The term trigeminal nerve refers to one of twelve cranial nerves that attach to the brain and pass out of the cranial cavity through openings at the base of the cranium. The nerve is composed of axons to and from several trigeminal nuclei in the midbrain and hindbrain. The bundles of axons located within the brain are referred to as trigeminal nerve fibers, tracts and roots. Defined on the basis of connectivity the trigeminal nerve includes both the trigeminal nerve fibers within the brain and the nerve trunks outside (See Models Where It Appears > Functional CNS Model - Rat below.)