The olfactory springs externally from the fissure of Sylvius, near the anterior perforated space. As it is close to the third frontal convolution, it follows that aphasia from affection of Broca's lobe on the left side may be associated with defective sensation of smell in the left nostril. The nerve may be completely destroyed by fracture of the anterior fossa or by malignant disease of the ethmoid, through the cribriform plate of which it passes to be distributed to the nasal mucous membrane.

The optic nerve arises from the geniculate bodies under the optic thalamus and corpora quadrigemina, the tracts winding over the crura cerebri and converging in front of the interpeduncular space to form the optic commissure. Here the fibres of the tracts divide, the outer half of each continuing to the outer side of the corresponding retina, while the inner half crosses to supply the inner half of the opposite retina. Behind these fibres are others, which run from one tract to the other, and are known as Gudden's commissure. If, therefore, the entire thickness of one tract be affected by the pressure of a tumour, the temporal side of the eye on the same side and the nasal side of the opposite eye will be rendered blind (hemianopsia). If the optic nerve be pressed on, however, as by a tumour of the orbit, there will result complete blindness of both sides of the affected eye, probably associated with affections of the third, fourth, and sixth nerves, which lie close to it.

The third nerve, or motor oculi, arises in front of the pons, near the posterior perforated spot, lies on the outer wall of the cavernous sinus, and, dividing into two branches, enters the orbit through the sphenoidal fissure between the heads of the external rectus. The action of the third nerve is considered in connection with the eye. It affords from its action on the pupil, which becomes contracted when the nerve is irritated, and dilated when it is paralyzed, a very delicate test for intracranial pressure arising from tumour, haemorrhage, or abscess in the region of the middle fossa. A lesion near the Sylvian aqueduct might produce a bilateral paralysis of the nerves.

The fourth nerve emerges just behind the corpora quadrigemina. Its course is similar to that of the third, and its action is discussed with the eye.

The fifth, or trigeminal, arises from the surface of the pons by a small motor and large sensory portions. These proceed forward in the posterior fossa of the base, pierce the dura at the attachment of the tentorium cerebelli to the superior border of the petrous, the sensory root then forming the large Casseri an ganglion, which is lodged in a small cavity of the dura, formed by a splitting of its layers, and called the cavum Meckelii. The ganglion lies in a depression on the apex of the petrous, and divides into its three branches-ophthalmic, superior, and inferior maxillary, the motor root joining the latter. The ophthalmic division pursues a course similar to the third nerve, and breaks up into frontal, nasal, and lachrymal branches, the nasal branch supplying the sensory root to the lenticular ganglion, which ganglion supplies the short ciliary nerves to the ciliary muscle and iris. Destruction of this division not merely destroys reflex blinking, but removes the trophic influence of the nerve on the parts supplied, ulceration being apt to occur in consequence.

The superior maxillary division passes through the foramen rotundum, crosses the spheno-maxillary fossa, and runs under the floor of the orbit, to emerge at the infra-orbital foramen. It supplies the skin in the temporal and malar regions, and the teeth and gums, lining of the antrum, and sensory branch to Meckel's ganglion. Meckel's ganglion supplies the nasal fossae, roof of mouth, upper part of pharynx, tonsil, gums, soft palate, and uvula, etc. ; while by its motor branch, derived from the vidian, it enervates the levator palati, azygos uvulae, palato-glossus, and pharyngeus.

The inferior maxillary division emerges through the foramen ovale, and divides into anterior and posterior trunks, of which the anterior is chiefly motor and supplies all the muscles of mastication except the buccinator (supplied by the seventh). The posterior trunk gives off the auriculotemporal, which supplies that region with sensation, as well as the temporo-maxillary joint and parotid gland, and gives a sensory root to the otic ganglion, which supplies the tensors tympani and palati. The trunk also gives off the gustatory nerve, which supplies sensation to the anterior two-thirds of the tongue. It is joined in the pterygoid region by the chorda timpani, through which probably taste fibres are derived, and later by branches of the hypoglossal. The inferior dental branch supplies the mylo-hyoid and anterior belly of the digastric, supplies sensation to the lower teeth, and gives off the mental branch, which emerges through the mental foramen to supply the skin of the lip and chin. Irritation of this nerve from dental caries may cause earache, while, conversely, placing cotton-wool with a little laudanum in the ear may ease toothache. The cold of an ether spray applied to the external auditory meatus may numb the nerve and enable a tooth to be extracted painlessly.

The Gasserian ganglion has frequently been removed successfully for intense trigeminal neuralgia. The neuralgia is believed to be due to contraction of the durai pocket, containing the ganglion, which is therefore crushed. The ganglion is best reached through the face, antrum of Highmore, sphenomaxillary fossa, and foramina rotundum and ovale (see under Middle Fossa of Skull), but may also be reached through the skull from above, or through the pterygoid region. Tic douloureux is a term frequently applied to trigeminal neuralgia, not necessarily involving all the divisions. Where only one division is affected, it may be due to peripheral irritation, such as dental caries, or inflammation at the point of exit from one of the osseous canals. In such cases a small peripheral operation may be sufficient.