Part 3

Near the union of the first pair of efferent branchial arteries a small _posterior vertebral artery_ arises from each, and runs anteriorly along the vertebral column.

Near the divisions of the common carotids two _anterior vertebral arteries_ arise from these vessels and pass posteriorly, often anastomosing with the posterior vertebral arteries. These vertebral arteries are vestiges of the former anterior part of the dorsal aorta (compare with Eugaleus, in which the dorsal aorta sends forward two vessels which join the common carotids).

An _oesophageal artery_ springs from the second efferent branchial, and passes back until it enters the wall of the oesophagus. It also gives off nutrient branches to the second, third, and fourth gill pouches. The nutrient artery of the first gill pouch arises directly from the first efferent branchial.

Near the point at which the fourth pair of efferent branchials join the aorta, two small _subclavian arteries_ leave the aorta and pass into the pectoral fins. There is some variation in regard to the point of origin of these vessels; it may be either in front of or behind the junction of the fourth efferent branchials with the aorta.

The _hypobranchial artery_ passes along the ventral ends of the gill pouches. It is either connected with the efferent branchial loops by short branches, or is formed, in part at least, by short vessels connecting these loops. The hypobranchials are important nutrient vessels, supplying the gill pouches and the muscles of the throat and the oesophagus by means of numerous small arteries; from the hypobranchials also arise small _posterior coronary arteries_ which pass to the ventral and posterior walls of the pericardium and the sinus venosus, and larger _anterior coronary arteries_ supplying the ventricle and conus arteriosus. The hypobranchials can frequently he followed along the dorsal side of the pericardium and then outward to junctions with the subclavian arteries.

The _coeliac artery_ (_coeliac axis_) arises from the aorta just back of the subclavians. Passing posteriorly and ventrad close to the right side of the stomach and reaching the gastro-hepatic omentum, it divides into two branches, the _gastro-hepatic_ and _anterior intestinal arteries_. The first gives off a small _hepatic artery_ to the liver and a large _gastric artery_ to the cardiac limb of the stomach. The _anterior intestinal artery_ supplies the pyloric limb of the stomach, the pancreas, duodenum, and right side of the large intestine.

Small _genital arteries_, supplying the reproductive glands, arise from the coeliac near its origin. (In Eugaleus the genital arteries arise from the anterior and posterior mesenteric arteries.)

At about the middle of the abdominal cavity two arteries arise close together from the aorta. The anterior of the two is the _anterior mesenteric artery_; it passes to the left side of the large intestine and its branches anastomose more or less with those of the anterior intestinal artery. The posterior vessel is the _lienogastric_; it goes to the spleen, pancreas, and loop of the stomach.

The _posterior mesenteric artery_ leaves the aorta a little distance back of the lienogastric and passes to the rectal gland, rectum, and cloaca.

Free the kidney from the body wall along its outer edge and turn it up so as to expose its dorsal surface. Observe the numerous _parietal arteries_ (going to the body wall) and _renal arteries_ (to the kidney), which spring from the dorsal aorta. Branches of the parietals also pass into the kidney.

A pair of small _iliac arteries_ pass into the pelvic fins.

_Oviducal arteries_, one or several on each side, arise from the aorta behind the coeliac artery and pass to the oviduct. Their size varies largely with the development and physiological condition of the oviduct.

The aorta is continued in the tail as the _caudal artery_.

DISSECTION OF THE HEART. Remove the heart together with the ventral aorta from the body and fasten it, dorsal side up, under water. Open the sinus venosus with scissors, wash it out, and observe the vertical slit-like opening into the auricle and the two membraneous valves which guard it.

Continue the cut through the _sinu-auricular aperture_ along the median dorsal line of the auricle; observe the thin walls of the auricle and their strengthening by an irregular mesh of muscles, the _musculi pectinati_; the shape and position of the _auriculo-ventricular aperture_; the flaps of the _auriculo-ventricular valve_. Press upon the sides of the ventricle and, if possible, observe the mode of action of the valve.

Cut across the ventricle from the auriculo-ventricular aperture. Carry another incision from this along the dorsal side of the conus arteriosus. Observe the small size of the cavity of the ventricle, the thickness of its walls, and the projecting network of muscles, the _columnae carneae_, some of which are attached to the edges of the auriculo-ventricular valves.

In the conus arteriosus observe the rows of three pocket-like valves each around the proximal end (_semilunar valves_), and a single row of three similar but larger valves at the junction of the conus and ventral aorta. There is some variability in the number of rows of valves in the conus of Squalus; there are always three rows of three valves each in that of Eugaleus.

In the aorta notice the apertures without valves which lead into the afferent branchial vessels.

HEPATIC PORTAL SYSTEM. The _hepatic portal vein_ is the large vein entering the liver alongside the hepatic artery and bile duct. It receives branches from the stomach, pancreas, spleen, intestine, and rectal gland.

At the surface of the liver it divides into two branches, which enter the two lobes of this organ. Within the liver the hepatic portal veins branch until a capillary system is formed from which the blood is collected by the hepatic veins and carried into the sinus venosus.

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.