A few weeks ago I posted a bone quiz that some of my friends have been finding a little trickier than usual. My hints were that:
(i) This feature is notable for arching laterally, and (ii) It is one that preserves well in archaeological contexts.
You can see one of the original images in the photo below, shown with a 3cm scale.
The fragment in question was the zygomatic process of the temporal bone, a feature that makes up part of the zygomatic arch that between the temporal bone and the malar bone. This portion of the temporal bone is highlighted in bright blue in the following diagram:
The reason this is also a palpable anatomy post is because it’s an easy feature to feel on your own skull. Press your fingers along the superior and posterior portion of your cheek bone, and nudge them towards your ear. You should feel a protruding ridge of bone running in an anterior to posterior direction. Your fingers will slide off above and below this ridge. In the photo below, I’ve lined my fingers up along the zygomatic arch – if you put yours in the same approximate location just in front of your ear, you’ll be able to feel the feature as well.
The reason I find the zygomatic process of the temporal bone so easy to recognize is because, to me, it looks like an arm. I see the jagged edge of the concisely named zygomaticotemporal suture as the fingers on a hand, and the chunkier, more posterior portion of the bone as the meatier forearm and elbow, as you can see in the convincingly organized photo below.
Once you visualize this parallel, the complete bone is easy to orient. Remember that the convex side is lateral, the thickest and roundest portion of the bone is posterior, and the sutural border is anterior. To double-check yourself all you need to do is hold out your own forearms and compare the orientation and curvature of the bone to the orientation and curvature of your upper limbs. (On a side note, when left to my own devices in lab I always look like I am doing some sort of bizarre, slow-motion yoga or interpretative dance, flipping my palms, stretching out my arms , palpating my mandible, etc., etc. It’s a good thing there’s only one other person working in the museum basement this summer).
These tricks will allow you to orient even the most fragmentary portion of this feature. If you don’t want to look as if you’re embracing a ghostly companion, simply place the bone in front of you so that the more concave side is facing down and the more convex side is facing up. The thinnest, straightest edge is superior, while the more angular border that likely has two different, identifiable flattened surfaces, is inferior. In this position, the jagged suture for the zygomatic bone points to the side the bone is from.
To test your skills, orient and side the fragmentary zygomatic processes shown in the following two photos. Pay attention to (i) the superior-inferior height of the fragment (is it ‘chunkier’ or more gracile?) (ii) whether the side shown is concave or convex, and (iii) the location of the zygomaticotemporal suture, if present.
Answers are posted below the jump after the image credits, with the fragments shown articulated with larger portions of temporal bones, and sides indicated in the alt text.
Fragment A: Two views, easier (Note that the fragment was flipped horizontally between the top photo and the bottom photo).
Fragment B: One view, more difficult.
Image Credits: First image of temporal bone highlighted in blue found here at studyblue.com. Second image of temporal bone found here. All other photos were taken at the Museo de Jaén in summer 2014.