There was a time when I visited a slaughterhouse twice a week to pick up twelve sheep brains. Like any dutiful medical school applicant, I had volunteered in a hospital, suffered through organic chemistry, and passed the Medical College Admissions Test. When a friend told me her professor needed a student to work in her lab, I jumped at the chance to fill in the remaining unwritten requirement for medical school acceptance: laboratory research.

Dr. S's lab studied oligodendrocytes, cells that provide electrical insulation to neurons in the brain. (Their function is crucial to our own: the debilitating neurologic illness Multiple Sclerosis results from oligodendrocytes' death.) Butchered animals are an inexhaustible source of the cells, but to be used for research, oligodendrocytes must be harvested immediately after death. Which is why, every Tuesday and Thursday morning, I rolled up to the back door of a slaughterhouse on Chicago's south side, a cooler and a malodorous plastic-lined cardboard box in the trunk of my car.

In the slaughterhouse entryway, I donned a paper suit and made my way to the killing floor amidst the hoots, snickers, and sideways glances of the workers. After their throats were cut (the sheeps', not the men's) the animals providing my cells glided toward me, hanging by their hindfeet from a conveyor belt. The man whose job it was to decapitate them knew me; when I put my box down at his feet, he tossed the next twelve heads into it. Then I carried the box--full of heads with tongues still moving and eyelashes still fluttering--a few yards away to a bandsaw. After sawing each head in half, muzzle to occiput, I pulled on a pair of gloves and scooped all twenty-four brain halves out of their skulls, dropping them in the two chilled beakers of salt solution I carried in the cooler. I then hurried outside and drove as fast as I could back to the lab, where we began the day-long process of harvesting oligodendrocytes.

I didn't stay at her lab long enough to learn if Dr. S ever isolated the protein she was looking for, and when I left I thought I never wanted to see another brain. But two years later, in medical school, our introductory neurobiology lecture was given by an oligodendrocyte expert. Just hearing the word brought back the sight of the sheep coming toward me, the smell of the slaughterhouse (raw meat, burned hair, a hint of manure), the particular queasiness I associate with being out of my element at the crack of dawn, and the warm slipperiness of the brains themselves.

In the rest of the course, we discussed where our brains store such intense memories and how a single word could elicit them. Slouched in my lumpy lecture-hall seat, juggling bagel, coffee, and notebook, I learned that to smell the slaughterhouse, actual molecules from the meat and manure had to enter my nose and bind to receptors in my olfactory nerve. The nerve then sent messages to a part of my brain known as the hippocampus, where memories are initially processed. (Although information from all the senses reaches the hippocampus, olfactory neurons have a more direct connection to it than neurons from the ears or eyes, which may explain why smells are so strongly associated with memory.) For a while--no one knows exactly how long, but probably on the order of years--our memories appear to live in the hippocampus, and can be lost with damage to it. Eventually, through an unknown process, the memories are apparently transferred to widely distributed areas in our brains and are no longer stored in or reliant on the hippocampus.

That an everyday process like memory was still so enigmatic surprised me. I became fascinated with what neuroscientists do and don't know, and with the methods they use to discover how our brains--slippery, wrinkled collections of cells that differ mainly in size from the sheep's brains I gathered for the lab--can give rise to our minds, capable of doing something so mysterious as wondering how our minds can do something so mysterious. I began writing my book to further explore such questions by visiting and speaking with the scholars investigating them. I travelled across the country, interviewing people studying the mind.

While I have never had more fun, what I found out was often disconcerting. I learned that injury has provided much of what we know about the human brain's function, and that the understanding we hope will cure devastating illnesses like Alzheimer's and Parkinson's disease is gained from the deaths of millions of lab animals. I was surprised at how arbitrary, unscientific, and inconsistent are many of the lines our society has drawn between brains worthy of legal protection and expendable brains. Through it all, I continually came up against one disturbing reality: The study of the mind and brain is a bloody, messy, morally ambiguous business, a fact that comes down to one inexorable truth, which is that we cannot separate our minds--our selves--from the meat of which we are made.