The olfactory nerve. The anterior surface of the olfactory lobe fills a large foramen in the anterior wall of the cranium and is pressed closely against the posterior surface of the nasal sac. Numerous small nerves, collectively forming the olfactory nerve, arise from the anterior face of the lobe, penetrate the membraneous wall of the olfactory organ, and are distributed to its highly folded surface.
The terminal nerve, Nervus terminalis, is a slender nerve running along the medial surface of the stalk of the olfactory lobe. Follow it backward to its origin on the anterior surface of the cerebral hemisphere, deep in the median fissure (in Eugaleus on the ventral surface). Trace it forward over the dorsal surface of the olfactory lobe to where it enters the nasal sac. The terminal nerve is a true cranial nerve which has escaped notice until recent years. It is associated with the olfactory nerve in vertebrates generally from fishes to men. The fibres of the terminal nerve remain distinct from those of the olfactory nerve, both in the olfactory organ and in the brain. Its function is unknown. There is still a division of opinion among authorities as to whether the terminal nerve should be considered to be a distinct cranial nerve, or a portion of the olfactory nerve.
The optic nerve can be seen at the bottom of the orbit between the eye and the skull, nearly under the superior oblique muscle. It arises from the ventral side of the diencephalon, passes outward, penetrates the orbit at its infero-medial angle, and continues directly outward to the eyeball.
The trochlear nerve, or patheticus, penetrates the wall of the orbit opposite the optic lobe. Follow it back to its origin from the dorsal surface of the brain in the depression between the optic lobes and the cerebellum. Then follow it from the skull to the superior oblique muscle, which it innervates.
The oculo-motor nerve arises from the ventral surface of the midbrain, passes outward, and penetrates the orbit on a level with and just anterior to the origins of the recti muscles. It divides immediately into three parts; two pass to the anterior and superior recti respectively, while the third passes downward along the posterior surface of the eyeball to the inferior rectus and inferior oblique muscles. In tracing this nerve the palatine process of the upper jaw will be seen projecting from below into the orbit.
The trigeminal, facial, and auditory nerves spring from the side of the medulla below the corpus restiformis. The roots, and some of the branches, of the trigeminal and facial nerves are so mingled as to be indistinguishable except by special neurological technique. The common root of the trigeminal and facial nerves shows a partial division into a dorsal and a ventral portion; the dorsal portion belongs to the facial nerve, while the ventral root is mixed. The root of the auditory nerve lies close behind the trigeminal-facial root, but can be distinguished fairly well. Both the trigeminal and facial nerves divide into several trunks, namely:
| Trigeminal | Facial |
|---|---|
| superficial ophthalmic | superficial ophthalmic |
| deep ophthalmic | buccal |
| maxillary | otic |
| mandibular | hyomandibular |
The superficial ophthalmic trunks of the two unite in a single nerve which passes along the inner wall of the orbit above the muscles of the eye to a foramen in the antero-medial angle of the orbit, through which it passes to the dorsal surface of the snout. The superficial ophthalmic nerve of Squalus is composed almost entirely of fibres of the facial nerve. The superficial ophthalmic trunk of the trigeminal gives rise to several small nerves leaving the common trunk near its origin and passing to the skin above the eye. The superficial ophthalmic trunk of the facial, nearly the whole of the common nerve, branches profusely to supply the sensory organs of the dorsal and lateral surfaces of the snout.
The superficial ophthalmic of Eugaleus rises from the dorsal part of the trigemino-facial root and leaves the cranium by a separate foramen above and anterior to the roots of the recti muscles.
Directly under the origin of the superficial ophthalmic will be found a comparatively slender nerve, which passes between the superior and posterior rectus muscles, and forward along the medial surface of the eyeball; it penetrates the anterior wall of the orbit by a separate foramen, and emerges under the superficial ophthalmic. It is distributed to the skin of the dorsal and lateral surfaces of the snout. This is the deep ophthalmic (ophthalmicus profundus) of the trigeminal nerve. A slender branch (posterior ciliary nerve) passes from the deep ophthalmic near its origin to the posterior surface of the eyeball. Farther forward the same trunk gives off an anterior ciliary nerve to the anterior part of the eyeball.