Cranial nerves

Cranial nerves are nerves that emerge from the brainstem instead of the spinal cord.

Names of nerves
In human anatomy, there are exactly 12 pairs of them, traditionally abbreviated by the corresponding Roman numerals.

Cranial nerves I and II are named as such, but are technically not nerves, as they are continuations of the central nervous system.

Note: technically the accessory nerve splits into the spinal accessory nerve and the cranial accessory nerve, but the latter quickly combines with the vagus nerve.

Cranial nerves in non-human vertebrates
Human cranial nerves are evolutionarily homologous to those found in many other vertebrates. The first ten pairs of cranial nerves arose in the common ancestor of tetrapods (amphibians, reptiles including birds, and mammals). Cranial nerves XI and XII evolved in the common ancestor to amniotes (non-amphibian tetrapods) thus totalling twelve pairs. These characters are synapomorphies for their respective clades. In some primitive cartilagenous fishes, such as the dogfish (Squalos acanthos), there is a terminal nerve numbered 0 (as it exits the brain before the first cranial nerve).

Thirteen cranial nerves?
Although twelve nerves are classically described in humans, there is a theory that finds favour with some authors, that humans really have thirteen cranial nerves (Andy Lelli, 1999). If the C1 spinal nerve were considered the thirteenth cranial nerve, C2 through C8 would be renamed as the first through seventh cervical nerves (though anatomists might change the term 'cervical' to 'nuchal' or some other term to avoid confusion between the two nomenclatures). This would make the numbering system consistent all the way through the spinal column - every nerve would exit below its corresponding vertebra, and the number of vertebrae would equal the number of spinal nerves.

Alternatively, C1 could be considered the spinal root of the hypoglossal nerve (CN XII). This would, again, make the numbering system consistent.