I was 19 years old when I first held a human brain. It was heavier than I had anticipated; grey, firm and laboratory-cold. Its surface was slippery and smooth, like an algae-covered stone pulled from a riverbed. I had a terror of dropping it and seeing its tight contours burst open on the tiled floor.

It was the start of my second year at medical school. The first year had been a helter-skelter of lectures, libraries, parties and epiphanies. We’d been asked to learn dictionaries of Greek and Latin terminology, strip a corpse’s anatomy to the bone and master the body’s biochemistry, along with the mechanics and mathematics of each organ’s physiology. Each organ, that is, except for the brain. The brain was for second year.

The Neuroanatomy Teaching Laboratory was on the second floor of the Victorian medical school building in central Edinburgh. To get there we had to walk through an archway inscribed ANATOMY, SURGERY, PHYSIC, climb some stairs, pass under the jaw of a blue whale and slip between the articulated skeletons of two Asian elephants. There was something reassuring in the dusty grandeur of these artefacts, their cabinet of curiosities oddity, as if we were being initiated into a fraternity of Victorian collectors, codifiers and classifiers. There was a second set of stairs, then some double doors, and there they were: forty brains in buckets.

Our lecturer was Icelandic and doubled as a welfare officer, so she was also the person you were sent to see if you found yourself unhappily pregnant, or had failed an exam more than once. Standing at the front she held up a half-brain, and began to point out its lobes and divisions. Seen in cross-section, the brain’s core was paler than the surface, like a great morel cut in two. Its outer surface was smooth but its interior was a complex series of chambers, nodules and fibrous bundles. The chambers, known as ‘ventricles’, were particularly intricate and mysterious.

I lifted a brain from its bucket, blinking back the fumes that rose from the preserving fluids. It was a beautiful object. As I cradled it in my hands I tried to think of the consciousness it had once supported, the emotions that had once crackled through its neurons and synapses. My dissection-mate had studied philosophy before switching to medicine. ‘Hand me that,’ she said, taking the brain in her hands. ‘I want to find the pineal body.’

‘What’s the pineal body?’

‘Have you never heard of Descartes? It’s the seat of the soul.’

She put her thumbs between the two hemispheres, as if to open the pages of a book. At the seam that ran through the middle she pointed out a little lump, a grey pea, towards the back. ‘There it is,’ she said. ‘The seat of the soul.’

Some years later I became a trainee neurosurgeon, and began to work with living brains every day. Each time I walked into the neurosurgical theatre I felt an urge to slip off my plastic clogs out of respect. The acoustics played a part in it: the clatter of a trolley or the whisper of an orderly seemed to echo and reverberate around the space. The room itself was a hemisphere, an upturned bowl of geodesic panels built in the 1950s. It looked the way I imagine Cold War radar domes, or Dounreay’s spherical nuclear reactor dome, appear from the inside. Its design seemed to embody that decade’s belief in technology’s promise of a future – an imminent future – without want or disease.

But there was still a lot of disease. I worked long days and nights with injured brains, and soon came to treat them as bruised or bloodied organs like any other. There were the victims of strokes, literally ‘struck’ dumb and paralysed by blood clots. There were creeping invasive tumours, wearing away at skulls and squeezing out personality. There were the comatose and catatonic, car-crashed and gun-shot, aneurysmal and haemorrhagic. There was little opportunity to think about theories of the mind, until one day the professor, my boss, asked me to help out on a special case.

By the time I had scrubbed in and put on my gown, the professor was already at work. ‘Come in, come in,’ he said, looking up from a heap of green cloth on a table. ‘You’re just in time for the fun part.’ I was dressed as he was; draped in the same green cloth as lay on the table, a surgical mask over my face and nose. The theatre lights flashed in the professor’s spectacles. ‘We’re just cutting the window in the skull.’ He turned back to his work, and resumed his conversation with the nurse opposite; they were discussing an American war movie. He began to cut into the skull with a saw. Smoke rose from the bone, together with a smell reminiscent of barbecued meat. The nurse sprayed water over the cutting surface, to catch the dust and keep the bone cool. She also held a suction tube to draw up the smoke, which threatened to cloud the professor’s view.

Seated to one side was the anaesthetist, who wore blue pyjamas instead of a green gown; he was doing a crossword, and occasionally reached under the pile of drapes. There were a couple of other nurses, standing back from the table, whispering to one another with their hands held behind their backs. ‘Stand over there,’ the professor said, and nodded to the space opposite. I jumped into position, and the nurse handed me the suction tube. I had already met the woman under the drapes – let’s call her Claire – and knew that she suffered from severe intractable epilepsy. Here, unusually, was someone affected not by tumour or trauma, but by a delicate shift in the electrical balance of her tissues. Her brain was structurally normal but functionally fragile, forever teetering on the edge of a slide into seizure. If normal cerebral activity – thought, speech, imagination, sensation – moves through the brain with the rhythms of music, seizures might be likened to a deafening blast of static. Claire had been so injured, frightened and handicapped by these seizures that she was prepared to risk her life to be free of them.

‘Suck,’ the professor said. He changed the position of the tube in my hands so that it hovered over his sawblade, then began to cut through more bone. ‘The physiologists tell me her seizures originate just under here.’ He tapped the exposed skull. ‘That’s the epileptogenic cortex.’

‘So we’ll cut out the source of the seizures?’

‘That’s the idea. Trouble is, the source is very close to the area responsible for speech. She won’t thank us if we make her mute in the process.’

Once he had sawn through the skull, the professor prised in little levers, similar to those used to take the tyre from a bicycle wheel, and lifted up a medallion of bone. He handed it to the nurse. ‘Don’t lose that,’ he laughed. The window was about five centimetres in diameter, and revealed the dura mater, the protective layer that lies beneath the skull, shiny and opalescent like the inside of a mussel shell. The professor removed that too, and I looked down on a disc of creamy pink matter, ribbed like sand at low tide, with blood vessels traced over its surface in filaments of purple and red. The brain was slowly pulsating, rising and falling with each beat of the patient’s heart.

And so to the ‘fun’ part. The dose of anaesthetic was slowly reduced, and Claire began to groan. Her eyes flickered and then opened. The drapes had been pulled back, and the steel pins fixed into her skull were now visible.

A speech therapist had arranged her chair so that she was able to bend forward, close to Claire’s face. She explained to her that she was in an operating theatre, that she couldn’t move her head, and that she would be shown a series of cards and should name each object and what could be done with it. Claire grunted, unable to nod, and they began. Her voice was drawling and disembodied – an effect of the sedatives. The cards showed images that could have been taken from a child’s storybook. ‘Clock,’ she said, ‘you tell the time with it.’ ‘Key,’ she said, ‘you open doors with it.’ The images of simple objects went on, drawing her back to her earliest linguistic memories. Her concentration was intense, eyebrows creased to arrowheads, forehead glistening with sweat.

Meanwhile, the professor had swapped his saw and scalpel for a nerve stimulator. He began to dab at the surface of the brain delicately, at first holding his breath. There were no hints of bravado now, no jokes or chat: his entire attention was concentrated on two steel points separated by a couple of millimetres. The electrical effect was minimal – it would barely be felt if applied to the skin – but on the sensitive surface of the brain its effect was overwhelming: it caused an electrical storm that obliterated normal function. The portion of the brain affected was small, but it was big enough to contain millions of nerve cells and their connections.

‘She carried on talking so that bit’s not eloquent,’ he said. ‘In other words we can cut it.’ He placed a numbered label, like a tiny stamp, over the place he had just touched with the stimulator. The number was carefully catalogued by one of the nurses, while he moved on to the next patch. The professor called this process ‘mapping’, as if each human brain were an uncharted country being opened to surgical discovery. He moved carefully over the surface contours, numbering and recording: it was methodical, patient work. I had heard stories of his standing at the operating table for 16 hours straight, reluctant to abandon the patient even to go to the toilet or eat a snack.

‘Bus, you can tra … tra …’

‘Speech arrest,’ the therapist said, looking up at us. ‘Shall we try that one again?’ She showed another card. ‘Knife, youah, aah …’

‘There we are,’ the professor said, pointing to the area he had just passed over with the electric current. ‘Eloquent brain.’ He placed another label carefully over the area, and moved on. I studied the eloquent brain carefully, willing it to appear in some way different from the rest of the tissue around it. Her vocal cords and throat might make the sound, but here was the wellspring of her voice. It was the connections between the neurons in that exact place, the patterns they made as they fired, that enabled speech and defined it as ‘eloquent’. But there were no distinguishing features, no sign that this patch of cortex was the channel through which Claire spoke to the world.

On one occasion at medical school a visiting neurosurgeon showed us slides of an operation to remove a brain tumour. Someone in the front row raised his hand and remarked that it didn’t look like a very delicate process. ‘People tend to think of brain surgeons as being very dextrous,’ the neurosurgeon replied, ‘but it’s the plastic surgeons and microvascular surgeons who do that meticulous stuff.’ He indicated the slide on the wall: a patient’s brain – an aerial array of steel rods, clamps and wires. ‘The rest of us just do gardening.’

Once Claire was asleep again, the professor removed a chunk of her brain – the ‘epileptogenic’ part – and dropped it into a bin. ‘What do you think that patch of cortex was responsible for?’ I asked him. He shrugged, perhaps a little defensively. ‘No idea,’ he said, ‘we just know it’s not eloquent.’

‘Will she notice any change?’

‘Probably not, the rest of the brain will adapt.’

There was a scar like a lunar crater by the time we’d finished. With her brain and mind once more anaesthetised, we cauterised the severed blood vessels, filled up the crater with fluid, and then sutured up the dura with neat embroidery stitches. We reattached the disc of bone by inserting little screws through strips of titanium mesh.

‘Don’t drop them,’ the professor said as he handed me each screw. ‘They cost about fifty quid each.’

We unrolled Claire’s scalp, which had been held out of the way with clips, and stapled it back in place. I met her again a couple of days later and asked her how she was feeling. ‘No seizures yet,’ she said. ‘You could have made a nicer job of the stapling, though.’ Her mouth unfurled into a ragged smile: ‘I look like Frankenstein’s monster.’

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