I’m reading up on mandibular rotation, which is the change in orientation of the mandibular corpus relative to the rest of the skull during growth (the corpus is the horizontal part of your jaw that holds up your teeth; check out the shape changes in the mandibles in the blog header). So far as I can tell, the original classic paper on the topic is by Bjork (1955). Growth was studied by implanting metal pins into the jaws, then seeing how they move across ontogeny via X-rays (which were once called “roentgenograms,” neat-o!) Here’s a picture of the procedure, from Bjork (1955):

HOLY GOD WHAT DID THAT KID DO TO DESERVE THIS?! And although there must be a third person there, it sorta looks like there’s a three-handed dentist wielding a hammer, a nail, and a kid’s face. No wonder so many people are afraid of the dentist.