Part 8

The otic nerve, passing from the root of the facial nerve to the postorbital canal, is not likely to be found in this dissection.

Observe the enlargement near the base of the hyomandibular trunk, and within the cartilaginous canal, the geniculate ganglion. The gasserian ganglion, a component of the trigeminal nerve, lies in the ventral portion of the trigeminal-facial root, and can now be located.

The auditory nerve arises close behind the ventral division of the trigemino-facial root. The root of the auditory nerve encloses a large auditory ganglion. A vestibular nerve arises from the anterior end of the auditory ganglion and passes into the ear capsule, innervating the upper part of the utriculo-sacculus and the ampullae of the anterior and horizontal canals. From the posterior part of the ganglion nerves pass to the ventral part of the sacculus and the ampulla of the posterior canal. Trace these branches as thoroughly as possible.

The abducens nerve emerges from the cranium under and close to the origin of the posterior rectus muscle, into which muscle it enters. To expose this nerve the trigeminal, facial and auditory nerves must be lifted and cut as they pass through the wall of the cranium. It can be traced obliquely backward and inward, through a long canal, to its origin near the midline of the ventral surface of the medulla.

The glossopharyngeal nerve passes through the base of the ear capsule from the side of the medulla to the upper end of the first branchial pouch. A ganglionic enlargement is found near where it emerges from the cartilage. Outside the cranium the glossopharyngeal divides into a pretrematic branch, passing down in front of the first gill pouch, and a posttrematic branch running behind the pouch. The pretrematic branch quickly sends off a pharyngeal nerve which runs antero-ventrally to the roof of the pharynx. The pretrematic and posttrematic branches can be followed along the gill arch to the ventral side of the pharynx. A fourth branch of the glossopharyngeal, the supratemporal, springs from the dorsal side of the ganglion; passing through the ear capsule it runs to the dorsal surface of the head, where it is distributed to the sense organs of a short section of the lateral line canal. This small nerve can be demonstrated by carefully separating the muscles and perichondrium from the posterior surface of the auditory capsule.

The vagus nerve (or pneumogastric) arises by an extensive series of roots from the side of the medulla. An easily distinguished ribbon-like portion of the root, the lateral line root, runs forward as far as the root of the glossopharyngeal. Note the canal by which the vagus leaves the cranium, and trace the nerve along the inner side of the anterior cardinal vein.

The principal branches of the vagus are:

1. The supratemporal branch, a small nerve running dorsal through the posterior part of the ear capsule to the lateral line canal and other sense organs of the head. It will be found near the supratemporal branch of the glossopharyngeal.

2. The lateral line branch, a large nerve which separates from the trunk of the vagus just outside the cranium and runs backward through the muscles, parallel to the vertebral column on a level with the lateral line. It sends off numerous small twigs to the sense organs of the lateral line canal.

3. Four branchial nerves, which can be seen through the floor of the anterior cardinal vein, leave the outer side of the vagus trunk. Each divides into a pretrematic and posttrematic branch; a pharyngeal branch, the last of which is the largest, arises from each posttrematic.

4. Beyond the branchial nerves the remainder of the vagus passes backward as the intestinal or visceral trunk, to the end of the pharynx, where it divides into a number of branches which are distributed chiefly to the wall of the stomach. Near the point of this last division the vagus is crossed by the hypobranchial nerve, which should be noted and preserved.

The occipital nerve penetrates the lateral wall of the cranium close behind the root of the vagus and enters the canal of the vagus, along which it passes. On emerging, it sends small branches to the nearby muscle, while the principal portion runs on to join the hypobranchial nerve. The occipital nerve will be found to arise from the ventral surface of the medulla, below and behind the root of the vagus, by two or more distinctly separated roots, which may represent distinct nerves.

SPINAL NERVES. The spinal nerves are those nerves which arise from the sides of the spinal cord. They differ from the cranial nerves not only in their origin outside the cranium, but also in that each spinal nerve arises by two roots which spring from the spinal cord near the dorsal and ventral surfaces. Each root passes through a foramen in the cartilaginous wall of the neural canal, the ventral a little anterior to the dorsal, after which they unite to form the spinal nerve. Between the junction of the roots and its foramen the dorsal root contains a mass of ganglion cells, which cause an enlargement known as the dorsal root ganglion. The typical course of a spinal nerve is around the body to the ventral surface, giving off branches to the muscles and skin of its segment. A short distance from the vertebral column the spinal nerves lie just outside the peritoneum, through which many of them can be seen and followed to about the level of the lateral vein. At this point they pass outward into the muscles of the body wall. To dissect any of the spinal nerves make a longitudinal incision along the lateral line and separate the dorsal muscle mass from the lateral muscles for some distance. The dorsal muscles can then be pressed toward the vertebral column and dissected away from the peritoneum. The spinal nerves, lying against the peritoneum, will be exposed and can be followed easily, first to their roots, next ventrally.

The hypobranchial nerve, to which attention was called at the point where it crosses the vagus, is formed by the union of the principal branches of the occipital and first two spinal nerves. The third spinal nerve receives a branch from the second, and itself accompanies the hypobranchial nerve closely without actually becoming a part of it. The union of nerves thus formed is known as a plexus. After crossing the vagus the hypobranchial nerve forks, one division passing medial to, the other lateral to the anterior cardinal vein; both run ventrally, following the last gill arch, and reunite on the lateral wall of the pericardium, forming a trunk which runs forward. At the anterior end of the pericardium this divides into a dorsal and a ventral branch which innervate the surrounding muscles. The hypobranchial nerve innervates the skin of the region immediately in front of the pectoral girdle, and the coraco-arcualis communis, coraco-mandibularis, coracohyoideus, and coraco-branchialis muscles.

The third, fourth, fifth and sixth spinal nerves pass backward and ventrad till they reach the level of the articulation of the pectoral fin with the girdle. Here they join to form a simple brachial plexus, from which arise branches proceeding to the musculature of the dorsal and ventral faces of the fin. The seventh to eleventh spinal nerves pass downward to the level of the fin, and then branch, one portion entering the muscles of the ventral body wall, while the other passes into the depressor muscles of the fin.

The pelvic fin is innervated by eight or nine spinal nerves which pass backward and downward along the medial edge of the septum between the myomeres, entering the dorsal side of the fin along its axis. No plexus is formed.

OLFACTORY ORGAN (NASAL SAC). Dissect away the skin and other tissues around the nostril so as to expose completely the olfactory organ; this will be found to be a dark-colored, nearly spherical mass, of half the diameter of the eye, firmly attached at its base. By cutting away the cartilage dorsal to the nasal sac its base will be exposed, and the olfactory bulb will be shown to be closely adherent to a considerable part of the postero-dorsal surface of the organ. Numerous short nerves can be demonstrated to pass from the olfactory bulb into the olfactory organ; all these nerves together are considered as the first cranial or olfactory nerve. Remove the olfactory organ from the head; divide it by a median longitudinal cut; observe the arrangement and structure of its double series of internal folds (lamellae), and the complete median septum.

VENTRAL SURFACE OF THE BRAIN. Cut the cord in two some distance back of the brain. Cut all cranial nerves just inside the cranium and carefully lift the brain out. Parts of the ventral portion of the brain lie in a recess beneath the mesencephalon and must be disengaged very gently.

Identify and examine the ventral parts of the brain. Note the considerable lateral compression of the mesencephalon. The optic nerves cross beneath the diencephalon, forming the optic chiasma. From the sides of the chiasma slightly elevated optic tracts, formed by the fibres of the optic nerves, can be traced into the optic lobes.

Back of the optic chiasma the projecting ventral portion of the diencephalon forms the hypothalamus. The posterior lobe of this structure is the hypophysis or pituitary body.

The oculo-motor nerves emerge over the posterior end of the hypothalamus.

The ventral portion of the mesencephalon is formed by the cerebral peduncles (crura cerebri), columns of fibres passing between the myelencephalon and telencephalon.

The abducens nerves arise on the ventral surface of the myelencephalon near the midline and just back of a line connecting the roots of the auditory nerves.

The internal carotid arteries reach the brain at the sides of the hypothalamus. Branches are sent upward and forward over the surface of the brain. Anastomoses between the vessels of the opposite sides are formed anterior to the optic chiasma. The main branches of the carotids pass backward along the sides of the hypothalamus and unite behind this organ. The median artery thus formed runs along the ventral surface of the myelencephalon and the spinal cord. Numerous transverse vessels are given off to the myelencephalon.

Identify the roots of the remaining cranial nerves.

CAVITIES OF THE BRAIN. Divide the brain into exactly equal halves by a vertical longitudinal cut.

Each lobe of the prosencephalon contains a large cavity. These are the prosocoels. They are commonly known either as the lateral ventricles, or the left cavity as the first ventricle and the right as the second ventricle. The prosocoels are continued into the olfactory lobes, these portions being known as rhinocoels.

The thalamocoel is the cavity within the diencephalon, often called the third ventricle. The prosocoels communicate with the thalamocoel by lateral openings, the foramina of Monro. The roof of the thalamocoel is very thin and is non-nervous; it is frequently torn during the early dissection. Where the lobes of the prosencephalon meet the dorsal wall of the diencephalon this thin roof is pushed into the prosocoels, carrying with it the pia mater and its blood vessels, and thus forms vascular ingrowths known as the choroid plexi. The thalamocoel continues above into the epiphysis and below into the hypothalamus.