In this article we will discuss about the dissection of garden lizard. Also learn about:- 1. Dissection of Circulatory System 2. Dissection of Venous System 3. Dissection of Arterial System 4. Dissection of Cranial Nerves 5. Dissection of Brain 6. Dissection of Urinogenital System 7. The Urinary (Excretory) System 8. The Genital System.
The garden lizard (Fig. 17.1) is a common reptile in India. It is an inhabitant of dry, sandy areas with sparse vegetation, though they are also common in other places of India.
Killing:
Killing is done with chloroform.
Dissection:
Expose the viscera following the procedure adopted for toad (Bufo melanostictus).
Dissection of Circulatory System:
The components of the circulatory system are heart, veins, arteries and capillaries. The heart is a part of both the venous and arterial systems.
The Heart:
To expose the heart remove the pericardium. The heart is a reddish brown, broadly conical, muscular structure with a broad anterior base (Fig. 17.2).
Sinus venosus:
A large thin walled, somewhat elongated, anteroposteriorly directed chamber situated dorsal to the right atrium (auricle) and on the right side of the base of the ventricle.
Atria (auricles):
Two thin walled muscular chambers in front of the ventricle. The right atrium receives the sinus venosus and the left atrium the pulmonary vein.
Ventricle:
A strongly muscular, thick walled, somewhat triangular organ posterior to the atria.
Dissection of Venous System:
(Fig. 17.3)
Veins arise from capillaries in the tissues. All veins and the heart constitute the venous system.
Remove the pericardium and displace the heart a little to the left to expose the major part of the sinus venosus, which is partially covered by the right side of the heart. The veins forming the two precavals are identical.
Trace a precaval and the veins taking part in its formation. Expose the veins received by the postcaval, and the veins forming the renal portal and hepatic portal systems. To trace the postcaval vein the right lobe of the liver is to be cut open as the vein remains embedded in it.
A. Precaval vein:
Each precaval receives following veins:
External jugular:
A superficial vein lying on the inner side and formed by a number of small veins from the buccopharynx, tongue, larynx and trachea.
Internal jugular:
A deep seated vein, formed by the union of a number of small veins from the head and mandible.
Subclavian:
It is formed by the joining of four veins.
Brachial from the fore limb.
Subscapular from the scapular region and skin and muscles of the back.
Pectoral from the pectoral region.
Vertebral from the anterior portion of the vertebral column.
B. Postcaval vein or Vena cava:
It is a large vein situated a little to the right of the middle line. The major part of it is embedded in the right lobe of the liver.
It arises in front of the kidneys and receives the following veins:
Renal:
Paired, spring from the kidneys. The two renals are continuous at the posterior extremity where they form a narrow loop.
Genital:
Paired, arise from the gonads— testes or ovaries.
Vertebral:
One or two from the vertebral column.
Hepatic:
A few, from both lobes of the liver.
Oesophageal:
A vein from the oesophagus.
Renal portal system:
Two renal portal veins with their tributaries constitute renal portal system. The portal veins end in the kidneys.
Caudal vein from the tail bifurcates to two renal portal veins in the coelom.
Renal portal veins run forward and enter the kidneys laterally.
Hepatic portal system:
It consists of veins opening in the liver.
Femoral from each hind limb enters the pelvic region and runs forward as pelvic vein.
Internal iliac from the pelvic region joins the pelvic vein at some distance from the posterior end of the kidney.
External iliac a short vein joining the pelvic with the renal portal vein.
Anterior abdominal formed by the union of two pelvic veins.
Hepatic portal formed by the union of veins from the stomach, intestine, rectum and spleen.
The hepatic portal and the anterior abdominal vein join together. The common vein divides into two branches and enter the lobes of the liver.
C. Pulmonary veins:
Small but wide vessels not connected with the major veins. Two veins from two lungs join to form a single vein. It passes over the dorsal side of the heart and opens into the left atrium (Figs. 17.2 & 17.3).
Dissection of Arterial System:
The arteries end in capillaries in the tissues. All the arteries and the heart constitute the arterial system (Fig. 17.4).
To bring the arteries in view some of the veins are to be removed. Remove the pericardium and identify the three aortic trunks arising from the anteroventrolateral border of the ventricle, a little right to the middle. Trace the branches of the vessels along their courses.
A. Pulmonary trunk:
It arises from the right side of the ventricle close to the middle of it. Running forward for a short distance it bends a little to the left and divides into two pulmonary arteries, the right and the left.
Pulmonary artery:
It loops over the heart anterodorsally, proceeds backward to end in the lung of the side.
Tracheal vein:
A vessel bringing deoxygenated blood from the trachea, oesophagus and larynx of the side joins the respective pulmonary artery.
B. Systemic trunks:
The origin of the left systemic trunk is a little anterior and right to that of the pulmonary trunk. The right systemic trunk is slightly mesial and anterior to that of the left one. The systemic trunks loop round the alimentary canal, turn dorsal and run posteromedially. Posteriorly they meet each other to form the dorsal aorta. The left systemic trunk twists around itself at the anterior end of the arch and forms a small loop. The branches of the systemic trunks are not similar.
Right systemic trunk (artery):
It sends following branches.
Coronary artery:
A small vessel, arises close to the origin of the trunk, turns backward and supplies blood to the heart.
Carotids:
The arteries arise close to each other and run by the sides of the oesophagus. They are known as right and left carotids. Near the throat each carotid divides into a pair of branches the external and internal carotid. At the junction of the branches the ductus caroticus joins the carotid artery.
External carotid:
Superficial in location and ends in throat and tongue.
Internal carotid:
Deep seated, supplies the head.
Ductus caroticus:
Arises from a point little behind the anteriormost end of the arch formed by the systemic trunk and runs forward to join the carotid artery.
Oesophageals:
Three to four small vessels posterior to the root of the ductus caroticus, supply the oesophagus.
Vertebral:
Posterior to the oesophageals, and ends in the region of cervical vertebrae through a number of branches.
Subclavian:
A pair, the right and left, arise close to each other and supply the right and left fore limbs.
Left systemic trunk (artery):
It sends only the ductus caroticus and oesophageal arteries, which are similar to those arising from the right systemic trunk.
Dorsal aorta:
It runs posteromedially and sends many branches of which the following are important.
Anterior gastric:
A few small branches supplying the stomach.
Parietals:
Small paired vessels supplying the trunk vertebrae, thoracic ribs and costal muscles.
Posterior mesenteric:
First large vessel of the dorsal aorta, runs backward and supplies the duodenum, caecum and colon.
Lienogastric:
Arises behind the posterior mesenteric and sends branches to the spleen, intestine and liver.
Anterior mesenteric:
Next to lienogastric and supplies small intestine and liver.
Genital:
Small, paired arteries supplying the gonad, testes or ovaries.
Renal:
Small, paired vessels ending in kidneys.
Iliac:
Large, paired arteries in the pelvic region. Each branches into external and internal iliacs and supply the hind limbs.
Caudal:
The dorsal aorta runs as caudal artery in the tail sending a series of small branches.
Dissection of Cranial Nerves:
The cranium or brain box is bony. To expose the brain and roots of the cranial nerves follow the technique adopted in case of Lata fish (Channa punctatus). Trace the nerves from their origin to distribution along their courses (Figs. 17.5 and 17.6). Cranial nerves are twelve pairs.
Carefully remove the eye-ball from the orbit to trace the branches of fifth and seventh cranial nerves.
Fifth (V) cranial nerve:
The fifth or trigeminal is the first nerve of the medulla oblongata, and arises from the ventrolateral side at its anterior end.
Immediately after its origin the nerve divides into three branches:
a. Ophthalmico pro-fundus:
A swelling, the pro-fundus ganglion is present at the base. It proceeds forward along the roof of the orbit, runs through the nasal area and innervates the snout region.
b. Maxillary:
The gasserian ganglion is present at its base. The nerve runs forward along the floor of the orbit and supplies the maxillary region and palate.
c. Mandibular:
It is posterior to the maxillary. It turns downward, runs forward and supplies the lower jaw.
Seventh (VII) cranial nerve:
The seventh or facial is the third nerve of the medulla oblongata. It arises from the side of the medulla behind the fifth nerve. At the base, the geniculate ganglion is present.
The nerve divides into two branches:
a. Palatine:
The slender, anterior branch runs forward along the floor of the orbit and innervates the roof of the mouth cavity.
b. Hyomandibular:
The stout posterior branch proceeds outward and backward round the tympanic region, bifurcates at the angle of the jaws and supply the post-tympanic area and lower jaw.
Eighth (VIII) cranial nerve:
The eighth or auditory is the fourth nerve of medulla oblongata. It arises from the side of the medulla at about its middle region. A large ganglion is present at its base. The nerve is small and sends branches to innervate the inner ear.
Ninth (IX) cranial nerve:
The ninth or glossopharyngeal nerve arises from the lateral side of the medulla oblongata immediately behind the auditory nerve. It is the fifth nerve of the medulla. The nerve runs posterolaterally for a short distance and bears petrosal ganglion. After emergence from the ganglion it divides into two branches.
The short, anterior branch runs forward and joins the hyomandibular branch of the VII or facial nerve. The large posterior branch runs downward, forward and innervates the tongue, pharynx and floor of the mouth cavity.
Tenth (X) cranial nerve:
The tenth or vagus nerve arises posterolaterally from the medulla oblongata. The vagus ganglion is present at its base.
It has four branches which course posteroventrally:
a. Laryngeal innervates the larynx.
b. Pulmonary innervates the lung.
c. Cardiac innervates the heart.
d. Gastric innervates the stomach.
Dissection of Brain:
Take out the brain (Figs. 17.7 & 17.8) following the technique adopted for Channa punctatus. Put it under water in a watch glass.
Dorsal View of the Brain:
Olfactory lobes:
Paired, anterior most outgrowths of the brain. These are small, lobular bodies joined to the cerebral hemispheres with a pair of olfactory peduncle. An olfactory nerve arises from each olfactory lobe.
Cerebral hemispheres:
Two large, nearly semicircular bodies with broad base and united mesially. They together form the cerebrum.
Diencephalon:
A small, laterally compressed, narrow area between the cerebrum and the optic lobes.
Pineal body:
A small, narrow structure springing from the mid-dorsal surface of the diencephalon.
Optic lobes:
Two large, almost round bodies, behind the diencephalon.
Cerebellum:
A small, somewhat pear- shaped body behind the optic lobes.
Medulla oblongata:
It is swollen in front and narrows posteriorly to continue as the spinal cord.
Ventral View of the Brain:
Optic chiasma:
It is really not a component of the brain. The optic nerves arising from the optic lobes cross each other in the posterior region of the cerebrum forming the optic chiasma.
Infundibulum:
It is a mesial, downward process from the ventral surface of the diencephalon behind the optic chiasma.
Pituitary body:
An ovoid body attached to the infundibulum. The olfactory lobes and peduncles, cerebral hemispheres, optic lobes and medulla oblongata are visible from ventral side also.
Dissection of Urinogenital System:
The urinary and genital systems in Calotes are not completely separate and it is customary to describe the two systems together as urinogenital system. The interdependence of the two systems are limited only to the posterior- most region of the genital and renal ducts in male.
The organs are located in the posterior region of the abdomen. Remove the alimentary system, heart and major vessels of the circulatory system except the posterior part of the dorsal aorta and the postcaval vein. The urinogenital system is exposed.
The Urinary (Excretory) System:
(Figs. 17.9 & 17.10)
Kidneys:
Two brownish, elongated, fleshy, slightly tabulated bodies, dorsomedian in position. Anteriorly, they are free and tapering. Posteriorly the two are joined and form a ploughshare-like vertical ridge at the end.
Ureter:
The urinary ducts arise from the ventral surface of the kidneys at some distance from the anterior end, and run backward along their mid-ventral surface. The ureter separates from the kidney before reaching the posterior end of the latter.
In male, the vas deferens of the side joins the ureter to form a short, common duct, the urinogenital passage, which opens into the cloaca on its dorsal surface through a pore on a urinogenital papilla. In females the ureters open directly into the cloaca.
Urinary bladder:
A small, thin walled sac in the pelvic region. It opens into the cloaca on its ventral surface through a narrow neck.
The Genital System:
Male Genital System:
(Fig. 17.9)
Testes:
White, ovoid, paired bodies in front of the kidneys, the right one being a little anterior than the left on. In a mature lizard the testes are large and pinkish in colour.
Vasa deferentia:
Paired, greatly elongated, convoluted tubes. Anteriorly each vas deferens forms a club-shaped epididymis connected with the testis by a number of small and narrow vasa efferentia.
Urinogenital passage:
Posteriorly the vas deferens joins the ureter and the two form a short, common duct, the urinogenital passage opening on the dorsal surface of the cloaca through a pore on a urinogenital papilla.
Female Genital System:
(Fig. 17.10)
Ovaries:
A pair of clusters of round germ cells Of different sizes in front of the kidneys constitute ovaries. They are slung to the dorsal wall of the coelom by mesentery.
Oviducts:
Two large convoluted tubes, lateral to the ovaries. The oviduct ends in a funnel at the anterior end, opening in the coelom in the vicinity of the ovary. Posteriorly, the ducts are wide and less convoluted and open separately into the cloaca on its dorsal surface through pores on genital papillae.