I’m a freak! I have an extra pair of floating ribs! And I’ve only just realized it. Turns out my 10th ribs are “false” — that is, they don’t merge with the costal cartilage of the 8th and 9th ribs, like well-behaved 10th ribs. Their tips float free..
This anatomical jiggery–pokery is reputedly common in the Japanese, a fact mentioned without support in several sources, though I assume they are referring to this abstract-less 1974 paper, the only result of a PubMed search for “floating tenth rib”. (Mysteriously, PubMed answers that search with just one other item: “Insect succession on a decomposing piglet carcass placed in a man-made freshwater pond in Malaysia,” which chronicles the lives of insects on said floating carcass for, you guessed it, ten days. “The carcass along with the maggots sunk on day tenth, leaving an oily layer on the water surface.” Search tech is tricky.)
Most anatomy texts do not depict a floating 10th … but the classic Gray’s Anatomy does! It could just be an inaccuracy, or perhaps it suggests that the variation isn’t all that rare.
Most humans have a dozen ribs, with the occasional extra one at the top of bottom. Stubby little 13th ribs are almost common, about 1% of the population — so there’s enough 13th ribbers out there to populate several megacities. But floating 10th ribbers? There is exactly zero data on our prevalence. (Rule of thumb: if you can barely Google it, it’s rare.)
But the real marvel is that I didn’t know. That wouldn’t be so surprising for most people, but I’ve been professionally keen on anatomy, massage therapy, and self-massage for 15 years. I’ve worked on myself a lot. And it’s not like I haven’t noticed the tips of my floating ribs before: they’re pretty obvious. How could I possibly have missed this? How could literally dozens of massage therapists?
Near as I can figure, whenever I felt the tip of a floating rib, I just assumed it was the tip of one of two of them … every single time. The only way to confirm it was to carefully, slowly move from the 10th to the 11th to the 12th — and then repeat several times. And then do quite lot of carefully landmarking and rib counting to confirm that they really are the 10th, 11th, and 12th and not the 11th, 12th, and 13th. Even after that, I still wasn’t 100% sure that the anomaly was false 10th as opposed to a 13th, and could only confirm that by comparing with normal anatomy: the positions of my putative 11th and 12th are identical to what you’d expect, while the 10th tip seems to be the weirdo.
In short, it was surprisingly difficult to confirm.