I’ll begin by admitting that ribs are some of my least favorite bones to deal with.
[Sidebar: I took a week-long forensic anthropology overview course once (the 25th year of this), and I was astonished at the extent to which forensic specialists rely on the ribs for estimating age. Isçan and colleagues provide one of the best-known methods for doing so, but using it requires you to identify the sternal end of the fourth rib. This sounds promising, until you realize that ribs from prehistoric contexts often look like this:
I asked the forensic anthropologists teaching the course how they identified the fourth rib and most of them said that they seriated the whole set in order to do so. I was momentarily taken aback until I realized that they more frequently handle relatively complete individuals, in situations where commingling is not an issue. The lesson here is that even when it comes to osteology, you have to choose your methods based on the materials at hand and your analytical goals. Later that day Hugh Berryman told us that he taught himself to identify, seriate and side ribs with his eyes closed in the darkness beneath his desk, so that he could pass his DABFA exam. Forensic anthropologists are a different breed.]
BUT I DIGRESS. The length and dimensions of ribs rarely allow for great preservation, so when you find them in archaeological contexts they tend to be extremely fragmentary. Unfortunately, in contrast to the shaft fragments of long bones which often come complete with nutrient foramina, muscle attachment sites or characteristic cross-sectional dimensions, shaft fragments of ribs are relatively homogenous. There are, however, a few tricks that you can use to help yourself orient and side ribs, even if you’re working with smaller portions.
As a quick guide to orientation in SAP, the most proximal portion of the rib, containing the head, neck and articular tubercle, (for ribs 1-10) articulates with the facets of your thoracic vertebrae, while the more distal, flatter and more regular portion of the rib articulates with the sternum or cartilage (or floats freely). The ribs are numbered from top to bottom, so 1 is the most superior and 12 is the most inferior. In terms of orientation, reach your hands out in front of you as if to hug someone standing in front of you. That’s effectively how your ribs are positioned. Importantly, the curvature of the ribs changes from flat to curving downwards as you move inferiorly down the rib cage. The first and second rib give something of an awkward “slow song at a middle-school dance” kind of a hug, while the lower ribs provide a more comfortable and self-assured embrace.
- Ribs 1-7 articulate posteriorly with with the demifacets and transverse processes of your thoracic vertebrae, and anteriorly with your sternum;
- Ribs 8-10 articulate with your thoracic vertebrae posteriorly, and cartilage connected to the sternum anteriorly;
- Ribs 11-12 articulate with your thoracic vertebrae posteriorly, and nothing anteriorly. They are appropriately known as the ‘floating ribs’.
1. Orienting the tubercles: If you’re attempting to side a rib based on an isolated head, the tubercle is posterior and somewhat inferior. The only exception to this rule is the first rib, where the tubercle is more superior. The first rib, just fyi, is kind of like the strepsirrhine primate of the rib cage – normal rules do not apply.
2. Orienting the sternal end: If all you’ve got is the sternal end of a rib, place it against your thumb. For ribs 8-12, this should slant downwards.
3. Figuring out which end is up (superior-inferior): A depression aptly named the costal groove runs along the underside of each rib. It is broader and more defined posteriorly (towards the head of the rib). In life, the costal groove houses a neurovascular bundle containing the intercostal vessels and intercostal nerve…and when I just looked this up to verify, I remembered having to learn the mnemonic VAN to describe them in Gross Anatomy (since moving superiorly to inferiorly you have the Vein, Artery, and Nerve).
4. Special ribs: Ribs 1, 2, 10, 11 and 12 are a little bit different to the “regular” ribs I’ve described above. Their size, orientation and curvature are unique due to their anatomical position. Just so you don’t get overwhelmed (and because I am lazy), I’ll save a post on them for sometime in the future…
Iscan, M., Loth, S., & Wright, R. (1984). Metamorphosis at the sternal rib end: A new method to estimate age at death in white males Metamorphosis at the sternal rib end: A new method to estimate age at death in white males., 65 (2), 147-156 DOI: 10.1002/ajpa.1330650206