped wide detection is flat-out amazing to me. But it also makes sense in a weird way. The ALL angles downward and forward from the lateral aspect of the distal femur to the anterior portion of the proximal tibia (hence anterolateral ligament), and in that position it is sandwiched between the patellar retinaculum, which lies deep to the ALL, and the iliotibial tract, which lies on top of it. Crucially, both the patellar retinaculum and the iliotibial tract are made of dense connective tissue, like the ALL, and they run in the same direction as the ALL.

I’ll bet that in the decades and centuries before the 2010s, hundreds if not thousands of surgeons and medical students saw the ALL and mistook it for part of either the patellar retinaculum or the iliotibial tract – structures that they were expecting to see in that region, also made of connective tissue, also running in the same direction.

If you only get to look inside the box, these two things appear identical. Modified from Staples et al. (2019: figure 6).

A similar thing probably happens with the aforementioned long cutaneous branch of the obturator nerve. In the two known cases, it was running with the great saphenous vein, in a position usually occupied by a branch of the saphenous nerve. I reckon that surgeons see the long cutaneous branch of the obturator nerve on a regular basis, but they have no way of knowing that it’s a weird variant because it sits where they were already expecting to see a nerve.

It’s hard to say how important this factor is, but I note that almost all the anatomical variants I’ve helped students present at conferences or publish are things that they found in complicated areas – nerve plexuses, bundles of tendons crossing a joint, and so on – where they could easily have escaped detection if people hadn’t really been on the ball. And of course I can only count the hits; I can’t tally all the variants that we missed because we mistook them for their distractors. Thoughts like that haunt me.

4. Some things are just hard to see

The plain fact is that some parts of the body are easier to investigate than others. I’ve written a lot about how the pneumatic diverticula of birds are under-documented, even in chickens, the most numerous and best-studied birds on the planet (whinge, whinge). But diverticula can be surprisingly tricky – when birds die, many of the diverticula empty out and collapse. The diverticula can look just like loose connective tissue, unless they’ve been injected with latex or resin, or re-inflated and CT scanned, and both the injection and the scanning take a lot more time and effort than a simple dissection. One handy thing about the paramedullary (or supramedullary) diverticula is that they’re unable to collapse; the bony walls of the neural canal keep them propped open whether they’re inflated or not.

An ostrich neck in cross-section, showing many of the pneumatic diverticula of the respiratory system. The neural canal is the bony tube around the spinal cord. From this post.

Speaking of, the neural canals of archosaurs host a whole zoo of anatomical novelties – big veins, pneumatic diverticula, odd joints, ligament scars, and, oh yeah, an entire novel balance organ. Although the big veins (in crocs and some birds) and pneumatic diverticula (in many birds) have been known to exist since the 1800s, they’ve really only started to be adequately documented in this century. The same goes for everything else on the list; to pick a timely example, the ligament scars were only described for the first time in archosaurs a couple of weeks ago. Why the delay? I think that neural canals, being relatively small-diameter bony tubes, are just that much harder to study than most other parts of the body, whether we’re talking about big-ass crocs or tiny hummingbirds. Heck, one of the most recently-discovered macro structures in the human body is the midline interlaminar ligament, only recognized for the first time in 2019 (Simonds et al.), which lies – you guessed it – along the roof of the neural canal.

So one way to make new discoveries is to simply look in inconvenient places. Sacral pneumaticity in dinosaurs is poorly understood because the sacral vertebrae are often inaccessible, but there are ways around that: studying the unfused sacral vertebrae of juvenile and subadult animals, looking at broken specimens, and staying alert for interesting opportunities. But now I’m getting ahead of myself – problem solving deserves a whole section.

What to do about it

Of the factors slowing down the pace of anatomical discovery that I numbered above, all but the first can be overcome with time, curiosity, patience, and determination. One of the biggest boosts is simply being aware that new discoveries are still being made, and staying on the lookout for them.

As for the first – the fact that not many folks get to dissect human bodies, and everyone who does is busy – I could fix that if I was sufficiently rich. If I was a multi-billionaire, I’d hire 1000 of the world’s best surgeons (in staggered waves, so I didn’t doom thousands of patients by pulling too many experts off the line at once), supply them with 10,000 ethically donated willed bodies representing as many geographic regions and genetic backgrounds of humankind as possible, and give each surgeon a couple of years to dissect their 10 bodies, ideally in labs with 50-100 bodies at a time so the small groups of surgeons could look at each other’s work without getting overwhelmed, or work in teams if they preferred. I’d also supply them with professional photographers to document everything they found, and a small army of research assistants to help them with library work and writing up. That wouldn’t be enough to declare the science of human anatomy a completed project, but we’d know a heck of a lot more than we do now.

I’m not a multi-billionaire, and no-one on the planet is ever going to fund the vast study I just described. I think we’ll still get to an equivalent level of knowledge, but it will take the next 500 to 1000 years, as those discoveries are made piecemeal, mostly by alert medical students who happen to do better than average dissections in their gross anatomy courses.

Turning to comparative anatomy, I’ll conclude this section with one of my favorite published sources. Baumel (1988) is a 123-page book on the anatomy of the tail of the pigeon. If a chunk of pigeon the size of the last digit of one of your fingers can bear over 100 pages of detailed examination – and it can, I have the book and I refer to it in my research – then we are not going to run out of new anatomy anytime soon (not least because there are the other 10,000+ species of birds that have not had their tails described in that level of detail).

But is it worth it?

Sure, people might say, some goobers might write boring-ass treatises about pigeon tails or chameleon tongues or frog pelvises, but isn’t that all just so much pointless stamp collecting? Does any of it really matter? Shouldn’t we funnel our limited support for science toward things that are going to make a practical difference?

I’d counter that science is a young enterprise and we are still exceptionally bad at determining in advance what kinds of things are going to be important in the future. Baumel’s book on pigeon tails has been cited just in this decade in fields as diverse as biomechanics, embryology, evolution – and, hey, by researchers investigating rudderless flight control for UAVs, a technology application that didn’t exist when the book was first published. The skin of sharks inspired wetsuits so efficient they’ve been banned at the Olympics, and the first-in-class COVID-19 medication remdesivir is one of hundreds of pharmaceuticals derived from the biochemistry of sea sponges. I think documenting the universe is a noble goal in itself, but we should probably keep researching All the Things because that’s where the new technology is going to come from. And the people – nations, states, businesses, inventors – smart enough to invest in basic science are going to get those discoveries before anyone else does.

And anyway, compared to most other fields of inquiry, anatomy research is dirt cheap. Embalmed human cadavers cost money, but I could still get the 10,000 cadavers I’d need for my dream project for less than the cost of a Marvel movie. Of course that project is never going to happen, but fortunately we can continue piggybacking human anatomical research on the vast anatomy education effort necessary to train physicians. For comparative anatomy and paleontology, we basically need to keep giving geeks a little research time and a ten-thousandth of a percent of the cost of the Large Hadron Collider so they can keep themselves busy when they’re not teaching or running museums, and they’ll keep doing the work. (That’s not to say that more support wouldn’t be appreciated, or speed things up a little.)

So if you like anatomy, come join the hunt. You probably won’t get rich, but you’ll stay busy doing interesting work, which is a different form of wealth. And if you stay alert, you will not run out of new things to find.

References

• Altounian, D., Tran, C.M., Tran, C., Spencer, A., Shendrik, A., Kraatz, B.P., and Wedel, M.J. 2015. A variant nerve that mimics the left recurrent laryngeal nerve: a case study in human anatomy. PeerJ PrePrints 3:e781v1. DOI: 10.7287/peerj.preprints.781v1
• Baumel, J.J. 1988. Functional morphology of the tail apparatus of the pigeon (Columba livia). Springer-Verlag, Heidelberg, Germany, 123pp.
• Claes, S., Vereecke, E., Maes, M., Victor, J., Verdonk, P. and Bellemans, J. 2013. Anatomy of the anterolateral ligament of the knee. Journal of Anatomy 223(4): 321-328.
• Lacovara, Kenneth J.; Ibiricu, L.M.; Lamanna, M.C.; Poole, J.C.; Schroeter, E.R.; Ullmann, P.V.; Voegele, K.K.; Boles, Z.M.; Egerton, V.M.; Harris, J.D.; Martínez, R.D.; Novas, F.E. (September 4, 2014). A Gigantic, Exceptionally Complete Titanosaurian Sauropod Dinosaur from Southern Patagonia, Argentina. Scientific Reports. doi:10.1038/srep06196.
• Morton, D.J. 1924. The peroneus tertius muscle in gorillas. Anatomical Record 27: 323-329. 
• Segond, P. 1879. Recherches cliniques et expérimentales sur les épanchements sanguins du genou par entorse. Progrès Médical, Paris.
• Simonds, E., Iwanaga, J., Ishak, B., Reina, M.A., Oskouian, R.J. and Tubbs, R.S. 2019. Discovery of a new ligament of the lumbar spine: the midline interlaminar ligament. The Spine Journal 20(7): 1134-1137.
• Standring, S. (editor). 2008. Gray’s Anatomy, 40th edition. 2008. Churchill Livingstone Elsevier, London.
• Staples, B., Ennedy, E., Kim, T., Nguyen, S., Shore, A., Vu, T., Labovitz, J., and Wedel, M. 2019. Cutaneous branch of the obturator nerve extending to the medial ankle and foot: a report of two cadaveric cases. Journal of Foot & Ankle Surgery 58: 1267-1272.
• Straus Jr, W.L. 1930. The foot musculature of the highland gorilla (Gorilla beringei). The Quarterly Review of Biology 5(3):261-317.
• Vincent, J. P., Magnussen, R. A., Gezmez, F., et al. 2012. The anterolateral ligament of the human knee: An anatomic and histologic study. Knee Surg Sports Traumatol Arthrosc. 20 (1): 147–52. doi:10.1007/s00167-011-1580-3

doi:10.59350/63r4z-32f49