exposed to the outside world

I’ve heard before that the sense of smell, unlike other senses -- vision, hearing, etc. -- is a sense operated directly by the brain. Not that I understood what that meant. Says Lewis Thomas in The Youngest Sciences: Notes of a Medicine-Watcher: “The olfactory receptor cells were known to be bona fide brain cells, the only proper neurones in the brain that are exposed to the outside world and act as their own receptors of information from the environment. … [T]he olfactory neurones [are always] replicating and replacing themselves in their positions at the surface of the olfactory mucosa, high up in the back of the nose. No other brain neurones have the property of multiplying or regenerating; once in place the neurones of the rest of the brain are there for the duration of life, and those which age and die off are not replaced.”

More recent research suggests this is not as uniformly true as Lewis makes out, that is, some other brain cells do regenerate.

“Another peculiarity … is that despite their exposure to the outside, and their location in a region of the air passages which is especially rich in bacteria and viruses of all sorts, the tissues in which they reside do not become infected. … It is now believed that the cells are somehow protected by the antimicrobial property of the mucus which is always present as a thin layer covering the cells.”

Didn’t the Egyptian embalmers pull the brain out through the nose? Maybe they knew it was already kinda sticking out there.