Two Brains in One Skull
The cerebrum is split into two hemispheres, left and right, joined by a thick band of some 200 million nerve fibers called the corpus callosum. Each hemisphere controls the opposite side of the body — the left hemisphere moves the right hand, and vice versa. This crossing is called contralateral control.
The hemispheres are also somewhat lateralized — specialized for different functions. In most people, the left hemisphere dominates language: two classic language areas, Broca's area (speech production) and Wernicke's area (language comprehension), sit in the left hemisphere, and damage to them produces distinct aphasias. The right hemisphere tends to specialize in spatial processing, face recognition, and aspects of emotional and musical perception.
The most dramatic evidence comes from split-brain patients, whose corpus callosum was surgically cut (historically to control severe epilepsy). With the bridge severed, the two hemispheres can no longer share information directly. An object shown only to the right hemisphere (via the left visual field) can be picked out by the left hand but cannot be named, because the language-dominant left hemisphere never received it.
Common pitfall: the pop-psychology myth of "left-brained" (logical) versus "right-brained" (creative) people. Lateralization means certain functions lean to one side — it does not mean individuals are dominated by one hemisphere. Both hemispheres are active in virtually everything a healthy brain does.
A top-down head: left and right visual fields cross to the opposite hemispheres; the corpus callosum bridge is drawn, then a scissors cut animates severing it.