An arm and a leg: The basics of animal anatomy

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The leaves are crunchy beneath my feet, the comforter is back on the bed and every single thing on the menu has been ruined with an overdose of pumpkin spice. Must be autumn. This is also the unusual time of year when we hang skeletons from our front doors. If you’re a normal person, they make you think of Halloween. If you’re me, they make you think of anatomy class.

Naturally, there are a lot of anatomic differences between people and their pets, many of which are invisible at a glance. Dogs and cats will never have to worry about appendicitis, for instance, because they don’t have an appendix. And we, mercifully and for similar reasons, don’t have to deal with our anal glands clogging up. Score one for being human.

You’d think that differences in skeletal anatomy, being on such overt display, would be easier to pick out. Anyone can see that our spines stop at our hips, while theirs carry on to form a tail. But some things aren’t what they seem, and it’s time to recognize that our four-legged friends don’t really have four legs at all.

Sure, their hind legs are very much like ours with a ball-in-socket hip joint and a knee topped by its eponymous cap. It would be easy to presume that the front legs are put together in the exact same way, but that isn’t the case. Those front legs are, in fact, arms. Dogs dig in dirt, cats destroy the couch and people post to Snapchat with the exact basic set of bones. They have the same evolutionary origin, but have been repurposed in different species. This is called homology, and you can see it well beyond your own pets. Whether it’s a bat’s wing or a penguin’s flipper, it’s the same limb with a few tweaks.

From this, it should be clear that dogs and cats (and countless other animals) don’t walk around with four knees. They’ve only got two, and they’re always in the rear. Now it does look like our pets have knees in the front, but those are actually wrist joints. Below that, their weight is balanced on the very tips of what would otherwise be fingers. And that’s not the weirdest thing about your pet’s front legs. While our arms connect to our skeletons by a collarbone, dogs and cats have nearly lost theirs entirely. Their forelimbs aren’t attached to the rest of their skeleton at all, except by muscle.

The hind limbs almost make familiar sense. The parts are the same as ours, but anatomists changed all the names to make it confusing. So while your pet does have a proper knee joint back there, someone decided it should be called a stifle instead. And the joint below that—the one that juts out sharply like an elbow—is technically their heel. But we don’t call it either of those things. Maddeningly, it’s a hock.

Why is that? I haven’t the slightest idea, but it’s enough to make any sensible person wash their hands of it all. Or their feet. Whichever.

Dr. Mike Fietz is a small-animal veterinarian at Georgetown Veterinary Hospital. He received his veterinary degree from Cornell University in 2003.