A System Built in Two Halves
The nervous system splits cleanly into two divisions with very different jobs. The central nervous system — the brain and spinal cord — is where information gets processed and decisions get made. The peripheral nervous system is everything else: the vast network of nerves, including twelve pairs of cranial nerves and thirty-one pairs of spinal nerves, that carries signals to and from the central system. Within the brain itself, the cerebrum — the large, wrinkled structure that makes up most of what people picture when they think of "the brain" — handles higher-level thinking, voluntary movement, and sensory processing, divided into frontal, parietal, temporal, and occipital lobes, each responsible for a different domain, from reasoning and personality in the front to vision at the very back.
Beneath the cerebrum sit smaller, older structures that quietly run the show. The cerebellum, tucked underneath and behind the cerebrum, coordinates balance and fine motor movement — and despite making up only about a tenth of total brain volume, it's estimated to contain more than half of the brain's total neurons. The thalamus acts as a relay station, routing nearly all incoming sensory information to the right part of the cortex, while the hypothalamus, just below it, regulates body temperature, hunger, and the release of hormones through its connection to the pituitary gland. The brainstem, at the very base of the brain, handles the functions too important to leave to conscious thought — heart rate, breathing, and the basic reflexes that keep a person alive without them ever having to think about it.
Two Cases That Rewrote the Textbook
Much of what anatomists know about which brain region does what comes not from careful experiment design, but from injury. In 1848, a railroad foreman named Phineas Gage survived an accident in which an iron rod was driven through his skull, destroying a significant portion of his frontal lobe. Gage kept his ability to walk and speak, but by all contemporary accounts his personality changed dramatically — a transformation that became one of the earliest pieces of evidence that the frontal lobe, and specifically the prefrontal cortex, governs personality and judgment rather than basic movement or sensation.
A century later, a patient known publicly only by the initials H.M. underwent surgery in the 1950s that removed his hippocampus on both sides of the brain to control severe seizures. The surgery worked, but it left H.M. unable to form new long-term memories for the rest of his life, even though he could still learn new physical skills through repetition and clearly recalled memories from before his operation. His case became one of the most studied in the history of neuroscience, providing direct evidence that the hippocampus is essential for converting short-term experience into lasting memory — and that different types of memory are stored using entirely different systems in the brain. Between Gage's damaged frontal lobe and H.M.'s missing hippocampus, two accidents of history did more to map the brain's function than decades of theory ever could.
Source: National Institutes of Health (NIH) and Encyclopaedia Britannica.