FACTORS ON WHICH THE FRICTION DEPENDS

Friction depends upon the following two factors. 

1. Normal Reaction 
2. Nature, of Surfaces 

NORMAL REACTION

Force of friction is directly proportional to the normal reaction R which acts in upward' direction against the weight of the body sliding on a surface i.e. 

NATURE OF SURFACES

Force of friction also depends upon the nature of two surfaces in contact with each other, it is constant for various pairs of surfaces in contact and this value is called co-efficient of friction. 

SELF ADJUSTING FORCE 

The force of friction' has the ability to increase its value with the increase of applied force till it reaches a maximum value..,Due to this strange nature of frictional forces we call them as the self adjusting force. It is denoted by Fs. 

LIMITING FRICTION  

Definition 

The maximum force of friction which just stops the body from moving is called limiting friction 

COEFFICIENT OF FRICTION 

Definition

 Coefficient of friction is the ratio between force of friction and perpendicular reaction. 

p is coefficient of friction. R is perpendicular reaction. F is force of friction. 

ROLLING FRICTION 

Definition :

Then a body rolls over a surface it experiences an opposing force that is called rolling friction. polling friction between two bodies is much less than the sliding friction because in rolling the contact S-Irface is much less than that in sliding. 

ADVANTAGES OF FRICTION  

• Frictional forces help us to move forward. 

• Nut and bolt can hold a body due to friction. 

• A nail stays in the wood because of friction. 

• We can not hold any thing without friction. 

• We can tie a knot because of friction. 

DISADVANTAGES OF FRICTION  

There are so many disadvantages of friction but some are as follows. 

In a car engine oil under pressure is supplied continuously to ball bearing surface. Failure of the oil supply will allow metal to metal contact and the resulting friction, often raises the temperature and due to this ball bearings and piston are sized up. 

MOMENTUM

Definition 

Momentum of a body is the product of its mass and velocity. 
                                                                                                   OR 
The quantity of motion of a body is called its momentum. 

Mathematically 

If "m" is the mass and "v" is the velocity of a body then Momentum P is given by: 
                                                                         P = mv 

Unit 

In M.K.S. system the unit of Momentum is kg-m/s or N-s 

Nature 

It is a vector quantity. Its direction is same as that of velocity, 

LAW OF CONSERVATION OF MOMENTUM 

Statement 

The total momentum of a system remains constant provided no external forces act on it. 
                                                                         OR 
The Momentum of an isolated system always remains constant. 

PROOF 

Consider a system consisting of two balls A and B of masses m₁ and m2 moving in a straight line, with velocities U1 and U₂. On colliding with each other they move with velocities V1 and V₂ respectively as shown in the figure. 
The total Momentum of the system before collision= m₁U1 + m₂U₂ 
The total Momentum of the system after collision = m_1V1 + m_2V2 
According to the law of conservation of Momentum Initial momentum = Final momentum 
                                                              m₁U1 + m₂U₂ = m1V1+ m₂V₂ 

Hence the Momentum of a system remains constant. 

FRICTION  

Definition 

When two bodies are in contact with each other and the force is applied to the upper body to make it move over the lower body, an opposing force is developed in the plane of contact which resists the motion. This force is called friction. 

                                                                          OR 

When one bod / slides over the surface of another body, an opposing force is set up to resist the motion. The force which opposes the motion is called friction. 

CAUSES OF FRICTION 

There is no ideal smooth surface present in universe. Each surface has elevations and depressions which causes to friction. 

When two surfaces are brought into physical contact. the elevations of one impacts into the depressions of first or vice versa which causes an inter locking with two surfaces. This interlocking opposes the relative motion of one surface on the other. 

Compulsory Military Training

There is a proverb that, "If you want to have peace, prepare for war." The best preparation for war is to keep man-power and munitions ready. Before the beginning of the Second World War, England and France were not prepared for war, while Germany and Japan were prepared for it. England and France wanted peace at any cost, but they forgot the above proverb.

Wars have become a necessary evil in modern times, and the only way to fight out this evil is through a preparation for war at the national level. In every country there are schools and colleges for military training. There are three branches of the army: ground force, naval force and air force. Military training is voluntary in almost all countries.

But in ancient Greece there was compulsory training. Germany and Japan had made military training compulsory till the end of the Second World War. Now there is compulsory military training in Russia, France, China, Italy and Germany and so many other have introduced compulsory military training.

In Pakistan, we are still dependent of our defence upon a professional army. It is with this objective in view that the Commission on National Education has recommended the introduction of compulsory military training in all the schools and colleges of our country. A few years ago a sort of military training called the N.C.C. force was introduced in some of the colleges. But now, after the recent Indo-Pak war, the importance of compulsory military training has been realised anew.

There are many advantages of military training. It develops the sense of orderliness and discipline in a nation. It makes people strong and courageous. The concept of new citizenship is incomplete without military training, it makes soldiers patriotic. It narrows the gap between civil life and military life. No other training can create in our young men the habit of self-obedience, team spirit and co-operation in a better than military training does. A compulsory military training is the only way to make our people smart and active. 

Write a note on the excretory system of frog with the help of a diagram.

EXCRETORY SYSTEM OF FROG:
In frogs, waste materials are excreted out in many ways e.g. skin, lungs, liver. digestive system. etc. It is the set of organs involved in the process of excretion--that is, the removal of metabolic waste- matters from the body. Besides these organs. nitrogenous waste materials are excreted through two kidneys, which are attached to the dorsal wall of the body cavity These are elongated in shape and composed of urinary tubules. Urinary tubules combine to form collecting ducts, which open into ureter. The urine collected by the kidneys comes into ureters. Each ureter starts from the edge of each kidney of its side and opens into cloaca. From here, the urine is excreted or stored in urinary bladder which is passed out of the body from the cloacal aperture. Carbon dioxide and water are excreted through skin and lungs while undigested food and some waste materials are excreted through liver and digestive system.

Write a note on electron microscope.

ELECTRON MICROSCOPE:

Electron microscope was invented in 1935. It has a resolving power 250 times or more than that of. a compound microscope. It uses a beam of electron as a source of illumination. The electron beam increases its resolving power. Modern electron microscope can achieve a resolution of .about 0.2 nm, a thousand times improvement over light microscope. The electron microscope uses electromagnet as lenses instead of glass lenses. Its image cannot be focused in human eye, therefore screen or photographic plates are used to review and focus these images. Electron microscope has a capacity to magnify an object up to 250,000 times of its original size. 

Electron microscopes reveal many organelles that are impossible to resolve with the light microscope.

In electron microscopy, chemical and physical methods are used to prepare samples which not only kill cells but also may introduce artifacts. A light microscope has many advantages especially for the study of live cells.

Write down the structure of an animal cell.

STRUCTURE OF AN ANIMAL CELL: 

Cell Membrane: This is the outermost boundary of an animal cell. It is also called the plasma membrane. In fact, a cell membrane surrounds the cytoplasm and nucleus in all types of cells. However, in plants and bacteria, cell membrane itself is surrounded by a cell wall. Cell membranes are composed of lipids and proteins (lipoproteins). A cell membrane is selectively or semi-permeable.

Fluid Mosaic Model of Cell Membrane:

Singer and Nicholson proposed a model of plasma membrane in 1972 According to this model:
(1) Lipids are in the form of a fluid bilayer.
(ii) Lipids are either phospholipids or glycolipids suspended in water.
(iii)• Protein molecules float like icebergs in the sea in the lipid bilayer.
(iv) Proteins are completely or partially embedded in the lipid bilayer forming a mosaic arrangement

Functions: 

(1) Cell membrane regulates selective movement of molecules into and out of the cell. It allows certain molecules to pass through it while others can not pass through it.

(2) Cell membrane takes in material by infolding in the form of vacuoles. It is called endocytosis.

Cytoplasm:

It is a viscous opaque (translucent) fluid portion of the cell which is present between the cell membrane and nuclear membrane. It consists of an aqueous ground substance called cytosol and granular portion called cytoplasmic organelles Its chemical analysis shows that it is composed of organic compounds such as proteins. carbohydrates, lipids (fats), enzymes and some inorganic compounds, e.g. water and salts. Some important cytoplasmic organelles found in eukaryotic cells are as under:
1. endoplasmic reliculum.
2. Golgi complex.
3. mitochondria.
4. plastids (in case of plant cells).
5. ceritrioles.
6. ribosomes.
7 vacuoles.

Functions: 

 
(1) It serves as a storehouse of vital chemicals.
(2) It is a site of metabolic reactions like protein synthesis, glycolysis, etc.
(3) Many reactions can occur at the same time in different regions of the cytoplasm.

Mitochondria (singular mitochondrion):

They are an important organelle of a eukaryotic cell because they provide energy to the cell and are  called the powerhouse of the cell. 

They are generally rod like or bean shaped bounded by double membrane. Their outer membrane is smooth while the inner membrane is folded. These infoldings are called cristae while the fluid present inside is called matrix. The number of mitochondria in a cells relates to its activities. In different animal cells, number of mitochondria differ. The cells which are more active e.g. liver cells have more than one thousand mitochondria while ear lobes have very small numbers only 50.

Function:

(1) Mitochondria play an important role in respiration.
(2) They contain enzymes which break down the food for the production of energy.

Endoplasmic Reticulum:

It is a network of channels extending throughout cytoplasm from plasma membrane to nuclear membrane. They are tube like. They are of two types.
(1) smooth endoplasmic reticulum (SER): It is nongranular because ribosomes are not attached to it.
(2) rough endoplasmic reticulum (RER): It is granular as ribosomes in the form of small granules are attached to it.

Function:

(1) Endoplasmic reticulum plays an important role in the transport of material from one part of the cell to the other.
(2) It is involved in the synthesis of proteins.
(3) It provides mechanical support to the cell so that its shape is maintained.
(4) It detoxifies the harmful drugs.

Nucleus (Karyon):

It was discovered by Robert Brown in 1831. It may be spherical or irregular in shape. In animal cells, it is usually present in the centre but in plant cells due to the presence of a large vacuole, it is pushed towards the cell membrane. Nucleus is surrounded by a double membrane called the nuclear membrane. This membrane has a large number of pores. Nucleus is filled with a gel like substance called nucleoplasm. The nucleoplasm contains nucleoli and a network of thread like structures called chromatin network. The threads of chromatin become prominent during cell division. Each thread is called chromosome. Chromosomes are made up of deoxyribonucleic acid (DNA) and proteins. DNA plays an important role in the inheritance of characters as well as in controlling and regulating the activities of cells. The number of chromosomes in the cells of a particular species always remains constant. It is the characteristic of that species.

Function:

Nucleus controls all the activities of cells.

GOIgi Complex:

They were discovered by Camillo Golgi and thus called Golgi complex or Golgi bodies or Golgi apparatus. They are a set of smooth membranes that are stacked into flattened, fluid filled sacs or vesicles containing carbohydrates, glyco-proteins and enzymes. They are especially prominent in glandular cells. In animal cells, usually a Golgi apparatus is found in each cell. In plant cells, there may be more.

Function:

(1) They are mainly concerned with cell secretions.
(2) In plants they synthesize cell walls.

Centrioles or Centrosome:

A rounded structure, the centrosome is present in the cytoplasm near the nucleus in animals cells. A centrosome contains two centrioles. Higher plants lack centrioles. Each centriole consists of a cylindrical. array of nine rows of microtubules. Both the centrioles lie perpendicular to one another.

Function:

They form fibrous protein spindles, which help in movement of chromosomes towards poles during animal cell division.

Ribosomes:

They are granules, which are rich in ribonucleic acid (RNA). They are found either freely suspended in the cytoplasm or attached to the surface of rough endoplasmic reticulum. They are the only organelle found in prokaryotic cells. They are the only organelle, which are not bound by a membrane.

Function:

They serve as sites for protein synthesis and hence called the protein factories of the cell.

Nucleolus:

It is found inside the nucleus. It has got no membrane. There may be one or more nucleoli in the nucleus. It disappears during the cell division and reappears later.

Functions:

1. The ribosomal RNA is synthesized and stored in the nucleolus.
2. It is the nucleolus where ribosomes are assembled and exported to cytoplasm via nuclear pores.

Vacuoles:

They are liquid filled cavities in the cytoplasm. They are surrounded by a membrane called tonoplast. In plant cells, there is usually one large permanent vacuole. In animal cells, there may be many small temporary vacuoles.

FUNCTIONS:

(1). They are connected with  the storage of cell sap.

(2) .In small organisms , water and waste materials are excreted through special vacoules called contractile vacoules and food is digested in food vacoules.

Essay On An Accident

Accidents are quite common in Karachi. The drivers are so ignorant of traffic law, and so careless about it, that they never like to look round while driving even on the main roads, or while taking sudden turns. Many a time I have noticed them stopping in the middle of the road, without looking back or sideways and without giving any signal.

It is rather unfortunate that public interest is easily ignored by the drivers. They drive very speedily and carelessly. The buses are often founded over-crowded. Even the traffic police are found helpless because they have failed to control the traffic. Although there are hundreds of buses on the roads, yet they are not sufficient for this huge population. Complaints of the public fall on deaf ears. The middle class travels by rickshaws and taxies. They are also tar more troublesome. They often charge double fare, hence cannot be availed by the poor. The taxi-drivers do not go to short distances. Public is continuously harassed by the all drivers because most of them are illiterate.

One day 1 was going to college with my friend, Amir, we were near the College. We heard a collision between a bus and a truck. The engine and the body of the bus were completely wrecked. The bus was overloaded. Some people were also hanging on the foot-board and some were on the roof of the bus.

The two drivers and six passengers were on the bus were killed instantly. Several of the passengers including a large number of women and children were seriously injured.

The Medical aid was rushed from AbdulSattar Edhi Welfare Centre. The Superintendent of the police and other officials of the Police Department also visited the scene of the accident. The seriously injured were taken to the Civil Hospital Karachi.

What is the significance of meiosis.

SIGNIFICANCE/IMPORTANCE OF MEIOSIS:

 The most significant features of meiosis are the halving of the chromosome number and the occurrence of chiasmata during the prophase of the first division. In the reproductive organs, certain cells give rise to gametes (e.g. sperms or ova) by a special kind of cell division which results in the gametes containing only half the number of chromosomes as compared to other cells of the organism. When the gametes fuse together during fertilization, the 2n chromosome number or diploid number is restored. There is a fixed number of chromosomes for each species, e.g. for man it is 46. 

The formation of chiasmata brings about a new combination of genes in the gametes. At each chiasma, an exchange of genetic material takes place between the maternal and paternal members of a homologous pair of chromosomes, the phenomenon is called crossing over. 

During fertilization, the male and female gametes unite. The offspring that results from this union will inherit a mixture of characters from both the parents. 

Thus meiosis keeps the chromosome number constant generation after generation and it provides a chance for genetic reshuffling so that a better species can result to suit the environment.

Write the external features of the Frog.

EXTERNAL FEATURES OF THE FROG:

A frog has greenish spots and dark lines on its skin which is kept moist and slimy due to the secretion of glands. Its ventral side is yellowish white in colour. The body of frog consists of head, trunk and limbs Neck and tail are absent Head directly joins with the trunk and therefore it can not move its head. The conical head has two large bulging eyes. .Behind each eye is a circular area called tympanic membrane. These membranes help in hearing At the tip of the snout, it has two openings called external nostrils through which the frog breathes. There is a membranous skin between its toes, which help in swimming. There are five toes in each foot but the hand has only four fingers because the thumb is rudimentary. In male frog, the first finger is thicker than the others. The eyes of frogs have two eyelids. An upper eyelid which is immovable and a lower eyelid which is slightly movable. A transparent membrane, the nictitating membrane is attached under the lower eyelid. This membrane can be drawn upwards over the surface of the eye to clean and moisten it when the frog is on land and to protect it when the frog is in water Frogs lay eggs in winter season in water. They hibernate (go to winter sleep) by burying themselves in the mud for the whole winter. 

Write a note on different parts of brassica flower.

PARTS OF BRASSICA FLOWER:

A flower is a reproductive part of the plant. With the maturation of age, the plant bears small yellowish flowers. Flowers grow on young branches in a special way. This branch is called pedicel and the arrangement is called inflorescence. The tip of the pedicel bears thalamus. The floral leaves are arranged in four whorls on the thalamus. They are as under:

(1) Calyx:

This is the outermost whorl and consists of four free sepals. The sepals are light greenish in young flowers. but as the flowers mature, their colour also becomes yellowish like that of the petals. In young flowers, sepals cover the inner parts of the flower. The basic function of calyx is to protect the inner parts of the flower.

(2) Corolla:

Corolla is the second whorl of floral leaves. It consists of four free yellow petals. Its yellow colour is conspicuous and can attracts insects, honey bees and butterflies which helps in pollination (transfer of pollen grains from another to stigma).

(3)  Androecium:

The androecium lies inside the petals. It makes the third whorl of the floral leaves. Its parts are not leaf like. The androecium consists of six free stamens, which are the male reproductive organs of the flower. In brassica flower, the stamens are arranged in two circles. The outer circle has two small stamens. The inner circle has four long stamens. Each stamen has two parts: a lower delicate stalk called the filament and an upper swollen part called the anther. Each anther contains numerous pollen grains. When the anther matures, a longitudinal slit appears in its walls from which pollen grains escape. At the base of the filament, four nectaries are present. When insects are attracted towards the flowers to collect this nectar, pollen grains get attached to their bodies and are transferred from one flower to another and pollination results.

(4) Gynoecium:

This is the inner most and the fourth whorl occupying a central position in the flower. Each part of the gynoecium is called carpel. Gynoecium makes the female reproductive organs of the plant. In Brassica, gynoecium is formed by the union of two carpels. Each carpel is divisible into three main parts. The lower swollen part is the ovary. Above the ovary, camel extends into a thin stalk, the style. The style has swollen tip which is called stigma. In the ovary, many ovules are present which become seeds. The ovary becomes a fruit. The fruit of brassica is a dry capsule.

Write a note on the structure of amoeba.

STRUCTURE OF AMOEBA:

Amoeba is a unicellular organism found in muddy water .at the bottom of ponds and ditches. It measures about 0.25 mm in size. It does not have a fixed shape. It keeps on changing its shape all the times. 

 1. Cell Membrane:

It is a thin membrane. It is also called plasmalemma. It is protective in function.

2. Cytoplasm:

It is differentiated into two parts:
(i) Ectoplasm: The outer portion of cytoplasm is clear and transparent.
(ii) Endoplasm:The inner, viscous, translucent and granular part of cytoplasm is called endoplasm. The endoplasm appears granular and darker in colour due to the presence of reserve food materials.

3. Food Vacuoles:

 The endoplasm contains food vacuoles of all sizes. There are many of them inside the endoplasm. Food vacuoles contain food particles. 4. Contractile Vacuole:
 There is also a large contractile vacuole inside the endoplasm. The contractile vacuole functions to remove excess water from the body. When it reaches its maximum size, it bursts and starts forming again.

5. Nucleus:

There is an oval shaped nucleus in the centre which controls all the metabolic activities of the organism.

6. Pseudopodia (singular pseudopodium):

Amoeba moves by producing finger like projections called pseudopodia. The pseudopodia are also used to capture food particles, which are then placed inside the food vacuole. Amoeba respires by exchanging gases with the surrounding water through its surface by diffusion.

Write a note on sense organs.

SENSE ORG4NS OF FROG:

Organisms like frogs have receptor or sense organs to get information about the environment. They are as under:

(1) Skin: 

It has different sensory nerve endings. Those that are located in the epidermis as free nerve endings sense temperature changes by thermoreceptors, which are important in body temperature homeostasis. They are also believed to sense pain and light touch. 

(2) Nose:

To detect smell, the olfactory cells are present in the nostril. The olfactory sensory cells may have minute hairs to receive the stimulus. TI-1 .animal with the most acute sense of smell is the male emperor moth. Using its antennae, it can detect a female emperor moth 11 km upwind. 

(3)Tongue:

Sensory cells. called gustatory cells. are grouped together in taste buds located in grooves in the tongue mucous membrane.
 Different tastes can be detected by these cells, e.g. sweet, sour, bitter or saltish.

Ear:

An ear is the sense organ for hearing and maintaining balance of the
body.

Eye:

 An eye is the organ of sight. Thus the five sense organs include: skin for touch, nose for smell, tongue for taste, ear for hearing and eye for seeing, 

Write a note on the ear of frog.

EAR OF FROG:

The organ of hearing of frogs like the other vertebrates is the ear. Its outermost part is called the tympanic membrane. On the inner side of tympanic membrane is a cavity known as tympanic cavity. The tympanic cavity contains rod like bones called the ossicles. One end of the ossicle bone is attached to the tympanic membrane and the other end with the internal ear.
The internal ear is a very delicate organ. It consists of three semi-circular canals. These canals are filled with a fluid. Sensory cells are present at special places in the semi-circular canals.

When sound waves strike the tympanic membrane, it is set into motion. This in turn vibrates the internal ear and thus sound waves stimulate the hearing receptors in the inner ear.
The internal ear, in addition to hearing also keeps the balance of the body too. 

Write a detailed note on the eye of frog.

EYE OF FROG:

The frog has two eyes one on each side of the head. 

(i) Eyeball: 

The eyeball is almost spherical. It has a wall composed of three layers or coats. 

(i) Outer Layer:

It is the hard, outer most layer of the eye. It consists of an almost opaque sclera or sclerotic layer. The anterior transparent part of the scelerotic layer is called cornea. which is connected, in a continuous ring. 

Function: 

Sclerotic layer provides shape to the eyeball. 

(ii) Middle Layer:

Its consists of a choroid layer , which is well supplied with blood vessels providing nutrients and oxygen. An eye deprived of oxygen will be irreparably damaged.A black or dark pigment makes the choroid layer totally opaque and lightproof

The circular iris, ciliary muscle and ciliary body from the front position of the middle layer.

The central perforation of the eye is called pupils of eye.

Function:

 (1) The choroid provides nourishment and oxygen to the eye. 

(2) The iris controls the amount of light entering the eye through pupil. 

(iii) Inner Layer:

 The innermost layer of the eyeball is called retina: Retina consists of sensory cells called the rods and cones. The rod cells are stimulated by low light intensities while the cone cells are stimulated by high light intensities. Only the cone cells are sensitive to coloured light. Function:

Retina acts as a screen, which receives the image of objects, focused by the lens. 

(iv) Aqueous Humour: 

The space between the lens and cornea is filled with aqueous humour, which is quite watery. 

(v) Lens: 

It is a clear biconvex lens held in position by suspensory ligaments connected to the ciliary body and muscles. Function: It focuses light on the retina. 

(vi) Vitreous Humour: 

A clear jelly like vitreous humour completely fills the eyeball behind the lens and ciliary muscles. 

Function: 

The spherical shape of the eye is maintained by constant pressure of the fluid components of the eye. 

(vii) Optic Nerve: 

Nerves connect with the retina sensory cells and join together at one position at the back of the eye to form an optic nerve. At the point called the blind spot where the optic nerve leaves the eyeball, there are no sensory cells. 

Function: 

Optic nerve connects the eye with the brain region that interprets sight.

Explain the reproductive system of frog.

REPRODUCTIVE SYSTEM OF FROG:

Male Reproductive System: 

It consists of a pair of testes and reproductive ducts. Testes lie on the ventral surface of the kidney of their side and are attached to them by a membrane. Fat bodies are present at their anterior ends. Each testis is composed of semniferous tubules in which sperms are produced. Sperms pass into kidney through fine tubes called vasa efferentia (singular vasa efferens). Sperms pass into cloaca through ureter and are discharged out in water. In males, ureters perform double functions, the excretion of urine and sexual material so it is called urinogenital duct and the urinary system and the genital system are associated into urinogenital system. 

Female Reproductive System: 

This system consists of two ovaries and reproductive ducts. Ovaries are located near the kidneys. At their ends, fat bodies are present. Each ovary has numerous follicles where eggs are formed: In the breeding season, ovaries increase in size. The ova are released from the ovary into the body cavity from where they enter the oviduct. The anterior end of oviduct is funnel like while the posterior end is broad and is called uterus, which opens into cloaca. Eggs are released into water through the cloaca. In water, a sperm fertilizes an egg and a zygote is formed. From zygote an offspring is formed. 

Describe the ventral and dorsal side of the frog's heart.

HEART OF FROG:

 The heart of frog is a muscular and conical organ located in the body cavity between the oesophagus and sternum. It is enclosed in a membrane called pericardium. It contracts and expands continuously throughout the life. This contraction and expansion of heart is called heartbeat due to which blood keeps circulating in the body. The heart of frog consists of three chambers: 
(i) Right Auricle or Atrium 
(ii) Left Auricle or Atrium 
(iii) Ventricle.
 The two thin walled atria form the broader anterior part of the heart. Right atrium is larger than the left atrium. Ventricle forms the conical, thick, uneven walled posterior part of the heart. A broad vessel, truncus arteriosus originates from the ventral side of the ventricle and divides into two branches near atria. Each branch divides into three arches (arteries). A thin walled triangular sac called sinus venosus opens into the right atrium. Both the truncus arteriosus and sinus venosus are not true chambers of the heart but often called accessory chambers. However, some biologists consider them as chambers of heart which beat in a rhythmic manner. First of all sinus venosus contracts then the two atria followed by the contraction of the ventricle and in the end truncus arteriosus contracts. Deoxygenated blood from the whole body except lungs is carried to the sinus venous by two precavals and one postcaval. Sinus venosus opens through an aperture into the right atrium. Oxygenated blood from lungs is carried to the left atrium by two pulmonary veins. Both the atria open jointly by an aperture guarded by a valve into the ventricle. 

• The two atria, the right one with deoxygenated blood and the left one with oxygenated blood contract simultaneously to push their blood collectively into the ventricle. In the middle of the ventricle, there is some mixing of the two types of blood whereas on the two sides the blood remains unmixed as the blood passes very fast from the ventricle. Ventricle contracts to push the blood into truncus arteriosus through an aperture guarded by a valve. From truncus arteriosus the blood enters (a) the pulmonary arteries which carry the blood to lungs for oxygenation and (b) the systematic arteries which supply the pure blood to all parts of the body and (c) the carotid arteries which supply blood to the brain. 

Explain the nervous system of frog.

NERVOUS SYSTEM OF FROG:

The set of organs which control and coordinate all the activities of the body is called the nervous system. It is composed of two parts: the central nervous system and the peripheral nervous system.

Central Nervous System:

It includes the brain and the spinal cord.

Brain:

The brain is protected in the brain case or cranium, which is the major portion of the skull. Brain has three parts which are called fore brain, mid brain and hind brain.

Fore Brain:

The most anterior region comprises the olfactory lobes. Immediately behind the olfactory lobes, there are two large outgrowths called the cerebral hemispheres.

Functions:

Olfactory lobes are associated with the sense of smell and transfer of olfactory sensation to the cerebral hemispheres which are the seats of intelligence and memory. The diencephalon receives a variety ofmessages from the internal and external environment of the body and also controls the secretions of hormones from the pituitary gland.

Mid Brain:

Behind the cerebral hemispheres are located two prominent outgrowths called the optic lobes. The part between the optic lobes and hemispheres is known as the diencephalon. On the dorsal surface of the diencephalon is present a pineal body while on its ventral surface the pituitary gland is attached.

Function:

This part is associated with the eyes and vision.

Hind Brain:

 The last part of the brain consists of the cerebellum and the medulla oblongata.

Function:

 This part controls and coordinates body movements and maintains balance of the body. The medulla also controls respiration. circulation and digestion.

Spinal Cord:

 The spinal cord starts from the posterior end of the medulla oblongata like a thick thread. It passes from the skull through a hole and enters the . canal of the vertebral column. The vertebral column protects the spinal cord. Brain and spinal cord are not solid. They have a system of canals. which is filled with a fluid

Function:

 The spinal cord controls the movement of the trunk area and many other functions independently. 

PERIPHERAL NERVOUS SYSTEM:

Peripheral nervous system consists of nerves, which connect the central nervous system with various parts of the body. They are of two types:

(1) Cranial Nervous: Nerves originating from brain are called cranial nerves. In frog. there are ten pairs of cranial nerves. 

(2) Spinal Nerves: Nerves originating from spinal cord are called spinal nerves. There are nine or ten pairs of spinal nerves. Nerves are of following types: 

 
i. Sensory Nerves: These nerves take messages from sensory organs to central nervous system. 

 
ii. Motor Nerves: These nerves take messages from central nervous system to glands and muscles. 

 
Mixed Nerves: They carry out both the above mentioned functions. Spinal nerves are all mixed nerves while cranial nerves are of all three types: 

Besides these nerves, some special nerves are present in the head and trunk region called the autonomic nerves. which work automatically. They control the internal organs of the body such as heart, lungs, stomach, smooth muscles of the intestine, vessels and glands.

Write a note on the arterial system of frog.

ARTERIAL SYSTEM OF FROG:

 Blood vessels carrying oxygenated blood from the heart to all parts of the body are called arteries, which together form a system called the arterial system.
The arterial system has two main branches:
 each of which further divides into three branches which are as follows.

1. Carotid Arch:

 It is composed of the arteries carrying oxygenated blood to various parts of the head region such as brain, tongue. head muscles, eyes, ears. lower jaw, etc.

2. Systemic Arch:

 It is composed of the arteries carrying oxygenated blood to all parts of the body except the head and lung. Right and left systemic arches join posteriorly to form dorsal aorta. The dorsal aorta is a major vessel of this system. It transports blood to various parts of the body.
 Prior to their union, each systemic arch gives out arteries supplying blood to vertebral column, oesophagus and forelimbs.

Dorsal Aorta:

 Dorsal aorta runs along the vertebral column towards the hind limbs.
Coeliacomesenteric artery originates from dorsal aorta and supplies blood to digestive system.
Renal arteries supply blood to kidneys and genital organs.
Posterior mesenteric artery carries blood to rectum Iliac arteries transport blood to hind limbs of their sides.

3. Pulmocutaneous Arch:

  It carries blood to lungs and skin. Pulmonary arteries carry deoxygenated blood to lungs for oxygenation. While cutaneous arteries supply blood to the skin.

 All the arteries after reaching their regions of supply branch and rebranch into smaller thin walled vessels called capillaries. 

Through the thin walls of capillaries exchange of materials between blood and tissue cells takes place. Capillaries join to form venules, which form veins and carry deoxygenated blood from all parts of the body to the heart.