Chapter II: Part 2

In general, the branches of origin of the hepatic portal vein follow closely the arteries of the digestive organs. Trace the following parts of the system: A _posterior intestinal vein_, from the rectal gland and rectum, the large intestine and spiral valve, across to the end of the pancreas, along the pancreas to the hepatic portal vein; an _anterior intestinal vein_, from the large intestine and spiral valve, along the duodenal lobe of the pancreas; _gastric_, _duodenal_, and _pyloric veins_ joining the veins already traced; a _splenic vein_ joining the posterior intestinal vein.

The liver, with the bile duct, may now be removed from the body if it is desired to trace the bile duct into the bladder or to trace the hepatic ducts. This can be done best by gently scraping away the soft liver tissue until the bladder and ducts are exposed.

THE NERVOUS SYSTEM.

Only the head and anterior part of the trunk will be required for the dissection of the nervous system. Cut across the body back of the pectoral fins; the posterior part of the body will not be required further unless it is desired to study the muscles and skeleton.

The manner of dissecting the brain depends somewhat upon the specimens at the disposal of the student. If a large head is to be used especially for the dissection of the cranial nerves, only the brain, eye and ear need be studied in the present specimen. But in most cases it will be found best for the student to dissect the first dogfish as thoroughly as possible, working out the cranial nerves as well as the brain, and reserving the second head for a thorough review of the entire nervous system. Chapter III of Herrick and Crosby’s “Laboratory Outline of Neurology” should be used in connection with such a review.

If a line be drawn over the dorsal surface of the head connecting the two spiracles, two small pores will be found near the middle. These are the external apertures of the ducti lymphatici. Cut carefully through the skin in a small circle around the pores, and remove the skin from the remainder of the dorsal surface of the skull without disturbing the small section containing the pores. The latter part should now be lifted gently; beneath it will be seen two delicate tubes passing from the pores to apertures in a depression of the skull below them. These tubes are the _ducti endolymphatici_, through which a passage exists between the internal ear and the exterior. As they cannot be preserved in the subsequent dissection, the pores by which they pass through the skull to the internal ear should be found now, and a memorandum-sketch made of the ducts themselves.

DORSAL SURFACE OF THE BRAIN. The roof of the skull should be removed from over the brain. Use a sharp scalpel and take very thin slices of cartilage. Do not cut beyond the brain at the sides. No attempt should be made at this time to expose more than the dorsal surface of the brain.

Above the anterior end of the brain there is a small median foramen through the skull, the _epiphysial foramen_. A strand of tissue, the _epiphysis_, leading from this to the surface of the brain, should be carefully observed and retained. The cartilage should also be cut away from above the portion of the spinal cord next the skull. Gently wash away any coagulated lymph.

The brain and spinal cord are invested by two membranes (_meninges_). The tough _dura mater_ lines the cavity in which they lie, clinging closely to the cartilage; in fact it forms the perichondrium of the internal surface of the cranium. The _pia mater_ envelops closely the brain and cord, and contains numerous blood vessels. Between the two is the _arachnoid space_, traversed by occasional fine threads of connective tissue and filled with lymph.

As the spinal cord passes forward into the skull it enlarges and merges with the posterior portion of the brain, the _medulla oblongata_ (_myelencephalon_). The roof of the medulla is extremely thin, and is broken if the cartilage has not been removed with extreme care, exposing a cavity within, the _fourth ventricle_.

In front of the medulla, and overlapping its anterior extremity, is a large oval organ, the _cerebellum_ (_metencephalon_). Ventral to the cerebellum, each side of the medulla is expanded in an ear-shaped lobe, the _corpus restiformis_. Anteriorly, the cerebellum overlaps a pair of rounded lobes, the _optic lobes_, which together form the dorsal portion of the _midbrain_ (_mesencephalon_).

In front of the optic lobes are two slightly larger lobes united in their posterior portions but separated anteriorly, the _cerebral lobes_ or _hemispheres_. Together they constitute the _prosencephalon_. (The prosencephalon is not divided in Eugaleus.) Between the mesencephalon and the prosencephalon is a depressed region belonging to the brain-stem, the _diencephalon_ (_thalamencephalon_), from which the epiphysis arises. The roof of the diencephalon also is very thin and is frequently broken during the exposure of the brain. The cavity seen within the diencephalon is the _third ventricle_.

Stalked bodies arising from the antero-lateral angles of the cerebral hemispheres are the _olfactory lobes_. The portion of the brain including the cerebral hemispheres and the olfactory lobes constitutes the _telencephalon_.

DISSECTION OF THE INTERNAL EAR. The structures composing this organ lie in the projecting cartilage at the side of the medulla (auditory capsule). Remove the cartilage of the auditory capsule in thin slices and bit by bit, following the ductus endolymphaticus to the membraneous labyrinth. Dissect away the surrounding cartilage leaving the membraneous canals in place, until the entire labyrinth is exposed. The _membraneous labyrinth_ consists of a large central sac (_utriculo-saccular chamber_) into which the endolymphatic duct opens, and three membraneous tubes (_semicircular canals_) external to the chamber but communicating with it in various ways. Two, one anterior and the other posterior to the sacculus, lie in a nearly vertical plane (_anterior and posterior semicircular canals_); one is external to the sacculus and lies in a nearly horizontal plane (_horizontal or external semicircular canal_). At the ventral ends of the vertical canals are nearly spherical enlargements called _ampullae_. The ampulla of the horizontal canal is at its anterior end. The dorsal ends of the vertical canals open near each other into the upper part of the utriculo-sacculus. The ventral extremity of the anterior vertical canal and the anterior extremity of the horizontal canal open beside each other into an anterior projection of the sacculus. The ventral extremity of the posterior vertical canal opens into the posterior and lower part of the sacculus. The posterior extremity of the horizontal canal opens into the posterior side of the sacculus. During life the utriculo-sacculus and the semicircular canals are filled with a lymphatic fluid, and the sacculus contains a large calcareous ear-stone (_otolith_), which is usually dissolved by the formalin used in preserving the dogfish.

Whitish patches of thickened sensory epithelium may be seen in the ampullae (_cristate acusticae_) and in the utriculo-saccular chamber (_maculae acusticae_). Branches of the eighth nerve can be followed to all these areas.

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.

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_.

The palato-quadrate and Meckelian cartilages are suspended from the hyomandibular by several strong ligaments, the direct attachments of the jaws to the cranium being of soft connective tissue only. Both the hyomandibular and ceratohyal cartilages bear slender rods (_branchial rays_) on their posterior edges, which support the anterior wall of the first gill pouch. Note the position of the spiracle between the mandibular and hyoid arches. The anterior wall of the spiracle is strengthened by two small, flat, vertical cartilages, probably homologous with the branchial rays of the gill arches.

The remaining five visceral arches differ little in their construction. Dorsally, each has a flat, sickle-shaped _pharyngo-branchial_ cartilage attached to the vertebral column by fibrous bands. The pharyngo-branchials of the last two arches are fused. Ventrad to each pharyngo-branchial is an _epibranchial_ cartilage. The next segment of each arch is formed by the _ceratobranchial_ cartilage. All the epibranchials and ceratobranchials except those of the fifth arch bear slender branchial rays. The ventral ends of the ceratobranchials articulate with each other, the first being attached to the ceratohyal by ligament. The second, third, and fourth arches have another more ventral series of cartilages, the _hypobranchials_. The lower ends of the hypobranchials are attached to a large median plate, the _basibranchial_. The fourth ceratobranchial joins the third hypobranchial, while the ceratobranchials of the fifth arch are attached to the basibranchial directly. The basibranchial is composed of two segments closely united by ligament; the anterior one narrow, the posterior broad and flat in front, tapering to a sharp point behind.

Short teeth of cartilage, called gill rakers, project into the pharynx from the inner edges of the arches.

A dorsal and a ventral series of _extra-branchial_ cartilages, thin, slender plates, lie on the external side of each gill arch.

PECTORAL GIRDLE AND FIN. Remove from the body the pectoral girdle, with the fins attached, and carefully scrape off the muscles from the cartilaginous parts. It will be found that the support of the fin is partly of cartilaginous plates and rods, partly of horny fibres (_dermal fin-rays_) which overlie the extremities of the cartilages and extend to the edges of the fin. These fibres are in two layers, one beneath the skin of each side. They are formed in the dermis. A similar arrangement of horny fibres is found in all the other fins.

The pectoral girdle passes across the ventral surface of the body and upward on each side to the level of the vertebral column. The stout ventral bar presents numerous facets for the origin and insertion of muscles. The articular surfaces for the pectoral fins are well up on the sides of the girdle. The slender dorsal end of each side of the girdle consists of a separate bar of cartilage, movably articulated to the lower portion. The ascending limb of the girdle, from the fin articulations to the base of the cartilage just mentioned, is called the _scapular portion_; the small bar is the _supra-scapular_; the ventral bar between the fin articulations is the _coracoid portion_.

The cartilaginous skeleton of the pectoral fin consists primarily of a row of three _basal_ cartilages, all articulating proximally with the girdle. The middle basal is much the largest. Distal to the basals are three rows of rod-like _radial_ cartilages, the proximal row being articulated to the basals.

PELVIC GIRDLE AND FIN. Remove the pelvic girdle from the body with the pelvic fins attached, and clean away the muscles.

The pelvic girdle consists of an almost straight bar of cartilage, slightly thicker at its middle than at its ends, which lies transversely in the ventral wall of the abdomen. To each end is attached a long _basal_ cartilage which lies in the fin, close to and parallel with its inner margin. A proximal series of slender _radial_ cartilages is attached to the lateral side of the basal; a distal series of very short radials lies outside of the first series, while the portion of the fin beyond these is supported by the dermal fin-rays.

FIRST DORSAL FIN. Remove the mass of muscles on both sides of the base of the fin down to the vertebral column. The principal cartilages of the fin lie in the median connective tissue septum which separates the dorsal musculature of the two sides of the body. The basal cartilages of the fin are attached to the vertebral column by means of this septum. It is best to remove the underlying portion of the column with the fin. The cartilages can then be scraped perfectly clean. The skeleton of the fin is composed of three rows of cartilages: (1) a basal row consisting of one very large, flat plate and two or three smaller ones posterior to it; (2) an intermediate row of several plates of nearly equal size; and (3) a distal row of several very small plates. The intermediate and distal rows extend beyond the body musculature into the base of the fin. The remainder of the fin is supported by the dermal rays. In front of the cartilages which have been mentioned is the strong spine of dentine (see p. 5), with its free portion sheathed by an enamel-like covering.

SECOND DORSAL FIN. Remove this from the body in the same manner as the first dorsal. Its structure follows the same general plan, the differences being minor ones of shape, size, and number of plates. Several thin cartilaginous plates are sometimes formed in the median septum in front of the spine.

CAUDAL FIN. Only one side of the caudal fin should be cleaned, as when both sides are cleaned there is danger of breaking the delicate cartilages. The cartilaginous skeleton of the caudal fin consists of a row of slender rods along the dorsal side of the vertebral column, extending to its tip. There are no cartilaginous elements in the fin ventral to the vertebral column. By far the greater part of the caudal fin is supported by the two layers of horny fin-rays only.

MUSCULATURE

Dissect the skin off the head, neck, and body to back of the pectoral fins. Observe first the musculature of the dorsal side of the neck and of the body back of the bases of the pectoral fins, noticing that it is composed of narrow, zigzag bands, called _myomeres_. Where these are fully developed they extend from the mid-dorsal to the mid-ventral line. Note carefully the relation of corresponding myomeres of the two sides, the exact course of a single myomere, and the direction of the muscle fibres in a typical myomere. Observe also that the muscles above the level of the vertebral column form a thick mass, which is frequently referred to as the dorsal musculature; the muscle below this level may be correspondingly referred to as the ventral musculature. As the muscles described below are dissected the mechanical effect of each should be determined.

MUSCULATURE OF THE HEAD AND NECK. On the lateral and ventral surfaces of the neck the primary relations of the myomeres are much modified by the development of numerous special muscles, yet here and there traces of the metameric arrangement still show. Immediately beneath the skin is a thin sheet of muscle covering most of the ventral and lateral surfaces of the throat as far back as the pectoral girdle. On the ventral surface a triangular space is left in front of the pectoral bar; on the sides of the neck the sheet extends back to the last gill-cleft; dorsally, it reaches to the upper extremities of the gill pouches. This is the _constrictor superficialis_ muscle. It is attached to fasciae dorsally and ventrally, and to the extra-branchial cartilages.

The constrictor superficialis consists of six metameric segments. The four posterior ones are distinctly limited by the gill-slits and extra-branchial cartilages. The second is anterior to the first gill slit, the largest of all, with distinct dorsal and ventral portions extending forward above and below the jaws. The first is recognized as consisting of two distinct parts, on the dorsal and ventral surfaces of the head. The dorsal portion is a small curved muscle on the anterior wall of the spiracle, extending from the external surface of the auditory capsule to the inner surface of the lower jaw. It lies close against the levator marillae superioris (see below). On the ventral surface of the throat the posterior constrictor muscles of the two sides are separated by a large triangular area. In front of this the ventral portions of the first and second constrictors meet in a median aponeurosis, from which their fibres extend transversely, those of the first to the mandibular cartilage, those of the second to the hyoidean cartilage. The first constrictor lies ventral or superficially to, and largely covers the second. Reflect the first constrictor from the aponeurosis outward, and demonstrate the two layers of muscle. It will be noted that the second to sixth constrictors consist of united dorsal, lateral and ventral portions, while the first is reduced to widely separated dorsal and ventral parts.

On each side of the head, just outside the angle of the mouth, is a large, thick muscle arising from the lateral surface of the cranium, and inserted upon the outer surface of the mandible, the _adductor mandibularis_.

In front of the small dorsal constrictor superficialis 1, and scarcely separated from it, is the strong _levator maxillae superioris_, which extends from the lateral surface of the auditory capsule to the dorsal edge of the palato-quadrate cartilage.

Eugaleus. The dorsal portion of the _constrictor superficialis 2_ reaches above the spiracle to the postorbital process. Reflecting it, the _adductor hyomandibulae_ is seen behind the spiracle, arising from the upper part of the side of the auditory capsule and inserted on the end of the hyomandibular cartilage. The _levator palpebrae inferioris_ arises under the origin of the levator hyomandibulae, and passes forward and downward between the spiracle and postorbital process, to insert in the posterior end of the lower eyelid. The _depressor palpebrae superioris_ arises from the fascia dorsal to the spiracle, passes mediad to the levator palpebrae inferioris, upward and forward, to insert in the posterior end of the upper eyelid. Remove these muscles. The infra-orbital canal passes mediad to the muscles of the eyelids. The _levator maxillae superioris_ lies between the spiracle and the orbit. Behind it is a small slip of muscle extending from the anterior wall of the spiracle to the lateral surface of the auditory capsule which seems to represent the _constrictor superficialis dorsalis 1_.

A thin sheet of muscle covers the anterior face of each inter-branchial septum. At the surface these pass into the constrictor superficialis, and are evidently portions of the latter muscle, though they are named the _musculi interbranchiales_.

Above the constrictor superficialis, lying on the side of the neck between it and the dorsal musculature, is a broad _trapezius_ muscle. It arises from the fascia covering the lateral surface of the dorsal musculature. Its fibres pass obliquely downward and backward, mediad to the posterior gill pouches, to insert upon the anterior edge of the scapular portion of the pectoral girdle. The anterior portion of the trapezius is also inserted upon the epibranchial of the fifth gill arch.

Just in front of the mouth is a pair of strong muscles (_levator labialis superioris_), each arising from the ventral surface of the cranium close to the median line. They pass into strong tendons which are inserted among the fibres of the ventral portion of the adductor mandibulae. The muscle mass in front of the mouth and the lower part of the adductor mandibularis thus form the two bellies of a digastric muscle, with the tendon between them.

[2]Remove the ventral portions of the first and second superficial constrictors and clear the mass of muscles lying between the coracoid portion of the pectoral girdle and the mandible. Immediately in front of the girdle are two large muscles, the _coraco-arcuales communes_, whose fibres run inward and forward. These muscles cover the ventral surface of the pericardium, to the wall of which their median fibres are attached, while the lateral fibres are attached around the ventral ends of the gill arches.

Footnote 2:

The coraco-mandibularis, coraco-hyodeus, coraco-arcualis communis, and
coraco-branchialis muscles should be dissected as a preliminary
operation to following the ventral aorta and its branches.