Tutorial 13: how to dissect a neck
March 30, 2011
This tutorial is based on all the things that I stupidly forgot to do along the way of tearing down the juvenile giraffe neck that Darren, John Conway and I recently got to take to pieces. At half a dozen different points in that process, I found myself thinking “Oh, we should have done X earlier on!” So it’s not a tutorial founded on the idea that I know how this should be done; it’s about how I am only now realising how it should be done, off the back of my dumb mistakes.
What you want is to get the maximum possible information out of your specimen. At each stage of preparation, information is lost — a necessary evil, because of course at the same time new information becomes available. So don’t miss anything early on.
The whole neck
If you’re lucky, you’ll get the complete, intact neck to work with. (Ours was not quite intact, having been skinned, and lost an indeteminate amount of superficial muscle and ligament in the process.) So before you start cutting, photograph the neck in dorsal, ventral, lateral, anterior and posterior aspects.
Next, you want to measure the neck:
- total mass
- total length, front of atlas to back of last centrum.
- maximum flexion (i.e. downwards bend)
- maximum extension (i.e. upwards bend)
- maximum deflection (i.e. lateral bend)
These last three are hard to do, because “maximum” flexion, extension and deflection are not exact things. You can always push or squeeze or bend a bit harder. These are the unpleasantly messy aspects of working with animals rather than robots — most kinds of tissue are flexible and resilient. You just have to do the best you can, and supplement your measurements with photographs of the neck bent in each direction.
Now you’re ready to start taking that baby apart. Get the skin off, then redo all your photos and redo all your measurements — yes, even total length, even though you “know” removing the skin can’t affect that. Because you don’t know what you don’t know. Does removing the skin affect the maximum range of movement? How much of the neck’s total mass was due to the skin? Weigh the skin as well: does its mass added to that of the deskinned neck add up to that of the intact neck? If not, is the discrepancy due to blood loss?
Once the skin is off, you can start removing muscles. Ideally, you want to identify each muscle as you go, and remove them one by one, so that you leave the major ligaments behind. In practice this is harder than it sounds, because the muscles in real necks are, inconveniently, not clearly delineated and labelled like the ones in books. Still, going slowly and carefully, it’s often possible to avoid cutting actual muscles but just the fascia between them, which allows you remove complete muscles. Done well, this can leave in place not only the nuchal ligament running along the top of all the neural spines, but the shorter ventral ligaments joining adjacent vertebrae.
As you’re doing this, you want to avoid damaging the intercentral joints and the zygapophyseal capsules, so far as possible. You’ll probably find it easy to preserve the former, which are tough, but harder not to accidentally damage at least some of the latter. You want to keep them intact as far as possible, so you can see how the react when you manipulate the neck. (Do these manipulations gently, or you’ll tear those capsules.)
Now that the skin and muscles are both off — at least, so far as you can remove the muscles, which will not be completely — you can redo all your photos and redo all your measurements again. Yes, all of them. Because you just can’t tell what you’re going to be interested in later, and curse yourself for missing.
Go right ahead. Remove the short ligaments, and do your best to get the nuchal ligament off all in one chunk — not quite as easy as it sounds, because it doesn’t just sit on top of the neural spines, but sort of encloses them. Measure the nuchal ligament at rest, then stretch it out as far as you can and measure it extended. Calculate how far it stretched as a proportion of the rest length. Compare this with what you learned from Alexander (1989:64-65). Hmm. Interesting, no?
You can guess what’s coming now: redo all your photos and redo all your measurements yet again. You should find that the total length is the same, but who knows what you might find about changing flexibility? Also, your progressive sequence of mass measurements will tell you what proportion of the whole neck was skin, muscle, ligament, etc.
Separating the vertebrae
This sounds like it should be easy, but it’s not. The zygapophyses will come apart very easily, but the centra will be held firmly together with very dense connective tissue which has be cut carefully away, piece by piece, with the blade of a scalpel worked between the condyle of one vertebra and the cotyle of the next. (I’m writing here about a giraffe neck, but I’m confident the same will be true of other artiodactyls and maybe most mammals; bird necks are different.)
Once you’ve got the vertebrae separate, photograph each vertebra separately, from each of the cardinal directions. Also, measure each vertebra separately — especially for centrum length, but you may as well get all the major measurements. These measurements will include the cartilage caps at the front and back of each centrum. (This is the step that I most regret missing out.)
Articulate the vertebrae in “neutral pose” by keeping the centra in full contact and rotating each intercentral joint about its midpoint until the corresponding zygapophyses are maximally overlapped. What does this pose look like? How does it compare to the animal’s habitual pose in life? (If possible, compare with the pose shown by an X-ray of the live animal, since necks lie.)
Articulate the vertebrae in positions of “maximal” flexion, extension and deflection by keeping the centra in full contact and rotating each intercentral joint about its midpoint until the corresponding zygapophyses are displaced to a degree of your choosing. Try it with the zygs allowed to slide until they are 50% disarticulated, then with 75% disarticulation, then displacing until they are just past the point of contacting each other. Photograph all these poses and measure their deflection. Compare these variant poses with those obtained when the vertebrae were still joined together, and when the ligaments, muscles and skin were still in place. What degree of zygapophyseal disarticulation best matches the whole-neck bending ability? How does this vary along the neck? How does that this compare with what you may have been led to expect in the literature. Hmm.
Using your earlier photos of the whole neck’s bending profile, arrange the vertebrae in the exact same pose. How much do the zygapophyses disarticulate in these poses? As you rotate the joints about the articulation of their centra, do the zygs just slide neatly past each other, or do they move far apart from each other as the neck bends? Interesting, yes?
Cleaning the vertebrae
Have you recorded all the information you need from the intact vertebrae with their cartilage in place? If you’re sure, then …
Lightly simmer the vertebrae for an hour or so, then remove the excess flesh by hand and using a toothbrush. Repeat as needed to get them clean. If you can do this really carefully — I couldn’t — you may be able to keep the cartilage firm, and firmly articulated with the bone. (Bugging the vertebrae is probably a better approach for this purpose, but I find it hard to be that patient.)
Once the vertebrae have dried out — and especially, once their cartilage is dry — re-measure each vertebra. Does the drying of the cartilage affect the centrum length?
Simmer the vertebrae again and gently peel off the cartilage caps at the front and back of each centrum. Re-measure the centra: how long are they now? What proportion of each centrum’s length was cartilage?
Articulate all the centra in a straight line, and measure the total length. How does this compare with the whole-neck length you started with? [Crib-sheet answer for our baby giraffe: 41 cm vs. a whole-neck length of 51 cm. Expect a closer match if you’re dealing with an adult animal,which will have proportionally less cartilage.]
Articulate the vertebrae in “neutral pose” as you did back when the individual vertebrae were complete. How does the new “neutral pose” compare with the old one? With habitual life posture? Huh. Makes you think, doesn’t it?
Nearly done …
Articulate the vertebrae in positions of “maximal” flexion, extension and deflection as you did before, and compare your results with those from when the vertebrae were complete with their cartilage caps. Well! Who’d have thought?
Now remember that the fossils we have of, say, sauropod cervicals are those of the dry bone only, with no cartilage. Think about how different the “neutral pose” and range of movement would be if we had the intact vertebrae with their cartilage.
Dammit all, I’ve given the game away
As I wrote this article, I found myself giving away more and more of a paper I’ve been planning to write, in which I go through essentially this process with a couple of necks, ideally from very different clades, and write up the results. Say, a giraffe, an ostrich and a croc. The extent to which the dry-bone postures and flexibility vary from those of the live animals would give us a reasonable starting point for thinking about how life postures and flexibility of sauropods might have varied from what we’d deduce from the dry bones alone.
Wouldn’t that be a great little paper?
Well, I might still write it when I find the time, but I won’t stand in the way of anyone else who wants to plough straight in and just get it done. (You might mention me in the acknowledgements if you do.)