Carl Zimmer in Discover Magazine:
There is nothing more humbling or more perception-changing than holding a human brain in your hands. I discovered this recently at a brain-cutting lesson given by Jean-Paul Vonsattel, a neuropathologist at Columbia University. These lessons take place every month in a cold, windowless room deep within the university’s College of Physicians and Surgeons. On the day I visited, there were half a dozen brains sitting on a table. Vonsattel began by passing them around so the medical students could take a closer look. When a brain came my way, I cradled it and found myself puzzling over its mirror symmetry. It was as if someone had glued two smaller brains together to make a bigger one.
Vonsattel then showed us just how weak that glue is. He took back one of the brains and used a knife to divide the hemispheres. He sliced quickly through the corpus callosum, the flat bundle of nerve fibers that connects the halves. The hemispheres flopped away from each other, two identical slabs of fleshy neurons.
Sometimes surgeons must make an even more extreme kind of slice in the brain of a patient. A child may suffer from epilepsy so severe that the only relief doctors can offer is to open up the skull and cut out the entire hemisphere in which the seizures start. After the surgery, the space soon fills with cerebrospinal fluid. It may take a child a year of physical therapy to recover from losing a hemisphere—but the fact that patients recover at all is stunning when you consider that they have only half a brain. It makes you wonder what good two hemispheres are in the first place.