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How does a frog separate blood in its ventricle?

How does a frog separate blood in its ventricle?


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Since a frog has two atria and one ventricle, how does it separate oxygenated blood from deoxygenated blood when both are mixed in the ventricle? Or does a frog simply pump a mixture of both to the heart and the lungs? I checked a few videos and sites for this, but was unable to find an answer.

https://www.youtube.com/watch?v=MhOCzvA4KhM&ab_channel=iWorxSystems%2CInc.

My best guess, based on the above linked simulation, is that it has to do with a difference in densities of oxygenated and deoxygenated blood, but I am not sure.


Animal Circulatory System

Many animals have a closed circulatory system, where the blood is maintained in vessels and pumped by a heart. Some organisms, such as many mollusks, have an open system, where the blood washes over and around tissues. Animals with a closed circulatory system tend to have higher blood pressure. The blood is also able to travel further than in an open system. These animals may have a one-, two-, three-, or four-chambered heart.

Frog Fish Earthworm
Open or closed circulatory system Closed Closed Closed
Number of circuits for circulation Three One One
Chambers in the heart Three Two One
Number of hearts in body One One Five


Nervous System of Frog (With Diagram) | Vertebrates | Chordata | Zoology

(iii) An autonomic nervous system made of two ganglionated sympathetic nerves.

The autonomic nervous system is often regarded as a part of the peripheral nervous system because the two are connected.

Central Nervous System:

A. Brain:

The brain of frog is elongated, bilaterally symmetrical, white coloured structure safely situated in the cranial cavity of the skull. It is surrounded by a thin, pigmented and vascular connective tissue membrane, the piamater, which is closely applied with the brain. Outside this membrane is a tough, fibrous membrane lining the interior of the cranial cavity called duramater. These two membranes are called meninges (singular, menix).

The space between the piameter and duramater is known as subdural which is filled with a kind of shock absorbing watery, clear lymphatic cerebro-spinal fluid. It is also found in the cavities of brain and central canal of spinal cord.

The brain of frog is divisible into three main parts:

(i) Forebrain or Prosencephalon

(ii) Midbrain or Mesencephalon

(iii) Hindbrain or Rhombencephalon.

It is the largest part of the brain consisting of a pair of anteriorly directed olfactory lobes, a pair of cerebral hemispheres, and a diencephalon.

(a) Olfactory Lobes:

The olfactory lobes are anterior small, spherical structures which are fused together in the median plane. Each lobe gives off an olfactory nerve and possesses a small cavity rhinocoel or olfactory autricle.

(b) Cerebral Hemispheres:

The two cerebral hemispheres are long, oval structures separated from olfactory lobes by a slight constriction. They are wider behind and narrower in front. They are separated from each other by a deep median longitudinal fissure. Each cerebral hemisphere has a large lateral ventricle or paracoel which is continuous anteriorly with the rhinocoel. Posteriorly the lateral ventricles communicate with each other and with the ventricle of diencephalon called diocoel by an opening, the interventricular foramen or foramen of Monro.

The nerve cell bodies form masses around the lateral ventricles and lie in layers. Fibres of the olfactory, tactile and optic impulse reach the cerebral hemisphere which may act as correlating centres but the hemispheres are largely olfactory in function.

On ventro-lateral side of each of the cerebral hemispheres there is a thick fibrous tract called the corpus striatum containing a network of white medullated nerve fibres and nerve cells. The corpora striata of two hemispheres are joined by a transversely running tract of fibres called anterior commissure and above which is another commissure (upper line) partly representing the hippocampal commissure of the brain of reptiles and mammals. They have a thick roof called the pallium in which more nerve cells have moved to the periphery.

(c) Diencephalon or thalamencephalon is a short unpaired structure of the forebrain situated behind the cerebral hemispheres. Its lateral walls are thick called optic thalami (singular thalamus) and its thick floor is called the hypothalamus. Its roof is thin and lined with a vascular membrane, the anterior choroid plexus.

Behind it arises a hollow, thin-walled stalk, called the pineal stalk which terminates dorsally at the brow spot. The pineal stalk or epiphysis, which originally was continuous with the brow spot, becomes constricted off from it in early larval life. On the ventral side of diencephalon is the optic chiasma or crossing of the optic nerves which go the eyes. Just behind the optic chiasma is a flattened bilobed infundibular lobe or infundibulum extending posteriorly and divided by a median longitudinal groove.

It is formed of nervous tissue and contains a cavity which is continuous with the III ventricle or diocoel of diencephalon. The hypothalamus cerebri or pituitary body lies behind and partly covered by infundibulum. The hypothalamus is an important centre regulating the whole endocrine system as well as other parts of the brain. It is composed of anterior and posterior parts. The anterior part of the hypophysis has no connection with the brain.

It is well developed consisting of two dorso-lateral large rounded optic lobes. The optic lobes are centres for impulses coming from the eyes. Their cavities are called optocoel or optic ventricles communicating with each other and the fourth ventricle behind through a narrow cavity, the iter or aqueduct of Sylvius.

Below the optic lobes are present two thick longitudinal bands of nerve fibres, called the crura cerebri. These connect diencephalon and medulla. These form the floor of midbrain. Lying transversely between the diencephalon and optic lobes is a band of nerve fibres called posterior commissure.

It consists of the cerebellum and the medulla oblongata:

(a) The cerebellum is a rudimentary narrow transverse solid band lying dorsally immediately behind the optic lobes. Its function is probably to regulate the vestibulo-oculomotor system controlling movements of the eyes,

(b) Medulla oblongata is short and somewhat triangular structure which is simply a widening of the spinal cord.

Its cavity is also triangular called fourth ventricle or metacoel which is joined in front to the iter but posteriorly it is continuous with the central cavity of the spinal cord. Its roof is thin, vascular and thrown into folds called the posterior choroid plexus.

From the sides of medulla arises several pairs of cranial nerves. Its lower surface is divided by a median fissure which is continuous with the ventral fissure of the spinal cord. On the dorsal surface of the medulla oblongata there is a triangular area of reddish brown colour which is called posterior choroid plexus.

B. Spinal Cord:

The medulla oblongata continues behind as spinal cord, lying in the neural canal of the vertebral column. It is short, somewhat flattened dorsoventrally and terminates behind the lumbar swelling in a tapering narrow thread called filum terminale lying in the urostyle.

The filum terminale with the nerve roots on either side is sometimes called cauda equina as it looks like a horse tail. It presents two enlargements during its course, one at the level of the forelimbs where nerves for arms arise and one far behind at the level of hindlimbs where nerves for hindlimbs arise.

The anterior enlargement of the spinal cord is known as brachial or cervical swelling, while the posterior as lumbar or sciatic swelling. It is traversed throughout its length by two longitudinal grooves, one on the dorsal side and the other on the ventral side, known as dorsal and ventral fissure respectively. These two fissures almost completely divide the spinal cord into two symmetrical halves.

Like the brain the spinal cord is also formed of gray matter with ganglion cells and non-medullated nerve cells surrounding the central canal, and white matter with ganglion cells and medullated nerve fibres surrounding the grey matter.

Grey matter is produced at the comers into dorsal and ventral columns or horns, while the white matter is divided by grey matter into four zones or funicules, the dorsal, the ventral and two lateral funicules. It is covered by two protective membranes the duramater lines the neural canal and the vascular, thin and pigmented piamater closely covers the cord.

Functions of Different Parts of the Brain:

Brain is the only centre for the immediate control of all vital activities as it receives impulses from different parts of the body through sensory nerves and issues orders through motor fibres to different parts of the body for appropriate action.

The pallium of cerebral hemispheres controls the activities of the olfactory, tactile and optic organs, whereas the cerebral hemispheres coordinate the activities of the neuro­muscular mechanism of the body, but these are supposed to be the seat of intelligence and voluntary control in higher animals.

ii. Diencephalon region of the brain controls the metabolism of the fats and carbohydrates and also regulates the genital functions.

iii. Optic lobes and optic thalami are supposed to be concerned with the sensation of sight and the control of the movement of the eye muscles.

iv. Cerebellum controls the mechanism of the automatic movements and also brings about coordination in movements of locomotion. It is in correlation with the medulla oblongata and regulates complex muscular movements of the body.

v. Medulla oblongata is an important nerve centre. It has nerve centres of all reflex functions and, thus, regulates particularly those functions of the body which are not directly under the control of the will like heart beating, respiration, swallowing, taste, hearing, sound production and secretions of various digestive juices.

Peripheral Nervous System:

The peripheral nervous system includes the nerves arising from the brain and spinal cord. Those nerves which arise directly from the brain are called cranial nerves, while those arising from the spinal cord are called spinal nerves.

Structure of Nerve:

The nerves are solid structures looking like white threads. Each nerve is composed of several bundles of nerve fibres called the funiculi (singular funiculus) and is covered externally by a sheath of loose connective tissue called the epineurium.

Each funiculus is also being enclosed by a thick covering of dense tissue called the perineurium. In each bundle or funiculus the individual nerve fibre is also covered by a connective tissue sheath called endoneurium which is continuous with the neurilemma of nerve fibres.

Types of Nerves:

A nerve may be afferent or sensory with sensory nerve fibres or efferent or motor with motor nerve fibres or mixed with both sensory as well as motor nerve fibres. Sensory nerves or nerve fibres are those which carry the impulses from the receptors to the central nervous system, whereas the motor nerves or fibres carry the impulses of appropriate order from the central nervous system to the effector organs.

In the body the following four types of fibres are recognised:

These are the fibres or nerves which carry the impulses from receptors such as skin, eyes, nose, body wall, muscles and joints to the central nervous system. The dendrons of these nerve fibres are very long, starting from the receptors pass to dorsal root ganglion in which their cell body lies.

(b) Somatic Motor:

Such fibres or nerves carry impulses from the central nervous system to the effector organs which include mainly the muscles. Their dendrites and cell bodies are always found in grey matter but their long axons pass through the ventral roots to the muscles (effector organ).

(c) Visceral Sensory:

Such fibres or nerves carry sensations from the receptors situated in the wall of alimentary canal to the central nervous system. Their dendrites start from the receptors located in the wall of alimentary canal and pass to the cell bodies situated in the dorsal root ganglia from where the axons pass into the grey matter.

(d) Visceral Motor:

These fibres are able to convey impulses from the central nervous system to the involuntary muscles of the alimentary canal, glands and other visceral organs. Their dendrites and cell bodies are found in the grey matter, while the axons pass out through the ventral root and end in the autonomic ganglia (preganglionic) from where the second neuron starts whose axon extends to the involuntary muscles or glands (post-ganglionic).

Cranial Nerves:

In an adult frog ten brain pairs of cranial nerves are found which emerge from the brain through various foramina of the cranium to supply the different organs of the body. Each cranial nerve originates from the brain by two roots, a dorsal and a ventral root, but these two roots do not unite with each other and, thus, look like separate nerves. These are numbered I to X, some of which are purely motor, some are sensory, while others are mixed.

The cranial nerves with their names are as follows:

It arises from the anterior end of olfactory lobe and innervates the cells of olfactory sac. It is sensory in nature.

Nerve fibres arise from the retina of the eye. The fibres of the two sides generally cross or decussate out the optic chiasma and then enter the optic thalamus of the opposite side, finally terminating in the thalamencephalon. It is also purely sensory.

III. Oculomotor Nerve:

It is small nerve arising from the ventral side of the midbrain (crura-cerebri). It divides into branches which supply the anterior, superior and inferior recti muscles and the inferior oblique muscle of the eye ball. It is exclusively motor.

IV. Trochlear or Pathetic Nerve:

It is also a small nerve arising from the dorsal side of the brain between the optic lobes and cerebellum and going to the superior oblique muscle of the eye ball, it is exclusively motor.

V. Trigeminal Nerve:

It is the largest of the cranial nerves arising from the sides of the anterior end of the medulla oblongata. Before it emerges from the skull it bears Gasserian ganglion.

It divides into three branches:

(a) Ophthalmic superficialis passes along the dorsal border of the orbit and goes to the skin of the snout. It is somatic sensory,

(c) Maxillary arise from a common stem and then separate.

The mandibular goes to the muscles of lower jaw and the maxillary forms the two branches going to the skin of the upper jaw and upper lip. Maxillary is a somatic sensory, whereas the mandibular is visceral motor nerve. Thus, trigeminal is a mixed nerve.

VI. Abducens Nerve:

It arises from the ventral side of the medulla oblongata and enters the orbit and goes to the posterior rectus muscle of the eye ball. It is a motor nerve.

VII. Facial Nerve:

It arises from the antero-lateral side of medulla oblongata close behind the fifth. It is mixed nerve as having both visceral sensory and visceral motor fibres.

It is divided into two branches:

(a) Palatine going to the roof of the buccal cavity,

(b) Hyomandibular going to the tongue and muscles of the lower jaw.

VIII. Auditory Nerve:

It is somatic sensory arising from the medulla oblongata behind the seventh and goes to internal ear.

IX. Glossopharyngeal:

It is mixed nerve arising from the lateral side of medulla and goes to the tongue, hyoid and pharynx. It does not bear pre-frematic and post-trematic branches to the first gill.

X. Vagus or Pneumogastric:

It is mixed nerve arising from the lateral side of medulla and goes as visceral branch to the larynx (laryngeal), oesophagus and stomach as gastric, heart (cardiac) and lungs (pulmonary).

Spinal Nerves:

In Indian frog, Rana tigrina usually 9 pairs of spinal nerves are found which arise from the spinal cord by two roots, a dorsal or sensory root and a ventral or motor root. Both the dorsal and ventral roots unite immediately after coming out of the neural canal through intervertebral foramen. Dorsal root has a ganglion of nerve cells.

The dorsal root entirely of afferent fibres which may be somatic sensory or visceral sensory which carry impulses from different body parts towards the spinal cord. The ventral root consists of somatic motor or visceral motor fibres of which the cell bodies together with dendrons are situated in the ventral part of the grey matter of spinal cord.

These carry responses from the spinal cord to the body tissues. Thus, in the spinal nerves these different kinds of fibres are mixed but as the roots are approached, they are segregated.

The dorsal root ganglia are covered by white soft chalky masses, calcareous bodies or glands of Swammerdam or periganglionic glands. It is probably the reserve supply of calcium to the body. Each nerve immediately after its origin gives off a short dorsal branch, ramus dorsalis to the dorsal skin and muscles of the back, a large ventral branch, ramus ventralis to the ventral skin and muscles of the body and a very short ramus communicans which unite to the nearest sympathetic ganglion.

Ramus dorsalis contains only somatic sensory fibres, whereas the ramus ventralis contains somatic motor fibres, while ramus communicans contains both visceral sensory and visceral motor fibres.

The first spinal nerve (hypoglossal) comes out between the first and second vertebrae and innervates the tongue and several muscles attached to the hyoid. The second spinal nerve is quite large and emerges between the second and third vertebrae. It receives the branches of the first and third spinal nerves to form a brachial plexus, then it proceeds as the brachial nerve to the skin and muscles of the forelimbs. The fourth, fifth and sixth nerves are small going obliquely to the skin and muscles of the abdomen.

The seventh, eighth and ninth spinal nerves pass almost directly backward and anastomose to form the sciatic or lumbo-sacral plexus from which a large sciatic nerve and several small nerves enter the hindlimb. The most important one is sciatic nerve which divides into tibialis and peroneus.

The tibialis gives branches to the gastrocnemius, tibialis posticus and numerous muscles of the plantar surface of the foot. The peroneus supplies the peroneus muscle tibialis. The tenth spinal nerve is not found in Rana tigrina but in other frogs it is present. When it is present it emerges from a foramen in the anterior part of the urostyle and goes to the cloaca and urinary bladder.

The roots of the last four pairs of spinal nerves are elongated forming a bundle of nerves called Cauda equina which lies inside the vertebral column along the filum terminale.

Autonomic Nervous System:

The autonomic nervous system is partly independent and not under voluntary control. Though it is involuntarily controlled by the nerve centres located in the central nervous system, it is also connected to spinal nerves and some cranial nerves. It is simply concerned with the intestinal regulation of the body with the central nervous system together with its spinal and cranial nerves and is concerned with the external regulations.

The autonomic nervous system consists of two delicate longitudinal chains of ganglia, lying one on either side of the backbone and the dorsal aorta from the brain to the end of the urostyle. Each chain at regular intervals has ten small metamerically arranged sympathetic ganglia. Each ganglion is connected with the corresponding spinal nerve by a small branch called ramus communicans.

Corresponding ganglia of both the cords are also connected together by small transverse commissure. Between the first and second ganglia the sympathetic chain of each side divides to form a loop around the subclavian artery which is known as annulus of Vieussens. Each sympathetic chain enters the skull along with the Xth cranial nerve through the vagus foramen and joins the vagus ganglion and then proceeds forward to join the Gasserian ganglion of the fifth cranial nerve.

Posteriorly each sympathetic chain ends by joining with one or more branches of the 9th spinal nerve. Sympathetic ganglia give nerves to the viscera, where they form plexuses, such as a solar plexus near the coeliaco-mesenteric artery and a cardiac plexus on the heart.

Kinds of Automatic Nervous System:

The autonomic nervous system is simply formed of visceral motor and visceral sensory fibres, which can be classified as follows:

The visceral sensory fibres have their cell bodies in the dorsal root ganglia and their dendrites lie in the organs which are not under voluntary control like the heart, blood vessels, different parts of the alimentary canal. Their axons are extended from the dorsal root ganglia to the grey matter of the central nervous system.

The visceral motor fibres have their cell bodies in the spinal cord forming synapses with neurons whose cell bodies are situated in the sympathetic ganglia.

Because of these synapses the visceral motor fibres are classified under two heads:

1. Preganglionic fibres which arise from the grey matter of the spinal cord and pass through spinal nerves and rami communicants and go to the corresponding sympathetic ganglia are medullated.

2. Postganglionic fibres are those whose cell bodies are in sympathetic ganglia and going in organs they supply are non-medullated.

Thus, sympathetic nerves are made of both visceral sensory and visceral motor fibres which on stimulation secrete a chemical substance called sympathy which generally stimulate the organs.

Parasympathetic system includes parasympathetic nerves and ganglia. The fibres of parasym­pathetic nerves come from cranial and spinal nerves, while parasympathetic ganglia are situated in the organs innervated by parasympathetic nerves. Their preganglionic fibres are very long and extend from the central nervous system to a ganglion in or near the organ which they innerverate. Here they are connected to the short postganglionic fibres.

These fibres on stimulation secrete a chemical substance called acetylcholine which has an inhibitory effect on the organs. The parasym­pathetic nerve fibres are included in oculomotor, trigeminal, facial and vagus cranial nerves.

The visceral and some other organs receive fibres both from sympathetic and parasympathetic nerves. These are antagonistic in effect as sympathin of sympathetic fibres brings an action, whereas acetylcholine of parasympathetic fibres retards it.

The autonomic nervous system simply regulates the functions of certain organs which are involuntary. It is not independent because it is intimately bound structurally and functionally with central and peripheral nervous system. Its nerve centres are located in the central nervous system, while its most fibres are parts of the peripheral nervous system.


Circulatory System of Toad (With Diagram) | Zoology

Circulatory or vascular system is the transport system by which materials such as food, oxygen and wastes are carried from one part of the body to other parts. The conveying medium is a liquid called blood. This is forced by a central pumping organ, the heart, through channels called blood vessels.

The large blood vessels carrying blood away from the heart are the arteries. These break up into smaller bran­ches as they pass farther away from the heart.

The smallest bran­ches penetrate into the tissues of the body and break up into very fine, thin-walled channels called capillaries (capillus = hair). The capillaries unite with one another to form a close network surround­ing the tissue cells.

The blood is collected from the other end of the network by another kind of vessels called veins. A vein receives other veins as tri­butaries, and thus increas­ing in size the veins bring the blood back to the heart.

Although the blood circulates through the tissues, yet it is not libe­rated into them because it is conveyed through a closed system of vessels. Only the liquid part of the blood carrying the mate­rial to be transported, passes out through the thin walls of the capillaries into the intercellular spaces.

This fluid is known as the lymph. It is collected by a special kind of vessels called lymphatics which ultimately return the lymph to the veins. The total volume of the blood is restored in this way (Fig. 25).

Composition of Circulatory System of Toad:

Thus the circulatory system is actually made up of two separate systems:

1. The blood vascular system and

The arteries carrying blood away from the heart, and the veins carrying the same towards the heart make up the blood vascular system.

The lymphatics carrying the lymph away from the tissue spaces constitute the lymphatic system. The arteries and veins differ net only in function, but they also differ in structure.

The wall of an artery consists of three layers:

(i) The outermost layer is composed of fibrous tissue and is, therefore, known as the fibrous coat or tunica adventitia

(ii) The middle or muscular coat consists of in­voluntary muscle fibres running round the vessel this is the tunica media

(iii) The innermost layer forms the tunica internal it is composed of elastic fibres and endothelium which make up the internal lining membrane of the artery.

A vein has all the three coats its fibrous coat is thicker and the muscular coat is thinner. Moreover, a vein has no elastic fibre in the tunica interna. Thus a vein has a delicate, thin and non-elastic wall, whereas an artery has a tough, thick and elastic wall.

The lumen of a vein is larger in diameter than the lumen of the corresponding artery. There are valves inside larger veins for regulating and directing the blood streams towards the heart.

1. Blood-Vascular System:

The blood-vascular system has three main components:

(c) The blood vessels, i.e. arteries and

The blood is a reddish opaque liquid flowing through the blood vessels. It consists of a straw-coloured ground substance called blood plasma and solid cells called blood corpuscles. The corpuscles are suspended in the plasma.

The blood plasma is mostly watery. Many substances are found in solution in the blood plasma. These include mineral substances, food, waste products, hormones, gases, etc. It is the chief transporting medium.

The corpuscles are of three kinds:

(a) Red blood cells or erythrocytes (erythros—red),

(b) White blood cells or leucocytes (leucos=white), and

(c) Blood platelets or thrombocytes.

The erythrocytes are oval, biconvex, nucleated cells about 15 to 20 micro in size (one micron = 1/1000 mm.). They carry the colour­ing matter of the blood, an iron-containing protein called haemo­globin.

The haemoglobin has a remarkable affinity for oxygen and is, therefore, useful in the transport of this gas during respiration. There are about 4 to 5 lakhs of erythrocytes in a cubic millimetre of blood which serve for transporting oxygen. The leucocytes are less numerous than the erythrocytes, numbering about 4 to 5 thousands in a cu. mm. of blood.

They are small, colour-less nucleated cells capable of changing their shape like an amoeba. They may creep out through the thin-walled capillaries to engulf and remove foreign bodies from the tissues. Thus the leucocytes are the scavengers of the body.

They destroy bacteria and protect the toad against invading mic­robes. Moreover, they can ingest fat globules from the small intestine and carry them into the circulating blood.

Leucocytes, therefore, are scavengers, soldiers and porters of the body. The thrombocytes are small spindle-shaped nucleated cells which are often designated as the blood platelets. They break down when the blood is shed, and release an enzyme which helps in the clotting or coagulation of blood.

Further bleeding is thus prevented when a blood vessels is injured. In the adult toad, the blood corpuscles are mainly manufactured by the bone-marrow.

The heart is the central pumping station for the circu­lation of blood. It is a hollow, pear-shaped, muscular organ which is situated in the anterior part of the body cavity, in front of the liver. It is completely enclosed in a transparent bag of membrane, the pericardium. The broad base of the heart is directed forwards, whereas the narrow apex points towards the posterior end and lies between the two main lobes of the liver.

The heart is mainly com­posed of three chambers: a thick-walled, conical ventricle and two thin-walled auricles, right and left. There are two other smaller chambers: a thin-walled triangular sinus venosus on the dorsal side, opening into the right auricle, and a thick-walled tubular conus arteriosus ventrally, connected to the base of the ventricle.

The sinus venosus is a thin-walled sac-like chamber which is situated on the dorsal side of the heart. It is more or less trian­gular in shape with the three caval veins opening into its three corners. These veins carry deoxygenated blood into the sinus.

It communicates with the right auricle through a slit-like sinuauricular aperture, the edges of which are guarded by the sinuauricular valves. The valves permit entry of blood from the sinus into the right auricle, but no backflow or regurgitation is allowed.

The two auricles, right and left, form the base of the heart. They are separated from the ventricle by a narrow groove, the coronary sulcus. The auricles are completely separated from one another by a strong vertical partition, the inter auricular septum. The right auricle is larger than the left it receives deoxygenated blood from the sinus venosus through the sinuauricular aperture.

The left auricle receives oxygenated blood from the lungs through a small opening, the aperture of the common pulmonary vein. The two auricles communicate with the ventricle by a common opening, the auriculo-ventricular aperture, which is guarded by the auriculo- ventricular valves.

The membranous cusps of this valve hang down like curtains to allow the passage of blood from the auricles into the ventricle, but not in a reverse direction. The free edges of the cusps are attached to the ventricular wall by means of a number of fine threads called chordae tendineae.

The ventricle is the thick-walled conical chamber which is situated behind the auricles. Its posterior bluntly pointed portion forms the apex of the heart. The ventricular cavity is greatly reduced by a number of interlacing muscle fibres which arise from its own wall.

The ventricle, therefore, is spongy and when it is cut, its interior looks like a honeycomb. The sponginess of the ventricle prevents admixture of the two kinds of blood in the ventricular cavity, the oxygenated kind from the left auricle and the deoxygenated kind from the right auricle.

The conus arteriosus is the stout tube which arises ventrally from the base of the ventricle and passes obliquely towards the left. It is continued forwards as the truncus arteriosus which is the base of the main artery for carrying the blood away. The truncus however is not a part of the heart it belongs to the arterial system. The conus is separated from the truncus by a set of pocket-like semilunar valves.

A similar set of three semilunar valves guards the opening between the ventricle and the conus. These valves prevent backflow into the ventricle. Inside the conus is a twisted, longitu­dinal flap, the spiral valve. This divides the cavity of the conus into a right channel, the cavum aorticum, and a left channel, the cavum pulmocutaneum.

The spiral valve, the semilunar valves and the spongy ventricular cavity co-operate with one another in guiding the two kinds of blood. The deoxygenated blood is pumped through the cavum pulmocutaneum and the oxygenated kind through the cavum aorticum. The two kinds of blood enter different arterial arches and are carried to different places.

The arteries and their branches form the arterial system. The truncus arteriosus is the main artery origi­nating from the conus. It at once divides into right and left bran­ches.

Each of these trunks splits into three arterial arches:

(ii) The systemic in the middle, and

(iii) The pulmocutaneous posteriorly.

Thus, there are three pairs of arterial arches for supplying blood to different parts of the body. The carotids supply the head region, the systemics supply the trunk and limbs, and the two pulmocutaneous supply the lungs and skin.

(i) Carotid Arches and their Branches:

The carotid arch, on each side, proceeds outward and forward. It soon bifurcates into an inner branch called external carotid artery and an outer branch, the internal carotid artery. Immediately near the point of bi­furcation is a small swelling called carotid labyrinth. Inside the labyrinth, the carotid arch breaks up into numerous minute vessels which reunite at the other end.

The carotid labyrinth offers an extra resistance to the passage of blood through the carotid arch. The external carotid artery supplies blood to the floor of the buccal cavity, tongue and outer side of the head.

The internal carotid artery turns backward and comes very close to the systemic arch of the same side, to which it is tied by a small amount of fibrous tissue, the carotid ligament. Finally, it turns forward to enter the skull through a foramen and is distributed to the brain and its coverings.

(ii) Systemic Arches and their Branches:

The systemic arch sweeps outward. It then curves round the oesophagus to reach the dorsal side, where it joins with its fellow of the opposite side to form a median artery called dorsal aorta. Thus, the two systemics form an arterial ring round the oesophagus.

Each syste­mic gives out the following branches:

(i) A short laryngeal artery to supply the voice-box

(ii) An occipito-vertebral artery which breaks up into oraches for supplying the pharynx, the back of the head, the vertebral column and the spinal cord

(iii) A stout sub­clavian artery which proceeds outward to supply the shoulder and forelimb of the same side

(iv) the left systemic gives off an addi­tional twig, the oesophageal artery, to the oesophagus. The right systemic has no oesophageal branch.

The dorsal aorta is formed in the mid-dorsal line by the union of the right and left systemic arches. It extends backward, lying in front of the vertebral column, and terminates at the posterior end of the body cavity by dividing into two iliac arteries. A stout coeliacomesenteric artery is given out from the commencement of the dorsal aorta.

This breaks up into a coeliac branch for sup­plying the stomach, pancreas, liver and gall-bladder, and a mesenteric branch for supplying the intestines, cloaca, spleen and mesentery. In passing through the space between the kidneys, the dorsal aorta gives off four to five pairs of renal arteries for supplying the urinogenital organs.

The iliac arteries are the terminal branches of the dorsal aorta. Each iliac gives off art epigastricovesical branch to supply the urinary bladder and the ventral body-wall. After this the iliac artery enters the hind limb of the same side where it divides into femoral and sciatic branches.

(iii) Pulmocutaneous Arches and their Branches:

Two pulmocutaneous arches carry deoxygenated blood to the lungs and skin for aeration. They are the hindermost and the shortest of the arterial arches. Each arch passes outward and then backward. A very slender branch is given out to supply the skin. This is the cutaneous artery. The main trunk then runs into and supplies the lung of the same side as the pulmonary artery.

The veins and their tributaries constitute the venous system. The arteries break up into smaller and smaller branches, and these branches merge into a network of thin-walled, hair-like vessels, called capillaries. The capillaries merge into small veins, which join to form larger veins.

The venous system of toad may be subdivided into three sepa­rate groups:

Two pulmonary veins carry oxygena­ted blood from right and left lungs. The right and left pulmonary veins unite to form a common pulmonary vein, which opens into the left auricle on the dorsal side.

These are represented by the three large veins or venae cavae draining into the corners of the triangular sinus venosus. They carry deoxygenated blood from all parts of the body except lungs. Two of the venae cavae are situated ante­riorly these are the right and left precavals. A single postcaval is found posteriorly, opening into the apex of the sinus venosus.

Each precaval vein is formed by the union of three tribu­taries:

(a) The external jugular vein is formed by the union of two tributaries—a lingual vein bringing blood from the tongue, and a faciomandibular vein from the jaws and snout.

(b) The innominate vein is also formed by the union of two tributaries—an internal jugular from the interior of the skull, and a subscapular from the back of the shoulder.

(c) The subclavian vein is similarly formed by the union of two veins—a brachial from the forelimb and a musculocuta­neous from the skin and muscles. In toad, the skin is an accessory respiratory organ. Hence the blood carried by the musculocuta­neous vein is comparatively rich in oxygen.

The postcaval vein has its roots in the kidneys. It is formed by the union of four to five pairs of renal veins which collect blood from the kidneys. The genital veins from the reproductive organs drain into some of the renal veins. The postcaval runs forward to enter the liver substance, and receives a pair of hepatic veins one from each lobe of the liver. It finally terminates by joining the posterior end of the sinus venosus.

A portal vein begins in capillaries and ends in capillaries before the blood which travels through it is re­turned to the heart.

There are two portal systems in the toad:

(A) Hepatic portal system, and

In the hepatic portal system, venous blood collected from capillaries at the poste­rior part and from the gut filters through capillaries in the liver on its way back to the heart. In the renal portal system, venous blood collected from capillaries in the posterior part of the body passes through capillaries in the kidneys on its way back to the heart.

(A) Hepatic Portal System.

In this, the two principal veins are the hepatic portal vein and the anterior abdominal vein. The hepatic portal vein is formed by the union of several small veins returning blood from the stomach, intestines, pancreas, spleen, etc.

The main trunk of the hepatic portal comes to the under surface of the liver, where it receives the anterior abdominal vein. Thus enlarged, it enters the liver substance and breaks up into hair-like sinusoids which finally drain into the hepatic veins.

The anterior abdominal Vein is formed in the following manner. Blood from the hind limb is returned by two large veins, the femoral and the sciatic. On entering the body cavity, the femoral vein gives off a median branch called pelvic vein.

The right and left pelvic veins unite in the mid-ventral line to form the anterior abdominal vein, which runs forward along the middle line under cover of the ventral body-wall. It receives several small tri­butaries from the ventral body-wall and from the urinary bladder.

At the level of the liver, it turns inward to join the hepatic portal vein. The conjoined trunk then divides into vessels which enter the lobes of the liver and break up into sinusoids. The hepatic veins originate at the other end of these networks and thus the blood is finally drained into the postcaval.

(B) Renal Portal System:

Blood is returned from the hind limbs by femoral and sciatic veins. The femoral vein comes from the front of the thigh. On entering the body cavity, it gives off the median pelvic branch which joins with its fellow of the oppo­site side to form the anterior abdominal vein. The main trunk of the femoral now receives the sciatic vein from the back of the thigh and proceeds towards the kidney as the renal portal vein.

At the outer margin of the kidney, the renal portal is joined by two or three dorsolumbar veins bringing blood from the body-wall. It now enters the kidney and breaks up into sinusoids which ultimately drain into the renal veins, thus returning the blood into the post­caval.

It is to be noted that the postcaval vein of the toad is the product of the renal portal and hepatic portal system, and the anterior abdominal vein is join­ing the two portal systems.

The heart continues to beat, taking short rest between successive contractions, and drives the blood into the arteries. The heart muscles have an innate tendency to contract and relax with a definite rhythm. Each period of contrac­tion or systole is followed spontaneously by a shorter period of relaxation or diastole.

The contraction starts at the sinus venosus, spreads to the auricles, then to the ventricle, and finally to the conus arteriosus. The heart of a freshly killed toad beats rhythmi­cally for a long time even when it is taken out.

During diastolic period, the sinus venosus receives deoxygenated blood from two precavals and the single postcaval vein. The left auricle, at the same time, is filled with oxygenated blood through the common pulmonary vein.

With the commencement of the systole, the sinus contracts and the deoxygenated blood is pumped into the right auricle, through the sinuauricular aperture. This is followed by the auricular systole. The two auricles contract simultaneously driving their contents into the single ventricle, through the auriculo-ventricular aperture.

Regurgitation, of deoxygenated blood into the sinus venosus is prevented by the closure of the sinuauricular valves.

Thus both kinds of blood enter the ventricle at the same time. As the ventri­cular cavity is spongy, there is not much mixing. During the very short stay in the ventricle the deoxygenated blood is stored on the right side, the oxygenated blood on the left side, and there is a mixed column of blood in the middle of the ventricular cavity.

The ventricle now contracts. Blood cannot flow back into the auricles because the auriculo-ventricular aperture is tightly closed by the auriculo-ventricular valves. Since the conus arises from the right side, the first blood to enter the conus will be the deoxygenated kind in the right side of the ventricle.

This is now guided by the spiral valve into the cavum pulmocutaneum and from there, through the pulmocutaneous arches, to the lungs and skin. Moreover, the pulmocutaneous arches are the shortest of the arterial arches.

They, therefore, offer least resistance to the entry of deoxygenated blood into them at the very beginning of the ventricular systole. The deoxygenated blood is aerated in the lungs and brought back to the left auricle through the pulmonary veins—thus completing the ‘pulmonary circuit’.

The portion of blood, which becomes oxygenated in the skin, is returned to the sinus venosus through the musculocutaneous vein. So this por­tion of oxygenated blood is mixed up with the deoxygenated blood stream before it is returned back to the heart.

As the force of the ventricular contraction is increased, the mixed blood in the middle of the ventricular cavity is pumped through the cavum aorticum into the systemic arches. This mixed blood cannot enter the carotid arches because of the high resist­ance offered by the carotid labyrinths. Thus the mixed blood is distributed to the trunk and limbs.

Lastly, the pressure exerted by the carotid labyrinths is over­come and the oxygenated blood in the left side of the ventricle is now forced into the carotid arches, through the cavum aorticum.

It is distributed to the head region of the animal. It is to be clearly understood that the spiral valve directs the various courses of the three kinds of blood streams. During ventricular diastole, the blood cannot flow back from the arterial arches because of the pocket-like semilunar valves at the root of the conus.

The systemic and carotid arches take the blood away to every part of the body, except lungs and skin. Food and oxygen are carried to the tissues in this way. Exchange of materials occurs by osmosis through the thin wall of the capillary networks.

The blood is finally brought back to the heart by the three caval veins—thus completing the ‘systemic circuit’. While returning, the blood coll­ects carbon dioxide and nitrogenous waste products from the tissues.

Further, the part of the blood stream which is returning from the alimentary canal becomes loaded with absorbed food. The carbon dioxide is mixed up with the general stream of sys­temic circuit and eliminated by the lungs and skin.

Blood rich in nitrogenous waste products is directed through the renal portal system into the kidneys for exertion. Lastly, the blood rich in food materials is diverted to the liver via the hepatic portal system. The liver cells are thus provided with a better chance of extracting food and storing the same for future use.

Vandervael and Foxon have experimentally demonstrated that there ii no separation of blood streams in the frog’s heart and the blood flowing out through the main arterial arches is nearly of the same composition. They in­jected a radio-opaque substance called thorotrast into the sinus venosus of a frog.

Subsequently a series of X-ray photographs were taken, which revealed that the opaque substance was uniformly distributed in all the three arterial arches. Similar results were obtained by injecting thorotrast through the common pulmonary vein.

It appears that there is no separation of the oxygenated and deoxygenated blood streams in the heart of frogs and toads. As aeration of blood occurs through the skin and mouth, the blood in the sinus venosus cannot be completely deoxygenated, because the cutaneous veins open into it.

Although a separa­tion of the oxygenated and deoxygenated streams would be far better and more efficient, toads and frogs can somehow manage without it.

2. Lymphatic System:

The blood is not directly poured into the tissues. While circulating through capillary networks, the blood plasma exudes into the inter­cellular spaces, coming into direct contact with the cells. This fluid is known as the lymph. It is a colourless liquid containing a few leucocytes, but no erythrocyte. The lymph is collected from the intercellular spaces by small vessels called lymphatics.

These drain into lymph sacs. Some of the lymph vessels are contractile and are known as lymph-hearts. The lymph-hearts pump the lymph back into the veins. There are a pair of lymph-hearts near the urostyle, opening into the femoral veins another pair, below the scapulae, drain into the subscapular veins.

The lymph transports food and oxygen from the blood to the cells of the body. Waste products are carried by the lymph into the blood stream.


Understanding Basic Rules of Circulation

There are some basic rules of circulation that apply to all living creatures. Understanding this will make it much easier to understand how the frog’s circulation is efficient in its makeup.

An efficient circulatory system has the following:

  • First, there must be a source of fluid to carry the nutrients, which in this case, would be blood.
  • Next, you need a system of vessels to carry the blood to its destinations.
  • Most importantly, a pump is needed to pump the blood through the system.

There must be exchange systems, or organs that serve as bridges between nutrients or pollution from the outside to the inside of the system. In this case, that would include the lungs and intestines, which add nutrients to the blood, and again the lungs and also the kidneys to help remove materials from the blood.

The most important part of a circulatory system besides the heart would be the lungs. This organ serves as a gas exchange center. It fulfills the demand for the transport of oxygen and carbon dioxide to and from a gas exchange organ.

All exchanges between the cells and the blood occur in the capillaries. This is always the case in a circulatory system. The force of the pump, or the heart in this case, pushes blood through the arteries. The blood dissipates as it flows through the capillaries. Because the capillaries are spread out all over the body, they end up being bigger than an artery in comparison. Although tiny in size, the individual area the capillaries cover is greater than the artery that supplies them the blood.


Frog Science Lesson

Frog Life Cycle

The lifecycle of a frog begins with a fertilized egg. The female frog usually lays eggs in water in a string or mass that sticks to vegetation. The male frog fertilizes the eggs as they are laid. The outer layer of a fertilized egg is a jelly-like material that swells in water, forming a protective coating. The fertilized egg is a single cell that rapidly divides, again and again, producing new cells that quickly differentiate into the organs of the frog embryo. Within 2 to 25 days, depending on water temperature, the egg hatches into a tadpole. The tadpole looks more like a fish at first than like a frog. As the tadpole develops, it forms gills that allow it to breathe efficiently underwater. Its tail grows longer and a fin forms, which allows the tadpole to swim effectively.

The tadpole continues to swim, eat and grow for several weeks before it matures to the next stage. The first sign of further development is the appearance of hind legs. Then front legs develop and the tail becomes shorter as it is resorbed. Internally, the tadpole’s gills are replaced with lungs until finally, the tadpole has become a frog. The young frog grows and matures to adulthood over a period of 2-4 years. The adult frogs then lay their eggs and begin the cycle again.

Anatomy of Adult Frogs

Some frogs are able to leap 20 times their body length! Their front legs are short and specially designed to absorb the impact of landing. Their muscular back legs also work well for swimming. Aquatic frogs have webbed rear feet, usually with five toes. Their front feet are not webbed and usually have four toes. Tree frogs have suction cups on their toes that allow them to cling to the bark of trees.

Frogs have large, bulging eyes that rotate in their socket, providing sight in almost any direction. Their nostrils are located on the top of the head to allow breathing while most of the head is submerged. Although frogs have a good sense of hearing, they don’t have typical external ears. Instead, frogs have a tympanic membrane behind each eye. These pick up sound waves and carry them into their internal ears. Frogs’ tongues are usually long and sticky and designed to be flicked out quickly to catch insects and other prey.

Frogs have skin that is specially designed to protect them from their enemies and to protect them from drying out. To hide from their enemies, frogs have camouflage skin colorings that help them to blend in with their surroundings. Special pigment cells in their skin control the camouflage pattern and colors. Also, some frogs have serous glands in their skin, which secrete a poison that will irritate the mouth of their predators. South American tree frogs secrete a deadly poison, but most are just irritating to humans. To help them keep from drying out, frogs have mucous glands that secrete a waterproof coating to keep their skin moist and slippery.

Have you ever wondered how frogs breathe? When underwater, frogs get their oxygen from water that passes through their skin. Capillaries take the oxygen from the skin into the bloodstream. On land, frogs usually get oxygen by taking air through their throats into saclike lungs. Frog hearts have three chambers.

To find out more about frogs, do research on one of these topics: what kinds of frogs live in your area? Can you find more than one species of tadpole locally? If so, compare them. What do local frogs eat? How would the mosquito population be affected if there were few or no frogs in a swampy region? Pick a frog or frog characteristic that is interesting to you, and see what you can find out about it. Look for close-up frog pictures in a magazine like National Geographic or on a website.


Frog – Definition, Facts

Frogs are a type of amphibians. Hence, they are a kind of primitive vertebrates. Frogs live in both land and water. They lay their eggs in moist environments. The larval stage of frogs lives in water. They do not have legs and their respiration occurs through gills. The adult stage moves onto the land by developing legs. Since frogs have a larval stage, which is morphologically different from the adult stage, they undergo complete metamorphosis. Since frogs are one of the first primitive vertebrates to migrate to the land, their skin is thin, soft, hairless, and porous. It may contain both mucous and poison glands. The aquatic, larval stage breathes through gills while the terrestrial, adult stage breathes through the lungs. However, a part of the breathing occurs through the skin. Furthermore, frogs are cold-blooded animals and depend on sunlight for the regulation of body temperature. A frog is shown in figure 1.

Figure 1: A Frog


The Circulatory System

The frog heart is the only organ contained within the coelom which has its own protective covering. This is the pericardium. There are two upper chambers of the heart, the right atrium and the left atrium. The frog heart, however, has only one lower chamber, a single ventricle. In humans, the lower heart chamber is divided into two compartments, the right ventricle and the left ventricle.

Oxygen-laden blood and oxygen-poor blood containing waste gases are present together in the frog ventricle at all times. The oxygen-laden and oxygen-poor bloods, however, do not mix. Such mixing is prevented by a unique arrangement of the frog’s heart. Instead of “perching” on top of the ventricle, the right atrium dips downward into the ventricle. This causes oxygen-poor blood entering the right atrium to pass all the way down to the bottom of the ventricle.

Meanwhile, oxygen-laden blood is received by the left atrium and enters the same single ventricle. The pool of oxygen-poor blood at the bottom of the ventricle holds up the oxygen-laden blood and prevents it from sinking to the bottom. When the oxygen-poor blood flows from the ventricle into vessels leading to the lungs, the oxygen-laden blood tries to “follow” it. The lung vessels, however, are filled with oxygen-poor blood, blocking the oxygen-laden blood and forcing oxygen-laden blood to detour into the arteries. These carry the oxygen-laden blood to the tissues.

Frog blood has both a solid and a liquid portion. The liquid plasma carries solid elements such as red blood cells and white blood cells.


Post Lab Questions

1. The membrane holds the coils of the small intestine together: _________________________
2.This organ is found under the liver, it stores bile: ___________________________
3. Name the 3 lobes of the liver: _____________________, ____________________, ____________________
4. The organ that is the first major site of chemical digestion: _______________________
5. Eggs, sperm, urine and wastes all empty into this structure: __________________________
6. The small intestine leads to the: _______________________________
7. The esophagus leads to the: ______________________________
8. Yellowish structures that serve as an energy reserve: _________________________
9. The first part of the small intestine(straight part): ____________________________
10. After food passes through the stomach it enters the: _________________________
11. A spiderweb like membrane that covers the organs: ___________________________
12. Regulates the exit of partially digested food from the stomach: _____________________
13. The digestive system ends at the opening called the:________________________
14. Organ found within the mesentery that stores blood: __________________________
15. The largest organ in the body cavity: __________________________


Circulatory System of Frogs vs. Circulatory Systems of Humans

The circulatory system of a human compared to that of a frog is different due to the number of chambers each contains. A frog’s heart has three chambers (two atria, and a single ventricle), whereas a human’s has four (two atria, and two ventricles). The atrium of a frog receives deoxygenated blood from the blood vessels that drain the various organs of the body. The left atrium receives oxygenated blood from the lungs and skin. Both atria empty into the single ventricle, which is divided into narrow chambers that reduce the mixing of the two bloods. The ventricle contracts, oxygenated blood from the left atrium is sent into the carotid arteries, thus taking blood to the head (and brain). Then, the deoxygenated blood from the right atrium is sent to the pulmocutaneous arteries, therefore taking blood to the skin and lungs where fresh oxygen can be picked up. The systemic circulation of a human is a loop from the heart to the various parts of the body, which works in contrast to the pulmonary circulation. In the systemic circulation, arteries collect the oxygen-rich blood from the heart and transport it to the body tissues. In the process, oxygen from the blood is diffused into the body cells, and carbon dioxide from the cells is diffused in the blood. The pulmonary circulation, however, is a loop from the heart to the lungs. Here, deoxygenated blood from the heart is carried to the lungs and then oxygenated blood is returned to the heart. The oxygen-depleted blood leaves the heart through the two pulmonary arteries and moves into the lungs. In the lungs, respiration takes place in which the red blood cells release carbon dioxide and absorbs oxygen. Oxygenated blood from the lungs is then carried back to the heart with the help of pulmonary veins.


About 360 million years ago, amphibians were the first vertebrates to live on land. Amphibians are diverse, widespread, and abundant group since the early diversification. There are about 4,500 species of amphibians. Frog is an amphibian and hence placed in the class Amphibia [Greek. Amphi – Both, bios –
life]. The largest order, with more than 3,900 species, is Anura, which includes the frogs and toads.

Rana hexadactyla is placed in the order Anura. Frogs live in fresh water ponds, streams and in moist places. They feed on small animals like insects, worms, small fishes, slugs, snails, etc. During its early development a frog is fully aquatic and breathes like a fish with gills. It is poikilothermic, i.e., their body temperature varies with the varying environmental temperature.

Morphology of Frog

The body of a frog is streamlined to help in swimming. It is dorso-ventrally flattened and is divisible into head and trunk. Body is covered by a smooth, slimy skin loosely attached to the body wall. The skin is dark green on the dorsal side and pale ventrally. The head is almost triangular in shape and has an apex which forms the snout. The mouth is at the anterior end and can open widely.

Differences between a Frog and Toad

External nostrils are present on the dorsal surface of the snout, one on each side of the median line (Figure 4.15). Eyes are large and project above the general surface of the body. They lie behind the external nostrils and are protected by a thin movable lower eyelid, thick immovable upper eyelid and a third transparent eyelid called nictitating membrane.

This membrane protects the eye when the frog is under water. A pair of tympanic membranes forms the ear drum behind the eyes on either side. Frogs have no external ears, neck and tail are absent. Trunk bears a pair of fore limbs and a pair of hind limbs. At the posterior end of the dorsal side, between the hind limbs is the cloacal aperature.

This is the common opening for the digestive, excretory and reproductive systems. Fore limbs are short, stumpy, and helps to bear the weight of the body. They are also helpful for the landing of the frog after leaping. Each forelimb consists of an upper arm, fore arm and a hand. Hand bears four digits. Hind limbs are large, long and consist of thigh, shank and foot.

Foot bears five long webbed toes and one small spot called the sixth toe. These are adaptations for leaping and swimming. When the animal is at rest, the hind limbs are kept folded in the form of letter ‘Z’. Sexual dimorphism is exhibited clearly during the breeding season.

The male frog has a pair of vocal sacs and a copulatory or nuptial pad on the ventral side of the first digit of each forelimb (Figure 4.16). Vocal sacs assist in amplifying the croaking sound of frog. Vocal sacs and nuptial pads are absent in the female frogs.

The Digestive System

The alimentary canal consists of the buccal cavity, pharynx, oesophagus, duodenum, ileum and the rectum which leads to the cloaca and opens outside by the cloacal aperture. The wide mouth opens into the buccal cavity.

On the floor of the buccal cavity lies a large muscular sticky tongue. The tongue is attached in front and free behind. The free edge is forked. When the frog sights an insect it flicks out its tongue and the insect gets
glued to the sticky tongue.

The tongue is immediately withdrawn and the mouth closes. A row of small and pointed maxillary teeth is found on the inner region of the upper jaw (Figure. 4.17) In addition vomerine teeth are also present as two groups, one on each side of the internal nostrils.

The lower jaw is devoid of teeth. The mouth opens into the buccal cavity that leads to the oesophagus through the pharynx. Oesophagus is a short tube that opens into the stomach and continues as the intestine, rectum and finally opens outside by the cloaca (Figure 4.18). Liver secretes bile which is stored in the gall bladder. Pancreas, a digestive gland produces pancreatic juice containing digestive enzymes.

Food is captured by the bifid tongue. Digestion of food takes place by the action of Hydrochloric acid and gastric juices secreted from the walls of the stomach. Partially digested food called chyme is passed from the stomach to the first part of the intestine, the duodenum.

The duodenum receives bile from the gall bladder and pancreatic juices from the pancreas through a common bile duct. Bile emulsifies fat and pancreatic juices digest carbohydrates, proteins and lipids. Final digestion takes place in the intestine. Digested food is absorbed by the numerous finger-like folds in the inner wall of intestine called villi and microvilli. The undigested solid waste moves into the rectum and passes out through the cloaca.

Respiratory System

Frog respires on land and in the water by two different methods. In water, skin acts as aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water gets, exchanged through the skin by diffusion. On land, the buccal cavity, skin and lungs act as the respiratory organs. In buccal respiration on land, the mouth remains permanently closed while the nostrils remain open.

The floor of the buccal cavity is alternately raised and lowered, so air is drawn into and expelled out of the buccal cavity repeatedly through the open nostrils. Respiration by lungs is called pulmonary respiration. The lungs are a pair of elongated, pink coloured saclike structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then to the lungs. During aestivation and hibernation gaseous exchange takes place through skin.

The Blood-Vascular System

Blood vascular system consists of a heart with three chambers, blood vessels and blood. Heart is covered by a double-walled membrane called pericardium. There are two thin walled anterior chambers called auricles (Atria) and a single thick walled posterior chamber called ventricle.

Sinus venosus is a large, thin walled, triangular chamber, which is present on the dorsal side of the heart. Truncus arteriosus is a thick walled and cylindrical structure which is obliquely placed on the ventral surface of the heart. It arises from the ventricle and divides into right and left aortic trunk, which is further divided into three aortic arches namely carotid, systemic and pulmo-cutaneous.

The Carotid trunk supplies blood to the anterior region of the body. The Systemic trunk of each side is joined posteriorly to form the dorsal aorta. They supply blood to the posterior part of the body. Pulmo-cutaneous trunk supplies blood to the lungs and skin.

Sinus venosus receives the deoxygenated blood from the body parts by two anterior precaval veins and one post caval vein. It delivers the blood to the right auricle at the same time left auricle receives oxygenated blood through the pulmonary vein. Renal portal and hepatic portal systems are seen in frog (Figure. 4.19 and 4.20).

The blood consists of plasma [60%] and blood cells [40 %] includes red blood cells, white blood cells, and platelets. RBCs are loaded with red pigment, nucleated and oval in shape. Leucocytes are nucleated, and circular in shape (Figure 4.21).

The Nervous System

The Nervous system is divided into the Central Nervous System [CNS], the Peripheral Nervous System [PNS] and the Autonomous Nervous System [ANS]. Peripheral Nervous System consists of 10 pairs of cranial nerves and 10 pairs of spinal nerves. Autonomic Nervous System is divided into sympathetic and parasympathetic nervous system. They control involuntary functions of visceral organs. CNS consists of the Brain and Spinal cord.

Brain is situated in the cranial cavity and covered by two meninges called piamater and duramater. The brain is divided into forebrain, midbrain and hindbrain. Fore brain (Prosencephalon) is the anterior most and largest part consisting of a pair of olfactory lobes and cerebral hemisphere (as Telencephalon) and a diencephalon. Anterior part of the olfactory lobes is narrow and free but is fused posteriorly. The olfactory
lobes contain a small cavity called olfactory ventricle.

The mid brain (Mesencephalon) includes two large, oval optic lobes and has cavities called optic ventricles. The hind brain (Rhombencephalon) consists of the cerebellum and medulla oblongata. Cerebellum is a narrow, thin transverse band followed by medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues as spinal cord, which is enclosed in the vertebral column (Figure 4.22).

Excretory system

Elimination of nitrogenous waste and salt and water balance are performed by a well developed excretory system. It consists of a pair of kidneys, ureters, urinary bladder and cloaca. Kidneys are dark red, long, flat organs situated on either sides of the vertebral column in the body cavity. Kidneys are Mesonephric. Several nephrons are found in each kidney.

They separate nitrogenous waste from the blood and excrete urea, so frogs are called ureotelic organisms. A pair of ureters emerges from the kidneys and opens into the cloaca. A thin walled unpaired urinary bladder is present ventral to the rectum and opens into the cloaca.

Reproductive system

The male frog has a pair of testes which are attached to the kidney and the dorsal body wall by folds of peritonium called mesorchium. Vasa efferentia arise from each testis. They enter the kidneys on both side and open into the bidder’s canal. Finally, it communicates with the urinogenital duct that comes out of kidneys and opens into the cloaca (Figure 4.23).

Female reproductive system (Figure 4.24) consists of paired ovaries, attached to the kidneys, and dorsal body wall by folds of peritoneum called mesovarium. There is a pair of coiled oviducts lying on the sides of the kidney.

Each oviduct opens into the body-cavity at the anterior end by a funnel like opening called ostia. Unlike the male frog, the female frog has separate genital ducts distinct from ureters. Posteriorly the oviducts dilated to form ovisacs before they open into cloaca. Ovisacs store the eggs temporarily before they are sent out through the cloaca. Fertilization is external.

Within few days of fertilization, the eggs hatch into tadpoles. A newly hatched tadpole lives off the yolk stored in its body. It gradually grows larger and develops three pairs of gills. The tadpole grows and metamorphosis into an air – breathing carnivorous adult frog (Figure 4.25). Legs grow from the body, and the tail and gills disappear. The mouth broadens, developing teeth and jaws, and the lungs become functional.