According to classical anatomical nomen-clature, there are 12 pairs of cranial nerves, although the first two pairs are not really peripheral nerves. The olfactory nerve (I) consists of the olfactory fibers, the bundledprocesses of sensory cells in the olfactory epithelium which enter the olfactory bulb(B8). The optical nerve (II) is a cerebral pathway; the origin of the opticalfibers, the retina, together with the pig-mented epithelium of the eyeball repre-sents an evagination of the diencephalon. Optic chiasm (B9), optical tract (B10).
The eye-muscle nerves are somato-motor nerves. The oculomotor nerve (III) leaves the brain on the floor of the inter-peduncular fossa (B11); thetrochlear nerve (IV) emerges at the dorsal surface of the midbrain and extends around the cerebral peduncles to the basal surface; the abducens nerve (VI) emerges from the lower border of the pons.
Five nerves have developed from the branchial arch nerves of lower vertebrates:the trigeminal nerve (V), the facialnerve (VII), the glossopharyngeal nerve (IX), the vagus nerve (X), and the accessory nerve (XI). The muscles supplied by these nerves are derived from the branchial arch muscles of the foregut. Hence, these nerves have originally been visceromotor nerves. In mammals, the branchial arch muscles have changed into the striated muscles of pharynx, oral cavity, and face. Unlike genuine striated muscles, they are not completely voluntary (e.g., facial expression in response to emotion).
The vestibulocochlear nerve (VIII) with its vestibular part represents a phylo-genetically old connection to the organ ofbalance already present in lower vertebrates. The trigeminal nerve (V) emerges from the lateral part of the pons. Its sensory root ex-tends to the trigeminal ganglion (semilunarganglion, Gasser’s ganglion) (B12); its motor root (B13) bypasses the ganglion. The facialnerve (VII) and the vestibulocochlear nerve (VIII) leave the medulla oblongata at the cerebellopontine angle. The taste fibers of the facial nerve emerge as an independent nerve, the intermediate nerve (B14). The glossopharyngeal nerve (IX) and the vagus nerve (X) emerge dorsal to the olive. Supe-rior ganglion of the vagus nerve (B15). Thecervical roots of the accessory nerve (XI) unite to form the spinal root (B16). The upper fibers originating from the medulla oblongata form the cranial root; they run a short course in the nerve and change over to the vagus nerve as internal branch (B17).
The hypoglossal nerve (XII) is a somatomotor nerve; in ontogenetic terms, it represents the remnants of several cervi-cal nerves that have become included in the brain region secondarily and now have only rudimentary sensory roots.
B18 Olfactory tract.
B19 Lateral olfactory stria.
B20 Anterior perforated substance.
B21 Hypophyseal stalk.
B22 Choroid plexus (flower spray of Bochdalek).