Seeing the complexity of the simple: Comparative anatomy of the scapula

We can do a lot of things with our arms that a quadruped can’t do with theirs. Throw spears at edible quadrupeds. Throw tomatoes at sopranos. Throw bums out of office.

Being a scientist means finding delight in things that look complicated but are actually governed by pretty simple principles, as well as in things that look pretty simple but are actually pretty complex.  Case in point: the scapula.

Common English: shoulder blade.

Technical term: scapula, plural scapulae or scapulas.

French: l’omoplate (nf), la scapula (WordReference)

A scapula looks simple: they’re mostly flat, with a protruberance here and there.  Unlike closely associated bones, they don’t get broken very often–a Swedish study found a rate of scapular fractures of 10 per 100,000 people, while another Swedish study found 50 clavicular fractures per 100,000 people.

And yet: comparing the scapula across species, you see all kinds of interesting shit.  The point of comparative anatomy is that you can understand something better if you compare it to other ways that it could be, but isn’t.  So: let’s compare some scapulae.

The most obvious thing about the scapula is that it is positioned differently in different species.  The basic situation for most living things with limbs is this: you’re a quadruped (i.e. have four legs), and the scapula is located on the side of the trunk.  In contrast: look at a human, and the scapula is on its back. Compare the position of the scapula in this lovely picture of a horse:

horse skeleton
The axis of the scapula is on the line between points A and B. Picture source:

…with the position of the scapula in this lovely picture of a human:

Picture source:

…and you see the difference.  It’s even more striking when you look at our closer relatives.  We are primates, and specifically, members of a group of primates known as apes, and even more specifically, of the great apes.  One of the biggest differences between us and our various and sundry primate relatives is that we are full-time bipeds.  Autrement dit: we walk upright, all the time.  In contrast, monkeys–which are primates, but not apes–are full-time quadrupeds.  Going along with this difference in locomotion is a difference in the position of the scapula: it’s on our back, but on a monkey’s side.  

Here’s a really nice view of a primate (left) and human (right) trunk.  Looking at the left side, labelled “monkey,” you see the typical quadruped architecture: the scapula is on the side of the chest cage.  On the right side, labelled “human,” it’s a different story: the scapula is on the back.

The arrows in this illustration make an important point: primates also have the typical quadruped chest cage, which is relatively narrow in comparison to its depth. In contrast, the human chest cage is sorta flat–relatively wide in comparison to its depth.  (Remember that the human skeleton in the picture is viewed from above, as if you had ripped someone’s head off in order to shit down their neck. (Sorry–a little sailor-talk there.  Unlike Trump, I served my country.)  In contrast, the monkey is being viewed from the front–I have no great analogy for you here.)

chest_compar (1)
Picture source:

Amongst primates in general, there is quite a bit of variability.  Why? Well, there’s quite a bit of variability in the extent to which they are quadrupedal versus bipedal. There’s quite a bit of variability in the capacity of the creature to do stuff with its hands over its head.  Here’s a nice layout that shows aspects of the shoulder anatomy across a range from true monkeys, to great apes, ending up with the ape-iest of us all: the anatomically modern human.  Start with the sacred monkey in the upper-left corner, and the scapula is clearly on the side of the thoracic cage.  End with the human in the bottom-right corner, and it’s clearly on the back.  In between…well, gibbons are (lesser) apes, while chimps and gorillas are great apes, like us.

Relative positions of the scapula in monkeys versus apes: the scapula is on the side in monkeys, on the back in apes.  Note the angle of the clavicle, too: the more dorsal the scapula is, the more perpendicular the clavicle is to the midline. Picture source:

What functional difference goes along with this structural difference? Well: the quadrupeds are really good at locomotion–it’s difficult to think of a quadruped that can’t outrun a human.  Try to catch your dog or your cat for a trip to the vet–good fucking luck, buddy.  But, quadrupeds also tend to have a big limitation: although their front limbs are very good at moving back and forth–see above about moving fast–they suck at anything else.  For example, we can make big circles with our arms; we can spread them.  We don’t have the speed of a quadruped, but we can do a lot of things with our arms that a quadruped can’t do with theirs. Throw spears at edible quadrupeds. Throw tomatoes at sopranos. Throw bums out of office.

There are two broad families of quadrupeds that have their scapula on their back, and they’re pretty fucking interesting. Come back next time for the details.

Position of the scapula in the dog. Picture source:

English notes

There are a lot of expressions that involve the shoulder.  For example, to stand shoulder to shoulder with someone has a literal meaning of standing close to someone (Merriam-Webster), and a figurative one of being united with someone, sharing a goal with them (Merriam-Webster). Example:

  • This resolution was offered in response to President Trump standing shoulder to shoulder with Putin while the Russian President offered the Special Counsel a chance to interview twelve Russian Military Intelligence Officers who’ve been indicted for conducting “large scale cyber operations to interfere with the 2016 presidential election” in exchange for politically-motivated Russian interrogations of U.S. citizens.  President Trump initially endorsed Putin’s cynical ploy as “an incredible offer” and during yesterday’s White House press briefing President Trump’s spokesperson said he was still considering it.

To give someone the cold shoulder is to intentionally treat them in a way that is cold or unsympathetic (Merriam-Webster); I would add the meaning of intentionally avoiding or ignoring them.  Example:

  • Most Twitter reactions seem to compare the president’s behavior to that of a child—which is pretty much on the money with what we’ve been saying since the start of this tale. Sure, it’s an improvement over the cold shoulder he gave German Chancellor Angela Merkel when she extended her hand for a handshake during her March White House visit. If the president doesn’t pout does he still earn, like, a half-star on the behavior chart? Even still, pushing is not encouraged, young man.

If you are the kind of person who would watch and carefully re-watch Dubstep Cat videos to see just how far back his arms do and don’t go: we should probably be planning our wedding.



2 thoughts on “Seeing the complexity of the simple: Comparative anatomy of the scapula”

  1. This a nice article and drew attention to several more issues of the scapula than I had thought of. I’d started out wondering about the position of mine relative to my dog’s. I looked for the next part couldn’t find it.

    Liked by 1 person

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