Cranial Nerve I: The cribriform plate of the ethmoid bone is the primary landmark to identify the location of CN I. It forms the roof of the nasal cavities. Small foramina in the cribriform plate allow passage of numerous olfactory nerves from the roof of the nasal cavity, superior portion of the nasal septum, and superior nasal concha. After passing through the cribriform plate, these nerves traverse the dura mater and subarachnoid space before ending in the olfactory bulb, which is situated under the basal surface of the frontal lobe.

Cranial Nerve II: The axons of the retinal ganglion cells in the eye are the basis of CN II which originates where they pass through the sclera. The nerve passes posteromedially beneath the superior rectus muscle and between the medial and lateral rectus muscles. It exits the orbit through the optic canal located in the sphenoid bone. Intracranially, the nerve passes superomedially to the ophthalmic artery and terminal segments of the internal carotid artery. The nerve ends at the optic chiasm which lies superior to the sella turcica, anterior to the pituitary stalk, and inferior to the supra-optic recess of the third ventricle. The central retinal artery and vein travel with the optic nerve within the meninges until they rejoin the ophthalmic artery and superior ophthalmic vein, respectively.

Cranial Nerve III: The oculomotor nerve originates from two nuclei (somatic and visceral motor) located in the midbrain at the level of the superior colliculus and anterior to the cerebral aqueduct. The nerve exits the midbrain into the interpeduncular fossa medial to the basilar artery. It immediately passes between the posterior cerebral artery and superior cerebellar artery. The nerve enters the cavernous sinus lateral to the internal carotid artery and superior to CN IV. It enters the orbit through the superior orbital fissure wherein it divides into the superior and inferior divisions to supply motor innervation to its target extraocular muscles. The inferior division carries the parasympathetic fibers to the ciliary and constrictor pupillae muscles.

Cranial Nerve IV: The trochlear nerve motor nucleus is in the periaqueductal gray matter of the midbrain at the level of the inferior colliculus. The nerve fibers emerge dorsally from the midbrain, travelling anterolaterally between the posterior cerebral artery and superior cerebellar arteries. It enters the lateral wall of the cavernous sinus inferolateral to CN III. To enter the orbit, the nerve passes medially through the superior orbital fissure below the anterior clinoid process and CN III.

Cranial Nerve V: The trigeminal nerve originates from 4 nuclei – 1 motor and 3 sensory. The locations and paths of these nuclei are beyond the scope of this chapter and can be reviewed elsewhere (Bathla and Hegde 2013; Moore and Dalley 1999). CN V emerges from the pons as essentially two separate nerves: a small motor and large sensory root. The collection of cranial nerve roots enters Meckel’s cave (Fig. 8) wherein the sensory branches coalesce into the trigeminal ganglion, while the motor root travels medially and rejoins the sensory portion of V₃ to create the mandibular nerve. The trigeminal ganglion splits into the 3 branches of CN V: the ophthalmic nerve (V₁), maxillary nerve (V₂), and the sensory component of the mandibular nerve (V₃). Both V₁ and V₂ pass through the lateral wall of the cavernous sinus inferolateral to CN IV and III.

Cranial Nerve VI: The abducens nerve arises in the medial aspect of the pons near the floor of the 4th ventricle and adjacent to the motor nucleus of CN VII. It emerges into the pontine cistern at the pontomedullary junction. It travels anteriorly along the basilar artery and enters the cavernous sinus through Dorello’s canal (Fig. 12) after bending over the petrous portion of the temporal bone. In the cavernous sinus it is immediately posterolateral to the internal carotid artery and medial to CN III and IV. It enters the orbit through the medial aspect of the superior orbital fissure where it innervates the lateral rectus muscle. The approximate pathway through the cavernous sinus is seen in Fig. 13.

Cranial Nerve VII: The facial nerve originates from two separate nerves exiting the brainstem at the cerebellopontine angle. The motor component arises from the motor nucleus in the dorsal pons. The sensory component, often called the nervus intermedius, arises from a combination of inputs from the spinal nucleus of CN V, the superior salivary nucleus, and the nucleus solitarius. The two nerves cross the cerebellopontine cistern and join at the internal acoustic meatus. The facial nerve travels with CN VIII until it enters the facial/fallopian canal and travels anterolaterally. At the geniculate ganglion the nerve abruptly changes direction forming the first genu. Here, the greater petrosal nerve branches off the facial nerve and travels through the greater petrosal foramen into the middle cranial fossa until it reaches the vidian canal where it forms the vidian nerve and finally reaches the pterygopalatine ganglion. The facial nerve travels posteriorly along the medial wall of the tympanic cavity wherein it gives rise to the nerve to the stapedius. At the end of the tympanic cavity is the second genu where the nerve turns 90° inferiorly and enters the mastoid bone. In this segment, the facial nerve gives rise to the chorda tympani which eventually joins the lingual nerve of CN V ₃ to supply taste to the anterior two-thirds of the tongue. The facial nerve then exits the cranium through the stylomastoid foramen and enters the substance of the parotid gland. The nerve terminates in six branches: the posterior auricular, temporal, zygomatic, buccal, mandibular, and cervical nerves. Additional information on the anatomy and path of the facial nerve can be found elsewhere (Gilchrist 2009; Veillona et al. 2010).