In this article we will discuss about the dissection of lata fish. Also learn about:- 1. The Alimentary System 2. The Arterial System 3. Afferent Branchial System 4. Efferent Branchial System 5. Dissection of Cranial Nerves 6. Dissection of Brain.
Lata (Fig. 15.1) is a bony fish. The procedure for dissection is same as in dog fish. It is suggested that the scales of the ventral surface be removed before dissection.
Live fishes are collected from the market.
The specimens may be killed following any of the two methods:
1. Wash the fishes with tap water and put them in a large cylindrical gas jar. Pour sufficient amount of hot water (not boiling) in the jar and the fishes are instantly killed. Quickly remove the specimens and put in cold water. Care should be taken that the fishes are not cooked.
2. Put the fishes washed with tap water in a large, cylindrical glass jar. Pour a measured quantity of chloroform running along the inner wall of the jar and quickly cover the jar with a glass lid. Care should be taken that the fishes do not jump out of the jar. Remove the fishes when their movement stops and wash with tap water.
For dissection follow the procedure adopted for dog fish.
The Alimentary System:
It is a large opening at the anterior end of the head and guarded by two jaws provided with pointed denticles.
A dorsoventrally compressed chamber with a small, immobile tongue on the floor.
A spacious chamber, perforated laterally by five pairs of vertical gill slits. The roof bears a pair of dental plates.
A thick walled tube, slightly narrows down posteriorly to open in the stomach.
The cardiac stomach is a fairly large, somewhat oval sac, slightly narrowed at the free end. The pyloric stomach is a narrow tube, close to the anterior end of the cardiac stomach.
A long, narrow tube, folded upon itself for a short distance near the posterior end.
A pair of blind diverticula, one long and the other short, close to the junction of the pyloric stomach and the intestine.
Slightly wider than the intestine and bulged at the middle. It opens through the anus.
A large, bilobed, brownish mass, the left lobe larger than the right one. The spherical gall bladder is located in the right side, close to the middle of the liver.
The Arterial System:
It has two components, the afferent and efferent branchial systems. The procedure for dissection is same as in dog fish. While removing the dental plates on the roof of the pharynx, care should be taken not to damage the underlying blood vessels.
Afferent Branchial System:
A median, somewhat -triangular chamber with the apex directed forward. The heart is lodged in it.
A muscular structure.
It has three chambers:
a. Sinus venosus:
A thin walled sac, dorsal in position and attached to the postero- dorsal wall of the pericardial cavity. It receives two ducti Cuvier and opens ventrally into the atrium.
b. Atrium (auricle):
A large, thin walled chamber, the lateral sides of which are drawn forward. It is ventral to sinus venosus.
A thick walled, somewhat oval chamber with a slightly narrowed anterior end. It is most ventral in position.
It arises from the anterodorsal end of the ventricle, the swollen base of which forms the bulbus aorta.
The ventral aorta sends laterally four pairs of afferent branchial arteries to four pairs of gills.
Afferent branchial arteries:
They supply deoxygenated blood to the gills for oxygenation. The posterior or the pair nearest to bulbous aorta are the fourth, and the farthest pair are the first afferent branchial arteries. The fourth and the third pair arise from one point in the ventral aorta.
The ventral aorta divides into two at its anterior end:
a. The fourth pair arise from the ventrolateral sides of the ventral aorta. Immediately after its origin it forms a loop around the third afferent branchial artery, becomes dorsal to it and courses posterolaterally to the fourth gill.
b. The third pair are lateral in origin and run to the third pair of gills.
c. The origin and course of the second pair are similar to those of the third pair. They supply the second pair of gills.
d. The first pair are formed by the bifurcation of the ventral aorta and supply the first pair of gills.
Efferent Branchial System:
The efferent branchial arteries (Fig. 15.4) collect oxygenated blood from the gills. The arteries arising from the first and second pairs of gills instead of joining the efferent branchial system supply oxygenated blood directly to the anterior region of the body.
a. Each of the third pair of efferent branchial arteries joins the carotid artery of the side. Anteriorly, the carotid sends branches to the head region. Posteriorly, the two carotids form a transverse vessel, which receives the fourth pair of efferent branchial arteries.
b. A branch on the left side of the transverse vessel receives the left fourth efferent branchial artery and sending a branch, the subclavian artery, it runs backward as dorsal aorta.
c. A branch on the right side of the transverse vessel receives the right fourth efferent branchial artery and sends two branches, subclavian and coeliacomesenteric artery.
Mid-dorsal in position and continues as caudal artery in the tail. In its course it sends a number of paired branches to the organs in the middle and posterior region of the body.
Dissection of Cranial Nerves:
Remove the skin from the dorsal surface of the head to expose the cranium. The cranium or brain box of lata fish is bony. Carefully drill a small hole on the mid-dorsal region of the cranium with the pointed end of a scalpel and clean the sutures of the cranium.
Push the tip of one arm of a pair of stout forceps through the hole in the cranium and break a small piece of bone. Working out in the same way remove the dorsal and lateral walls of the cranium.
Care should be taken not to damage the cranial nerves in the process. Remove the covering sheath or meninges either with an eye forceps or with a brush. The brain and the roots of the cranial nerves are exposed. Trace the nerves along their courses up to their distribution to the organs (Figs. 15.5-15.7).
Fifth (V) and seventh (VII) cranial nerve:
Carefully remove the eye ball from the orbit to trace the nerves and their branches (Figs. 15.5, 15.6, and 15.10).
The fifth or trigeminal is the first nerve of the medulla oblongata. It arises from the ventrolateral side of the medulla, at its anterior end in the form of two laterally compressed, broad bundles of nerves. Four more bundles of nerves arise from the posterior aspect of the trigeminal roots, which are actually roots of the facial nerve.
The three roots of the facial nerve along with two of the trigeminal coalesce to form a complex mass. The mass receives a short connective from the remaining facial nerve bundle and form the trigeminofacial ganglionic complex. Three main nerve trunks arise from the trigeminofacial ganglionic complex—the supra orbital, the infraorbital and the hyomandibular, in addition to ramus palatinus.
a. Supraorbital trunk (V):
The dorsolateral branch with a swollen ganglionic base.
It divides into two branches:
i. Ramus ophthalmicus superficialis facialis, a thick dorsal branch, running straight forward to innervate the snout area.
ii. Ramus ophthalmicus superficialis trigemini, a thin ventral branch, innervating the orbital surface bordering its rim and the skin.
b. Infraorbital trunk (V):
A solitary branch from the ganglionic base and running posterior to orbit divides into three branches:
i. Ramus maxillaris trigemini:
A flattened nerve, moves downward and forward along the posteroventrai part of orbit and innervates the upper lip.
ii. Ramus mandibularis trigemini:
A thick, compressed nerve, turns backward making a sharp angle at the maxillomandibular junction, courses forward to the quadratomandibular joint and innervates the mandibular muscles.
iii. Ramus buccalis facialis:
Emerging from the base of the infraorbital trunk it moves outward and then downward to innervate the posterior section of infraorbital sensory canal.
c. Hyomandibularis trunk (VII):
Leaving the cranium through a foramen of the hyomandibular bone, it divides into:
i. Ramus hyoideus:
It moves postero-ventral and then turns forward to innervate the operculi and hyoideus adductores muscles, preopercular and mandibular sensory canal.
ii. Ramus mandibularis facialis innervates the intra-mandibularis muscle.
d. Ramus palatinus (VII):
A slender nerve, arises from the ventromesial aspect of the trigeminofacial ganglionic complex and innervates the roof of the buccal cavity.
Ninth (IX) cranial nerve:
The ninth or glossopharyngeal, the fifth nerve of the medulla oblongata, arises from the lateral side of the medulla behind the auditory nerve through a single root (Figs. 15.5, 15.7 and 15.8). Turning backward, it creeps upward along the dorsal plane of the supra-branchial organ. The ganglion, petrosal ganglion, is formed over the suprabanchial chamber, from which arise two branches, one anterior and the other lateral.
a. The anterior branch innervates the wall of the supra-branchial chamber.
b. The lateral branch descends along the lateral part of the first gill arch and finally terminates in the ventral branchial muscles.
Tenth (X) cranial nerve:
The tenth or vagus is the last cranial nerve. It arises by two roots from the lateral surface of the medulla oblongata, posterior to the origin of the glossopharyngeal nerve. The roots meet outside the cranium to form vagus ganglion (Figs. 15.5, 15.7 and 15.8). Three groups of nerves, the ramus lateralis, the ramus visceralis and the ramus branchialis arise from the ganglion.
a. Ramus lateralis:
The outermost group of nerves, arise from the lateral part of vagus ganglion. The main trunk is a broad, flat nerve. It enters deep into the body to reach the vertebral column and runs straight up to the caudal end.
i. Ramus supratemporalis:
Arises from the base of the lateralis trunk and innervate temporal and supra-temporal sensory canals.
b. Ramus visceralis:
A single, stout nerve, emerges from the vagus ganglion, moves downward and divides into two branches:
i. The dorsal branch innervates the retractor dorsalis muscle.
ii. The middle branch enters deep into the visceral cavity and bifurcates. The dorsal branch moves straight backward for a long distance along the ventral surface of the kidney and innervates the air bladder.
The ventral branch sends a tributary, the gastric to the stomach and continues as the cardiac branch, which is thin and most ventral. It bifurcates and sends branches to sphincter oesophagi muscle and heart.
c. Ramus branchialis:
A group of four branchial nerves.
i. First branchialis arises independently from the vagus ganglion, moves outward and bifurcates. The thinner pretrematic branch innervates the first gill arch. The stouter posttrematic branch innervates the ventral trough of the second gill arch.
ii. Second branchialis arising independently bifurcates as pre and post trematic branches supplying second and third gill arches respectively. The third and fourth branchialis arise from a common trunk at the base of the visceralis branch.
iii. Third branchialis:
The pre and post trematic branches innervate the third and fourth gill arches respectively.
iv. Fourth branchialis:
At the very onset, it sends a fine pretrematic branch to innervate the fourth gill arch and runs anteriorly along the posteroventral part of the branchial basket as a flat nerve, innervating the adjoining structures.
Dissection of Brain:
Follow the procedure adopted to expose the brain in the dissection of cranial nerves. Fully expose the anterior end of the brain by removing the bones forming the anterior border of the brain box. Proceeding posteriorly expose the junction of the medulla and spinal cord. Cut the roots of all the cranial nerves starting from olfactory to vagus.
Cut the medulla transversaly at its junction with the spinal cord. Carefully lift the brain from the posterior end by holding the cut end of the medulla lightly with a pair of fine forceps. Cut off any attachment between the brain and the cranium. Proceed anteriorly working in the same way till the brain is separated from the cranium (Fig. 15.10). Put it under water in a watch glass.
Dorsal View of the Brain:
Two round structures at the anterior end of the brain. Olfactory nerves arise from the lobes.
A pair of fairly large semicircular bodies, posterior to the olfactory lobes. Each is called cerebral hemisphere. They are united mesially.
A small mass posteromedian to the cerebrum.
Two large, almost round lobes, a little broader posteriorly. They project considerably beyond the lateral borders of the brain.
A large, nearly round median body.
It is stout anteriorly and posteriorly narrows down a little to continue as the spinal cord.
Ventral and Lateral View of The Brain:
(Figs. 15.9 & 15.10)
The olfactory lobes, the cerebrum and the optic lobes are visible from the ventral and lateral side also.
After their origin from the Pituitary body. A median, small and optic lobes the two nerves run forward side by side for some distance and cross each other to follow opposite courses. They are really no components of the brain.
A small downward process from the ventral surface at the junction of cerebrum and optic lobes.
A median, small and oval structure attached to the infundibulum.
Paired, somewhat oval bodies, broader anteriorly, and dorsolateral to saccus vasculosus in position.
A narrow, median structure posterior to the penial body.