Stupid sesamoid bones…

Today, I had an osteology midterm. As midterms go it was pretty easy. There was, however, one bone that completely stumped me (and I hope, the rest of the class).

This guy. In isolation, obviously.

Give up? It’s the pisiform of a bear. This might mean nothing to you. But if you do happen to know what it is, you’re probably thinking, “That’s a dinky-ass little bone in the hand. There are tons of other bones your professor could have given to you!” You’d be right. But in other animals, it’s not so dinky. To give you some perspective:

The pisiform in the human wrist is marked with a p

One can clearly see how much larger the bear’s pisiform is relative to the rest of the hand while the other carpals and metacarpals seem about the same relative size. Even in the great apes, the pisiform varies greatly in size (with humans having BY FAR the smallest). Not bad for a little sesamoid bone. To understand why this drastic variation in size and shape, we need think about what exactly this bone is doing in the hand. At least in hominoids (us apes) the pisiform serves three major functions: it is one of the “high points” on the palmar side of the carpals for the flexor retinaculum (thus helping to create the “carpal tunnel” that all the important stuff goes through), it is the insertion point for the flexor carpi ulnaris (a muscle, one of the major flexors of the wrist), and it is the origin of the abductor digiti minimi (also a muscle. it allows you to move your pinky from side to side. so…important). In our close relatives the pisiform is long and rod-shaped instead of eponymously pea-shaped as it is in humans. This serves two major purposes for our more suspensory kin: it allows the carpal tunnel to be much larger (for the more massive digital flexors that they’re gonna need), but more importantly it allows for a more efficient lever arm for the flexor carpi ulnaris to act against. This is really important when you’re hanging from trees and perhaps trying to pull yourself up.

But why is it so large in the bear? They’re not hanging from trees by their digits that often. But think about what needs to happen if you’re walking on all fours with your palms on the ground. In order to push off from the substrate, you’re going to have to flex your wrist. And powerfully, if you’re an animal the size of a bear.

So… yeah. I probably should have not gotten it wrong, especially as it only had one articular surface. And it sure as shit wasn’t a distal phalanx of ANYTHING. But this post is to remind me, and you, to always keep the pisiform under consideration. You never know.


About alexclaxton

Paleoanthropology grad student, pop culture obsessive
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