Part 4

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.

The _myelocoel_ is the large cavity of the myelencephalon. It also is frequently apparently open to the exterior at the posterior end by the accidental breaking of the thin, non-nervous dorsal wall of this region. The myelocoel is also known as the fourth ventricle.

The thalamocoel and myelocoel are connected by a narrow passage through the mesencephalon, the _aqueduct of Sylvius_ (_iter, mesocoel_).

The optocoels are large cavities within the optic lobes which open into the aqueduct of Sylvius.

A large metacoel in the metencephalon opens into the myelocoel. The myelocoel is also continued into the corpora restiforma; posteriorly it joins the _central canal_ which extends down the center of the spinal cord.

DISSECTION OF THE EYE. Remove one of the eyes from its orbit, and divide it into inner and outer halves by an equatorial cut around the eyeball (not directly through it, as this tears the lens from its fastenings). Place the halves under water and observe:

In the inner half:

The _posterior chamber_, the cavity of the eyeball which has been opened. During life it is filled by a gelatinous substance, the _vitreous humor_.

The _retina_, a delicate yellowish-white membrane lining the interior of the eye, loosely attached to the outer coats except at the point of entrance of the optic nerve.

The _choroid coat_, a thin, black membrane outside the retina. It can be pulled away from the outer coat quite easily except near the optic nerve.

The _sclerotic coat_, the outer coat of the eye. This is composed of connective tissue having an almost cartilaginous consistency, is only slightly pigmented, and is somewhat translucent. The muscles of the eye are inserted upon the sclerotic.

In the outer half:

The _ora serrata_, an irregular line along which the retina ends.

The _iris_, a fold of the choroid extending inward like a shelf, and perforated centrally to form the pupil. Around the iris the choroid is folded radially into the _ciliary processes_.

The _lens_, a spherical body, transparent and elastic during life, but opaque and hard in preserved specimens. It projects into the pupil and is suspended from the ciliary processes by a delicate membrane, the _suspensory ligament_.

The _anterior chamber_, in front of the iris and lens, filled with a watery fluid, the _aqueous humor_.

The transparent _cornea_, forming the outer side of the eyeball, continuous with the sclerotic.

Take out the other eye and cut it in two by a section through the pupil and optic nerve. Review the relation of the parts.

THE SKELETON

There seems to be no easy way of cleaning the skeleton of dogfish which have been preserved in formalin or alcohol, the only procedure being to cut, pick, and scrape the flesh away from the skeleton. Time and patience are required, but if these are allowed there is no reason why all the parts of the skeleton cannot be thoroughly studied. Specimens which have been preserved in brine are more easily skeletonized.

The skeleton is entirely composed of cartilage which, in large species of elasmobranchs and in old individuals of small species, becomes impregnated with lime salts, in some cases to such an extent as to resemble soft bone.

The parts of the skeleton are frequently grouped under two heads: the _axial skeleton_, comprising the skull and vertebral column; and the _appendicular skeleton_, including the pectoral and pelvic girdles and the skeleton of the fins.

VERTEBRAL COLUMN. The vertebral column is divided into two regions, thoracic and caudal, distinguished by the slightly different character of the vertebrae. Remove the muscle and connective tissue from the vertebral column for a short distance anterior to the first dorsal fin. Care is required not to cut away small cartilages occupying the positions of ribs. Now remove from the body about two inches of the portion of the column exposed with any cartilaginous parts which may be attached to the vertebrae. The vertical column is made up of segments, called _vertebrae_. Each vertebra consists of a large ventral mass, the centrum, and an arch, the neural arch, roofing over the dorsal surface of the centrum; the arch is composed of several small plates of cartilage. The opening enclosed by each centrum and its neutral arch is the vertebral foramen; the joined vertebral foramina form the neural canal, which is occupied by the spinal cord.

Separate one of the vertebrae from the rest. The centrum is deeply concave at each end; such a centrum is termed amphicoelous. At the middle of the centrum the concavities meet and thus a canal is formed through it. This canal and the spaces between the ends of adjoining vertebrae are filled by the remains of the notochord, a rather pulpy structure extending from end to end of the vertebral column.

The concave faces of the vertebrae consist of much firmer cartilage than the remaining portions, sometimes even calcified. Make a transverse section through the middle of a centrum and observe the relations of the parts.

On each side of the centrum, near the ventral edge, is a plate-like projection, the transverse process. Attached to the extremity of this is a slender cartilaginous _rib_.

Each _neural arch_ is made up of two distinguishable sets of plates. The first consists of a pair of broad _neural plates_ extending upward from each side of the centrum and uniting with each other dorsally. Between the neural plates of two successive vertebrae is a pair (one on each side) of _intercalary plates_ which also unite over the neural canal. The intercalary plates are over the joint between the centra. Neural and intercalary plates together make the lateral and dorsal walls of the neural canal. The relations of these plates can sometimes be seen best when the neural arch is cleaned, then cut away from the centrum, and looked through toward the light.

In the lower part of each neural plate is a small foramen which allows the passage of the ventral root of the spinal nerve. A foramen for the dorsal root is found at about the middle of the intercalary plate.

Clean and remove some of the caudal vertebrae from the region just back of the cloaca. In general they have nearly the same structure and relations as the thoracic vertebrae, but have no transverse processes and the plates of the neural arches are not so distinct. There is also added a ventral arch similar in form to the neural arch. This is the _haemal arch_, in which lie the caudal aorta and vein. Its roof is the surface of the centrum, the sides are formed by pairs of plates which correspond in number to the centra, and unite with each other ventrally. Between the successive plates are openings for the passage of branches of the artery and vein.

In this region foramina for the roots of the spinal nerves are found only in every other pair of neural and intercalary plates. Toward the tip of the vertebral column the relation of the neural and intercalary plates to the centra becomes very irregular.

In Eugaleus the roof of the neural arch is formed by a row of small, diamond-shaped plates which fit in between the other two sets. As these plates correspond morphologically to the neural spines of higher vertebrates, they may receive that name here. It is probable that the dorsal portion of the arch in Squalus is composed of similar neural spine elements which have become fused with the neural and intercalary plates of each side.

SKULL. The skull is entirely cartilaginous, and comprises three principal divisions: (1) the _cranium_, an undivided mass of cartilage lodging the brain and the organs of smell, sight, and hearing; (2) the _jaws_; (3) the _visceral arches_, or skeletons of the gill arches.

(1) The cranium. A blunt prolongation of the anterior extremity of the cranium forms the _rostrum_, which supports the soft tissues of the snout. At each side of the base of the rostrum the cranium widens abruptly. On the anterior face of the widened portion and below the posterior angles of the rostrum is a pair of protruding _olfactory capsules_, in which the olfactory sacs are enclosed. An oval aperture in the posterior wall of each capsule opens into the braincase and permits the passage of the olfactory nerve through the cranium.

Back of the olfactory capsules are large lateral cavities, the _orbits_. The dorsal edge of the orbit makes an overhanging ledge, known as the _supra-orbital crest_. The projecting anterior and posterior angles of the orbit are distinguished as the prae- and postorbital processes.

The portions of the cranium back of the orbit and at the sides of the braincase form large lateral projections (_auditory capsules_) containing the organs of hearing.

At the center of the nearly vertical posterior surface of the cranium is a large opening, the _foramen magnum_, through which the spinal cord passes.

At either side of and below the foramen magnum is a smooth articulatory surface (_occipital condyle_) articulating with the centrum of the first vertebra.

The flattened ventral surface of the posterior part of the cranium forms the roof of the mouth, or palate.

In the mid-dorsal line of the cranium, between the prae-orbital processes, is a small aperture opening into the brain cavity, the _epiphysial foramen_. It is closed during life by a tough, fibrous membrane. The stalk of the epiphysis extends to the under surface of this membrane.

Between the auditory capsules is a deep depression in the roof of the cranium in the floor of which can be seen the two small pores through which the ducti endolymphatici pass into the capsules. Close behind them are two larger openings for the perilymph ducts.

A pair of foramina passes through the inner edge of the prae-orbital process; these permit the passage of the ophthalmic branches of the trigeminal and facial nerves to the dorsal surface of the snout. Near the bottom of the inner wall of the orbit is the foramen of the optic nerve. In the postero-ventral angle of the orbit is the large trigemino-facial foramen for the exit of branches of the trigeminal and facial nerves; in front of it is the small oculo-motor foramen. The extremely small foramen of the trochlear nerve is almost directly above the optic foramen, near the top of the inner wall of the orbit. Close below the trigemino-facial foramen is the small passage for the abducens nerve. Below the abducens foramen is the transbasal canal. Behind and below the trigemino-facial foramen are two foramina, through which pass the hyomandibular branches of the facial nerve. The foramen of the vagus nerve is close to the foramen magnum, upon the posterior surface of the cranium. The foramen of the glossopharyngeal nerve is lateral to that of the vagus, near the postero-lateral angle of the cranium.

The cranium of Eugaleus is much like that of Squalus, except that the rostrum is formed by three rods, two dorsal and one ventral, which arise from the front of the braincase and converge anteriorally until they meet and fuse. The olfactory capsules are much larger and of heavier cartilage than in Squalus. The auditory region similarly is more prominent.

(2) The jaws. The jaws in reality are the first pair of visceral or gill arches, and in spite of the modification which has taken place this relation can be seen easily in the adult shark. The upper jaw consists of a pair of _palato-quadrate_ cartilages, united medially by ligament, and bearing the upper series of teeth. A large hooked palatine process extends from each palato-quadrate cartilage upward along the inner wall of the orbit. The lower jaw likewise consists of a pair of _Meckel’s cartilages_, united medially (the union is called the symphysis), and bearing the lower series of teeth. A pair of small labial cartilages, which support the edges of the labial pockets, lie at each corner of the mouth.

(3) Visceral arches. The first of the visceral arches is much larger and heavier than the rest. It is known as the _hyoid arch_. Each side of the arch consists of two rods of cartilage: (1) the _hyomandibular cartilage_, which articulates with a distinct facet on the lateral surface of the auditory capsule, and extends from here downward, outward, and backward; (2) the _ceratohyal_ cartilage, which is movably articulated to the hyomandibular and extends downward, forward and inward. The ventral ends of the ceratohyals are united by a median, plate-like _basihyal_.