A sequestrum is a piece of dead bone that has become separated during the process of necrosis from normal or sound bone. It is a complication of osteomyelitis; the pathological process is as follows: infection in the bone leads to an increase in intramedullary pressure due to inflammatory exudates the periosteum becomes stripped from the osteum, leading to vascular thrombosis bone necrosis follows due to lack of blood supply sequestra are formedThe sequestra are surrounded by sclerotic bone, avascular. Within the bone itself, the haversian canals become blocked with scar tissue, the bone becomes surrounded by thickened periosteum. Due to the avascular nature of this bone, antibiotics which travel to sites of infection via the bloodstream poorly penetrate these tissues, hence the difficulty in treating chronic osteomyelitis. At the same time as this, new bone is forming. Openings in this involucrum allow debris and exudates to pass from the sequestrum via sinus tracts to the skin. A sequestrum may turn out to be an osteoid osteoma, a rare tumor of the bone
X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen who discovered these on November 8, 1895, credited as its discoverer, who named it X-radiation to signify an unknown type of radiation. Spelling of X-ray in the English language includes the variants x-ray, X ray. Before their discovery in 1895 X-rays were just a type of unidentified radiation emanating from experimental discharge tubes, they were noticed by scientists investigating cathode rays produced by such tubes, which are energetic electron beams that were first observed in 1869. Many of the early Crookes tubes undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below.
Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. This voltage accelerated the electrons coming from the cathode to a high enough velocity that they created X-rays when they struck the anode or the glass wall of the tube; the earliest experimenter thought to have produced. In 1785 he presented a paper to the Royal Society of London describing the effects of passing electrical currents through a evacuated glass tube, producing a glow created by X-rays; this work was further explored by his assistant Michael Faraday. When Stanford University physics professor Fernando Sanford created his "electric photography" he unknowingly generated and detected X-rays. From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as studied by Heinrich Hertz and Philipp Lenard.
His letter of January 6, 1893 to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner. Starting in 1888, Philipp Lenard, a student of Heinrich Hertz, conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air, he built a Crookes tube with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it. He found that something came through, that would cause fluorescence, he measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were X-rays. In 1889 Ukrainian-born Ivan Pulyui, a lecturer in experimental physics at the Prague Polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes.
Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his announcement, it was formed on the basis of the electromagnetic theory of light. However, he did not work with actual X-rays. In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds. After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, as well as Crookes tubes. On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard tubes and Crookes tubes and began studying them, he wrote an initial report "On a new kind of ray: A preliminary communication" and on December 28, 1895 submitted it to Würzburg's Physical-Medical Society journal. This was the first paper written on X-rays. Röntgen referred to the radiation as "X"; the name stuck.
They are still referred to as such in many languages, including German, Danish, Swedish, Estonian, Japanese, Georgian and Norwegian. Röntgen received the first Nobel Prize in Physics for his discovery. There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a reconstruction by his biographers: Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a fluorescent screen painted with barium platinocyanide, he noticed a faint green glow from the screen, about 1 meter away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow, he found they could pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper. Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays.
The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death."The discovery of X-rays stimul
Sport includes all forms of competitive physical activity or games which, through casual or organised participation, aim to use, maintain or improve physical ability and skills while providing enjoyment to participants, in some cases, entertainment for spectators. Hundreds of sports exist, from those between single contestants, through to those with hundreds of simultaneous participants, either in teams or competing as individuals. In certain sports such as racing, many contestants may compete or consecutively, with one winner; some sports allow a "tie" or "draw". A number of contests may be arranged in a tournament producing a champion. Many sports leagues make an annual champion by arranging games in a regular sports season, followed in some cases by playoffs. Sport is recognised as system of activities which are based in physical athleticism or physical dexterity, with the largest major competitions such as the Olympic Games admitting only sports meeting this definition, other organisations such as the Council of Europe using definitions precluding activities without a physical element from classification as sports.
However, a number of competitive, but non-physical, activities claim recognition as mind sports. The International Olympic Committee recognises both chess and bridge as bona fide sports, SportAccord, the international sports federation association, recognises five non-physical sports: bridge, draughts, Go and xiangqi, limits the number of mind games which can be admitted as sports. Sport is governed by a set of rules or customs, which serve to ensure fair competition, allow consistent adjudication of the winner. Winning can be crossing a line first, it can be determined by judges who are scoring elements of the sporting performance, including objective or subjective measures such as technical performance or artistic impression. Records of performance are kept, for popular sports, this information may be announced or reported in sport news. Sport is a major source of entertainment for non-participants, with spectator sport drawing large crowds to sport venues, reaching wider audiences through broadcasting.
Sport betting is in some cases regulated, in some cases is central to the sport. According to A. T. Kearney, a consultancy, the global sporting industry is worth up to $620 billion as of 2013; the world's most accessible and practised sport is running, while association football is its most popular spectator sport. The word "sport" comes from the Old French desport meaning "leisure", with the oldest definition in English from around 1300 being "anything humans find amusing or entertaining". Other meanings include. Roget's defines the noun sport as an "activity engaged in for relaxation and amusement" with synonyms including diversion and recreation; the singular term "sport" is used in most English dialects to describe the overall concept, with "sports" used to describe multiple activities. American English uses "sports" for both terms; the precise definition of what separates a sport from other leisure activities varies between sources. The closest to an international agreement on a definition is provided by SportAccord, the association for all the largest international sports federations, is therefore the de facto representative of international sport.
SportAccord uses the following criteria, determining that a sport should: have an element of competition be in no way harmful to any living creature not rely on equipment provided by a single supplier not rely on any "luck" element designed into the sport. They recognise that sport can be physical mind, predominantly motorised co-ordination, or animal-supported; the inclusion of mind sports within sport definitions has not been universally accepted, leading to legal challenges from governing bodies in regards to being denied funding available to sports. Whilst SportAccord recognises a small number of mind sports, it is not open to admitting any further mind sports. There has been an increase in the application of the term "sport" to a wider set of non-physical challenges such as video games called esports due to the large scale of participation and organised competition, but these are not recognised by mainstream sports organisations. According to Council of Europe, European Sports Charter, article 2.i, "'Sport' means all forms of physical activity which, through casual or organised participation, aim at expressing or improving physical fitness and mental well-being, forming social relationships or obtaining results in competition at all levels."
There are opposing views on the necessity of competition as a defining element of a sport, with all professional sport involving competition, governing bodies requiring competition as a prerequisite of recognition by the International Olympic Committee or SportAccord. Other bodies advocate widening the definition of sport to include all physical activity. For instance, the Council of Eu
Bone density, or bone mineral density, is the amount of bone mineral in bone tissue. The concept is of mass of mineral per volume of bone, although clinically it is measured by proxy according to optical density per square centimetre of bone surface upon imaging. Bone density measurement is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk, it is measured by a procedure called densitometry performed in the radiology or nuclear medicine departments of hospitals or clinics. The measurement involves low radiation exposure. Measurements are most made over the lumbar spine and over the upper part of the hip; the forearm may be scanned if the lumbar spine are not accessible. There is higher probability of fracture. Fractures of the legs and pelvis due to falls are a significant public health problem in elderly women, leading to much medical cost, inability to live independently and risk of death. Bone density measurements are used to screen people for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.
Bone density tests are not necessary for people without risk factors for weak bones. Unnecessary testing is more to result in superfluous treatment rather than discovery of a true problem; the following are risk factors for low bone density and primary considerations for the need for a bone density test. Females age 65 or older males age 70 or older people over age 50 with any of the following: previous bone fracture from minor trauma rheumatoid arthritis low body weight a parent with a hip fracture Individuals with vertebral abnormalities. Individuals receiving, or planning to receive, long-term glucocorticoid therapy. Individuals with primary hyperparathyroidism. Individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy. Individuals with a history of eating disordersOther considerations that are related to risk of low bone density and the need for a test include smoking habits, drinking habits, the long-term use of corticosteroid drugs, a vitamin D deficiency.
For those people who do have bone density tests, two conditions which may be detected are osteoporosis and osteopenia. The usual response to either of these indications is consultation with a physician. Results are reported in 3 terms: Measured areal density in g cm−2 Z-score, the number of standard deviations above or below the mean for the patient's age and ethnicity T-score, the number of standard deviations above or below the mean for a healthy 30-year-old adult of the same sex and ethnicity as the patient While there are many different types of BMD tests, all are non-invasive. Most tests differ according to; these tests include: Dual-energy X-ray absorptiometry Dual X-ray Absorptiometry and Laser Quantitative computed tomography Quantitative ultrasound Single photon absorptiometry Dual photon absorptiometry Digital X-ray radiogrammetry Single energy X-ray absorptiometry DXA is the most used, but quantitative ultrasound has been described as a more cost-effective approach to measure bone density.
The DXA test works by measuring a specific bone or bones the spine and wrist. The density of these bones is compared with an average index based on age and size; the resulting comparison is used to determine risk for fractures and the stage of osteoporosis in an individual. Average bone mineral density = BMC / W BMC = bone mineral content = g/cm W = width at the scanned line Results are scored by two measures, the T-score and the Z-score. Scores indicate. Negative scores indicate lower bone density, positive scores indicate higher; the T-score is the relevant measure. It is the bone mineral density at the site, it is a comparison of a patient's BMD to that of a healthy 30-year-old. The US standard is to use data for a 30-year-old of the same sex and ethnicity, but the WHO recommends using data for a 30-year-old white female for everyone. Values for 30-year-olds are used in post-menopausal women and men over age 50 because they better predict risk of future fracture; the criteria of the World Health Organization are: Normal is a T-score of −1.0 or higher Osteopenia is defined as between −1.0 and −2.5 Osteoporosis is defined as −2.5 or lower, meaning a bone density, two and a half standard deviations below the mean of a 30-year-old man/woman.
The Z-score is the comparison to the age-matched normal and is used in cases of severe osteoporosis. This is the number of standard deviations a patient's BMD differs from the average BMD of their age and ethnicity; this value is used in premenopausal women, men under the age of 50, in children. It is most useful. In this setting, it is helpful to scrutinize for coexisting illnesses or treatments that may contribute to osteoporosis such as glucocorticoid therapy, hyperparathyroidism, or alcoholism. Use of BMD has several limitations. Measurement can be affected by the size of the patient, the thickness of tissue overlying the bone, other factors extraneous to the bones. Bone density is a proxy measurement for bone strength, the resistance to fracture and the significant characteristic. Although the two are related, there are some circumstances in which bone density is a poorer indicator of bone strength. Reference standard
A shoe is an item of footwear intended to protect and comfort the human foot while the wearer is doing various activities. Shoes are used as an item of decoration and fashion; the design of shoes has varied enormously through time and from culture to culture, with appearance being tied to function. Additionally, fashion has dictated many design elements, such as whether shoes have high heels or flat ones. Contemporary footwear in the 2010s varies in style and cost. Basic sandals may be sold for a low cost. High fashion shoes made by famous designers may be made of expensive materials, use complex construction and sell for hundreds or thousands of dollars a pair; some shoes are designed for specific purposes, such as boots designed for mountaineering or skiing. Traditionally, shoes have been made from leather, wood or canvas, but in the 2010s, they are made from rubber and other petrochemical-derived materials. Though the human foot is adapted to varied terrain and climate conditions, it is still vulnerable to environmental hazards such as sharp rocks and temperature extremes, which shoes protect against.
Some shoes are worn as safety equipment, such as steel-soled boots which are required on construction sites. The earliest known shoes are sagebrush bark sandals dating from 7000 or 8000 BC, found in the Fort Rock Cave in the US state of Oregon in 1938; the world's oldest leather shoe, made from a single piece of cowhide laced with a leather cord along seams at the front and back, was found in the Areni-1 cave complex in Armenia in 2008 and is believed to date to 3500 BC. Ötzi the Iceman's shoes, dating to 3300 BC, featured brown bearskin bases, deerskin side panels, a bark-string net, which pulled tight around the foot. The Jotunheimen shoe was discovered in August 2006. Archaeologists estimate that the leather shoe was made between 1800 and 1100 BC, making it the oldest article of clothing discovered in Scandinavia, it is thought that shoes may have been used long before this, but because the materials used were perishable, it is difficult to find evidence of the earliest footwear. By studying the bones of the smaller toes, it was observed that their thickness decreased 40,000 to 26,000 years ago.
This led archaeologists to deduce that wearing shoes resulted in less bone growth, resulting in shorter, thinner toes. These earliest designs were simple in design mere "foot bags" of leather to protect the feet from rocks and cold, they were more found in colder climates. Many early natives in North America wore a similar type of footwear, known as the moccasin; these are tight-fitting, soft-soled shoes made out of leather or bison hides. Many moccasins were decorated with various beads and other adornments. Moccasins were not designed to be waterproof, in wet weather and warm summer months, most Native Americans went barefoot; as civilizations began to develop, thong sandals were worn. This practice dates back to pictures of them in ancient Egyptian murals from 4000 BC. One pair found in Europe was made of papyrus leaves and dated to be 1,500 years old, they were worn in Jerusalem during the first century of the Common Era. Thong sandals were made from a wide variety of materials. Ancient Egyptian sandals were made from papyrus and palm leaves.
The Masai of Africa made them out of rawhide. In India they were made from wood. In China and Japan, rice straw was used; the leaves of the sisal plant were used to make twine for sandals in South America while the natives of Mexico used the Yucca plant. While thong sandals were worn, many people in ancient times, such as the Egyptians and Greeks, saw little need for footwear, most of the time, preferred being barefoot; the Egyptians and Hindus made some use of ornamental footwear, such as a soleless sandal known as a "Cleopatra", which did not provide any practical protection for the foot. The ancient Greeks viewed footwear as self-indulgent and unnecessary. Shoes were worn in the theater, as a means of increasing stature, many preferred to go barefoot. Athletes in the Ancient Olympic Games participated barefoot – and naked; the gods and heroes were depicted barefoot, the hoplite warriors fought battles in bare feet and Alexander the Great conquered his vast empire with barefoot armies. The runners of Ancient Greece are believed to have run barefoot.
Pheidippides, the first marathoner, ran from Athens to Sparta in less than 36 hours. After the Battle of Marathon, he ran straight from the battlefield to Athens to inform the Athenians of the news; the Romans, who conquered the Greeks and adopted many aspects of their culture, did not adopt the Greek perception of footwear and clothing. Roman clothing was seen as a sign of power, footwear was seen as a necessity of living in a civilized world, although the slaves and paupers went barefoot. Roman soldiers were issued with chiral footwear. There are references to shoes being worn in the Bible. A common casual shoe in the Pyrenees during the Middle Ages was the espadrille; this is a sandal with braided jute soles and a fabric upper portion, includes fabric laces that tie around the ankle. The term comes from the esparto grass; the shoe originated in the Catalonian region of Spain as early as the 13th century, was worn by peasants in the farming communities in the area. Many medieval shoes were made using the turnshoe method of construction, in which the upper was turned flesh side out, was lasted onto t
In science and engineering, the weight of an object is related to the amount of force acting on the object, either due to gravity or to a reaction force that holds it in place. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as the magnitude of the gravitational force. Others define it as the magnitude of the reaction force exerted on a body by mechanisms that keep it in place: the weight is the quantity, measured by, for example, a spring scale. Thus, in a state of free fall, the weight would be zero. In this sense of weight, terrestrial objects can be weightless: ignoring air resistance, the famous apple falling from the tree, on its way to meet the ground near Isaac Newton, would be weightless; the unit of measurement for weight is that of force, which in the International System of Units is the newton. For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, about one-sixth as much on the Moon.
Although weight and mass are scientifically distinct quantities, the terms are confused with each other in everyday use. Further complications in elucidating the various concepts of weight have to do with the theory of relativity according to which gravity is modelled as a consequence of the curvature of spacetime. In the teaching community, a considerable debate has existed for over half a century on how to define weight for their students; the current situation is that a multiple set of concepts co-exist and find use in their various contexts. Discussion of the concepts of heaviness and lightness date back to the ancient Greek philosophers; these were viewed as inherent properties of objects. Plato described weight as the natural tendency of objects to seek their kin. To Aristotle and levity represented the tendency to restore the natural order of the basic elements: air, earth and water, he ascribed absolute weight to earth and absolute levity to fire. Archimedes saw weight as a quality opposed to buoyancy, with the conflict between the two determining if an object sinks or floats.
The first operational definition of weight was given by Euclid, who defined weight as: "weight is the heaviness or lightness of one thing, compared to another, as measured by a balance." Operational balances had, been around much longer. According to Aristotle, weight was the direct cause of the falling motion of an object, the speed of the falling object was supposed to be directly proportionate to the weight of the object; as medieval scholars discovered that in practice the speed of a falling object increased with time, this prompted a change to the concept of weight to maintain this cause effect relationship. Weight was split into a "still weight" or pondus, which remained constant, the actual gravity or gravitas, which changed as the object fell; the concept of gravitas was replaced by Jean Buridan's impetus, a precursor to momentum. The rise of the Copernican view of the world led to the resurgence of the Platonic idea that like objects attract but in the context of heavenly bodies. In the 17th century, Galileo made significant advances in the concept of weight.
He proposed a way to measure the difference between the weight of a moving object and an object at rest. He concluded weight was proportionate to the amount of matter of an object, not the speed of motion as supposed by the Aristotelean view of physics; the introduction of Newton's laws of motion and the development of Newton's law of universal gravitation led to considerable further development of the concept of weight. Weight became fundamentally separate from mass. Mass was identified as a fundamental property of objects connected to their inertia, while weight became identified with the force of gravity on an object and therefore dependent on the context of the object. In particular, Newton considered weight to be relative to another object causing the gravitational pull, e.g. the weight of the Earth towards the Sun. Newton considered space to be absolute; this allowed him to consider concepts as true velocity. Newton recognized that weight as measured by the action of weighing was affected by environmental factors such as buoyancy.
He considered this a false weight induced by imperfect measurement conditions, for which he introduced the term apparent weight as compared to the true weight defined by gravity. Although Newtonian physics made a clear distinction between weight and mass, the term weight continued to be used when people meant mass; this led the 3rd General Conference on Weights and Measures of 1901 to declare "The word weight denotes a quantity of the same nature as a force: the weight of a body is the product of its mass and the acceleration due to gravity", thus distinguishing it from mass for official usage. In the 20th century, the Newtonian concepts of absolute time and space were challenged by relativity. Einstein's equivalence principle put all observers, accelerating, on the same footing; this led to an ambiguity as to what is meant by the force of gravity and weight. A scale in an accelerating elevator cannot be distinguished from a scale in a gravitational field. Gravitational force and weight thereby became frame-dependent quantities.
This prompted the abandonment of the concept as superfluous in the fundamental sciences such as physics and chemistry. Nonetheless, the concept remained important in the teaching of physics; the ambiguities introduced by relativity led, starting in the 1960s, to considerable debate in the teaching community as how to define weight for their s