The skull is perhaps the most important protective bone in your body. It’s thick, hard, and nearly continuous, save the small exit holes it provides for nerves and blood vessels to communicate with the rest of the body. It protects you—your brain, a delicate ball of cellular mush—from all the bangs, bumps, and knocks you experience day in and day out. It’s safe to say that the skull, functioning properly, does not want to be opened.
Our cadaver’s skull was built in 1911. For a hundred years, it kept the brain of a huge, fridge-bodied man safe from the dangers of everyday life. And in death, this man’s skull did all it could to save his brain from the peering eyes of four first year medical students.
In theory, extraction of the brain from the skull is rather straight forward. The idea is to make a circular cut around the skull that is low enough to provide an opening for the brain (the skull is actually egg shaped, so the trick is to make the cut so that you have the widest possible opening). While this sounds easy, it can be complicated by several other constraints. First, the skull isn’t uniform in its thickness or shape; it’s actually composed of many bones that fuse together at irregular, suture-like joints early in life. Some of these bones are quite thin, such as the porous cribiform plate of the ethmoid bone that allows your olfactory bulb to project smell receptor neurons directly into your nasal cavity. Others are extremely thick. The petrous bone is one of these.
It’s a thick slab of bone that conducts your external ear canal into the depths of your head. Laying just at the level of the ear canal, the second trick to taking out the brain is to make sure your cut is also above the petrous bone, which is thick enough to resist even some of the most aggressive sawing. This becomes complicated when certain landmarks are missing, such as when an ear (or both) has been removed in a previous lab. With nothing more than a small external ear hole to set your landmarks, it’s easy to make trace an erroneous cut-path and not realize until it’s too late.
The tool we use to cut into bones is a wedge-bladed plaster saw. Rather than using a rotary blade, it vibrates a wedge shaped saw in a back and forth motion. This supposedly allows one to cut dense materials (like bone) and without damaging softer tissues (like brain). In practice, however, it acts a bit different. As the saw is meant to cut through plaster casts, cutting through a denser material like bone requires a lot more force to be applied to make the cut. For cylinder shaped bones (like in the leg and rib), it’s easy to see what lies beneath the bone and when your cut is through. This is not the case for the skull.
The first pass over was delicate; we wanted to try our best to avoid damaging the dura (a leathery covering over the brain) and the various cerebral structures below. After a few minutes, however, we found that this barely made a dent in the skull’s surface. Okay then, a little more pressure. As the saw makes its way deeper, you first see the smoke produced from hot metal grinding on white bone. It smells of burnt hair—which isn’t so bad, until you realize it’s small particles of a dead man’s bone that is literally wafting up to your own olfactory receptors. As you go deeper chips of white fly out. The sounds the saw produces become lower in frequency and feel more labored. Shielding your eyes is a good idea, to make sure any stray chips don’t make their way into you your cornea. A second pass through, much deeper, but the skull still does not budge. Now a third pass, to ensure nothing is missed. There’s a distinctive change in sound as the saw passes through the last of the bone and plunges into the dura and brain matter; it doesn’t belong there. Third pass complete, and still, it does not come off.
Other labs are finished at this point. We get the chisel and hammer, a set of tools that provides more force at the cost of specificity. We chisel around, separating most of the skull except for one part. It’s clear we’ve run ashore of the petrous bone, but by now there’s little that can be done. More banging, larger bone chips. I can see where we’ve pierced the dura now, clearly, along the edges of the entire brain. That wasn’t how it was supposed to go. With a few final blows the petrous is separated, along its most resistant portion, and the skull is completely free. Well, sort of.
The dura, the leathery tissue I mentioned before, also attaches to the inside of the skull to anchor the brain. The idea is to cut through the bone just to the level of the dura so that, once the sawing is done, you can use a small flat tool to scrape the dura off of the inside of the skull. Since we had cut the entire dura during our sawing and chiseling, there was nothing to get underneath. Another cut had to be made, a small wedge, up, to try and get an edge of dura for us to pry under. More sawing, more delicately, more time consuming. The dura gets cut, again.
The only other option is to probe under the edge of the skull, blindly, and hope to get an edge of the dura. It’s like fishing under your couch for a lost remote; you’re not sure exactly where it is, and you touch a lot of other weird things, but once you get it, you know. We finally get an edge and start prying the dura off of the skull cap. This also takes time and if you go to fast you risk ripping the dura. With some luck, we finally get the skull cap off, leaving the brain exposed but still seated in the bottom half of the skull. It’s a truly odd look.
More dura prying is required to remove the brain from the bottom of the skull, as the dura wraps around the brain completely. There are also nerves and arteries and veins at the base, the brains only life-lines to the outside body from it’s cozy, isolated cavern. These have to be carefully revealed and cut, as they are some of the most important anatomy to be studied about the brain. Then, there is the spinal cord, which also sits attached to the caudal (rear) portion of the brain. A laminectomy has to be performed, which means cutting into the vertebral bones just below the bottom of the skull. More bone sawing and chiseling. Only after transecting the spinal cord is the brain truly free and able to be removed.
It’s an extremely strange feeling, holding an object so delicate, intricate, and mysterious in the palm of your hands, exposed to air and light under which it has never, ever been before.
Tomorrow I begin my medical training—kind of weird huh? All the premed classes, MCAT studying, primaries, secondaries, interviews, and post-interview letters lead to this. I feel like Homer in that episode of The Simpsons, getting to what seems to be the peak of my climb only to find out it’s just the beginning. Only up from here.
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