Cranial Nerves

The cranial nerves are twelve pairs of nerves that sprout directly from the underside of the brain (and the brainstem just below it), rather than from the spinal cord like most other nerves. Think of the brain as a control tower and these twelve cables as the direct, short-haul wiring that reaches the face and head: they carry the senses of smell, sight, taste, hearing, and balance inward, and they send commands outward to move the eyes, chew, smile, speak, and swallow. Because there is a left and a right of each, there are twelve pairs in all, and they are traditionally numbered I through XII from front to back.

Each nerve has its own job, and many do more than one thing. Some are purely about sensation (for example, the optic nerve for vision), some are purely about movement (for example, the nerves that steer the eyeball), and some are mixed, carrying both sensation and motor commands. A few of them—most famously the tenth, the vagus nerve—travel far beyond the head to help run internal organs like the heart, lungs, and gut, quietly slowing the heart and aiding digestion. This is why cranial nerves matter so much in medicine: a problem like a drooping eyelid, a crooked smile, double vision, or trouble swallowing can point a doctor to exactly which nerve has been injured.