The Olfactory System
The Olfactory System is basically the body’s system of smell and it begins at the roof of the mouth and nasal cavity.

Tiny molecules of aroma are inhaled by the nose and are trapped in the
nose by hair like nerve endings that pass the aroma on to receptors.
These are then carried to the Olfactory Bulb within the limbic region of
the brain. As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons).

The olfactory region of each of the two nasal passages in humans is a small area of about 2.5 square centimetres containing in total approximately 50 million primary sensory receptor cells.