Out of the roughly 206 bones in our body, the hyoid bone is the only one that is entirely separated from the rest of the skeleton. Vesalius in the 16th century recognized its isolation and depicted the hyoid resting on the plinth like a set of false teeth. Being U-shaped, it is appropriately named after the Greek letter upsilon. The hyoid bone resides immediately under the jawbone, just above the thyroid cartilage (Adam’s apple in men). Here 12 muscles attach that are critical for swallowing and vocalizing. In this well-protected position, it doesn’t get much notice and is rarely broken. Strangulation is the exception, so the coroner in your detective novel might want a look at it.
In humans, and uniquely so, the hyoid bone is moveable, which has been one of the evolutionary changes important for speech development. Its mobility, however, also contributes to obstructive sleep apnea. As the attached base-of-tongue muscles fall backwards in relaxation, they narrow the airway in the throat. Otolaryngologists counter this by drawing the hyoid approximately an inch forward and tethering it with strong sutures either to the inside of the jawbone or to the top edge of the thyroid cartilage. I would think that this might affect speech or swallowing, but the research papers say otherwise.
Don’t confuse the hyoid bone with the wishbone, which is present in some birds and their dinosaur relatives. The wishbone is farther down the neck, is a fusion of those animals’ collar bones, and contributes to efficient flying in some species. (See previous post: Twelve No-Fail Conversation Starters About Wishbones.) By contrast, all vertebrate animals have a hyoid bone, although it varies widely in shape and function. Here are two examples.
The hyoid bone helps a woodpecker pound its beak 22 times a second, 12,000 times a day, without apparently giving itself headaches. The beak is a bit flexible, which absorbs some of the shock, as does a thick pad of spongy bone that separates the beak from the skull. The hyoid bone handles the rest. It starts in the neck, like in the rest of us, but then curls around behind and over the skull to attach next to a nostril, thus bypassing the brain. It is thin and fragile looking.
Not so for the endangered howler monkey. Its hyoid bone is about the size and general shape of a 4-ounce, round-bottomed measuring cup. That is almost half the size of the monkey’s skull. The hyoid is instrumental in the howler’s ability to project its voice two miles.
Given the relative numbers of howlers and humans, it’s good that we are not so well-voiced.
Published in abbreviated form in NYT Book Review Letters, March 21, 2021
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