The human brain's most direct connection to the world outside the skull is through the nose.

Some 40 million nerves run from two small olfactory bulbs deep inside the brain to the roofs of the nasal cavity, protected from outside influences only by a thin layer of mucus, write Time-Life Books editors in The Mysteries of the Human Body.

One reason why smell, of all the five senses, is the most emotionally evocative may be that the olfactory centers are surrounded by the limbic region, the part of the brain that controls emotions and plays a key role in the formation of memories.