Think about it

If you were designing from scratch the cutaneous innervation of the head and neck, it might strike you as logical to get down on all fours like a quadruped and tilt your head back so that your face was the most anterior part of you. In this position it makes sense that the dorsal aspect of the neck and head should be supplied by dorsal rami of spinal nerves, and the ventral aspect of the neck and head (under the chin) by ventral rami. This leaves the entire anterior aspect of the face, which, in a quadruped, goes first into new environments, with a cutaneous nerve all to itself – the trigeminal. This is exactly how it is. All you have to do is remember that because we are upright bipeds, the relative positions of the head and trunk have changed as compared with the quadruped. Think about it.

Sensory information from the face and scalp is carried back to the trigeminal sensory nuclei (Section 4.4) in neurons with cell bodies in the trigeminal ganglion (except for proprioceptive neurons), and it is relayed to various centres within the brain. Examples of these central connections can be illustrated by what happens when we wash our face in the morning.