The large intestine known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces before being removed by defecation; the colon is the largest portion of the large intestine, so many mentions of the large intestine and colon overlap in meaning whenever anatomic precision is not the focus. Most sources define the large intestine as the combination of the cecum, colon and anal canal; some other sources exclude the anal canal. In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve, it continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about 1.5 metres long, about one-fifth of the whole length of the gastrointestinal tract.
The colon is the last part of the digestive system. It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a major role in absorption of foods and nutrients. About 1.5 litres or 45 ounces of water arrives in the colon each day. The length of the adult human male colon is 65 inches or 166 cm, on average, for females it is 155 cm. In mammals, the colon consists of five sections: the cecum plus the ascending colon, the transverse colon, the descending colon, the sigmoid colon, the rectum. Sections of the colon are: The ascending colon including the cecum and appendix The transverse colon including the colic flexures and transverse mesocolon The descending colon The sigmoid colon – the s-shaped region of the large intestine The rectumThe parts of the colon are either intraperitoneal or behind it in the retroperitoneum. Retroperitoneal organs in general do not have a complete covering of peritoneum, so they are fixed in location.
Intraperitoneal organs are surrounded by peritoneum and are therefore mobile. Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, transverse colon and sigmoid colon are intraperitoneal; this is important as it affects which organs can be accessed during surgery, such as a laparotomy. The average inner diameter of sections of the colon in centimeters are cecum 8.7, ascending colon 6.6, transverse colon 5.8, descending/sigmoid colon 6.3 and rectum near rectal/sigmoid junction 5.7. The cecum is the first section of the colon and involved in the digestion, while the appendix which develops embryologically from it, is a structure of the colon, not involved in digestion and considered to be part of the gut-associated lymphoid tissue; the function of the appendix is uncertain, but some sources believe that the appendix has a role in housing a sample of the colon's microflora, is able to help to repopulate the colon with bacteria if the microflora has been damaged during the course of an immune reaction.
The appendix has been shown to have a high concentration of lymphatic cells. The ascending colon is the first of four sections of the large intestine, it is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs upwards through the abdominal cavity toward the transverse colon for eight inches. One of the main functions of the colon is to remove the water and other key nutrients from waste material and recycle it; as the waste material exits the small intestine through the ileocecal valve, it will move into the cecum and to the ascending colon where this process of extraction starts. The unwanted waste material is moved upwards toward the transverse colon by the action of peristalsis; the ascending colon is sometimes attached to the appendix via Gerlach's valve. In ruminants, the ascending colon is known as the spiral colon. Taking into account all ages and sexes, colon cancer occurs here most often; the transverse colon is the part of the colon from the hepatic flexure known as the right colic, to the splenic flexure known as the left colic.
The transverse colon hangs off the stomach, attached to it by a large fold of peritoneum called the greater omentum. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesentery known as the transverse mesocolon; the transverse colon is encased in peritoneum, is therefore mobile. The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of the superior mesenteric artery, while the latter third is supplied by branches of the inferior mesenteric artery; the "watershed" area between these two blood supplies, which represents the embryologic division between the midgut and hindgut, is an area sensitive to ischemia. The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. One function of the descending colon in the digestive system is to store feces that will be emptied into the rectum, it is retroperitoneal in two-thirds of humans. In the other third, it has a mesentery.
The arterial supply comes via the left colic artery. The descending colon is called the distal gut, as it is further along the gastrointestinal tract than the proximal gut. Gut flora are dense
Gerhard Rudolph Edmund Meyer-Schwickerath was a German ophthalmologist, university lecturer and researcher. He is known as the father of light coagulation, the predecessor to many eye surgeries. Gerhard Rudolph Meyer was born as the son of Edmund Meyer and Josephine Meyer B. Schmitz in Elberfeld, Germany. In 1935, the family adopted surname of Edmund Meyer's mother, Julie Schwickerath and henceforth Gerhard Rudolph Meyer was stylized as Meyer-Schwickerath. One year after Gerhard's birth, his younger brother, Klaus Meyer-Schwickerath, was born, who went on to study law and become a politician. After graduating from high school, Meyer-Schwickerath decided not to be a lawyer, contrary to the family tradition, because he did not want to defend Nazism, he opted to become a physician instead. He began his medical studies in 1940. During the World War II he worked as a medic. A knee injury saved him from working on the front lines. Shortly after the war, Meyer-Schwickerath moved to Hamburg, where he worked as an assistant physician at the University of Hamburg-Eppendorf's eye clinic until 1952.
In 1953, he received his post-doctoral degree and the right to professorship at the University of Bonn. In 1959, he worked as senior physician with Paul Mikat and Kurt Biedenkopf to transform Essen's municipal hospital into the Essen University Hospital. From 1959 to his retirement in 1985, Meyer-Schwickerath served as the Director of the Ophthalmology Center at the Essen University Hospital. In 1964, he took the position of professor at the University of Münster, he was an honorary president of the German Ophthalmological Society. One of his most famous patients was Leonard Bernstein. Meyer-Schwickerath examined many patients whose retinas were damaged following total solar eclipse of July 9, 1945, he noticed. In 1946/1947, Meyer-Schwickerath determined that a progressive retinal detachment could be halted through precision scars. According to some accounts, the idea of producing a scar by means of light came to him following a sleepless night, in which, for fear of forgetting, he had recorded the two words "light" and "coagulation" on a note.
In 1946, he started conducted the first experiments on light coagulation. In 1949, he performed the first successful treatment of a retinal detachment with a light beam by with a self-constructed device on the roof of the ophthalmic clinic at the University of Hamburg-Eppendorf; this first device focused sunlight through a telescope and utilized a series of mirrors leading into the operating room and into the eye of a Patients. Since sunlight is not always reliable due to cloud coverage, this method proved to be unsatisfactory in the long run to Meyer-Schwickerath. In the 1950s, he collaborated with the Zeiss Labs to develop the high pressure xenon gas discharge lamp, which eliminated the need for sunlight and produced a stronger beam for coagulation. "His method of photo- or light coagulation has now been replaced by the application of the laser, but nothing has changed in the principle of the treatment of pre-stages of retinal detachment, of tumors and vascular diseases, of diabetic eye changes."The Deutschmuseum Bonn is the loan of the optical museum of the company Carl Zeiss in Oberkochen, the original part of the sunlight coagulator developed by Meyer-Schwickerath from 1949 under the inventory number 1994 - 11,000.
In 1945, after the war and graduation, Meyer-Schwickerath married 22-year-old Berta Steinbicker in Münster. They had one daughter. With the development of the light coagulation technique and his work, Meyer-Schwickerath acquired an international reputation. At the 2007 DOG congress, Charles P. Wilkinson, president of the American Academy of Ophthalmology, counted Meyer-Schwickerath among the pantheon of German medical figures. Wilkinson said, "I can assure you that the names von Graefe, Leber, all the way up to Custodis, Meyer-Schwickerath – these legendary names are known to every resident who has trained in America." Gerhard Meyer-Schwickerath received honorary doctorate different universities. His name was proposed for the Nobel Prize three times, he regarded his greatest prize of the Order Pour le Mérite for the arts. Other awards: 1960: Graefe Prize of the German Ophthalmological Society 1969: Election in the German Academy of Natural Scientists Leopoldina 1978: Pour le Mérite 1981: Large Federal Service Cross with Stern 1986: Order of Merit of North Rhine-Westphalia 1986: Graefe Medal of the German Ophthalmological Society 1989: State Prize of the State of North-Rhine Westphalia
OCLC Online Computer Library Center, Incorporated d/b/a OCLC is an American nonprofit cooperative organization "dedicated to the public purposes of furthering access to the world's information and reducing information costs". It was founded in 1967 as the Ohio College Library Center. OCLC and its member libraries cooperatively produce and maintain WorldCat, the largest online public access catalog in the world. OCLC is funded by the fees that libraries have to pay for its services. OCLC maintains the Dewey Decimal Classification system. OCLC began in 1967, as the Ohio College Library Center, through a collaboration of university presidents, vice presidents, library directors who wanted to create a cooperative computerized network for libraries in the state of Ohio; the group first met on July 5, 1967 on the campus of the Ohio State University to sign the articles of incorporation for the nonprofit organization, hired Frederick G. Kilgour, a former Yale University medical school librarian, to design the shared cataloging system.
Kilgour wished to merge the latest information storage and retrieval system of the time, the computer, with the oldest, the library. The plan was to merge the catalogs of Ohio libraries electronically through a computer network and database to streamline operations, control costs, increase efficiency in library management, bringing libraries together to cooperatively keep track of the world's information in order to best serve researchers and scholars; the first library to do online cataloging through OCLC was the Alden Library at Ohio University on August 26, 1971. This was the first online cataloging by any library worldwide. Membership in OCLC is based on use of services and contribution of data. Between 1967 and 1977, OCLC membership was limited to institutions in Ohio, but in 1978, a new governance structure was established that allowed institutions from other states to join. In 2002, the governance structure was again modified to accommodate participation from outside the United States.
As OCLC expanded services in the United States outside Ohio, it relied on establishing strategic partnerships with "networks", organizations that provided training and marketing services. By 2008, there were 15 independent United States regional service providers. OCLC networks played a key role in OCLC governance, with networks electing delegates to serve on the OCLC Members Council. During 2008, OCLC commissioned two studies to look at distribution channels. In early 2009, OCLC negotiated new contracts with the former networks and opened a centralized support center. OCLC provides bibliographic and full-text information to anyone. OCLC and its member libraries cooperatively produce and maintain WorldCat—the OCLC Online Union Catalog, the largest online public access catalog in the world. WorldCat has holding records from private libraries worldwide; the Open WorldCat program, launched in late 2003, exposed a subset of WorldCat records to Web users via popular Internet search and bookselling sites.
In October 2005, the OCLC technical staff began a wiki project, WikiD, allowing readers to add commentary and structured-field information associated with any WorldCat record. WikiD was phased out; the Online Computer Library Center acquired the trademark and copyrights associated with the Dewey Decimal Classification System when it bought Forest Press in 1988. A browser for books with their Dewey Decimal Classifications was available until July 2013; until August 2009, when it was sold to Backstage Library Works, OCLC owned a preservation microfilm and digitization operation called the OCLC Preservation Service Center, with its principal office in Bethlehem, Pennsylvania. The reference management service QuestionPoint provides libraries with tools to communicate with users; this around-the-clock reference service is provided by a cooperative of participating global libraries. Starting in 1971, OCLC produced catalog cards for members alongside its shared online catalog. OCLC commercially sells software, such as CONTENTdm for managing digital collections.
It offers the bibliographic discovery system WorldCat Discovery, which allows for library patrons to use a single search interface to access an institution's catalog, database subscriptions and more. OCLC has been conducting research for the library community for more than 30 years. In accordance with its mission, OCLC makes its research outcomes known through various publications; these publications, including journal articles, reports and presentations, are available through the organization's website. OCLC Publications – Research articles from various journals including Code4Lib Journal, OCLC Research, Reference & User Services Quarterly, College & Research Libraries News, Art Libraries Journal, National Education Association Newsletter; the most recent publications are displayed first, all archived resources, starting in 1970, are available. Membership Reports – A number of significant reports on topics ranging from virtual reference in libraries to perceptions about library funding. Newsletters – Current and archived newsletters for the library and archive community.
Presentations – Presentations from both guest speakers and OCLC research from conferences and other events. The presentations are organized into five categories: Conference presentations, Dewey presentations, Distinguished Seminar Series, Guest presentations, Research staff
Cauterization is a medical practice or technique of burning a part of a body to remove or close off a part of it. It destroys some tissue in an attempt to mitigate bleeding and damage, remove an undesired growth, or minimize other potential medical harm, such as infections when antibiotics are unavailable; the practice was once widespread for treatment of wounds. Its utility before the advent of antibiotics was said to be effective at more than one level: To prevent exsanguination To close amputationsCautery was believed to prevent infection, but current research shows that cautery increases the risk for infection by causing more tissue damage and providing a more hospitable environment for bacterial growth. Actual cautery refers to the metal device heated to a dull red glow, that a physician applies to produce blisters, to stop bleeding of a blood vessel, for other similar purposes; the main forms of cauterization used today in the first world are electrocautery and chemical cautery—both are, for example, prevalent in the removal of unsightly warts and stopping nosebleeds.
Cautery can mean the branding of a human, either recreational or forced. Cauterize is a Middle English word borrowed from the Old French cauteriser, from Late Latin cauterizare "to burn or brand with a hot iron", from Greek καυτηριάζειν > kauteriazein, from καυτήρ, kauter, "burning or branding iron", καίειν, "to burn". Cauterization has been used to stop heavy bleeding since antiquity; the process was described in the Edwin Smith Papyrus and Hippocratic Corpus. It was used to control hemorrhages those resulting from surgery, in ancient Greece. Archigenes recommended cauterization in the event of hemorrhaging wounds, Leonides of Alexandria described excising breast tumors and cauterizing the resulting wound in order to control bleeding; the Chinese Su wen recommends cauterization as a treatment for various ailments, including dog bites. Indigenous peoples of the Americas, ancient Arabs, Persians used the technique. Tools used in the ancient cauterization process ranged from heated lances to cauterizing knives.
The piece of metal was applied to the wound. This caused tissues and blood to heat to extreme temperatures, causing coagulation of the blood and thus controlling the bleeding, at the cost of extensive tissue damage. In rarer cases, cauterization was instead accomplished via the application of cauterizing chemicals like lye. Cauterization continued to be used as a common treatment in medieval times. While employed to stop blood loss, it was used in cases of tooth extraction and as a treatment for mental illness. In the Arab world, scholars Al-Zahrawi and Avicenna wrote about techniques and instruments used for cauterization; the technique of ligature of the arteries as an alternative to cauterization was improved and used more by Ambroise Paré. Electrocauterization is the process of destroying tissue using heat conduction from a metal probe heated by electric current; the procedure stops bleeding from small vessels. Electrocautery applies high frequency alternating current by a bipolar method, it can be a continuous waveform to cut tissue, or intermittent to coagulate tissue.
The electrically produced heat in this process inherently can do numerous things to the tissue, depending on the waveform and power level, including cauterize, coagulate and dry. Thus electrocautery, electrocoagulation, electrodesiccation, electrocurettage are related and can co-occur in the same procedure when desired. Electrodesiccation and curettage is a common procedure. In unipolar cauterization, the physician contacts the tissue with a single small electrode; the circuit's exit point is a large surface area, such as the buttocks. The amount of heat generated depends on the size of contact area, power setting or frequency of current, duration of application, waveform. A constant waveform generates more heat than intermittent; the frequency used in cutting the tissue is higher than in coagulation mode. Bipolar electrocautery passes the current between two tips of a forceps-like tool, it has the advantage of not disturbing other electrical body rhythms and coagulates tissue by pressure. Lateral thermal injury is greater in unipolar than bipolar devices.
Electrocauterization is preferable to chemical cauterization, because chemicals can leach into neighbouring tissue and cauterize outside of intended boundaries. Concern has been raised regarding toxicity of the surgical smoke electrocautery produces; this contains chemicals that, through inhalation, may harm medical staff. Ultrasonic coagulation and ablation systems are available. Many chemical reactions can destroy tissue, some are used in medicine, most to remove small skin lesions such as warts or necrotized tissue, or for hemostasis; because chemicals can leach into areas not intended for cauterization and electrical methods are preferable where practical. Some cauterizing agents are: Silver nitrate is the active ingredient of the lunar caustic, a stick that traditionally looks like a large match, it is pressed onto the lesion for a few moments. Trichloroacetic acid Cantharidin is an extract of the blister beetle that causes epidermal necrosis and blistering, it is used to treat warts. Frequent nosebleeds are most caused by an exposed blood vessel in the nose one in Kiesselbach's plexus.
If the nose is not bleeding at the time, a physician may cauterize it to prevent future bleeding. Cauterization methods include burning the affected area with acid, hot metal, o
An ion laser is a gas laser that uses an ionized gas as its lasing medium. Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike helium–neon lasers, the energy level transitions that contribute to laser action come from ions; because of the large amount of energy required to excite the ionic transitions used in ion lasers, the required current is much greater, as a result all but the smallest ion lasers are water-cooled. A small air-cooled ion laser might produce, for example, 130 milliwatts of output light with a tube current of about 10 amperes and a voltage of 105 volts. Since one ampere times one volt is one watt, this is an electrical power input of about one kilowatt. Subtracting the light output of 130 mW from power input, this leaves the large amount of waste heat of nearly one kW; this has to be dissipated by the cooling system. In other words, the power efficiency is low. A krypton laser is an ion laser using ions of the noble gas krypton as its gain medium.
The laser pumping is done by an electrical discharge. Krypton lasers are used in scientific research, in commercial uses, when the krypton is mixed with argon, it creates a "white-light" lasers, useful for laser light shows. Krypton lasers are used in medicine, for the manufacture of security holograms, numerous other purposes. Krypton lasers can emit visible light close to several different wavelengths 406.7 nm, 413.1 nm, 415.4 nm, 468.0 nm, 476.2 nm, 482.5 nm, 520.8 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm. The argon-ion laser was invented in 1964 by William Bridges at the Hughes Aircraft Company and it is one of the family of ion lasers that use a noble gas as the active medium. Argon-ion lasers are used for retinal phototherapy and the pumping of other lasers. Argon-ion lasers emit at 13 wavelengths through the visible and ultraviolet spectra, including: 351.1 nm, 363.8 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, 528.7 nm, 1092.3 nm. However, the most used wavelengths are in the blue-green region of the visible spectrum.
These wavelengths have the potential for use in underwater communications because seawater is quite transparent in this range of wavelengths. Common argon and krypton lasers are capable of emitting continuous-wave output of several milliwatts to tens of watts, their tubes are made from nickel end bells, kovar metal-to-ceramic seals, beryllium oxide ceramics, or tungsten disks mounted on a copper heat spreader in a ceramic liner. The earliest tubes were simple quartz followed by quartz with graphite disks. In comparison with the helium–neon lasers, which require just a few milliamperes of input current, the current used for pumping the krypton laser is several amperes, since the gas has to be ionized; the ion laser tube produces much waste heat, such lasers require active cooling. The typical noble-gas ion-laser plasma consists of a high-current-density glow discharge in a noble gas in the presence of a magnetic field. Typical continuous-wave plasma conditions are current densities of 100 to 2000 A/cm2, tube diameters of 1.0 to 10 mm, filling pressures of 0.1 to 1.0 Torr, an axial magnetic field of the order of 1000 gauss.
William R. Bennett, a co-inventor of the first gas laser, was the first to observe spectral hole burning effects in gas lasers, he created the theory of "hole burning" effects in laser oscillation, he was co-discoverer of lasers using electron-impact excitation in each of the noble gases, dissociative excitation transfer in the neon–oxygen laser, collision excitation in several metal-vapor lasers. Ar/Kr: A mix of argon and krypton can result in a laser with output wavelengths that appear as white light. Helium–cadmium: blue laser emission at 442 nm and ultraviolet at 325 nm. Copper vapor: yellow and green emission at 578 nm and 510 nm. Xenon Iodine Oxygen Confocal laser scanning microscopy Surgical Laser medicine High speed typesetters Laser light shows DNA sequencers Spectroscopy experiments Pumping dye lasers Semiconductor wafer inspection Direct write high density PCB lithography Fiber Bragg Grating production Long coherence length models can be used for holography Laser List of laser types List of plasma articles
University of Hamburg
The University of Hamburg is a comprehensive university in Hamburg, Germany. It was founded on 28 March 1919, having grown out of the previous General lecture system and the Colonial Institute of Hamburg as well as the Akademic Gymnasium. In spite of its short history, six Nobel Prize Winners and serials of scholars are affiliated to the university; the University of Hamburg is the biggest research and education institution in Northern Germany and one of the most extensive universities in Germany. The main campus is located in the central district of Rotherbaum, with affiliated institutes and research centres spread around the city state; the institution is classified as a global top 200 university by cited ranking systems such as the Times Higher Education Ranking, the Shanghai Ranking and the CWTS Leiden Ranking, placing it among the top 1% of global universities. On a national scale, U. S. News & World Report ranks UHH 7th and QS World University Rankings 14th out of a total of 426 German institutions of higher education.
At the beginning of the 20th century, wealthy individuals made several petitions to the Hamburg Senate and Parliament requesting the establishment of a university, however those were made to no avail. Although for a time, senator Werner von Melle supported the merger of existing institutions into one university, this plan failed because of the parliaments composition due to the effects of class voting. Much of the establishment wanted to see Hamburg limited to its dominant role as a trading center and shunned both the costs of a university and the social demands of the professors that would have to be employed. Progress was made however, since proponents of a university founded the Hamburg Science Foundation in 1907 and the Hamburg Colonial Institute in 1908; the former institution supported the recruitment of scholars for the chairs of the General lecture system and funding of research cruises, the latter was responsible for all education and research questions concerning overseas territories.
In the same year, the citizenry approved a construction site on the Moorweide for the establishment of a lecture building, which opened in 1911 and became the main building of the university. However, the plans for the foundation of the university itself had to be shelved, following the outbreak of the First World War. After the war, the first elected senate chose von Melle as mayor, he and Rudolf Ross made a push for education reform in Hamburg, their law establishing the university and an adult high school went through. On March 28, 1919 the University of Hamburg opened its gates; the number of full professorships in Hamburg was increased from 19 to 39. Both the Colonial Institute and the General lecture system were absorbed into the university; the first faculties created by the university were Law and Political Science, Medicine and Natural Sciences. During the Weimar Republic, the university grew into importance. Several thousand students were continuously enrolled, it drew scholars like Albrecht Mendelssohn Bartholdy, Aby Warburg and Ernst Cassirer to Hamburg.
The number of full professors had by 1931 grown to 75. Because many students were suffering due to the bad economic situation that prevailed in the early republic, the Hamburg Association of Student Aid was founded in 1922. Ernst Cassirer became principal of the university in 1929, one of the first Jewish scholars with that role in Germany; the academic situation shifted after the general election in March 1933. On May 1 of that year – the university held a ceremony to honor Adolf Hitler as its leader. Massive political influence by the Nazis followed, including the removal of books from the libraries and harassment against alleged enemies of the people. About fifty scientists, including Ernst Cassirer and William Stern, had to leave the university. At least ten students working with the White Rose in Hamburg were arrested. In the foyer of the lecture hall a design by Fritz Fleer commemorative plate was taken in 1971 in memory of the four resistance fighters. Once the Second World War was over, the university was reopened in the winter of 1945 with 17,800 employees.
Out of the 2,872 students who were enrolled at the University of Hamburg in the first postwar semester of 1945/46, 601 had been admitted at the Philosophical, 952 at the Medical and 812 to the Faculty of Law and Political Science. The smallest number joined the Faculty of Mathematics and Natural Sciences with 506 students in total; the first student association during this period was elected in 1946 under British supervision, it formed the foundation of the AStA in 1947. During the West German era, new departments were added to the university, most notably the Faculty of Theology as well as the Faculty of Economic and Social Sciences in 1954; the late 1950s and early 1960s saw a lot of construction: the Auditorium and the Philosopher's Tower where inaugurated near the Von-Melle-Park, while the Botanical Institute and Botanical Garden were relocated to Flottbeck. The university grew from 12,600 students in 1960 to 19,200 in 1970. A wave of protests during the student movements of 1968 sparked a reform of the university structure, in 1969 the faculties were dissolved in favor of more interdisciplinary departments.
Student and staff involvement in the administration was strengthened, the office of Rektor abolished in favor of a university president. However, parts of the reform were rescinded in 1979. Further construction in the 1970s built up the remaining space on the main campus of Rotherbaum quarter, with the Geomatikum
The fovea centralis is a small, central pit composed of packed cones in the eye. It is located in the center of the macula lutea of the retina; the fovea is responsible for sharp central vision, necessary in humans for activities where visual detail is of primary importance, such as reading and driving. The fovea is surrounded by the parafovea belt, the perifovea outer region; the parafovea is the intermediate belt, where the ganglion cell layer is composed of more than five rows of cells, as well as the highest density of cones. The perifovea contains an more diminished density of cones, having 12 per 100 micrometres versus 50 per 100 micrometres in the most central fovea. This, in turn, is surrounded by a larger peripheral area that delivers compressed information of low resolution following the pattern of compression in foveated imaging. Half of the nerve fibers in the optic nerve carry information from the fovea, while the remaining half carry information from the rest of the retina; the parafovea extends to a radius of 1.25 mm from the central fovea, the perifovea is found at a 2.75 mm radius from the fovea centralis.
The term fovea comes from the from Latin foves, meaning'pit'. Fovea is the depression in the inner retinal surface, about 1.5 mm wide, the photoreceptor layer of, cones and, specialized for maximum visual acuity. Within the Fovea is a region of 0.5mm diameter called the foveal avascular zone. This allows the light to be sensed without any loss; this anatomy is responsible for the depression in the center of the fovea. The foveal pit is surrounded by the foveal rim; this is the thickest part of the retina. The fovea is located in a small avascular zone and receives most of its oxygen from the vessels in the choroid, across the retinal pigment epithelium and Bruch's membrane; the high spatial density of cones along with the absence of blood vessels at the fovea accounts for the high visual acuity capability at the fovea. The center of the fovea is the foveola – about 0.35 mm in diameter – or central pit where only cone photoreceptors are present and there are no rods. The central fovea consists of compact cones and more rod-like in appearance than cones elsewhere.
These cones are densely packed. Starting at the outskirts of the fovea, rods appear, the absolute density of cone receptors progressively decreases; the anatomy of the foveola was reinvestigated, it was discovered that outer segments from the central foveolar cones of monkeys are not straight and twice as long as those from the parafovea. The size of the fovea is small with regard to the rest of the retina. However, it is the only area in the retina where 20/20 vision is attainable, is the area where fine detail and colour can be distinguished. Anatomical macula / macula lutea / area centralis: Diameter = 5.5mm Demarcated by the superior and inferior temporal arterial arcades. Has an elliptical shape horizontally. Histologically the only region of the retina where GCL has >1 layer of ganglion cells Yellowish appearance = luteal pigments (xanthophyll and beta-carotenoid in the outer nuclear layers inward. Anatomical perifovea: Region between parafovea and edge of macula GCL has 2-4 layers of cells.
12 cones / 100 um Anatomical parafovea: Diameter = 2.5mm. GCL has >5 layers of cells, highest density of cones Anatomical fovea / fovea centralis Area of depression in the centre of the macula lutea. Diameter = 1.5mm Foveal avascular zone Diameter = 0.5mm Approximately equal to the foveola Anatomical foveola Diameter = 0.35mm the central floor of depression of fovea centralis 50 cones / 100 um Highest visual acuity Anatomical umbo Represents the precise center of the macula Diameter = 0.15mm Corresponds to the clinical light reflex In the primate fovea the ratios of ganglion cells to photoreceptors is about 2.5. Therefore, the acuity of foveal vision is limited only by the density of the cone mosaic, the fovea is the area of the eye with the highest sensitivity to fine details. Cones in the central fovea express pigments that are sensitive to red light; these cones are the'midget' pathways that underpin high acuity functions of the fovea. The fovea is employed for accurate vision in the direction.
It takes up over 50 % of the visual cortex in the brain. The fovea sees only the central two degrees of the visual field. If an object is large and thus covers a large angle, the eyes must shift their gaze to subsequently bring different portions of the image into the fovea. Since the fovea does not have rods, it is not sensitive to dim lighting. Hence, in order to observe dim stars, astronomers use averted vision, looking out of the side of their eyes where the density of rods is greater, hence dim objects are more visible; the fovea has a high concentration of the yellow carotenoid pigments zeaxanthin. They are concentrated in the Henle fiber layer and to a