Medical doctors often specialize in a particular area, and those specialties feature English that’s heavily populated with Latin- and Greek-originated words. For example, there might be a “thoracic” specialist focusing on the torso — that specialty comes from “thorax”, which comes from the Greek word for the breastplate part of a suit of armor. An “osteopath” focuses on bones, again from a Greek root “osteon” (bone) — it migrated into Latin as “osseous” (bony) before entering English around the 1400s.
But the medical profession has missed some good possibilities too, just based on the available terms. For instance, as far as I know there’s no such thing as a rachidist, even though I’m pretty sure there are physicians who specialize in the spine. And it’s not like they’ve never encountered the word; they’re probably familiar with a rachiometer, which is a device for measuring the spine. Years ago they might have looked out for rachitogenic factors in a young patient, because those would be things that could cause rickets (a childhood bone disease). Both of those things could be called rachidian because they have to do with the spine, or (to use a more obscure term), the rachis.
In addition to referring to the spine, “rachis” — which comes from a Greek word for “ridge” — can also mean the central stem of a feather. A feather does, after a fashion, somewhat resemble a spinal column with ribs attached.
Although “rachis” means spine, and has spawned a set of related words all beginning with “rach-“, “rach” itself is also a word. But it has nothing to do with spines or feathers — a rach is a dog that hunts by scent. As sometimes happens, English words that are very similar (even identical) can arrive there by completely different paths, even though they’re really unrelated. That’s what happened here. The “rach” that refers to a dog has no connection to Greek or Latin. “Rach” the dog comes from Old English (raecc), which in turn came from the Old Norse word for dog, “rakki.”
You might be thinking of yet another word for a thing that has at least a passing resemblance to ribs extending from a spine and also sounds a bit like “rach” and “rachis”: “rake.” But “rake” represents yet another lineage; it comes from the Old English word “racu.” That word came from the Old German “rekho,” which, like “racu,” pretty much meant the implement you use to gather up fallen leaves in the autumn. When you’re working on that task, by the way, be careful not to hurt your rachis.
