The Brain

Zoom in and the folded tissue resolves into cells — somewhere in the order of tens of billions of neurons, the exact count revised downward in recent years from the old textbook figure of a hundred billion. A neuron is a strange-looking cell: a central body, a spray of short branches called dendrites that receive signals, and one long fibre, the axon, that sends them. Sablonnière describes it less as a wire than as a tiny decision-maker, constantly tallying the signals arriving on its dendrites and firing only when the sum crosses a threshold.

The firing itself is electrical. A pulse travels down the axon as a brief, self-propagating change in charge — the action potential — all-or-nothing, like a line of dominoes rather than a dimmer switch. But where one neuron meets the next, the electricity usually stops. The two cells don't touch; there's a microscopic gap, the synapse, and the message crosses it chemically.

Everyone has seen the diagram: the brain painted in zones, each coloured for its job. Vision at the back, movement along a central strip, language camped in two patches on the left. The map isn't invented — those associations are real and were mapped the hard way, often by observing what people lost when a particular region was injured. The patients named Broca and Wernicke studied in the nineteenth century really did reveal that specific language abilities live in specific places.

But Sablonnière's point is that the map flattens something the brain does constantly: it works in networks, not territories. Reading this sentence lights up visual areas, language areas, memory, attention and more, firing together across distances, trading signals through that white-matter cabling. No single spot 'does' reading. A capacity emerges from a conversation between regions, and if you only ever photograph the regions, you miss the conversation.

If one idea runs underneath the whole guided tour, it's that this organ is not finished. For most of the twentieth century the working assumption was that the brain was built in childhood and fixed by adulthood — you were handed your neurons, wore some out, and that was the hand you played. Sablonnière spends his closing pages dismantling that assumption, because the evidence of the last few decades points the other way. The brain is plastic, meaning it rewires itself in response to what it does.

Plasticity happens mostly at the synapses. Connections that get used repeatedly grow stronger and more numerous; connections left idle weaken and are pruned away. This is the physical face of learning — when you practise an instrument or a language, you are not filling a container, you are reshaping tissue. Sablonnière points to the well-known finding that intensive navigation work physically enlarges the hippocampal region tied to spatial memory. Use carves the organ.

Come back to the grapefruit in the hand. It looked, at the start, like a finished object — a thing you could weigh, slice, label. By the end of Sablonnière's tour it looks more like a process that happens to have a shape: billions of cells trading electrical pulses and chemical messages, organised into networks that overlap and share work, running on a greedy, fragile blood supply, and quietly reorganising themselves around everything the body experiences.