In this article we will discuss about the dissection of dog fish. Also learn about:- 1. Dissection of Alimentary System 2. Dissection of Arterial System 3. Dissection of Efferent Branchial System 4. Dissection of Cranial Nerves 5. Dissection of Eye Muscles 6. Dissection of Internal Ear.
The dog fish (Fig.14.1) is an elasmobranch. It is marine. The specimens supplied for laboratory work should be well preserved.
Put the dog fish on its back on the dissecting tray. Fix the specimen on the tray keeping the ventral surface up by pushing pins through the fins, and if necessary also through the muscles of the lateral body wall. Pierce the ventral body wall close and anterior to the cloacal aperture with the pointed arm of a pair of stout scissors and make a small cut.
Take out the pointed arm of the scissors, push the blunt arm through the cut and give an incision along the mid-ventral line proceeding anteriorly up to the pectoral girdle. The scissors should be held in a near-horizontal position to avoid damage to the organs of the viscera. Don’t cut the pectoral girdle.
Make two lateral cuts, one on each side, along the posterior border of the girdle up to the lateral body wall. Carefully separate the structures from the pectoral cartilages with a sharp arrow head needle. Continue the mid-ventral incision up to the posterior border of the lower jaw, without damaging the pectoral cartilage. Don’t damage the pharynx.
Starting from the anterior end of the cut give two posterolateral incisions, one on either side, along the lateral borders up to the base of the pectoral fin. Make two lateral cuts of the ventral body wall up to the posterior limit of the coelome.
Alternately turn the two flaps of the ventral body wall outward and pin them down on the tray. The coelom or body cavity is exposed. The visceral organs are lodged in it and held in position with mesentery.
Dissection of Alimentary System:
Expose the alimentary canal by cutting the mesentery as required. Shift it to one side to bring in view the different zones of the gut and the glands attached to it (Fig. 14.2). Pin it down in proper position.
Remove the pericardium with the heart, and other adjacent structures to expose the oesophagus and ventral wall of the pharynx.
It is horseshoe-shaped, ventral in position and fairly posterior to the snout. Both the jaws bear sharp pointed teeth.
A dorsoventrally compressed chamber. A small, immobile tongue is present on the floor.
A large chamber, perforated laterally by five pairs of vertical gill slits.
It is a short, thick walled tube.
It is J-shaped. The cardiac stomach is sac-like and the pyloric stomach, a narrow tube. A blind sac, the caecum is present at the junction of the two.
The pyloric stomach opens in it. It is a thick walled small chamber leading to intestine.
An almost straight tube, divisible into an anterior duodenum and a posterior ileum. A spiral valve is present in the ileum.
Short and opens in the cloaca.
A large, brownish yellow, bilobed gland. A V-shaped gall bladder is embedded in the right lobe. The bile duct opens into the duodenum.
A whitish, laterally compressed gland lying in the loop formed by the stomach and duodenum. The pancreatic duct opens in the latter.
Dissection of Arterial System:
It has two components, the afferent branchial system and the efferent branchial system:
Afferent Branchial System:
A tough membrane, the septum transversum divides the coelom into a small, anterior pericardial chamber and a large abdominal chamber. The pericardium is lodged in the pericardial chamber. Carefully expose the pericardium by removing muscles and membranes adjacent to it.
Proceed anteriorly and remove the skin and muscles covering the ventral aorta, up to the posterior border of the lower jaw. Cut the pericardium to expose the heart. Remove the membrane covering the ventral aorta and blood vessels. Expose the origin of the afferent branchial arteries from the ventral aorta. Trace the vessels till their entrance into the gills (Fig. 14.3).
A median, triangular chamber with the apex directed forward. The heart is lodged in it.
A muscular tube, bent on itself and divided into four compartments by transverse constrictions:
a. Sinus venosus:
A transversely oriented, thin walled, triangular chamber attached to the posterodorsal wall of the pericardial cavity. Laterally it receives two ducti cuvieri (singular, ductus) and opens anteriorly into the atrium.
b. Atrium (auricle):
A large, dorsally placed, thin walled, triangular sac communicated with the ventricle, which is located ventrally.
A thick walled sac opening anteriorly into conus arteriosus.
d. Conus arteriosus:
A muscular tube in front of the ventricle. Anteriorly it continues as ventral aorta.
It runs anteriorly up to the hyoid arch. Laterally it sends five pairs of afferent branchial arteries to the five pairs of gills.
Afferent branchial arteries:
They supply deoxygenated blood to the gills for oxygenation. The posterior or proximal pair are called fifth and the anterior or distal pair are first afferent branchial arteries.
a. The fourth, third and second pairs run to the fourth, third and second gill arches respectively.
b. Anteriorly the ventral aorta divides into two, the innominate, each of which has two branches. The posterior one ends in the first gill arch and the anterior one supplies the gill filaments of the hyoidean hemi-branch.
Dissection of Efferent Branchial System:
The efferent branchial arteries are located in the roof of the buccal cavity and pharynx. Cut the right corner of the mouth. Proceed posteriorly along a line ventral to the level of external gill slits up to the pectoral girdle. Cut the floor of the pharynx transversely behind the level of the heart.
Turn the flap left (your right side) and pin it down. The roof of the buccal cavity and pharynx are exposed. By giving an incision along the left side, following the procedure adopted for the right side, the floor of the pharynx may be separated.
The blood vessels are located dorsal to the mucous membrane lining the roof of the buccal cavity and pharynx. Remove the membrane and adjacent muscles and tissues to expose the arteries (Fig. 14.3). The efferent branchial arteries collect oxygenated blood from the gills.
Efferent branchial arteries:
Nine pairs. On each side two arteries emerging from each of the four gill arches unite to form a collecting or efferent loop. The ninth vessel arising from the anterior border of the fifth branchial cleft ends in the loop of the fourth cleft.
The four epibranchials of each side arise from the four efferent loops. They open in the dorsal aorta.
Mid-dorsal in position. Anteriorly it divides into several branches. Posteriorly it runs up to the tail sending branches to the organs in the trunk and to the tail region. The head region obtains blood supply partly from the dorsal aorta and partly from the first efferent branchial artery. The vessels taking part are buccals, carotids, efferent spiracular and hyoidean epibranchial.
Dissection of Cranial Nerves:
Remove the skin from the dorsal surface of the head to expose the cranium. The cranium or brain box of dog fish is cartilaginous. The dorsal and lateral sides of the cranium can be removed either by scraping it or cutting off in small bits with a sharp scalpel. Care should be taken not to damage the nerves coming out of the cranium.
Following the exposure of the dorsal and lateral sides of the brain remove the tissues or meninges around the brain, and the roots of the cranial nerves will be exposed. Trace the nerves along their course up to their distribution to the organs (Figs. 14.4 & 14.5).
Ninth (IX) cranial nerve:
The root of the ninth or the glossopharyngeal nerve is on the lateral side of the medulla oblongata immediately behind the auditory nerve. It is the fifth nerve of the medulla. It courses dorsal to the first gill cleft and divides into a pre- and post trematic branches running in the anterior and posterior walls of the cleft.
Tenth (X) cranial nerve:
The tenth or vagus is the last cranial nerve. It arises from the side of the medulla, behind the ninth nerve. A ganglion is present at its base.
Immediately after its origin it divides into three branches:
Courses posteroventrally and divides into four branches. Each again has two branches, the pre- and post trematic supplying two walls of the second to fifth gill clefts.
Runs backward and ventral, enters the coelom and sends branches-cardiac to heart and gastric to stomach.
Runs posteriorly sending branches to the lateral line sense organs along its course.
Dissection of Eye Muscles:
Remove the skin around the orbit to expose it. Remove other tissues to expose the muscles attached to the eye-ball. The other ends of the muscles are inserted to the skull. There are six eye muscles which bring about the movement of eye (Fig. 14.6).
Superior oblique muscle:
Attached to anterodorsal surface of the eye-ball.
Inferior oblique muscle:
Ventral to the superior oblique muscle. Attached to the anteroventral surface of the eye-ball.
Attached to the eye-ball, slightly posteroventral to the superior oblique muscle.
Attached to the anterior face of the eye-ball. It is ventral to the superior rectus.
Attached to the anterior surface of the eye-ball.
Attached to the posterior face of the eye-ball.
Dissection of Internal Ear:
The internal ear may be exposed in auditory capsule or taken out. The internal ear or membranous labyrinth is embedded in a mass of cartilage, the auditory capsule, behind the orbit in the skull. Two small apertures, the openings of the aqueductius vestibuli (endo-lymphatic ducts) are present in a mesial depression on the upper surface of the posterior portion of the skull.
Remove the skin of the head and underlying muscles to expose the auditory capsules. The positions of the semicircular canals of the membranous labyrinth in the auditory capsule are identified by three prominent ridges on the dorsal surface of the capsule.
Carefully remove the cartilage over the ridges bit by bit with an arrowhead needle or a pair of forceps. The semicircular canals are exposed. If it is necessary to take out the internal ear, remove the cartilage surrounding the canals in the same way, and it will be free.
Take out the ear and put it under water in a watch glass. The membranous labyrinth consists of a vestibule and three semicircular canals arising from it (Fig. 14.7). The ear is filled with endolymph.
A triangular chamber with a dorsal utricle and a ventral saccule. The lagena is a posteroventral outgrowth of the saccule.
Three in number, two vertical, at right angle to each other, and one horizontal. Each canal bears an ampulla at the lower end.