Read Ebook: A Guide for the Dissection of the Dogfish (Squalus Acanthias) by Griffin Lawrence Edmonds

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Ebook has 119 lines and 22141 words, and 3 pages

Trace all veins by passing a flexible probe or guarded bristle along them and then opening the vein with the probe as a guide. All smaller vessels emptying into those described should be noted.

In Eugaleus this connecting vein between the lateral vein and the cardinal sinus is wanting, the pectoral vein opening directly into the lateral.

THE EFFERENT BRANCHIAL ARTERIES AND DORSAL AORTA. Commencing at the mouth, cut through the floor of the pharynx close to the left side of the ventral aorta and the heart. The cut should leave the gill arches uninjured, and may be continued into the oesophagus.

Examine the interior of the mouth and pharynx, observing particularly the form and arrangement of the teeth, the spiracular and branchial clefts, the gill rakers, and the character of the mucous coat of the pharynx.

In Eugaleus the dorsal aorta extends forward beyond the union of the first pair of efferent branchials and then divides into small right and left branches which pass forward and outward to unite with the common carotid arteries.

If the dissection is made with care, the branches of the internal carotid can all be found without cutting any important nerves. The branches passing to the eye and brain are best traced to their terminations in connection with the dissection of the nervous system.

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.

In the conus arteriosus observe the rows of three pocket-like valves each around the proximal end , 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.

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.

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.

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.

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

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 . 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.

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.

Trigeminal Facial

superficial superficial ophthalmic ophthalmic

deep ophthalmic buccal

maxillary otic

mandibular hyomandibular

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.

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.

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 principal branches of the vagus are:

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.

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.

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

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.

Identify the roots of the remaining cranial nerves.

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

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

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 . Place the halves under water and observe:

In the inner half:

In the outer half:

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.

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.

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.

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.

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

At either side of and below the foramen magnum is a smooth articulatory surface 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.

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.

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