Remember when I told you how brains are freaking amazing? They tell us everything we know about the world, they contain all our thoughts and feelings, they build and repair themselves all on their own. Yeah, they’re basically just all-around awesome.
But most people don’t usually get to appreciate their awesomeness up close. Even neuroscientists like me. As you guys probably know by now, I study fruit fly brains. Believe it or not, fly brains are really frickin small. You can barely even see them with the naked eye, let alone make out different brain structures. And you can’t just grab a scalpel and slice one open.
So it was pretty cool to get to dissect sheep brains with a sixth grade class that I’m co-teaching. Like, it wasn’t just cool for the kids—it was cool for ME. I’ve seen brains from all kinds of different animals, but only from the outside. Even having studied neuroscience for over a decade, I’ve never gotten to cut open a decent sized brain myself and uncover all the parts that we neuroscientists talk about all the time.
Kids with brains
Bringing the sheep brains out to the students immediately provoked a slew of questions.
“Why is it so small?”
“Why isn’t it bloodier? I wish there was more blood.”
“Can I squish it?”
“Can I eat it?”
Sheep brains are about the size of your fist, so they’re smaller than what you might expect. The brains were chemically preserved, which changes their texture. Live brains are about the consistency of jello. Our preserved sheep brains were still squishy but much sturdier than jello, which was a good thing because they had to withstand the force of rambunctious sixth graders.
At first the students were a bit squeamish, poking tentatively at the brains with a finger or two. But once they realized there’s nothing to be afraid of, they started picking them up, turning them over, squeezing them, even trying to crush them.
“Does your brain ever get crushed like this?” asked the kid who was treating his sheep brain like a stress ball.
That’s actually not a bad question. Thanks to our thick skulls, our brains are usually pretty well protected. In addition, our brains float in a liquid called cerebrospinal fluid, which provides cushioning. Without this fluid, the bottom of the brain would get crushed by its own weight (about 3 pounds in an adult human).
Folding the brain
The most striking features of the outside of the brain are the folds and grooves that cover its surface. As we explained to the students, these folds are there for a simple, kinda boring reason—to increase the surface area of the brain. It’s like what happens when you crumple up a piece of paper: you can fit it into a smaller, more compact space, and you end up with a ball that has lots of folds. If you were to unfold a human brain and lay it out flat, it would be about the size of a large pizza. Which wouldn’t be very convenient to carry around.
Not all animals’ brains have folds. Mouse brains, for example, are perfectly smooth. A folded brain is a sign of a big brain, all wadded up to fit inside someone’s head. Some animals need a lot of brain simply because they have big bodies, like elephants and whales. Other animals aren’t particularly large but have a lot of brain because of their greater intelligence, such as humans and other primates.
Sheep brains have a modest amount of folding. They’re not perfectly smooth like a mouse brain, but not as folded as a human or elephant brain. What was interesting was that the folds of some sheep brains seemed much more obvious than in others. The kids noticed this too, which immediately led to typical sixth grader one-upmanship:
“My brain has the most folds! My sheep must have been the smartest!”
“How smart was MY sheep? Was it smarter than his sheep?”
Me: “Guys, I was not personally acquainted with any of your sheep.”
Slicing and dicing
The first cut we made was straight down the middle, separating the two symmetrical hemispheres of the brain. The students got their first glimpse of the inside of a real brain, in all its guts and glory. The inner workings of the organ that controls our every thought and action. Their reaction was nearly unanimous:
“It looks like chicken!”
They weren’t wrong. The whitish, fibrous structure of the brain did give it a passing resemblance to some kind of white meat. Even my stomach started to rumble.
The cool thing about cutting into the brain was immediately seeing the grey matter and white matter. You’ve probably heard of this distinction, as have all of us neuroscientists of course, but I never thought it would be something you could just see with your own eyes after crudely chopping open a brain. I always figured it was one of those things that you’d have to be an anatomy expert to see, with the rest of us squinting at it for awhile before hesitantly declaring, “Yeah, I guess I kinda see it…”. Like a magic eye or something.
But nope, seeing white and grey matter is as clear as night and day (or white and grey, I suppose). The white matter is myelin. Myelin is a fatty substance that coats axons, which are the long processes of brain cells that transmit information to other neurons. So the white matter represents information being relayed to other areas, often traversing long distances across the brain.
The most obvious white matter tract is the corpus callosum, which transmits information between the two brain hemispheres. Without this tract the two hemispheres are essentially separate, controlling the different sides of your body independently. Your left hand literally wouldn’t know what your right hand was doing.
Grey matter represents everything else, like the cell bodies of neurons and their dendrites, through which they receive signals from other brain cells. The white and grey matter beautifully marble the outer layer of the brain, called the cerebral cortex. The cortex is believed to be the highest part of the brain, where complex thought and decision-making place. When you look at an intact brain, almost all you can see is the cortex. All those brain folds that we talked about? That’s all cortex.
It’s only when you cut the brain in half, like we did, that you can see that the cortex is only the outermost layer of the brain. In fact, cortex literally means “bark”. Beneath the cortex, deeper into the brain, you can observe all sorts of other structures (see photo above). The corpus callosum, for example, and the thalamus, which collects information from your senses and relays it to the cortex. You can even see the tiny pineal body, thought by Descartes to represent the seat of the soul. (Its real function is to regulate sleep and circadian rhythms by producing melatonin.)
We next cut one of our brain hemispheres perpendicularly to the first cut, revealing new hidden areas. The most exciting structure was the hippocampus, where most learning takes place. “Hippocampus” means “seahorse”; it was named for its thin, curved shape resembling the animal.
Initially the students had some trouble finding the hippocampus, which is buried deep within other structures. One kid grumbled, “I don’t see no seahorse!” There were several murmurs of agreement, myself included.
But as soon as one of my co-teachers showed us a good example of a hippocampus, we were able to hunt it down. She showed us how you can even pop the hippocampus right out of the brain using your finger. This is pretty crazy—it’s not like you can just pop out the cortex or thalamus or most other brain structures. I assume the hippocampus must be special because it’s so highly interconnected with itself and relatively less connected to the rest of the brain. But don’t go trying this at home: without a hippocampus you wouldn’t be able to form new memories, like the famous patient H.M. or the guy from the movie Memento.
The “little brain”
Our final cut was through the cerebellum, which is the ball-like structure at the back of your brain that’s responsible for motor coordination.
Without a cerebellum you would still be able to move all your limbs, but you wouldn’t be able to coordinate their movement. Occasionally people are born without a cerebellum, like this guy; they struggle to walk normally and perform other daily tasks, but amazingly they can sometimes live on their own. This woman didn’t even discover that she lacked a cerebellum until she was 24!
“Cerebellum” means “little brain”, which is kind of like what it looks like. I think many people would agree that the cerebellum is the most beautiful part of the brain. Its folds and fibers are far more intricate than those of the cortex. When you cut it open, the white and grey matter form a beautiful leaf-like pattern. The cerebellum actually contains more brain cells than the rest of your brain combined—several times more, in fact. This amazing density of cells gives rise to its exquisite structure.
The cerebellar cut was the final part of our dissection, not counting the kids who continued to cut up their brains into smaller and smaller pieces for no apparent reason (scalpels are fun!). We then poked toothpicks into our brains to label different areas, making them appear even more like chicken hors d’oeurves.
So that’s what it was like to dissect sheep brains with sixth graders. Beforehand, my main criterion for judging the activity a success was if no one got stabbed with a scalpel. But it turned out that most of the students appreciated the brain dissection for the amazing opportunity that it was. Kids who had never seemed engaged in any of our previous classes were actually participating and asking questions. They learned a lot about the organ that makes us who we are—and so did I.