A barrier is something, either real or immaterial, that obstructs or prevents one thing from accessing another thing. A fence is a barrier in that it obstructs people from accessing what is on the opposite side of the fence. A cattle grid is a barrier to cloven-hooved animals, precenting their passage over the grid, while allowing vehicles and other animals free passage. Barrier creams are substances which prevent germs, water, the sun's rays or other substances from reaching the skin or a wound. In abstract terms, language can be a barrier to communication, if two people do not speak a common language then communication between them is seriously impaired and obstructed. Research Barrier
In geography coral reef is a barrier, lying at or just below the surface of the sea, built up of the skeletons of immense numbers of small creatures called coral polyps. Research Coral Reef
Dunes are low hills of sand accumulated on the sea-coasts of Holland, Britain, Spain, and other countries, in some places encroaching on and covering what once was cultivated land, but in others serving as a natural barrier to protect the country from the destructive encroachments of the sea. Research Dunes
Lakes are accumulations of water in hollows on the earth's surface. When they are drained by rivers their waters are fresh, but when they have no outlet they are salty, e.g. the Dead Sea, Sea of Aral, etc.
Lakes may owe their origin to:
Barriers across a river valley hold back the water, which forms a lake. Such barriers may be of various types. (a) Sometimes artificial barriers of concrete and masonry are built across a valley so as to make a lake which can act as a reservoir for the water-supply of a large city, e.g. LakeVyrnwy for Liverpool. (b) A glacier may deposit a mass of morainic material across a valley. In this way the lakes of the Lake District and many of the Scottish lakes were formed. (c) A landslip may occur. A lake was formed thus in the Upper GangesValley in 1892. Two years later the landslip dam gave way, and disastrous floods occurred downstream. (d) Oxbow lakes are formed from the meanders of rivers. The deposition of silt at the two ends of the 'oxbow' closes the channel between the main river and its old loop. Many oxbow lakes border the River Murray in Australia, and the lower Mississippi. (e) Sometimes a lavastream may flow across a valley and cause the formation of a lake, e.g. LakeTaupo in New Zealand. (f) Sometimes large estuaries are partially filled with silt. In the portions not so filled are large shallow lagoons. Such lagoons are found in deltaic areas. The NorfolkBroads are portions of an old river estuary. (g) When a silt-laden stream enters a lake its speed is checked and a barrier or delta is built across the lake splitting it into two portions. This has happened in the Lake District, where Keswick stands in the alluvial flats between Lakes Bassenthwaite and Derwentwater, and in Switzerland, where Interlaken is situated in the flats between Lakes Thun and Brienz. (h) The action of the sea often causes an accumulation of sand and pebbles which cuts off a lagoon of sea water. The Fleet in Dorset is such a lagoon, cut off from the sea by Chesil Bank, a long pebble beach which joins Portland Island to the mainland.
The nehrungs of East Prussia are sand-spits which enclose the shallow salt-water lagoons or halls, such as Kurische Haff. Earth movements cause lakeformation when subsidence occurs. This is most easily seen in rift valleys. Examples of riftvalley lakes are the Dead Sea, Lakes Nyasa and Tanganyika in Africa, and LakeTorrens in Australia. These are all long, narrow, and very deep lakes.
In Cheshire, the removal of underground beds of salt has caused subsidence resulting in the 'meres' of the Weaver Valley. The 'folding' of the earth across the line of a river valley may partially block a river and help to form a lake. The study of a good physical map will reveal the connection between mountain building and the formation of LakeGeneva and LakeConstance in Switzerland. Where there are large areas of depressed lowland wide and shallow lakes are formed in the lowest part of the depression, for example the Sea of Aral in Asiatic Russia, LakeBalaton in Hungary, and LakeEyre in Australia. Ice sheets and valley glaciers may scoop out hollows to form 'rock basins'. Mountain tarns and corrie lakes in North Wales and Scotland have been formed in this way. Water also accumulates in the hollows of unevenly- distributed glacial drift. Such are the lakes of East Prussia, and also those of the Cheshire-Shropshire borders near Ellesmere. Subsidence of the land surface and consequent lakeformation may be directly related to volcanic action. Lough Neagh in Northern Ireland is a shallow lake formed by subsidence of this type. Lakes are often formed by the accumulation of water in the craters of extinct volcanoes, for example the Laachersee in the Eifel region of Germany. Research Lakes
Offa was king of the East Saxons in 700 until he became a monk at Rome.
Offa was King of Mercia from 757 until his death in 796. He seized the throne after a civil war, and established supremacy over many lesser kings. He consolidated his position by marrying his daughters to the kings of Wessex and Northumbria, and was the first ruler to be called 'king of the English'.
Offa ruthlessly overcame strong opposition in southern England. By the end of his reign, Offa was master of all England south of the Humber; he married his daughters to the kings of Wessex and Northumbria. He had a frontierbarrier (known as Offa's Dyke) built; this continuous ditch and bank ran 149 miles along the boundary between the Mercian and Welsh kingdoms 'from sea to sea'. Offa had dealings with the emperor Charlemagne (a proposed a dynastic marriage between their children came to nothing), and he visited Rome in 792 to strengthen his links with the papacy. The English penny (silvercurrency) was introduced during Offa's reign. In the first recorded coronation in England,
Offa's son Ecgfrith was consecrated in 787 in Offa's lifetime in an attempt to secure the succession. However, Ecgfrith died childless months after Offa. Offa's success in building a strong unified kingdom caused resistance in other kingdoms. The Mercians' defeat at the hands of Egbert of Wessex at the battle of Ellendun in 825 meant that supremacy passed to Wessex. Research Offa
The placenta is formed from the chorionic villi, small finger-like projections that cover the outer cells of the blastocyst. After implantation of the fertilized ovum, the chorionic villi burrow into the lining of the uterus seeking nourishment; those which have penetrated deepest erode some of the small uterine blood vessels and become bathed in the mother's blood. At this point the burrowing stops and the villi start to multiply and form branches. It is these villi which form the basis of the placenta. The placenta is responsible for the transfer of nourishment from the mother to the fetus, and of the waste products the fetus produces to the mother so that they can be excreted. Two layers of cells keep the fetal circulation in the placenta separate from the maternal blood. Through these cells the vital exchangefunction of the placenta takes place. Carbon dioxide, waste products and hormones pass from the fetus to the mother. Oxygen, nutrients and hormones are transferred in the opposite direction. The placenta also acts as a barrier to protect the fetus against potentially harmful substances. Research Placenta
The human body is continually exposed to disease producing organisms, called pathogens, and other harmful substances in the environment. The immune system is the body's defense against these harmful invaders. The body's ability to counteract the effects of pathogens and other harmful agents is called resistance and it is dependent on a variety of defense mechanisms.
The immune system is made up of billions of special cells called white blood cells, lymphocytes, unique proteins called antibodies, chemicals that mediate immune response, and special organs that replenish and integrate the whole immune process. All of these defense mechanism must act together and are designed to react rapidly to provide protection against disease-producing organisms and their toxins. There are two aspects of the immune system's response to disease: innate and acquired. Natural, or innate, immunity is present from birth and is the first line of defense against the vast majority of infectious agents. Innate immunity involves barriers that keep harmful material from entering the body. The skin provides an impenetrable barrier. The eyes use fluids, such as tears, and the presence of enzymes, such as lysozyme, that destroy bacteria. The respiratory system utilizes cilia, mucus, and coughing to get rid of foreign materials. If infection-causing organisms gets past these defenses, the body produces fever, inflammation, and other reactions designed to conquer the unwelcome invader.
Inflammation causes an increase in the local blood supply so that large numbers of white blood cells can be brought to the area to fight the infection. Some of these white blood cells are phagocytes and macrophages that literally eat the invading microorganism. In most cases of minor infection, these cells solve the problem. If the pathogen succeeds in passing this barrier, a more complex process, involving other cells of the immune system, is necessary. When a virus enters the body an immune response begins automatically. A scavenger macrophage will eat the virus and display the viral antigen on its surface. Anything that can trigger an immune response is called an antigen. An antigen can be a germ such as a virus, or even a part of a virus. Other white blood cells in your body called 'helper T-cells' will see the viral antigen and produce toxins that will destroy it. The helper T- cells then send chemical messages that activate lymphocytes called B-cells which make antibodies that recognize the viral antigen. These cells ' remember' the specific disease organism and divide into many more cells. The resulting 'clone' of identical cells starts producing very large numbers of antibodies that bind to all the organisms of that disease and destroy them. This process is called acquired immunity. It is a learning process of the immune system that develops either through exposure to microorganisms. It is estimated that the body has more than 100 million different kinds of antibodies, each one custom-built to identify a particular pathogen. If the body is exposed a second time, no symptoms occur because the organism is destroyed quickly - the bofy is immune to that particular pathogen. Research The Immune System
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Barrier
