Part 7

A projection of the ventral wall of the utriculo-sacculus is the lagena, the rudiment from which the cochlea of higher animals developed. It also contains a macula acustica.

EXTERNAL FEATURES OF THE EYE. Observe the transparent cornea covering the external surface of the eye; the dark ring of the iris; the central opening in the iris, the pupil; the conjunctival sac surrounding the external half of the eyeball. Cut away sufficient of the upper wall of the cartilaginous orbit to expose the eyeball and its muscles. Note the considerable amount of soft connective tissue around the eye and explore the orbital sinus (p. 13). Take notice of the following nerves, in order to ensure their preservation until the time comes to trace them more completely. A large nerve crossing the medial side of the orbit, the superficial ophthalmic; a nerve leaving the cranium opposite the optic lobe, passing under the superficial ophthalmic to the anterior muscle of the eyeball, the trochlear; several long ciliary nerves passing to the eyeball; several other nerves visible in the deep angle of the orbit.

Six muscles move the eye. Four of these arise close together at the deep postero-medial angle of the orbit. Diverging, they are inserted upon four sides of the eyeball, and from the position of their insertions are named the superior, posterior, inferior, and anterior recti. Two muscles arise from the antero-medial angle of the orbit, the superior and inferior oblique muscles.

Between the recti muscles can be seen a mushroom-shaped stalk of cartilage, the ophthalmic peduncle; the eyeball rests against its expanded end. (There is no peduncle in Eugaleus.)

THE CRANIAL NERVES. The cranial nerves are twelve pairs of nerves arising from the brain, and thus distinguished from the spinal nerves which arise from the sides of the spinal cord. They are distributed chiefly to the head and neck, though branches of the vagus nerve go to the viscera and to the sense organs of the lateral line. Since the nerves are all paired, the distribution of both nerves of a pair being alike, the descriptions will mention but one nerve of a pair. As the cranial nerves are traced dissect away the sides of the cranium down to the foramina penetrated by the nerves, and follow each nerve from its origin on the brain to the parts innervated by it. Features of the dissection which are not found in tracing the nerve of one side should be sought on the other side.

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 superficial ophthalmic 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.

A large nerve which crosses the floor of the orbit, beneath the eyeball, consists of the maxillary trunk of the trigeminal and the buccal trunk of the facial nerve. These remain associated, even into the small branches. Near the anterior margin of the orbit the maxillary-buccal trunk divides into three parts; the smallest and outer one passes to the surface lateral and anterior to the eye. The other two dip downward and pass in front of the jaw to the ventral surface of the snout. Reflect the skin of the ventral surface of the snout, and by dissection expose these nerves as they emerge from the orbit. The larger branch runs forward close to the median line of the snout, giving off numerous twigs; the other, which appears to be a pure trigeminal branch, is distributed near the angle of the mouth. The fibres of the maxillary trunk supply the skin, while those of the buccal go to the canal organs and ampullae of Lorenzini.

The mandibular trunk of the trigeminal nerve arises beneath and behind the maxillary. It passes outward in front of the levator maxillae superioris muscle, sending a few twigs into this muscle, and turns downward over the palato-quadrate cartilage. It divides here, one part entering the adductor mandibularis muscle, the other passing downward along the edge of the mandible, innervating the skin of the lower jaw and the first ventral superficial constrictor muscle.

The mandibular and maxillary-buccal trunks of Eugaleus are united until they approach the edge of the orbit, and the palatine branch is much larger; otherwise the trigemino-facial branches are much as in Squalus.

The hyomandibular trunk of the facial nerve can be found just beneath the skin behind and close to the spiracle. From here it can be followed back to the brain. It arises from the ventral part of the trigemino-facial root, emerging from the cranium through the hyomandibular canal. It divides into a number of branches just beyond the spiracle:

1. The external mandibular branch consists of two portions, a small anterior nerve extending antero-ventrally to the canals above and below the angle of the mouth, and a larger nerve which passes laterally and suddenly breaks up into a brush of twigs which innervate the hyoidean group of ampullae.

2. The internal mandibular branch arises at about the same level as the external mandibular, but under it, passes inward around the edge of the hyoid cartilage, under the adductor mandibularis muscle, and then forward along the mandibular cartilage.

3. The hyoid branch separates from the hyomandibular trunk at about the same level as the preceding nerves, and then passes, deep in the tissues, around the angle of the jaw to the ventral side where it is distributed to the superficial constrictor muscles. Several nerves pass from the hyomandibular trunk and the hyoid branch to the dorsal superficial constrictors.

4. The palatine branch springs from the base of the hyomandibular trunk inside the hyomandibular canal. It passes outward and forward, dividing into numerous branches which innervate the mucous membranes of the mouth. It can be traced completely later.

One or more small nerves proceeding to the pseudobranch and anterior wall of the spiracle arise near the point of origin of the palatine branch.