Introduction' Definition And Major Branches Of Chemistry .

Chemistry,
                                                                    

study of the composition, structure, properties, and interactions of matter. Chemistry arose from attempts by people to transform metals into gold beginning about ad 100, an effort that became known as alchemy (see Chemistry, History of). Modern chemistry was established in the late 18th century, as scientists began identifying and verifying through scientific experimentation the elemental processes and interactions that create the gases, liquids, and solids that compose our physical world. As the field of chemistry developed in the 19th and 20th centuries, chemists learned how to create new substances that have many important applications in our lives.
Chemists, scientists who study chemistry, are more interested in the materials of which an object is made than in its size, shape, or motion. Chemists ask questions such as what happens when iron rusts, why iron rusts but tin does not, what happens when food is digested, why a solution of salt conducts electricity but a solution of sugar does not, and why some chemical changes proceed rapidly while others are slow. Chemists have learned to duplicate and produce large quantities of many useful substances that occur in nature, and they have created substances whose properties are unique.
Much of chemistry can be described as taking substances apart and putting the parts together again in different ways. Using this approach, the chemical industry produces materials that are vital to the industrialized world. Resources such as coal, petroleum, ores, plants, the sea, and the air yield raw materials that are turned into metal alloys; detergents and dyes; paints, plastics, and polymers; medicines and artificial implants; perfumes and flavors; fertilizers, herbicides, and insecticides. Today, more synthetic detergent is used than soap; cotton and wool have been displaced from many uses by artificial fibers; and wood, metal, and glass are often replaced by plastics.
 A living organism is a complex chemical factory in which precisely regulated reactions occur between thousands of substances. 
Chemistry is often called the central science, because its interests lie between those of physics (which focuses on single substances) and biology (which focuses on complicated life processes). A living organism is a complex chemical factory in which precisely regulated reactions occur between thousands of substances. Increased understanding of the chemical behavior of these substances has led to new ways to treat disease and has even made it possible to change the genetic makeup of an organism. For example, chemists have produced strains of food plants that are hardier than the parent strain.
Because the field of chemistry covers such a broad range of topics, chemists usually specialize. Thus, chemistry is divided into a number of branches, some of which are discussed at the end of this article. Nevertheless, the process of learning the properties of a substance and of taking it apart is fundamental to nearly all of chemistry.
The first step in investigating a complex material is to try to break it down into simpler substances. Sometimes this is easy. A mixture of brass and iron tacks, for instance, could be sorted with a magnet or even by hand. Getting the salt out of brine or seawater is a little harder, but the water can be evaporated, leaving the salt. Changes of this sort, which do not alter the fundamental nature of the components of the mixture but do modify their physical condition, are called physical changes. Grinding a rock, hammering a metal, or compressing a gas causes physical changes. Another example of physical change is the melting of ice, in which water changes from the solid to the liquid state.
Salt and water may not only be separated when in solution, but each may be broken down into other substances. This, however, involves a different kind of change—one that usually requires more energy than a physical change and that alters the fundamental nature of the material. This type of change is called a chemical change. By applying electrical energy, water can be broken down into two gases, hydrogen and oxygen. Hydrogen is a light gas that burns; oxygen is a gas that is necessary to sustain animal life. Salt can be broken down by melting it, then passing an electric current through it. This produces a pungent yellow-green gas called chlorine and a soft, silvery metal called sodium, which burns readily in air.
Some materials can be broken down simply by heating them. Other materials yield to attack by another substance; for example, iron oxide ore heated with coke yields metallic iron.

II  ELEMENTS AND COMPOUNDS
More than 100 chemical elements—substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means—are known to exist in the universe. However, several of these elements, such as the so-called transuranium elements, have not been found in nature and can only be produced artificially.
Russian chemist Dmitri Ivanovich Mendeleyev and German physicist Julius Lothar Meyer independently developed the periodic law of the chemical elements at about the same time in the late 19th century. Mendeleyev is generally credited with the findings, because he established the periodic law in 1869, and Meyer established this chemical law in 1870. Both discovered that arranging the elements in order of increasing atomic mass produced a table of chemical properties and reactivity patterns that were regularly repeated. This phenomenon—known as the periodic law—is most often represented in the periodic table of the elements (see Atom).
A  Elements
Hydrogen, oxygen, chlorine, sodium, and iron are examples of elements. Elements cannot be resolved into simpler substances by ordinary heat, light, electricity, or attack by other substances. To say that elements can never be broken down would not be accurate, but breaking them down takes millions of times more energy than can be applied by ordinary means. It requires either special equipment, such as a particle accelerator, or temperatures like those in the interior of the sun. An element can therefore be defined as a substance that cannot be broken down into simpler substances by ordinary means.
Ninety elements are known to occur in nature, and 22 more have been made artificially. Out of this limited number of elements, all the millions of known substances are made.
Abbreviating the names of the elements is often convenient. For each element, a symbol has been chosen that consists of one or two letters. The symbols are derived from the names of the elements; for example, H stands for hydrogen, He for helium, C for carbon, and so on. The abbreviations are not always derived from the English names, however. The symbol Fe for iron comes from the Latin ferrum, and W for tungsten comes from the German wolfram. These symbols are internationally recognized and are used even by people whose native languages do not use the Roman alphabet, such as Russian and Japanese.
B  Compounds Salt, water, iron rust, and rubber are examples of compounds. A compound is made up of elements, but it looks and behaves quite differently, as a rule, from any of its component elements. Iron rust, for example, does not look and feel like its components: oxygen gas and iron metal. Some synthetic fabrics, with fibers made from coal, air, and water, do not feel at all like any of the components that make them up. This individuality of properties, as well as other qualities, distinguishes a compound from a simple mixture of the elements it contains. Another important characteristic of a compound is that the weight of each element in the compound always has a fixed, definite ratio to the weight of the other elements in the compound. For example, water always breaks down into 2.016 parts of hydrogen by weight to 16.000 parts of oxygen by weight, which is a ratio of about 1 to 8, regardless of whether the water came from the Mississippi River or the ice of Antarctica. In other words, a compound has a definite, invariable composition, always containing the same elements in the same proportions by weight; this is the law of definite proportions.
Many elements combine in more than one ratio, giving different compounds. In addition to forming water, hydrogen and oxygen also form hydrogen peroxide. Hydrogen peroxide has 2.016 parts of hydrogen to 32 parts of oxygen; that is, 1.008 parts of hydrogen to 16 parts of oxygen. Water, as stated above, has 2.016 parts of hydrogen to 16 parts of oxygen. The figure 2.016 is twice 1.008. This example illustrates the law of multiple proportions: When two elements combine to form more than one compound, the element whose mass varies combines with a fixed mass of the second element weights in a simple whole-number ratio such as 2:1, 3:1, or 3:2.
C  Atoms and Molecules
The concepts of atoms and of the groups of linked atoms called molecules are the foundation of all chemistry (see Atom). An atom is the smallest unit of an element that has the properties of the element; a molecule is the smallest unit of a compound or the form of an element in which atoms bind together that has the properties of the compound or element.
The idea of atoms is an old one. Greek philosopher Leucippus and his student Democritus appear to have originated the idea during the 4th and 5th centuries bc. According to them, matter consisted of small, indivisible particles called atoms. All atoms were made of the same basic material, but neither philosopher stated what this material was. The atomic theory was developed further by another Greek philosopher, Epicurus, who added the property of weight to the atoms and attributed a horizontal, as well as a vertical, motion to them in order to explain how atoms combine to form matter. These ideas were restated by Roman poet Lucretius in the 1st century bc.
In the 18th century ad, English schoolmaster John Dalton developed his well-known atomic theory, which explained the laws of definite and multiple proportions. Convincing proof that atoms exist, however, has only been generated since 1900. Much, but not all, of this proof came from the study of radioactivity and of energetic particles. When Lucretius watched dust particles dancing in a sunbeam and said that they were being battered by the invisible blows of restless atoms, he was basically right. True, most of the dancing was caused by air currents, yet even in still air, specks of dust or smoke are in constant motion, as are minute particles suspended in water. This constant random movement of particles is the so-called Brownian motion. Two thousand years after Lucretius, French scientist Jean-Baptiste Perrin, armed with a microscope and, more importantly, a mathematical theory, measured the random motions of suspended dye particles and calculated the number of the invisible molecules whose collisions were causing the visible dye particles to move. This way of counting molecules helped substantiate the existence of atoms and molecules.
Chemistry, Organic, branch of chemistry in which carbon compounds and their reactions are studied. A wide variety of classes of substances—such as drugs, vitamins, plastics, natural and synthetic fibers, as well as carbohydrates, proteins, and fats—consist of organic molecules. Organic chemists determine the structures of organic molecules, study their various reactions, and develop procedures for the synthesis of organic compounds. Organic chemistry has had a profound effect on modern life: It has improved natural materials and it has synthesized natural and artificial materials that have, in turn, improved health, increased comfort, and added to the convenience of nearly every product manufactured today.
The advent of organic chemistry is often associated with the discovery in 1828 by the German chemist Friedrich Wöhler that the inorganic, or mineral, substance called ammonium cyanate could be converted in the laboratory to urea, an organic substance found in the urine of many animals. Before this discovery, chemists thought that intervention by a so-called life force was necessary for the synthesis of organic substances. Wöhler's experiment broke down the barrier between inorganic and organic substances. Modern chemists consider organic compounds to be those containing carbon and one or more other elements, most often hydrogen, oxygen, nitrogen, sulfur, or the halogens, but sometimes others as well.
Chemistry, Physical, field of science that applies the laws of physics to elucidate the properties of chemical substances and clarify the characteristics of chemical phenomena. The term physical chemistry is usually applied to the study of the physical properties of substances, such as vapor pressure, surface tension, viscosity, refractive index, density, and crystallography, as well as to the study of the so-called classical aspects of the behavior of chemical systems, such as thermal properties, equilibria, rates of reactions, mechanisms of reactions, and ionization phenomena (see Chemical Reaction; Heat; Heat Transfer; Ionization). In its more theoretical aspects, physical chemistry attempts to explain spectral properties of substances in terms of fundamental quantum theory; the interaction of energy with matter; the nature of chemical bonding; the relationships correlating the number and energy states of electrons in atoms and molecules with the observable properties shown by these systems; and the electrical, thermal, and mechanical effects of individual electrons and protons on solids and liquids.

Chemistry, Inorganic, study of the structure, properties, and reactions of the chemical elements and their compounds. Inorganic chemistry does not include the investigation of hydrocarbons—compounds composed of carbon and hydrogen that are the parent material of all other organic compounds. The study of organic compounds is called organic chemistry.
Inorganic chemists have made significant advances in understanding the minute particles that compose our world. These particles, called atoms, make up the elements, which are the building blocks of all the compounds and substances in the world around us. Just as the entire English language is constructed from combinations of the 26 letters in the alphabet, all chemical substances are made from combinations of the 112 chemical elements found on the periodic table (see Periodic Law).
Ninety elements are known to occur in nature, and 22 more have been made artificially. Elements—which include substances such as oxygen, nitrogen, and sulfur—cannot be broken into more elementary substances by ordinary chemical means. The elements are arranged in the periodic table in rows from the lightest element (hydrogen) to the heaviest (ununbium). These rows are split so that elements with similar chemical properties fall in the same columns (for more information, see the Periodic Law section of this article).
The smallest representative unit of an element is an atom (see Atom). (For example, the smallest representative of the element helium (He) is a helium atom.) When atoms that come in close contact have a sufficiently large attractive force, a chemical bond, or binding link, forms between them. The combination of two or more atoms bonded together is called a molecule. A molecule is the smallest particle of a substance possessing the specific chemical properties of that substance. For example, an atom of oxygen (O) combines with two atoms of hydrogen (H) to form a water molecule (H2O). While molecules of H2O possess the properties of water, individual oxygen and hydrogen atoms do not.
Much of chemistry can be described as breaking substances apart and putting chemical components together to form new substances. This process is accomplished by breaking chemical bonds between atoms and creating new bonds, a process known as a chemical reaction.

Analytical Chemistry, one of the major branches of modern chemistry. It is subdivided into two main areas, qualitative and quantitative analysis. The former involves the determination of unknown constituents of a substance, and the latter concerns the determination of the relative amounts of such constituents. See Chemical Analysis.

About Brain And It,s Functions,Human Brain And Much More.

Brain,

 portion of the central nervous system contained within the skull. The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. All human emotions—including love, hate, fear, anger, elation, and sadness—are controlled by the brain. It also receives and interprets the countless signals that are sent to it from other parts of the body and from the external environment. The brain makes us conscious, emotional, and intelligent.                                         

Human Brain                                                                                                                                                 

The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (center). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay center for electrical impulses traveling to and from the cerebral cortex.

 II  ANATOMY

The adult human brain is a 1.3-kg (3-lb) mass of pinkish-gray jellylike tissue made up of approximately 100 billion nerve cells, or neurons; neuroglia (supporting-tissue) cells; and vascular (blood-carrying) and other tissues.

Between the brain and the cranium—the part of the skull that directly covers the brain—are three protective membranes, or meninges. The outermost membrane, the dura mater, is the toughest and thickest. Below the dura mater is a middle membrane, called the arachnoid layer. The innermost membrane, the pia mater, consists mainly of small blood vessels and follows the contours of the surface of the brain.

A clear liquid, the cerebrospinal fluid, bathes the entire brain and fills a series of four cavities, called ventricles, near the center of the brain. The cerebrospinal fluid protects the internal portion of the brain from varying pressures and transports chemical substances within the nervous system.

From the outside, the brain appears as three distinct but connected parts: the cerebrum (the Latin word for brain)—two large, almost symmetrical hemispheres; the cerebellum (“little brain”)—two smaller hemispheres located at the back of the cerebrum; and the brain stem—a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. Two other major parts of the brain, the thalamus and the hypothalamus, lie in the midline above the brain stem underneath the cerebellum.

The brain and the spinal cord together make up the central nervous system, which communicates with the rest of the body through the peripheral nervous system. The peripheral nervous system consists of 12 pairs of cranial nerves extending from the cerebrum and brain stem; a system of other nerves branching throughout the body from the spinal cord; and the autonomic nervous system, which regulates vital functions not under conscious control, such as the activity of the heart muscle, smooth muscle (involuntary muscle found in the skin, blood vessels, and internal organs), and glands.

A  Cerebrum

Most high-level brain functions take place in the cerebrum. Its two large hemispheres make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter. The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortex—about 1.5 m2 (16 ft2) in an adult—to fit within the cranium. The pattern of these convolutions is similar, although not identical, in all humans.

The two cerebral hemispheres are partially separated from each other by a deep fold known as the longitudinal fissure. Communication between the two hemispheres is through several concentrated bundles of axons, called commissures, the largest of which is the corpus callosum.

Several major sulci divide the cortex into distinguishable regions. The central sulcus, or Rolandic fissure, runs from the middle of the top of each hemisphere downward, forward, and toward another major sulcus, the lateral (“side”), or Sylvian, sulcus. These and other sulci and gyri divide the cerebrum into five lobes: the frontal, parietal, temporal, and occipital lobes and the insula.
The frontal lobe is the largest of the five and consists of all the cortex in front of the central sulcus. Broca's area, a part of the cortex related to speech, is located in the frontal lobe. The parietal lobe consists of the cortex behind the central sulcus to a sulcus near the back of the cerebrum known as the parieto-occipital sulcus. The parieto-occipital sulcus, in turn, forms the front border of the occipital lobe, which is the rearmost part of the cerebrum. The temporal lobe is to the side of and below the lateral sulcus. Wernicke's area, a part of the cortex related to the understanding of language, is located in the temporal lobe. The insula lies deep within the folds of the lateral sulcus.

The cerebrum receives information from all the sense organs and sends motor commands (signals that result in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side across the top of the cerebrum just in front of the central sulcus. The sensory cortex, a parallel strip of cerebral cortex just in back of the central sulcus, receives input from the sense organs.

Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering. Cortical regions known as associative cortex are responsible for integrating multiple inputs, processing the information, and carrying out complex responses.

B  Cerebellum

The cerebellum coordinates body movements. Located at the lower back of the brain beneath the occipital lobes, the cerebellum is divided into two lateral (side-by-side) lobes connected by a fingerlike bundle of white fibers called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum, the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fiber bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem—the midbrain, the pons, and the medulla oblongata.

The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.

C  Thalamus and Hypothalamus

 The thalamus and the hypothalamus lie underneath the cerebrum and connect it to the brain stem. The thalamus consists of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres. The thalamus is the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory input to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.

The hypothalamus lies beneath the thalamus on the midline at the base of the brain. It regulates or is involved directly in the control of many of the body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity. It also controls the function of internal body organs by means of the autonomic nervous system, interacts closely with the pituitary gland, and helps coordinate activities of the brain stem.

D  Brain Stem

The brain stem is evolutionarily the most primitive part of the brain and is responsible for sustaining the basic functions of life, such as breathing and blood pressure. It includes three main structures lying between and below the two cerebral hemispheres—the midbrain, pons, and medulla oblongata.

D1  Midbrain

 The topmost structure of the brain stem is the midbrain. It contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers—pathways carrying sensory (input) information and motor (output) commands. Relay and reflex centers for visual and auditory (hearing) functions are located in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focusing the lens. A second pair of nuclei, called the inferior colliculus, control auditory reflexes, such as adjusting the ear to the volume of sound. At the bottom of the midbrain are reflex and relay centers relating to pain, temperature, and touch, as well as several regions associated with the control of movement, such as the red nucleus and the substantia nigra.

D2  Pons

Continuous with and below the midbrain and directly in front of the cerebellum is a prominent bulge in the brain stem called the pons. The pons consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.

D3  Medulla Oblongata

 The long, stalklike lowermost portion of the brain stem is called the medulla oblongata. At the top, it is continuous with the pons and the midbrain; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibers connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half of the brain communicates with the right half of the body, and the right half of the brain with the left half of the body.

D4  Reticular Formation

Running up the brain stem from the medulla oblongata through the pons and the midbrain is a netlike formation of nuclei known as the reticular formation. The reticular formation controls respiration, cardiovascular function (see Heart), digestion, levels of alertness, and patterns of sleep. It also determines which parts of the constant flow of sensory information into the body are received by the cerebrum.

E  Brain Cells

There are two main types of brain cells: neurons and neuroglia. Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system. Each neuron is composed of a cell body called a soma, a major fiber called an axon, and a system of branches called dendrites. Axons, also called nerve fibers, convey electrical signals away from the soma and can be up to 1 m (3.3 ft) in length. Most axons are covered with a protective sheath of myelin, a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.

Neuroglial cells are twice as numerous as neurons and account for half of the brain's weight. Neuroglia (from glia, Greek for “glue”) provide structural support to the neurons. Neuroglial cells also form myelin, guide developing neurons, take up chemicals involved in cell-to-cell communication, and contribute to the maintenance of the environment around neurons.

F  Cranial Nerves

Twelve pairs of cranial nerves arise symmetrically from the base of the brain and are numbered, from front to back, in the order in which they arise. They connect mainly with structures of the head and neck, such as the eyes, ears, nose, mouth, tongue, and throat. Some are motor nerves, controlling muscle movement; some are sensory nerves, conveying information from the sense organs; and others contain fibers for both sensory and motor impulses. The first and second pairs of cranial nerves—the olfactory (smell) nerve and the optic (vision) nerve—carry sensory information from the nose and eyes, respectively, to the undersurface of the cerebral hemispheres. The other ten pairs of cranial nerves originate in or end in the brain stem.

III  HOW THE BRAIN WORKS

 The brain functions by complex neuronal, or nerve cell, circuits (see Neurophysiology). Communication between neurons is both electrical and chemical and always travels from the dendrites of a neuron, through its soma, and out its axon to the dendrites of another neuron.

Dendrites of one neuron receive signals from the axons of other neurons through chemicals known as neurotransmitters. The neurotransmitters set off electrical charges in the dendrites, which then carry the signals electrochemically to the soma. The soma integrates the information, which is then transmitted electrochemically down the axon to its tip.

At the tip of the axon, small, bubblelike structures called vesicles release neurotransmitters that carry the signal across the synapse, or gap, between two neurons. There are many types of neurotransmitters, including norepinephrine, dopamine, and serotonin. Neurotransmitters can be excitatory (that is, they excite an electrochemical response in the dendrite receptors) or inhibitory (they block the response of the dendrite receptors).

One neuron may communicate with thousands of other neurons, and many thousands of neurons are involved with even the simplest behavior. It is believed that these connections and their efficiency can be modified, or altered, by experience.

Scientists have used two primary approaches to studying how the brain works. One approach is to study brain function after parts of the brain have been damaged. Functions that disappear or that are no longer normal after injury to specific regions of the brain can often be associated with the damaged areas. The second approach is to study the response of the brain to direct stimulation or to stimulation of various sense organs.

Neurons are grouped by function into collections of cells called nuclei. These nuclei are connected to form sensory, motor, and other systems. Scientists can study the function of somatosensory (pain and touch), motor, olfactory, visual, auditory, language, and other systems by measuring the physiological (physical and chemical) changes that occur in the brain when these senses are activated. For example, electroencephalography (EEG) measures the electrical activity of specific groups of neurons through electrodes attached to the surface of the skull. Electrodes inserted directly into the brain can give readings of individual neurons. Changes in blood flow, glucose (sugar), or oxygen consumption in groups of active cells can also be mapped.

Although the brain appears symmetrical, how it functions is not. Each hemisphere is specialized and dominates the other in certain functions. Research has shown that hemispheric dominance is related to whether a person is predominantly right-handed or left-handed (see Handedness). In most right-handed people, the left hemisphere processes arithmetic, language, and speech. The right hemisphere interprets music, complex imagery, and spatial relationships and recognizes and expresses emotion. In left-handed people, the pattern of brain organization is more variable.

Hemispheric specialization has traditionally been studied in people who have sustained damage to the connections between the two hemispheres, as may occur with stroke, an interruption of blood flow to an area of the brain that causes the death of nerve cells in that area. The division of functions between the two hemispheres has also been studied in people who have had to have the connection between the two hemispheres surgically cut in order to control severe epilepsy, a neurological disease characterized by convulsions and loss of consciousness.

A  Vision

The visual system of humans is one of the most advanced sensory systems in the body (see Vision). More information is conveyed visually than by any other means. In addition to the structures of the eye itself, several cortical regions—collectively called primary visual and visual associative cortex—as well as the midbrain are involved in the visual system. Conscious processing of visual input occurs in the primary visual cortex, but reflexive—that is, immediate and unconscious—responses occur at the superior colliculus in the midbrain. Associative cortical regions—specialized regions that can associate, or integrate, multiple inputs—in the parietal and frontal lobes along with parts of the temporal lobe are also involved in the processing of visual information and the establishment of visual memories.

B  Language

Language involves specialized cortical regions in a complex interaction that allows the brain to comprehend and communicate abstract ideas. The motor cortex initiates impulses that travel through the brain stem to produce audible sounds. Neighboring regions of motor cortex, called the supplemental motor cortex, are involved in sequencing and coordinating sounds. Broca's area of the frontal lobe is responsible for the sequencing of language elements for output. The comprehension of language is dependent upon Wernicke's area of the temporal lobe. Other cortical circuits connect these areas.

C  Memory

Memory is usually considered a diffusely stored associative process—that is, it puts together information from many different sources. Although research has failed to identify specific sites in the brain as locations of individual memories, certain brain areas are critical for memory to function. Immediate recall—the ability to repeat short series of words or numbers immediately after hearing them—is thought to be located in the auditory associative cortex. Short-term memory—the ability to retain a limited amount of information for up to an hour—is located in the deep temporal lobe. Long-term memory probably involves exchanges between the medial temporal lobe, various cortical regions, and the midbrain.

D  The Autonomic Nervous System

The autonomic nervous system regulates the life support systems of the body reflexively—that is, without conscious direction. It automatically controls the muscles of the heart, digestive system, and lungs; certain glands; and homeostasis—that is, the equilibrium of the internal environment of the body (see Physiology). The autonomic nervous system itself is controlled by nerve centers in the spinal cord and brain stem and is fine-tuned by regions higher in the brain, such as the midbrain and cortex. Reactions such as blushing indicate that cognitive, or thinking, centers of the brain are also involved in autonomic responses.

IV  BRAIN DISORDERS
The brain is guarded by several highly developed protective mechanisms. The bony cranium, the surrounding meninges, and the cerebrospinal fluid all contribute to the mechanical protection of the brain. In addition, a filtration system called the blood-brain barrier protects the brain from exposure to potentially harmful substances carried in the bloodstream.

Brain disorders have a wide range of causes, including head injury, stroke, bacterial diseases, complex chemical imbalances, and changes associated with aging.

A  Head Injury

Head injury can initiate a cascade of damaging events. After a blow to the head, a person may be stunned or may become unconscious for a moment. This injury, called a concussion, usually leaves no permanent damage. If the blow is more severe and hemorrhage (excessive bleeding) and swelling occur, however, severe headache, dizziness, paralysis, a convulsion, or temporary blindness may result, depending on the area of the brain affected. Damage to the cerebrum can also result in profound personality changes.

Damage to Broca's area in the frontal lobe causes difficulty in speaking and writing, a problem known as Broca's aphasia. Injury to Wernicke's area in the left temporal lobe results in an inability to comprehend spoken language, called Wernicke's aphasia.

An injury or disturbance to a part of the hypothalamus may cause a variety of different symptoms, such as loss of appetite with an extreme drop in body weight; increase in appetite leading to obesity; extraordinary thirst with excessive urination (diabetes insipidus); failure in body-temperature control, resulting in either low temperature (hypothermia) or high temperature (fever); excessive emotionality; and uncontrolled anger or aggression. If the relationship between the hypothalamus and the pituitary gland is damaged (see Endocrine System), other vital bodily functions may be disturbed, such as sexual function, metabolism, and cardiovascular activity.

Injury to the brain stem is even more serious because it houses the nerve centers that control breathing and heart action. Damage to the medulla oblongata usually results in immediate death.

Information About Different Cats Of The World,

Cat,

 small, mainly carnivorous animal, Felis catus, member of the family Felidae, popular as a household pet, and valuable for killing mice and rats. Like other members of the cat family, the domestic cat has retractile claws; keen hearing and smell; remarkable night vision; and a compact, muscular, and highly supple body. Cats possess excellent memory and exhibit considerable aptitude for learning by observation and experience. The natural life span of a domestic cat is about 15 years.
Most authorities believe that the shorthaired breeds of domestic cat are derived from the Caffre cat, Felis libyca, a species of African wildcat domesticated by the ancient Egyptians perhaps as early as 2500 bc and transported by the Crusaders to Europe, where it interbred with the indigenous smaller wildcats. The longhaired breeds may have sprung from the Asian wildcat, Felis manul. Over the centuries, cats have remained virtually the same in size, weighing about 3.6 kg (about 8 lb) when full-grown, and have preserved their instinct for solitary hunting.
A Physiology of the Cat
The body of a domestic cat is extremely flexible; its skeleton contains more than 230 bones (the human skeleton, although much larger, contains 206 bones), and its pelvis and shoulders are more loosely attached to its spine than in most other quadrupeds. The cat's great leaping ability and speed are due in part to its powerful musculature. Its tail provides balance when jumping or falling.
The cat's claws are designed for catching and holding prey. The sharp, hooked, retractile claws are sheathed in a soft, leathery pocket at the end of each toe, and are extended for fighting, hunting, and climbing. The cat marks its territory by scratching and scenting trees or other objects; its claws leave visible scratch marks, and the scent glands on its paw pads leave a scent mark.
The cat's teeth are designed for biting, not for chewing. Its powerful jaw muscles and sharp teeth enable the cat to deliver a killing bite to its prey.
B  Senses
The cat's vision is exceptionally well adapted for hunting, especially at night. It has excellent night vision; extensive peripheral vision; and binocular vision, which enables it to accurately judge distances. The cat's daylight vision is not as good as that of humans; cats see movement much more easily than detail, and are thought to see only a limited range of colors.
The cat's hearing is extremely sensitive. It can hear a wide range of sounds, including those in the ultrasonic range. Its ears are less sensitive to lower frequencies, which may explain why some domestic cats are more responsive to female voices than to male voices. The cat can turn its ears to focus on different sounds.
The cat has a highly developed sense of smell, which plays a vital role in finding food and in reproduction. Many of the social signals of domestic cats take the form of scents; for example, male cats can apparently smell a female cat that is receptive to male cats from a distance of hundreds of meters or yards.
The cat's sense of taste is peculiarly specialized: it has little ability to detect sweetness, but is extremely sensitive to slight variations in the taste of water. The cat's tongue is covered with rough protuberances, or papillae, that it uses to rasp meat from bones. It also uses its tongue to groom itself.
The cat's whiskers, or vibrissae, are extremely sensitive to the slightest touch, and are used for testing obstacles and sensing changes in the environment. In extremely dim light, a cat may feel its way by using its whiskers.
C  Reproduction
The domestic cat usually reaches puberty at around nine or ten months of age. A sexually mature female cat goes into heat, or estrus, several times a year; during estrus, she is both receptive to, and attractive to, male cats. The gestation period of the cat is about 65 days; the average litter consists of 4 kittens. Kittens are born blind, deaf, and helpless. Their eyes open at 8 to 10 days of age, and they begin to be weaned about 6 weeks after birth.
D  Coat Colors
The domestic cat's original coat color was probably grayish-brown with darker tabby stripes, a color that provides excellent camouflage in a variety of environments. All other coat colors and patterns are the result of genetic mutations; for example, solid coat colors such as black and blue are the result of a gene that suppresses tabby stripes; an orange coat is the result of a gene that transforms black pigment to orange; and a solid white coat is the result of a gene that completely suppresses all formation of pigment.
Two pigments, black and orange, form the basis for all coat colors in the modern domestic cat. These pigments may be combined with each other or with white (the absence of pigment). A single gene, the O (Orange) gene, determines whether a cat's coat contains black or orange pigment. The O gene can be thought of as a switch that is either on (orange) or off (black). The gene is located on the X chromosome, so its inheritance is sex-linked.
III  CAT BREEDS
About 40 varieties, or breeds, of domestic cats are recognized internationally. Although the various cat breeds often differ dramatically in coat length and overall look, they vary less in size than do dog breeds. The smallest cat breeds weigh about 2 to 3 kg (about 5 to 7 lb) when full-grown; the largest weigh about 7 to 9 kg (about 15 to 20 lb). So far, attempts to develop miniature or giant domestic cat breeds have been unsuccessful.
A  Breed Origins
 Many domestic cat breeds, including the Maine coon, Manx, Russian blue, and Siamese, began as a naturally occurring variety of domestic cat native to a specific geographic area. Others, such as the Himalayan, are artificially created breeds, the result of generations of careful breeding for a desired look. Some relatively new breeds, including the curly-coated Rex breeds, the hairless Sphynx, the fold-eared Scottish fold, and the curl-eared American curl, began with a genetic mutation and were then developed by selective breeding into a distinct breed.
B  Breed Standards
For each domestic cat breed, there is an official standard of perfection registered with different cat associations that describes the ideal cat of that breed and its distinctive features; lists desirable and undesirable characteristics; and mentions faults that, in a cat show, could result in penalty or disqualification. For example, in the Siamese breed standard, the eyes are described as almond-shaped and slanting toward the nose; a tendency to squint is penalized, and crossed eyes are a disqualifying fault.
Breed standards differ slightly from cat association to cat association, and not all cat associations recognize every breed. To become recognized in a particular cat association, a breed must first be accepted for provisional status by that association. To become recognized for championship competition, the breed must complete a rigorous set of requirements that differ from association to association.
IV  THE CARE OF CATS
Cats are known for their ability to fend for themselves in the wild, but household pets, dependent on human beings for care and feeding, require considerable attention. Educational materials on the care of cats and responsible cat ownership are available through bookstores and local humane societies.
A  General Care
Although cats have a reputation for being relatively independent, domestic cats require love and attention from their owners. A balanced daily diet, such as that provided by high-quality commercial cat food, is essential for health and longevity, as is a regular supply of fresh water. Regular cleaning of litter pans is necessary to prevent disease; some cats will refuse to use a badly soiled litter pan. Cats' claws should be trimmed frequently. To prevent damage to furniture, cats that live indoors should be provided with a scratching post, preferably covered with a rough material such as sisal rope. Cats use their tongues to clean their coats, and they normally swallow any loose hair. All cats, including shorthairs, should be brushed weekly to remove loose hair; this will help prevent hairballs from forming in their stomachs. A few longhaired breeds, such as the Persian and the Himalayan, require daily combing to prevent their long, soft fur from matting.
B  Neutering or Spaying
Every year hundreds of thousands of unwanted domestic cats and kittens are destroyed because homes cannot be found for them. To avoid contributing to this problem, a cat should be altered (surgically treated to make it incapable of reproducing) unless it is a registered, pedigreed member of a responsible breeding program. A female cat is spayed (altered by removing the uterus and ovaries); a male cat is neutered (altered by removing the testicles). Cats that have been altered are healthier and easier to live with. Unaltered females may be susceptible to uterine infections and ovarian cysts; unaltered cats of both sexes may mark their territory by spraying urine. Some veterinarians recommend altering cats as young as 12 weeks of age, while others recommend waiting until the animal reaches sexual maturity (at six to ten months of age). Current veterinary research indicates that early altering has little negative effect on a cat's health; a low-quality diet, however, can cause serious urinary tract problems.
C  Indoors vs. Outdoors
Some domestic cat owners choose to keep their cats indoors; others permit their cats to go outdoors some or all of the time. The decision of whether to allow a cat outdoors is a personal one; cats that have been declawed, however, and those that have not been altered, should not be allowed outdoors unless confined to a covered enclosure.
Cats that are allowed outside have some degree of freedom and independence, and may enjoy hunting small animals and interacting with other cats; they get plenty of exercise and are unlikely to become bored or lonely. The outdoors, however, poses many hazards to cats, even in rural areas. An outdoor cat may be struck by a car, poisoned by common pesticides, or injured by other animals (other cats, dogs, and, in some areas, wild animals such as coyotes). In addition, the cat may be exposed to the fatal feline diseases that are endemic in the stray cat population. According to some authorities, a cat that is permitted outdoors has an average life expectancy of 2 to 3 years; conversely, the average life expectancy of an indoor cat is about 15 years.
Although an indoor cat does not enjoy the same freedom as an outdoor cat, many indoor cats live happy and complete lives. It is easier to keep a cat indoors if it has not become accustomed to going out. Indoor cats need exercise just as outdoor cats do. Some cats can be trained to use a harness leash. Often, the easiest way to provide an indoor cat with exercise and stimulation is to provide a feline companion.
D  Cat Diseases
Domestic cats are susceptible to a variety of viral and bacterial diseases. Fortunately, many common feline diseases can be controlled by a regular system of inoculation. Cats may also suffer from external parasites such as fleas and mites, and from intestinal parasites (worms). Cats can contract rabies from infected prey or other infected animals, but such instances are rare.
Upper respiratory infections are a common feline illness and can sometimes be fatal, especially in young kittens. Vaccines provide some protection against the following upper respiratory diseases: feline viral rhinotracheitis (FVR), feline calicivirus (FCV), and chlamydia (feline pneumonitis).
Panleukopenia (feline infectious enteritis) is a highly contagious, often fatal disease characterized by a sudden onset and severe gastrointestinal symptoms such as vomiting and diarrhea. Vaccination is the only effective way to control the disease.
Feline leukemia virus (FeLV) is a fatal, contagious disease that is spread by direct contact. A cat with feline leukemia may have a variety of symptoms, including general malaise, weight loss, anemia, and fever. An infected cat may transmit the disease to other cats before it develops clinical symptoms itself. A blood test can detect whether a cat has been infected. Although a vaccine is available, the most reliable way to prevent a cat from contracting feline leukemia is to keep it from coming into contact with FeLV-positive cats.
Feline infectious peritonitis (FIP) is an inflammation of the peritoneum (lining of the abdomen). Although FIP is contagious, some cats appear to develop a natural immunity to it. An infected cat may be a symptomless carrier. Once a cat develops symptoms, the disease is invariably fatal. There is no reliable blood test for FIP, but a vaccine is now available.
E  Inoculations
Cats can be successfully inoculated against many serious feline diseases. Kittens should be inoculated against rhinotracheitis, calicivirus, panleukopenia, and, optionally, chlamydia. Most veterinarians recommend a series of two or three inoculations, given every 3 weeks starting at 6 weeks of age. After 12 weeks of age, a kitten may also be inoculated against rabies, feline leukemia, and feline infectious peritonitis. Inoculations should be repeated annually to maintain immunity.
V  SHOWING AND JUDGING CATS
Many owners, even those of mixed-breed cats, enjoy exhibiting their cats at shows. Judges at cat shows must be trained and certified. Purebred cats are judged on health, temperament, and how well they fit the official standard for their breed. Mixed-breed cats are judged on health, temperament, and general appearance. All cats are expected to be amenable to handling; a cat may be disqualified if it bites or otherwise injures a judge.
A  Cat Associations
A cat association is an organization that registers cats and kittens, selects cat show judges, and schedules cat shows. There are various cat associations in the United States, including the Cat Fanciers' Association (CFA), The International Cat Association (TICA), and the American Cat Fanciers Association (ACFA). The largest of these groups, the CFA, registers more than 80,000 cats and kittens annually. All of the cat associations operate independently; cat clubs, breeders, and exhibitors choose which associations they wish to join and whose breed standards and rules they wish to follow.
B  Cat Shows
An increasing number of local, regional, and national cat shows are held throughout the year in the United States, with hundreds of cats competing for awards. Owners show their cats for fun and to gain a reputation among other exhibitors and breeders. Cat shows typically do not award monetary prizes, and the entry fees and travel expenses can be expensive.
Although exact show rules and procedures vary from association to association, the general format is the same. There are four categories of competition: purebred kittens, purebred adults, purebred alters (cats that have been neutered or spayed), and household pets (mixed-breed cats or kittens).
A single cat show may have as many as 20 or more different judges; usually, a cat is judged by every judge in the show. At cat shows in the United States, each judge has his or her own ring—an area consisting of 10 to 15 numbered cages and a judging table. Cats wait in cages in another area of the show hall, called the benching area. The owners bring the cats to the ring when called and place them in the judging cages. The judge takes each cat out of its cage in turn, places it on the judging table, and examines the cat carefully to make sure that it is healthy and meets the standard for that breed. After judging each cat within a particular class or breed, the judge gives out preliminary awards, such as Best of Color or Best of Breed. After judging all the cats in a category, the judge gives top awards to the ten best cats in that category. Each judge works independently, and judges' opinions sometimes differ markedly.
VI  CAT LORE
Cats and humans have interacted for thousands of years. These animals have figured in the history of many nations, are the subject of much superstition and legend, and are a favorite subject of artists and writers.
A  History and Legend
Cats became objects of worship in ancient Egypt because of their ability to keep down the rodent population in the country's economically important grain fields along the Nile. The Egyptian cat goddess Bast, or Bastet, became associated with fertility and childbearing. Egyptian cats were also used for sport by their owners. Attached to leashes, these animals hunted birds for the family table; a boomerang flung by the master brought the birds down and the cats, unleashed, would retrieve them. Because they were economically useful and were believed to ensure many children for a family, cats were so revered that they were mummified and buried either with their owners or in specially designated cemeteries.
Despite Egyptian laws that forbade the removal of the sacred cats, Phoenician sailors smuggled them out of the country. Cats were traded along with other treasures from the Middle East and in antiquity could be found throughout the Mediterranean area. Archaeological evidence indicates that the Romans were the first to bring cats to the British Isles.
Throughout much of the Middle Ages, cats were feared and hated. Because of their nocturnal habits, they were believed to consort with the devil. This association with witchcraft has been responsible for many acts of cruelty toward cats through the centuries. The Renaissance, in contrast, was the golden age for cats. Almost everyone owned one, from members of royal families and their staffs down to the peasantry.
The first domestic cats in North America arrived with the colonists and were employed to keep the rodent population under control in the settlers' fields, barns, and homes. Cats are said to have played an important part in keeping rats out of the California gold mines.
In India cats often played an important part in religious or occult ceremonies. In South America the Inca revered sacred cats; cats are represented in pre-Columbian Peruvian artifacts. Cats continue to be worshiped as deities in countries such as Thailand and China.
B  Cats in Art and Literature
Egyptian tomb paintings and sculpture are the earliest representations of the domestic cat. Images of cats appear on Greek coins of the 5th century bc; cats were later depicted in Roman mosaics and paintings and on earthenware, coins, and shields. The 8th-century Irish manuscript of the Gospels, the Book of Kells, has a representation of cats and kittens in one of its illuminations. Later artists, such as the Italian artist Leonardo da Vinci and his German contemporary Albrecht Dürer, are among the many who included cats in their works.
Although the Old Testament makes no mention of cats, the Babylonian Talmud tells of their admirable qualities and encourages the breeding of cats “to help keep the houses clean.” Memorable literary cats include the British writer Rudyard Kipling's “Cat That Walked by Himself” (one of the Just So Stories, 1902), the delightful cats of Old Possum's Book of Practical Cats (1939) by the Anglo-American poet T. S. Eliot, and the Cheshire Cat, joint creation of the English writer Lewis Carroll and the illustrator Sir John Tenniel in the children's classic Alice's Adventures in Wonderland (1865). Many contemporary comic strips and animated cartoons also contain feline characters that delight ailurophiles (lovers of cats) of all ages.

The Sun And It,s Introduction, Find The Best Online Information.

Sun,


                                                   

 closest star to Earth. The Sun is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth and the other planets in the solar system in orbit. The Sun’s light and heat influence all of the objects in the solar system and allow life to exist on Earth.
The Sun is an average star—its size, age, and temperature fall in about the middle of the ranges of these properties for all stars. Astronomers believe that the Sun is about 4.6 billion years old and will keep shining for about another 7 billion years.
For humans, the Sun is beautiful and useful, but also powerful and dangerous. As Earth turns, the Sun rises over the eastern horizon in the morning, passes across the sky during the day, and sets in the west in the evening. This movement of the Sun across the sky marks the passage of time during the day (see Sundial). The Sun’s movement can produce spectacular sunrises and sunsets under the right atmospheric conditions. At night, reflected sunlight makes the Moon and planets bright in the night sky.
The Sun provides Earth with vast amounts of energy every day. The oceans and seas store this energy and help keep the temperature of Earth at a level that allows a wide variety of life to exist. Plants use the Sun’s energy to make food, and plants provide food for other organisms. The Sun’s energy also creates wind in Earth’s atmosphere. This wind can be harnessed and used to produce power.While it lights our day and provides energy for life, sunlight can also be harmful to people. Human skin is sensitive to ultraviolet light emitted from the Sun. Earth’s atmosphere blocks much of the harmful light, but sunlight is still strong enough to burn skin under some conditions (see Burn). Sunburn is one of the most important risk factors in the development of skin cancers, which can be fatal. Sunlight is also very harmful to human eyes. A person should never look directly at the Sun, even with sunglasses or during an eclipse. The Sun influences Earth with more than just light. Particles flowing from the Sun can disrupt Earth’s magnetic field, and these disruptions can interfere with electronic communications.
Characteristics of the Sun, The distance between the sun and the earth varies because the earth travels in an elliptical rather than circular orbit. The distance is roughly 100 times the sun’s diameter. Turbulence in the photosphere forms granules of various sizes and sunspots. Temperature is a measure of kinetic energy. The dense plasma in the center of the sun is roughly 2500 times hotter than the surface. Gases in the corona have escaped from the sun’s surface and have a very high velocity. The sun’s spectral type, G2, indicates that it is composed of hydrogen, helium, calcium,iron and other metals.
II PHYSICAL CHARACTERISTICS
The Sun is large and massive compared to the other objects in the solar system. The Sun’s radius (the distance from its center to its surface) is 695,508 km (432,169 mi), 109 times as large as Earth’s radius. If the Sun were hollow, a million Earths could fit inside it. The Sun has a mass of 1.989 × 1027 metric tons. This number is very large. Written out, it would be the digits 1989 followed by 24 zeroes. The Sun is 333,000 times as massive as Earth is. Despite its large mass, the Sun has a lower density, or mass per unit volume, than Earth. The Sun’s average density is only 1.409 g/cu cm (1.188 oz/cu in), which is a quarter of the average density of Earth.
The Sun produces an enormous amount of light. It generates 3.83 × 1026 watts of power in the form of light. In comparison, an incandescent lamp emits 60 to 100 watts of power. The temperature of the outer, visible part of the Sun is 5500°C (9900°F).
From Earth the Sun looks small, because it is far away. Its average distance from Earth is 150 million km (93 million mi). Light from the Sun takes about eight minutes to reach Earth. This light is still strong enough when it reaches Earth, however, to damage human eyes when viewed directly. The Sun is much closer to Earth than any other star is. The Sun’s nearest stellar neighbor, Proxima Centauri (part of the triple star Alpha Centauri), is 4.3 light-years from our solar system, meaning light from Proxima Centauri takes 4.3 years to reach the Sun. The Sun is so much closer to Earth than all other stars are that the intense light of the Sun keeps us from seeing any other stars during the day.

A  Importance to Earth
Earth would not have any life on it without the Sun’s energy, which reaches Earth in the form of heat and light. This energy warms our days and illuminates our world. Green plants absorb sunlight and convert it to food, which these plants then use to live and grow. In this process, the plants give off the oxygen that animals breathe. Animals eat these plants for nourishment. All plant and animal life relies on the Sun’s presence
The Sun also provides—directly or indirectly—much of the energy on Earth that people use for fuel (see Solar Energy). Devices called solar cells turn sunlight into electricity. Sunlight can heat a gas or liquid, which can then be circulated through a building to heat the building. The energy stored in fossil fuels originally came from the Sun. Ancient plants used sunlight as fuel to grow. Animals consumed these plants. The plants and animals stored the energy of sunlight in the organic material that composed them. When the ancient plants and animals died and decayed, this organic material was buried and gradually turned into the petroleum, coal, and natural gas people use today. The Sun’s energy produces the winds and the movements of water that people harness to produce electricity (see Wind Energy; Water Power). The Sun heats Earth’s oceans and land, which in turn heat the air and make it circulate in the atmosphere as wind. The Sun fuels Earth’s water cycle, evaporating water from the oceans, seas, and lakes. This water returns to the ground in the form of precipitation, flowing back to the oceans through the ground and in rivers. The energy of water’s motion in rivers can be harnessed with dams.

B  Role in the Solar System

The Sun’s gravitational pull holds the solar system together. The planets, asteroids, comets, and dust that make up our solar system are strongly attracted to the Sun’s huge mass. This gravitational attraction keeps these bodies in orbit around the Sun. The Sun also influences the solar system with its diffuse outer atmosphere, which expands outward in all directions. This expanding atmosphere fills the solar system with a constant flow of tiny, fast, electrically charged particles. This flow is called the solar wind. The region through which the solar wind blows is called the heliosphere. Estimates vary about the extent of the heliosphere, ranging from about 86 to about 100 times the distance between Earth and the Sun. Interstellar winds may give the heliosphere an egg shape. The solar wind spreads out as it leaves the Sun. The point at which the solar wind is so diffuse that it stops having an effect on its surroundings is called the heliopause. The heliopause marks the outer edge of the solar system. Within the heliosphere, the Sun provides most of the heat and light that are present, and the particles in the solar wind interact with the planets and satellites in the solar system. The solar wind causes auroras—displays of colored light—in the atmosphere of Earth’s polar regions. The solar wind also carries remnants of the Sun’s magnetic field, which affect the magnetic fields of the planets and larger satellites. The solar wind pushes the planets’ magnetic fields away from the Sun, turning them into elongated, windsock shapes. For more information, see the Solar Wind section of this article.

III  THE SUN AS A STAR
The Sun is extremely important to Earth and to our solar system, but on the scale of the galaxy and the universe, the Sun is just an average star. It is one of hundreds of billions of stars in our galaxy, the Milky Way, which is just one of more than 100 billion galaxies in the observable universe
The Sun’s Place in the Milky Way
The Milky Way Galaxy contains about 400 billion stars. All of these stars, and the gas and dust between them, are rotating about a galactic center. Stars that are farther away from the center move at slower speeds and take longer to go around it.

The Sun is located in the outer part of the galaxy, at a distance of 2.6 × 1017 km (1.6 × 1017 mi) from the center. The Sun, which is moving around the center at a velocity of 220 km/s (140 mi/s), takes 250 million years to complete one trip around the center of the galaxy. The Sun has circled the galaxy more than 18 times during its 4.6-billion-year lifetime.

B  Comparisons with Other Stars
A star is a ball of hot, glowing gas that is hot enough and dense enough to trigger nuclear reactions, which fuel the star. In comparing the mass, light production, and size of the Sun to other stars, astronomers find that the Sun is a perfectly ordinary star. It behaves exactly the way they would expect a star of its size to behave. The main difference between the Sun and other stars is that the Sun is much closer to Earth.Most stars have masses similar to that of the Sun. The majority of stars’ masses are between 0.3 to 3.0 times the mass of the Sun. Theoretical calculations indicate that in order to trigger nuclear reactions and to create its own energy—that is, to become a star—a body must have a mass greater than 7 percent of the mass of the Sun. Astronomical bodies that are less massive than this become planets or objects called brown dwarfs. The largest accurately determined stellar mass is of a star called V382 Cygni and is 27 times that of the Sun.The range of brightness among stars is much larger than the range of mass. Astronomers measure the brightness of a star by measuring its magnitude and luminosity. Magnitude allows astronomers to rank how bright, comparatively, different stars appear to humans. Because of the way our eyes detect light, a lamp ten times more luminous than a second lamp will appear less than ten times brighter to human eyes. This discrepancy affects the magnitude scale, as does the tradition of giving brighter stars lower magnitudes. The lower a star’s magnitude, the brighter it is. Stars with negative magnitudes are the brightest of all. Magnitude is given in terms of absolute and apparent values. Absolute magnitude is a measurement of how bright a star would appear if viewed from a set distance away. By convention, this distance is 10 parsecs, or 32.6 light-years. Apparent magnitude measures how bright a star appears from Earth. The Sun’s absolute magnitude is 4.8. The brightest known stars have absolute magnitudes of about -9 (lower magnitudes mean brighter stars), and the dimmest known stars have absolute magnitudes of about 20. The apparent magnitude of the Sun is -26.72. The apparent magnitude of the brightest star in Earth’s night sky, Sirius, is -1.46. The dimmest stars that can be seen from Earth with unaided eyes have apparent magnitudes of about 6.
Astronomers also measure a star’s brightness in terms of its luminosity. A star’s absolute luminosity or intrinsic brightness is the total amount of energy radiated by the star per second. Luminosity is often expressed in units of watts. The Sun’s absolute luminosity is 3.86 × 1026 watts. The absolute luminosity of stars ranges from one thousandth of the luminosity of the Sun to 10 million times that of the Sun.Another way of measuring brightness is to measure the amount of light that reaches an observer. This measurement is called apparent brightness or apparent luminosity. Apparent luminosity depends on the absolute luminosity of a star and the distance from the star to the observer. Apparent luminosity becomes smaller as distance from the star to the observer becomes larger. From Earth, the apparent luminosity of the Sun is 10 billion times greater than the apparent luminosity of the next brightest star, Sirius, because the Sun is so much closer to Earth. The radius of the Sun is about average among stars. The radii of most stars fall between 0.2 and 15 times the Sun’s radius, although some giant stars are hundreds of times larger than the Sun. Larger stars usually have larger absolute luminosities.We receive much more energy from the Sun than from other stars, because the Sun is so nearby. The Sun’s proximity also allows scientists to study its face in detail. A modest telescope can resolve solar structures that are 700 km (400 mi) across—about the distance from Boston, Massachusetts, to Washington, D.C. That level of detail is comparable to seeing the features on a coin from 1 km (0.6 mi) away. Other stars are so distant that the details on their surfaces remain unresolved with even the largest telescopes.

C  Composition of the Sun

The Sun is a second-generation star, meaning that some of its material came from former stars. Some stars in our galaxy are nearly as old as the expanding universe, which scientists believe originated in the big bang explosion about 14 billion years ago (see Big Bang Theory). In contrast, the Sun is only 4.6 billion years old.The first stars were composed only of the hydrogen and helium produced in the early universe. These stars are called first-generation stars. Although hydrogen is also the main ingredient of the Sun, it contains heavier elements, such as carbon, nitrogen, and oxygen, as well. These elements formed inside first-generation stars that lived and died before the Sun was born. When these massive, short-lived stars used up their internal fuel, they exploded and ejected the heavier elements into interstellar space. The Sun formed from this material, making it a second-generation star.

D  The Sun’s Remote Past and Distant Future
The Sun and planets in our solar system formed when a rotating cloud of dust and gas in space collapsed, or condensed, due to the gravitational attraction between the particles in the cloud. A nearby supernova explosion may have triggered the collapse, or a random fluctuation in the density of the cloud may have started the process. The Sun formed at the center of the spinning cloud, while the debris that condensed into planets formed a flattened disk revolving around the Sun. When the Sun reached its present size about 4.6 billion years ago, it was hot enough inside to ignite the nuclear reactions that make it glow.
The Sun cannot shine forever, because it will eventually use up its present fuel. The nuclear fusion reactions that make the Sun glow (for more information, see the section entitled The Sun’s Energy in this article) depend on the element hydrogen, but the hydrogen in the Sun’s core will eventually run out. Nuclear reactions have converted about 37 percent of the hydrogen originally in the Sun’s core into helium. Astronomers estimate that the Sun’s core will run out of hydrogen in about 7 billion years.
The Sun will grow steadily brighter as time goes on and more helium accumulates in its core. Even as the supply of hydrogen dwindles, the Sun’s core must keep producing enough pressure to keep the Sun from collapsing in on itself. The only way it can do this is to increase its temperature. The increase in temperature raises the rate at which nuclear reactions occur and makes the Sun brighter. In 3 billion years, the Sun will be hot enough to boil Earth’s oceans away. Four billion years thereafter, the Sun will have used up all its hydrogen and will balloon into a giant star that engulfs the planet Mercury. At this point in its life, the Sun will be a red giant star. The Sun will then be 2,000 times brighter than it is now, and hot enough to melt Earth’s rocks. At this time the outer solar system will get warmer and more habitable. The icy moons of the giant planets may warm enough to be covered by water instead of ice.
When the giant Sun uses up its fuel, it will no longer be able to support the weight of its inner layers, and they will begin to collapse toward the core, eventually producing a small, dense, cool star called a white dwarf. The Sun will then have about the same radius as Earth has, but it will be much denser and more massive than Earth. The Sun will become a white dwarf star about 8 billion years from now. After it becomes a white dwarf, it will cool slowly for billions of years, eventually becoming so cool that it will no longer emit light.

IV  THE SUN’S ENERGY

The Sun produces an amazing amount of light and heat through nuclear reactions (Nuclear Energy). The process that produces the Sun’s energy is called nuclear fusion. In nuclear fusion, two atoms come together to produce a heavier atom. Fusion reactions release energy and tiny elementary particles.

A  Scale of the Sun’s Energy 

In just one second the Sun emits more energy than humans have used in the last 10,000 years. The Sun has been shining relatively steadily for 4.6 billion years. Until the early 20th century, humans did not know of any process that could explain the energy production of the Sun. Even if a fire, such as those that occur on Earth, were as large as the Sun, the fire would consume the mass of the Sun in a few thousand years.

Scientists now know that the Sun is mainly composed of hydrogen, the lightest and most abundant element in the universe. The Sun contains an enormous amount of hydrogen, however, which makes the Sun very massive. All matter inside the Sun is gravitationally attracted to all the other matter in the Sun, and this attraction tends to pull the Sun’s mass together. This inward pull creates high pressures and temperatures inside the Sun.

The center is so violent and hot that collisions between atoms break the hydrogen atoms apart into their subatomic ingredients. A hydrogen atom is made up of a nucleus that contains a positively charged proton, and a negatively charged electron that orbits the nucleus. In the Sun, collisions separate the electron from the nucleus, freeing each to move about the solar interior. The positively charged nuclei, or protons, are called ions. A gas in which particles are ionized, or have electric charges, is called plasma. Scientists often consider plasma, such as the material inside the Sun, to be a fourth state of matter—the three more familiar states of matter are gas, liquid, and solid. See also Atom.

Best Information About Rabbits And Their Types,

Rabbits and Hares,

 common name for certain small, furry mammals with long ears and short tails. Although the names rabbit and hare are often used interchangeably, in zoological classification the species called rabbits are characterized by the helplessness of their offspring, which are born naked and with closed eyes, and by their gregarious habit of living in colonies in underground burrows. (The exception is the cottontail of North America, which does not dig burrows; its nest is on the surface, usually in dense vegetation, and it is not social.) Species designated zoologically as hares are born furred and with open eyes, and the adults merely construct a simple nest and rarely live socially. Furthermore, the hare is generally larger than the rabbit and has longer ears with characteristic black markings. Moreover, the skulls of rabbits and hares are distinctly different.
Distributed throughout the world, hares and rabbits have many common characteristics. Both groups breed prolifically, bearing four to eight litters a year, with three to eight young in each litter, have a period of gestation lasting about a month, reach sexual maturity in about six months, and have a life span of about ten years. These animals, which weigh from about 1 to 5 kg (about 2 to 11 lb) and attain a length of about 30 to 60 cm (about 12 to 24 in), feed mainly on herbs, tree bark, and vegetables. They prefer to live in regions where the soil is loose and dry and where brushwood offers shelter. Although rabbits and hares are valued as game by hunters, as food, and for their fur, they often are pests to farmers whose trees and crops they destroy. The species commonly found in the United States are the cottontail, the snowshoe rabbit, the jackrabbit, and the domestic rabbit.

II  COTTONTAIL

The chief wild rabbit of North America is the cottontail. Its name is derived from the white undersurface of its short tail, which resembles a puff of cotton. The cottontail is noted for remaining motionless to avoid notice when it senses danger. The rabbit, which swims well, also evades enemies by plunging into lakes or streams.

III  SNOWSHOE

The varying hare, known popularly as the snowshoe rabbit, is distributed widely throughout North America. In winter it is pure white except for black ear tips, and in summer it is reddish-brown. The young snowshoe weighs 85 g (3 oz) at birth and develops so rapidly that it crawls on the second day after birth and hops on the third day. Adult males, called bucks, fight one another with their teeth when they court the same females, which are known as does. Although largely herbivorous, the adult snowshoe rabbit may eat mice and carrion.

IV  JACKRABBIT One species of jackrabbit, the black-tailed jackrabbit, is found in the western parts of the United States and Canada. The fastest of the rabbits and hares, jackrabbits achieve speeds of about 70 km/h (about 45 mph) and can bound some 4.5 to 6 m (some 15 to 20 ft) in a single jump. Because this species competes with grazing animals for food, livestock owners in the western United States have undertaken great drives to reduce the hare population, which has been estimated to be as high as 3100 per sq km (8000 per sq mi). Jackrabbits may carry tularemia, a bacterial disease that can be fatal to humans.
The varying hare, known popularly as the snowshoe rabbit, is distributed widely throughout North America. In winter it is pure white except for black ear tips, and in summer it is reddish-brown. The young snowshoe weighs 85 g (3 oz) at birth and develops so rapidly that it crawls on the second day after birth and hops on the third day. Adult males, called bucks, fight one another with their teeth when they court the same females, which are known as does. Although largely herbivorous, the adult snowshoe rabbit may eat mice and carrion.
V  DOMESTIC RABBIT
At least 66 varieties of the domesticated rabbit are derived from a wild rabbit native to Europe and Africa. Some varieties are Angora, Belgian, Dutch, Himalayan, lop, Siberian, Patagonian, silver-tip, Polish, and Flemish. The domesticated rabbit has extremely diverse characteristics, varying in color through every grade, shade, and mixture, from pure white to all black; in coat from very short to long, silky hair capable of being woven; and in style of ears from the prick ear—erect, small and almost as stiff as metal—to the floppy, broad, soft-skinned lopped ear, which hangs to the ground. Domestic rabbits warn one another of danger by thumping on the ground with their hind feet. They are bred as pets, for genetic studies, for laboratory experimentation, and for their meat and furs; domestic rabbits' furs are sold under the trade names of arctic seal, clipped seal, and lapin.Rabbits have been introduced to South America, Java, Australia, New Zealand, and various oceanic islands around the world. A significant instance of the rapid distribution of rabbits can be found in the present abundance of rabbits in Australia and New Zealand. In New Zealand, seven rabbits were first turned out near Invercargill, apparently about 1860. They soon spread to both countries and multiplied so rapidly that rabbit control became a serious problem. In Australia a virus deadly only to true rabbits was developed, and in 1951 decimation of the rabbit population began through the artificial promotion of this virus infection, known as myxomatosis. The project met with success only in areas with sufficient water to serve as breeding grounds for mosquitoes, which transmit the virus. However, the disease spread to Europe, killing rabbits in Britain, Belgium, and France, where the animal serves useful purposes.

Scientific classification: Rabbits and hares belong to the family Leporidae of the order Lagomorpha. Cottontails are classified in the genus Sylvilagus. The snowshoe rabbit is classified as Lepus americanus, the black-tailed jackrabbit as Lepus californicus, and the wild rabbit of Europe and Africa as Oryctolagus cuniculus.

Best Information About Titanic Disaster,

Titanic Disaster:                           

 one of the worst maritime disasters in history. The British luxury liner Titanic (46,000 gross tons) of the White Star Line, on its maiden voyage from Southampton to New York City, struck an iceberg about 153 km (about 95 mi) south of the Grand Banks of Newfoundland just before midnight on April 14, 1912. Of the more than 2220 persons aboard, about 1513 died, including the American millionaires John Jacob Astor, Benjamin Guggenheim, and Isidor Straus.
The ship had been proclaimed unsinkable because of its 16 watertight compartments. Nevertheless, the iceberg sufficiently damaged the Titanic to make it sink in less than three hours. Subsequent investigations found that the ship had been steaming too fast in dangerous waters, that lifeboat space had been provided for only about half of the passengers and crew, and that the Californian, close to the scene, had not come to the rescue because its radio operator was off duty and asleep. These findings led to many reforms, such as lifeboat space for every person on a ship, lifeboat drills, the maintenance of a full-time radio watch while at sea, and an international ice patrol.
The sinking of the Titanic has been the subject of several books and films, but not until September 1985 was the actual wreck found resting under about 3800 m (about 12,000 ft) of water. The area was photographed by a joint French-United States expedition through the use of robot submersibles equipped with television cameras (see Deep-Sea Exploration, Submersible Craft). In July 1986 the U.S. researchers explored the Titanic in the three-person Alvin submersible; they took pictures of the interior, but recovered no artifacts. The following year a controversial French salvage effort retrieved dishes, jewels, currency, and other artifacts, which were exhibited in Paris in September 1987.
Although these visits brought back haunting photographs and artifacts, no one had ever been able to thoroughly assess the damage caused by the Titanic's collision with the iceberg. Historians had long thought the sinking of the vessel resulted from massive damage to the ship's steel hull. Experts have since found evidence, however, that it was the location, rather than the extent, of the damage that caused the ship to sink. An expedition in August 1996 set out to examine the wreckage and to locate and study the damage to the hull. Sonar experts, naval architects (including one from the shipyard that built the Titanic), a microbiologist, and historians of the shipwreck were among the experts accompanying the expedition. The purpose was to determine whether the ship broke apart on the surface, to what extent flaws in the steel used to build the Titanic's hull contributed to the disaster, and how long the wreck is likely to survive on the ocean floor. The expedition also attempted to raise a 28 sq m (about 300 sq ft) section of the Titanic's hull, but a storm and a broken rope sent the piece plunging back to the ocean bottom.
According to the 1996 expedition's experts, the bow portion of the Titanic had struck the seabed at an angle and slid across it, plowing up sediments that covered the damaged area of the hull. To overcome this obstacle, the 1996 expedition used sophisticated sonar equipment, known as a sub-bottom profiler, to determine the extent and nature of the damage. Sonar uses reflected sound waves instead of light waves to “see” objects. Because sound waves can both penetrate material and reflect off material of differing density in different ways, the sonar was able to provide an image of what lay behind the sediments.
The Titanic was considered practically unsinkable because its hull was divided into 16 watertight compartments. The ship was designed to stay afloat with any two adjacent compartments or the front four compartments (which were smaller in volume) flooded. As a result, many authors of books on the disaster thought that only a huge tear, perhaps 90 m (300 ft) long, could have caused the 269 m (882 ft) ship to sink. But Edward Wilding, a naval architect, testified in the wake of the disaster that the total area damaged by the iceberg was small and probably did not exceed 1 sq m (about 12 sq ft). Others, however, did not believe that so large a ship could be undone by so little damage, and so the myth of the huge gash began. Previous expeditions found no sign of a gash, however, and the latest sonar findings confirmed Wilding's belief that the damage was slight: six thin breaches spread out along a 35 m (110 ft) section of the hull with a total surface area of about 1 sq m (about 12 sq ft). The ruptures punctured six watertight compartments and were spread strategically along riveted seams.
A 1991 expedition had also retrieved samples of the Titanic's steel for analysis. Tests determined that the steel's poor resistance to impact, a quality known as impact strength, combined with its chemical makeup, made the steel brittle. This problem was compounded by the fact that the Titanic was operating in unusually cold waters for that time of year. When exposed to near-freezing temperatures, tests showed that the steel became extremely brittle. The August 1996 expedition confirmed these findings and applied them to the question of whether the Titanic broke apart before sinking.
At the time of the Titanic's sinking there were conflicting reports as to whether the ship broke up at the surface or sank intact. All of the ship's surviving officers said the Titanic sank intact. A number of passengers, however, said that the ship broke up at the surface. Earlier expeditions established that the ship was in two pieces on the ocean floor, but some experts had theorized that the ship broke up on its way to the bottom. There was even a claim that there might be a third piece.
Based on the new findings about the nature of the damage sustained by the Titanic and the quality of the steel used in the hull, naval architects set out to determine the stresses that might have prevailed as the ship sank. The architects used a computer simulation of stresses in the hull, known as a finite element model. The simulation showed that the weight of the waterlogged bow would have generated enough stress to cause failures in the Titanic's steel plates as the ship sank, confirming reports that the ship broke apart before sinking. Also, the 1996 expedition located a third piece of the ship, indicating that the ship broke in two places.
Another series of tests performed in 1998 on some iron rivets brought back from the site found excess amounts of slag, a metal waste product added in small amounts to give iron strength. However, too much slag makes iron brittle, and there is evidence that weak rivets may have also contributed to the ruptures.
Great forces conspired to sink the Titanic, but scientists found that tiny ones will cause it to collapse and eventually disappear. In the 85 years since the Titanic sank, iron-eating microbes have slowly sapped the strength from the Titanic's structure. Eventually the wreck will no longer be able to support its own weight.
Disaster, in this article, a sudden, accidental event that causes many deaths and injuries. Most disasters also result in significant property damage. Common natural causes of disasters include earthquakes, floods, hurricanes and typhoons, and tornadoes. Tsunamis (popularly, but incorrectly, known as tidal waves), volcanic eruptions, wildfires, and landslides and avalanches rank among the other natural forces that sometimes create disasters.
Not all disasters are produced by the forces of nature. Many modern-day disasters involve accidents aboard passenger-carrying airplanes, ships, or railroads. Other “man-made” disasters can be traced to the collapse of buildings, bridges, tunnels, and mines, as well as to explosions and fires unintentionally triggered by humans.

Information About Different And Famous Oceans Of The World.

Oceans,

 Pacific Ocean, largest and deepest of the world's four oceans, covering more than a third of the earth's surface and containing more than half of its free water. It is sometimes divided into two nominal sections: the part north of the equator is called the North Pacific; the part south of the equator, the South Pacific. The name Pacific, which means peaceful, was given to it by the Portuguese navigator Ferdinand Magellan in 1520.
BOUNDARIES AND SIZE
The Pacific Ocean is bounded on the east by the North and South American continents; on the north by the Bering Strait; on the west by Asia, the Malay Archipelago, and Australia; and on the south by Antarctica. In the southeast it is arbitrarily divided from the Atlantic Ocean by the Drake Passage along 68° west longitude; in the southwest, its separation from the Indian Ocean is not officially designated. Apart from the marginal seas along its irregular western rim, it has an area of 166 million sq km (64 million sq mi), substantially larger than the entire land surface of the globe. Its maximum length north to south is 15,500 km (9,630 mi) from the Bering Strait to Antarctica, and its greatest width is 17,700 km (11,000 mi) from Panama to the Malay Peninsula. Its average depth is 4,280 m (14,040 ft). The greatest known depth in any of the world's oceans is 11,033 m (36,198 ft) in the Mariana Trench off Guam.
III  GEOLOGIC FORMATION AND STRUCTURAL FEATURES
The Pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at 200 million years. The major features of the basin and rim have been shaped by the phenomena associated with plate tectonics. The coastal shelf, which extends to depths of 180 m (600 ft), is narrow along North and South America but is relatively wide along Asia and Australia. The East Pacific Rise, a midocean ridge system, extends 8,700 km (5,400 mi) from the Gulf of California to a point 3,600 km (2,240 mi) west of the southern tip of South America, and rises an average of 2.1 m (1.3 ft) above the ocean floor. Along the East Pacific Rise molten rock upwells from the earth's mantle, adding crust to the plates on both sides of the rise. These plates, which are huge segments of the earth's surface, are thus forced apart, causing them to collide with the continental plates adjacent to their outer edges. Under this tremendous pressure, the continental plates fold into mountains, and the oceanic plates downbuckle, forming deep trenches, called subduction zones, from which crust is carried back into the mantle (see Earth: Plate Tectonics). The stresses at the areas of folding and subduction are responsible for the earthquakes and volcanoes that give the rim of the Pacific basin the name “ring of fire” (see Ocean and Oceanography).
IV  ISLANDS
The Pacific Ocean contains more than 30,000 islands; their total land area, however, amounts to only one-quarter of one percent of the ocean's surface area. The largest islands, in the western region, form volcanic island arcs that rise from the broad continental shelf along the eastern edge of the Eurasian Plate. They include Japan, Taiwan, the Philippines, Indonesia, New Guinea, and New Zealand. The oceanic islands, collectively called Oceania, are the tops of mountains built up from the ocean basin by extruding molten rock. The mountains that remain submerged are called seamounts. In many areas, particularly the South Pacific, the land features above the sea surface are accretions of shell material (see Coral Reef). Along the eastern edge of the Pacific, the continental shelf is narrow and steep, with few island areas. The major groups are the Galápagos at the equator, which rise from the Nazca Plate, and the Aleutians in the north, which are part of the North American continental shelf.
V  CURRENTS
The driving forces for ocean currents are the earth's rotation, wind friction at the surface of the water, and variations in seawater density due to differences in temperature and salinity. The interaction between wind and current has a major effect on climate and is studied for long-range weather prediction and for sea travel.
The surface currents of the North Pacific consist of two gyres, or circular systems. In the extreme north the counterclockwise Subarctic Gyre encompasses the westward-flowing Alaska Current and the eastward-flowing Subarctic Current. The main body of the North Pacific, however, is dominated by the huge North Central Gyre, which circulates clockwise. It encompasses the North Pacific Current, flowing east; the California Current, flowing southeast; and the Kuroshio Current (or Japan Current), flowing north up the coast of Japan. The California Current is cold, broad, and slow-moving; the Kuroshio is warm, narrow, and rapid, similar to the Gulf Stream. Close to the equator at 5° north latitude, the eastward-flowing Equatorial Countercurrent separates the North and South Pacific systems but sends most of its waters into the North Equatorial Current. The South Pacific is dominated by the counterclockwise-moving South Central Gyre, which encompasses the South Equatorial Current flowing east and south, the South Pacific Current flowing west, and the Mentor Current flowing north, parallel to South America. Located in the extreme south is the Antarctic Circumpolar Current (West Wind Drift), which encircles the globe, merging the waters of the Pacific, Atlantic, and Indian oceans. It is the most important source of deep-sea circulation. From it flows the broad, cold Peru, or Humboldt, Current, which travels north along the coast of South America and sends its waters into the South Equatorial Current.
VI  WIND SYSTEMS
The outstanding wind systems of the Pacific Ocean are the twin belts of westerlies, which blow from west to east between 30° and 60° latitude, one in the northern hemisphere and one in the southern hemisphere. These winds vary in seasonal patterns. The stormy and unpredictable westerly of the North Central Pacific is being studied for its possible controlling effect on global weather patterns. Between the westerlies are the much more steady trade winds, which move from the east in the northern hemisphere and from the west in the southern hemisphere. Violent tropical storms, called typhoons in the western Pacific and hurricanes in the southern and eastern Pacific, originate in the trade wind belt in late summer and early autumn. At the equator are the equatorial doldrums, light winds with seasonal cyclonic activity. At the highest latitudes of the Pacific, the winds have little direct effect on climate and water currents.
RESOURCES
Much of the plant and animal life of the Pacific Ocean is concentrated along its margins. Nutrient-rich waters from the deep Antarctic Circumpolar Current upwell to the surface in the Peru Current along the coast of Chile and Peru, and the area sustains a large population of anchovetas that is of great importance as a world food resource. A large guano industry has been established from droppings of the seabirds that feed upon the anchovetas. The northwestern Pacific, including the Sea of Japan (East Sea) and the Sea of Okhotsk, is another major world fishery. Coral reefs rich with sea life reach their peak in the Great Barrier Reef, which extends for 2,010 km (1,250 mi) along the northeastern coast of Australia. Tuna is another important Pacific resource, bringing fleets of many nations in search of the schools that migrate over much of the ocean. The Pacific has also begun to be exploited for its vast mineral resources. The continental shelves off the coasts of California, Alaska, China, and the Indonesian area are known to contain large reserves of petroleum. Patches of the ocean floor are covered with “manganese nodules,” potato-sized concretions of iron and manganese oxides that sometimes also contain copper, cobalt, and nickel. Programs are under way to examine the feasibility of mining these deposits. See also Deep-Sea Exploration.
Arctic Ocean, smallest of the four world oceans. The Arctic Ocean extends south from the North Pole to the shores of Europe, Asia, and North America.
BOUNDARIES AND SIZE
The surface waters of the Arctic Ocean mingle with those of the Pacific Ocean through the Bering Strait, by way of a narrow and shallow channel, which has a depth of 55 m (180 ft). More importantly, the Arctic waters mix with those of the Atlantic Ocean across a system of submarine sills (shallow ridges) that span the great distances from Scotland to Greenland and from Greenland to Baffin Island at depths of 500 to 700 m (1,600 to 2,300 ft). Emptying into the Arctic Ocean are the Ob’, Yenisey, and Lena rivers in Asia and the Mackenzie River in North America. The total surface area of the Arctic Ocean is 14.1 million sq km (5.4 million sq mi). The major subdivisions of the Arctic Ocean include the Norwegian, Barents, Kara, Laptev, and Beaufort seas.
III  STRUCTURAL FEATURES
Approximately one-third of the Arctic Ocean is underlain by continental shelf, which includes a broad shelf north of Eurasia and the narrower shelves of North America and Greenland. Seaward of the continental shelves lies the Arctic Basin proper, which is subdivided into a set of three parallel ridges and four basins (also known as deeps). These features were discovered and explored beginning in the late 1940s. The Lomonosov Ridge, the major ridge, cuts the Arctic Basin almost in half, extending as a submarine bridge 1,800 km (1,100 mi) from Siberia to the northwestern tip of Greenland. Parallel to it are two shorter ridges: the Alpha Ridge on the North American side, defining the Canada and Makarov basins, and the Arctic Mid-Ocean Ridge on the Eurasian side, defining the Nansen and Amundsen basins. The average depth of the Arctic Ocean is only 1,300 m (4,300 ft) because of the vast shallow expanses on the continental shelves. The deepest point in the Arctic Ocean is 5,450 m (17,880 ft).
IV  ISLANDS
The islands of the Arctic Ocean lie on the continental shelves. To the northeast of Norway lies the archipelago of Svalbard (formerly known as Spitsbergen); to the east are Franz Josef Land, Novaya Zemlya, Severnaya Zemlya, the New Siberian Islands, and Wrangel Island, all of which are located north of Russia. The numerous islands of the Canadian Arctic Archipelago extend north and east from the Canadian mainland to Greenland, the largest island of the Arctic Ocean.
V  ICE
Three forms of ice are found in the Arctic Ocean: land ice, river ice, and sea ice. Land ice enters the ocean in the form of icebergs, which are created when pieces of glaciers break off. In the Arctic Ocean, icebergs are created primarily along the coasts of Greenland. The freezing of fresh water, and its subsequent transport into the ocean by rivers, produces nearshore concentrations of river ice over small areas of the Siberian and North American shelves. Sea ice is formed by the freezing of seawater. It is the most extensive form of ice in the Arctic Ocean. In winter a permanent cap of sea ice covers all of the ocean surface, except for the area northeast of Iceland and north of Scandinavia. In summer the ice cover shrinks to expose narrow bands of relatively open water along the coasts of most of Siberia, Alaska, and Canada. The ice cap is composed of pack ice—that is, pieces of ice that pile up and are pressed in ridges or hummocks that may be more than 10 m (30 ft) in depth.
VI  RESOURCES
Fish, in commercially exploitable quantities, are found only in the warmer marginal seas of the Arctic Ocean, notably in the Barents Sea (primarily cod). Sea mammals, including various species of seal and whale, were hunted to near extinction before being protected by quotas set during the 1900s. Tin is actively mined off the coast of eastern Siberia, and petroleum and natural gas are extracted north of Alaska and Canada.
Indian Ocean, third largest of Earth's four oceans, bounded on the west by Africa, on the north by Asia, on the east by Australia and the Australasian islands, and on the south by Antarctica. No natural boundary separates the Indian Ocean from the Atlantic Ocean, but a line 4,000 km (2,500 mi) long on the 20th meridian east of Greenwich, connecting Cape Agulhas at the southern end of Africa with Antarctica, is generally considered to be the boundary.
The total area of the Indian Ocean is 73.4 million sq km (28.4 million sq mi). The ocean narrows toward the north and is divided by the Indian peninsula into the Bay of Bengal on the east and the Arabian Sea on the west. The Arabian Sea sends two arms northward, the Persian Gulf and the Red Sea. The average depth of the Indian Ocean is 3,900 m (12,800 ft), or slightly greater than that of the Atlantic, and the deepest known point is 7,725 m (25,344 ft), off the southern coast of the Indonesian island of Java. In general, the greatest depths are in the northeastern sector of the ocean, where 130,000 sq km (50,000 sq mi) of the ocean floor lie at a depth of more than 5,500 m (18,000 ft).
The Indian Ocean contains numerous islands, the largest of which are Madagascar and Sri Lanka. Smaller islands include the Maldive group and Mauritius. From Africa the ocean receives the waters of the Limpopo and Zambezi rivers, and from Asia those of the Irrawaddy, Brahmaputra, Ganges, Indus, and Shatt al Arab rivers. As a rule, the winds over the Indian Ocean are gentle, with frequent extended periods of calm. Tropical storms occur occasionally, however, particularly near Mauritius, and the ocean is notable for seasonal winds called monsoons.
Atlantic Ocean, the second largest of the earth's four oceans and the most heavily traveled. Only the Pacific Ocean is larger, covering about twice the area of the Atlantic Ocean. The Atlantic is divided into two nominal sections: The part north of the equator is called the North Atlantic; the part south of the equator, the South Atlantic. The ocean's name is derived from Atlas, one of the Titans of Greek mythology.
 BOUNDARIES AND SIZE
The Atlantic Ocean is essentially an S-shaped north-south channel, extending from the Arctic Ocean in the north to the Antarctic continent in the south and situated between the eastern coast of the American continents and the western coasts of Europe and Africa. The Atlantic Ocean proper has a surface area of 82 million sq km (32 million sq mi). Including its marginal seas—the Gulf of Mexico-Caribbean Sea, the Arctic Ocean, and the North, Baltic, Mediterranean, and Black seas—the total area is 106 million sq km (41 million sq mi).
The boundary between the North Atlantic and the Arctic Ocean is arbitrarily designated as lying along a system of submarine ridges that extend between the land masses of Baffin Island, Greenland, and Scotland. More clearly defined is the boundary with the Mediterranean Sea at the Strait of Gibraltar and with the Caribbean Sea along the arc of the Antilles. The South Atlantic is arbitrarily separated from the Indian Ocean on the east by the 20° east meridian and from the Pacific on the west along the line of shallowest depth between Cape Horn and the Antarctic Peninsula.
III  GEOLOGIC FORMATION AND STRUCTURAL FEATURES
The Atlantic began to form during the Jurassic period, about 150 million years ago, when a rift opened up in the supercontinent of Gondwanaland, resulting in the separation of South America and Africa. The separation continues today at the rate of several centimeters a year along the Mid-Atlantic Ridge. Part of the midoceanic ridge system that girdles the world, it is a submarine ridge extending north to south in a sinuous path midway between the continents. Roughly 1,500 km (930 mi) wide, the ridge has a more rugged topography than any mountain range on land, and is a frequent site of volcanic eruptions and earthquakes. The ridge ranges from 1 to 3 km (0.6 to 2 mi) above the ocean bottom.
Along the American, Antarctic, African, and European coasts are the continental shelves—embankments of the debris washed from the continents. Submarine ridges and rises extend roughly east-west between the continental shelves and the Mid-Atlantic Ridge, dividing the eastern and western ocean floors into a series of basins, also known as abyssal plains. The three basins on the American side of the Mid-Atlantic Ridge are 5,000 m (16,000 ft) deep: the North American Basin, the Brazil Basin, and the Argentina Basin. The Eurafrican side is marked by several basins that are smaller but just as deep: the Iberia, Canaries, Cape Verde, Sierra Leone, Guinea, Angola, Cape, and Agulhas basins. The large Atlantic-Antarctic Basin lies between the southernmost extension of the Mid-Atlantic Ridge and the Antarctic continent.
The Atlantic Ocean has an average depth of 3,600 m (11,810 ft). At its deepest point, in the Puerto Rico Trench, the bottom is 8,605 m (28,231 ft) below the surface.
IV  ISLANDS
The largest islands of the Atlantic Ocean lie on the continental shelves. Newfoundland is the principal island on the North American shelf; the British Isles are the major island group of the Eurafrican shelf. Other continental islands include the Falkland Islands (Islas Malvinas), the only major group on the South American shelf, and the South Sandwich Islands on the Antarctic shelf.
Oceanic islands, usually of volcanic origin, are less common in the Atlantic Ocean than in the Pacific. Among these are the island arc of the Antilles (including Puerto Rico, Hispaniola, Jamaica, and Cuba). In the eastern Atlantic, the Madeiras, Canaries, Cape Verde, and the São Tomé-Príncipe group are the peaks of submarine ridges. The Azores, Saint Paul's Rocks, Ascension, and the Tristan da Cunha group are isolated peaks of the Mid-Atlantic Ridge system; the large island of Iceland is also the result of volcanic action at the Mid-Atlantic Ridge. Bermuda rises from the floor of the North American Basin, and Saint Helena from the Angola Basin.
V  CURRENTS
The circulatory system of the surface waters of the Atlantic can be depicted as two large gyres, or circular current systems, one in the North Atlantic and one in the South Atlantic. These currents are primarily wind driven, but are also affected by the rotation of the earth. The currents of the North Atlantic, which include the North Equatorial Current, the Canaries Current, and the Gulf Stream, flow in a clockwise direction. The currents in the South Atlantic, among which are the Brazil, Benguela, and South Equatorial currents, travel in a counterclockwise direction. Each gyre extends from near the equator to about latitude 45°; closer to the poles are the less completely defined counterrotating gyres, one rotating counterclockwise in the Arctic regions of the North Atlantic and one rotating clockwise near Antarctica in the South Atlantic. See Ocean and Oceanography: Ocean Currents.
The Atlantic receives the waters of many of the principal rivers of the world, among them the St. Lawrence, Mississippi, Orinoco, Amazon, Paraná, Congo, Niger, and Loire, and the rivers emptying into the North, Baltic, and Mediterranean seas. Nevertheless, primarily because of the high salinity of outflow from the Mediterranean, the Atlantic is slightly more saline than the Pacific or Indian oceans.
VI  TEMPERATURES
The Atlantic Ocean may be described as a bed of water colder than 9° C (48° F)—the cold-water sphere—within which lies a bubble of water warmer than 9° C—the warm-water sphere. The warm-water sphere extends between latitude 50° north and latitude 50° south and has an average thickness of 600 m (2,000 ft). The most active circulation is found in the uppermost layer of warm water. Below this, circulation becomes increasingly sluggish as the temperature decreases.
Surface temperatures range from 0° C (32° F), found year-round at the Arctic and Antarctic margins, to 27° C (81° F) in the broad belt at the equator. At depths below 2,000 m (6,600 ft), temperatures of 2° C (36° F) are prevalent; in bottom waters, below 4,000 m (13,200 ft), temperatures of -1° C (30° F) are common.
VII  MARINE RESOURCES
The Atlantic Ocean contains some of the world's most productive fisheries, located on the continental shelves and marine ridges off the British Isles, Iceland, Canada (especially the Grand Banks off Newfoundland), and the northeastern United States. Upwelling areas, in which the nutrient-rich waters of the ocean depths flow up to the surface, as in the vicinity of Walvis Bay off southwestern Africa, also have abundant sea life. Herring, anchovy, sardine, cod, flounder, and perch are the most important commercial species. Tuna is taken off northwestern Africa and northeastern South America in increasing numbers. The catch per unit area is much higher in the Atlantic than in the other oceans.
A remarkable example of plant life is found in the Sargasso Sea, the oval section of the North Atlantic lying between the West Indies and the Azores and bounded on the west and north by the Gulf Stream. Here extensive patches of brown gulfweed (Sargassum) are found on the relatively still surface waters.
Actively mined mineral resources in the Atlantic include titanium, zircon, and monazite (phosphates of the cerium metals), off the eastern coast of Florida, and tin and iron ore, off the equatorial coast of Africa. The continental shelves and slopes of the Atlantic are potentially very rich in fossil fuels. Large amounts of petroleum are already being extracted in the North Sea and in the Caribbean Sea-Gulf of Mexico region; lesser amounts are extracted off the coast of Africa in the Gulf of Guinea.