Nerve · Head

Olfactory nerve

Nervus olfactorius

Smell has its own cranial nerve, and it barely looks like one. Rather than a single trunk it is a spray of about twenty fine bundles of unmyelinated fibres, each running only a couple of centimetres from the smell-sensing cells in the roof of the nasal cavity, up through the perforated bone above them, to the olfactory bulb sitting on the underside of the frontal lobe. It is the shortest cranial nerve and the only sensory pathway that reaches the cortex without first relaying through the thalamus — which is part of why a smell can reach memory and emotion so directly. Because the fibres are tethered where they thread through that perforated plate, they are physically fragile: a shearing blow to the front of the head can tear them where bone holds them fast.