Asked by Joey to Lindsay on 21 May 2015.
Keywords: rib, shape
That’s a fantastic question, since it’s one of the things I love most about ribs—their shape!
Most people have 24 ribs (12 on each side), and they all have a similar crescent, blade-like shape. However, as you progress down from your 1st to your 12th rib, that shape changes, as well as the rib’s principal orientation. So, you see that each is a variation on a theme, rather than the same shape over and over. We call this serial (repeated), but modified, patterning metamerism. Metameric structures usually have a similar developmental origin (back from when they’re part of the embryo), and they perform similar, though sometimes different functions. This is true of the ribs because they do share a developmental origin, but they are each slightly different from the one above.
The first rib starts out tightly curved, but relatively flat top to bottom, and oriented kind of like a horseshoe. As you move down, this shape starts to open up at the end closest the sternum (breastbone) and turn so that it’s oriented more up and down. By the time that you get to the last rib, the orientation has shifted completely so that instead of lying flat, it instead points more straight out. You can tell at a glance where a human rib falls by paying attention to its curvature and how the body is oriented relative to its head(s) (the part that attaches to the ribs).
We further categorize the ribs into the true (1-7, having a direct cartilage attachment to the sternum), false (8-10, having an attachment to the sternum via the cartilage above) and floating ribs (11-12, having no cartilaginous attachment, but little nibs on the ends). Depending on region, they have different actions that assist primarily in breathing. The upper ribs make a pump handle movement up and down, while the lower ones have a pincer-like movement out and in. Their individual shapes help them to perform these movements efficiently, and also give our rib cage the barrel shape that is characteristic of the genus Homo.
You can see the shapes of the individual ribs here (they’re kinda hard to describe):
You can see an animation of rib movement in breathing here:
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