A vertebra from the neck of a turkey, sanded down to reveal the honeycomb of air spaces inside.
Credit: Mathew Wedel
In 1758, British anatomist John Hunter was doing bad things to birds. He cut off the wing of a live chicken at the middle of the upper wing bone, and immediately tied a string tightly around its windpipe to strangle it.
A young hawk got similar treatment, only instead of losing a wing, it had one of its legs lopped off at mid-thigh.
Hunter wasn't pointlessly torturing the birds. Although gruesome by modern standards, Hunter's experiments were designed to solve a mystery more than 2000 years old.
Since at least the time of the ancient Greeks, observant naturalists had noted that many of the bones in the skeletons of birds were filled with air rather than bone marrow. This raised a perplexing question: how did the air get into the bones?
Some people had hypothesised that the air spaces in the bones communicated with the lungs, but no one had been able to show a connection.
Hunter had found another way to test the hypothesis: both of his mutilated birds survived, at least for a bit, and were able to breathe despite having their windpipes tied off.
In a 1774 account of his experiments, he wrote that "the air passed to and from the lungs by the canal in [the] bone" - the bones being the upper wing bone of the chicken and the thighbone of the hawk.
It wasn't easy on the birds. Breathing through holes in their bones was so difficult "as to render it impossible for the animal to live longer, than to prove evidently, that he did breathe through the cut bone." (One imagines that the trauma of having a limb severed without anesthesia and then being strangled might also have added to the birds' distress.)
Although the import of the discovery was no doubt lost on his test subjects, Hunter was the first person in history to successfully show that the air-filled bones of birds are indeed connected to the lungs.
Air-filled bones may seem exotic, but you have been carrying around a set for most of your life: the sinuses in your forehead and cheeks and behind your nose and ears. During fetal development and early childhood, the bones that house your sinuses are invaded by air-filled sacs, or pneumatic diverticula, that develop from your nose and middle ear.
These diverticula are lined by the same delicate mucous membranes as the inside of your nose. Even after they are done forming, the sinuses all retain tiny openings to your respiratory airways. Simply by having sinuses, you have personal experience with one of the most important aspects of air-filled bones: to form and be maintained, the air cells inside the bones have to maintain a constant connection to the outside.
