Ophthalmology

src: static1.squarespace.com

Ophthalmology ( or ) is the branch of medicine that deals with the anatomy, physiology and diseases of the eyeball and orbit. An ophthalmologist is a specialist in medical and surgical eye disease. Their credentials include an M.D. or D.O. degree in medicine, followed by an additional four years of residency. Additional training may be sought through a fellowship in a particular specialty of eye pathology. Ophthalmologists are allowed to medically treat eye disease, implement laser therapy, and perform incisional surgery when warranted.

Video Ophthalmology

Etymology

The Greek roots of the word ophthalmology are ???????? (ophthalmos, "eye") and -?o??? (-logia, "study, discourse"), i.e., "the study of eyes". The discipline applies to all animal eyes, whether human or not, since the practice and procedures are quite similar with respect to disease processes, while differences in anatomy or disease prevalence, whether subtle or substantial, may differentiate the two.

Maps Ophthalmology

History

Ancient India

The Indian surgeon Sushruta wrote Sushruta Samhita in Sanskrit in about 800 BC which describes 76 ocular diseases (of these 51 surgical) as well as several ophthalmological surgical instruments and techniques. His description of cataract surgery was more akin to extracapsular lens extraction than to couching. He has been described as the first cataract surgeon.

Before Hippocrates

The pre-Hippocratics largely based their anatomical conceptions of the eye on speculation, rather than empiricism. They recognized the sclera and transparent cornea running flushly as the outer coating of the eye, with an inner layer with pupil, and a fluid at the centre. It was believed, by Alcamaeon and others, that this fluid was the medium of vision and flowed from the eye to the brain by a tube. Aristotle advanced such ideas with empiricism. He dissected the eyes of animals, and discovering three layers (not two), found that the fluid was of a constant consistency with the lens forming (or congealing) after death, and the surrounding layers were seen to be juxtaposed. He and his contemporaries further put forth the existence of three tubes leading from the eye, not one. One tube from each eye met within the skull.

Rufus

Rufus of Ephesus recognised a more modern eye, with conjunctiva, extending as a fourth epithelial layer over the eye. Rufus was the first to recognise a two-chambered eye, with one chamber from cornea to lens (filled with water), the other from lens to retina (filled with an egg white-like substance). The Greek physician Galen remedied some mistakes including the curvature of the cornea and lens, the nature of the optic nerve, and the existence of a posterior chamber.

Though this model was a roughly correct modern model of the eye, it contained errors. Still, it was not advanced upon again until after Vesalius. A ciliary body was then discovered and the sclera, retina, choroid, and cornea were seen to meet at the same point. The two chambers were seen to hold the same fluid, as well as the lens being attached to the choroid. Galen continued the notion of a central canal, but he dissected the optic nerve and saw that it was solid. He mistakenly counted seven optical muscles, one too many. He also knew of the tear ducts.

Middle Eastern ophthalmology

Medieval Islamic Arabic and Persian scientists (unlike their classical predecessors) considered it normal to combine theory and practice, including the crafting of precise instruments, and therefore found it natural to combine the study of the eye with the practical application of that knowledge. Hunain ibn Ishaq, and others beginning with the medieval Arabic period, taught that the crystalline lens is in the exact center of the eye. This idea was propagated until the end of the 1500s.

Ibn al-Haytham (Alhazen), an Arab scientist with Islamic beliefs, wrote extensively on optics and the anatomy of the eye in his Book of Optics (1021).

Ibn al-Nafis, an Arabic native of Damascus, wrote a large textbook, The Polished Book on Experimental Ophthalmology, divided into two parts, On the Theory of Ophthalmology and Simple and Compounded Ophthalmic Drugs.

17th and 18th centuries

In the 17th and 18th centuries, hand lenses were used by Malpighi, and microscopes by van Leeuwenhoek, preparations for fixing the eye for study by Ruysch, and later the freezing of the eye by Petit. This allowed for detailed study of the eye and an advanced model. Some mistakes persisted, such as: why the pupil changed size (seen to be vessels of the iris filling with blood), the existence of the posterior chamber, and of course the nature of the retina. In 1722, van Leeuwenhoek noted the existence of rods and cones, though they were not properly discovered until Gottfried Reinhold Treviranus in 1834 by use of a microscope.

Georg Joseph Beer (1763-1821) was an Austrian ophthalmologist and leader of the First Viennese School of Medicine. He introduced a flap operation for treatment of cataracts (Beer's operation), as well as popularizing the instrument used to perform the surgery (Beer's knife).

In North America, indigenous healers treated some eye diseases by rubbing or scraping the eyes or eyelids.

Ophthalmic surgery in Great Britain

The first ophthalmic surgeon in Great Britain was John Freke, appointed to the position by the Governors of St Bartholomew's Hospital in 1727. A major breakthrough came with the appointment of Baron Michael Johann Baptist de Wenzel (1724-90), a German who became oculist to King George III of England in 1772. His skill at removing cataracts legitimized the field. The first dedicated ophthalmic hospital opened in 1805 in London; it is now called Moorfields Eye Hospital. Clinical developments at Moorfields and the founding of the Institute of Ophthalmology (now part of the University College London) by Sir Stewart Duke Elder established the site as the largest eye hospital in the world and a nexus for ophthalmic research.

20th century

The prominent opticians of the late 19th and early 20th centuries included Ernst Abbe (1840-1905), a co-owner of at the Zeiss Jena factories in Germany where he developed numerous optical instruments. Hermann von Helmholtz (1821-1894) was a polymath who made contributions to many fields of science and invented the ophthalmoscope in 1851. They both made theoretical calculations on image formation in optical systems and had also studied the optics of the eye.

Central Europe

Numerous ophthalmologists fled Germany after 1933 as the Nazis began to persecute those of Jewish descent. A representative leader was Joseph Igersheimer (1879-1965), best known for his discoveries with arsphenamine for the treatment of syphilis. He fled to Turkey in 1933. As one of eight emigrant directors in the Faculty of Medicine at the University of Istanbul, he built a modern clinic and trained students. In 1939, he went to the United States, becoming a professor at Tufts University. German ophthalmologist, Gerhard Meyer-Schwickerath is widely credited with developing the predecessor of laser coagulation, photocoagulation. 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 (light coagulation) by with a self-constructed device on the roof of the ophthalmic clinic at the University of Hamburg-Eppendorf.

Polish ophthalmology dates to the 13th century. The Polish Ophthalmological Society was founded in 1911. A representative leader was Adam Zamenhof (1888-1940), who introduced certain diagnostic, surgical, and nonsurgical eye-care procedures and was shot by the Nazis in 1940. Zofia Falkowska (1915-93) head of the Faculty and Clinic of Ophthalmology in Warsaw from 1963 to 1976, was the first to use lasers in her practice.

src: www.sullivanostoicheye.com

Professional requirements

Ophthalmologists are physicians (MD/MS after MBBS or D.O./DOMS/ DNB, not OD or BOptom) who have completed a college degree, medical school, and residency in ophthalmology. Ophthalmology training equips eye specialists to provide the full spectrum of eye care, including the prescription of glasses and contact lenses, medical treatment, and complex microsurgery. In many countries, ophthalmologists also undergo additional specialized training in one of the many subspecialties. Ophthalmology was the first branch of medicine to offer board certification, now a standard practice among all specialties.

Australia and New Zealand

In Australia and New Zealand, the FRACO/FRANZCO is the equivalent postgraduate specialist qualification. It is a very competitive speciality to enter training and has a closely monitored and structured training system in place over the five years of postgraduate training. Overseas-trained ophthalmologists are assessed using the pathway published on the RANZCO website. Those who have completed their formal training in the UK and have the CCST/CCT are usually deemed to be comparable.

Bangladesh

In Bangladesh to be an ophthalmologist the basic degree is an MBBS. Then they have to obtain a postgraduate degree or diploma in specialty ophthalmology. In Bangladesh, these are Diploma in Ophthalmology, Diploma in Community Ophthalmology, Fellow or Member of the College of Physicians and Surgeons in ophthalmology, and Master of Science in ophthalmology.

Canada

In Canada, an ophthalmology residency after medical school is undertaken. The residency lasts a minimum of five years after the MD degree which culminates in fellowship of the Royal College of Surgeons of Canada (FRCSC). Subspecialty training is undertaken by about 30% of fellows (FRCSC) in a variety of fields from anterior segment, cornea, glaucoma, vision rehabilitation, uveitis, oculoplastics, medical and surgical retina, ocular oncology, ocular pathology, or neuro-ophthalmology. About 35 vacancies open per year for ophthalmology residency training in all of Canada. These numbers fluctuate per year, ranging from 30 to 37 spots. Of these, up to ten spots are at French-speaking universities in Quebec. At the end of the five years, the graduating ophthalmologist must pass the oral and written portions of the Royal College exam in either English or French.

Finland

In Finland, physicians willing to become ophthalmologists must undergo a five-year specialization which includes practical training and theoretical studies.

India

In India, after completing MBBS degree, postgraduate study in ophthalmology is required. The degrees are Doctor of Medicine, Master of Surgery, Diploma in Ophthalmic Medicine and Surgery, and Diplomate of National Board. The concurrent training and work experience is in the form of a junior residency at a medical college, eye hospital, or institution under the supervision of experienced faculty. Further work experience in form of fellowship, registrar, or senior resident refines the skills of these eye surgeons. All India Ophthalmological Society and various state-level ophthalmological societies hold regular conferences and actively promote continuing medical education.

Nepal

In Nepal, to become an ophthalmologist, three years postgraduate study is required after completing MBBS degree. The postgraduate degree in ophthalmology is called MD in Ophthalmology. This degree is currently provided by Tilganga Institute of Ophthalmology, Tilganga, Kathmandu, BPKLCO, Institute of Medicine, TU, Kathmandu, BP Koirala Institute of Health Sciences, Dharan, Kathmandu University, Dhulikhel and National Academy of Medical Science, Kathmandu. Few Nepalese citizen also study this subject in Bangladesh, China, India, Pakistan and other countries. All the graduates have to pass Nepal Medical Council Licensing Exam to become a registered Ophthalmology in Nepal. The concurrent residency training is in the form of a PG student (resident) at a medical college, eye hospital, or institution according to the degree providing university's rules and regulations. Nepal Ophthalmic Society holds regular conferences and actively promote continuing medical education.

Ireland

In Ireland, the Royal College of Surgeons of Ireland grants Membership (MRCSI (Ophth)) and Fellowship (FRCSI (Ophth)) qualifications in conjunction with the Irish College of Ophthalmologists. Total postgraduate training involves an intern year, a minimum of three years of basic surgical training and a further 4.5 years of higher surgical training. Clinical training takes place within public, Health Service Executive-funded hospitals in Dublin, Sligo, Limerick, Galway, Waterford, and Cork. A minimum of 8.5 years of training is required before eligibility to work in consultant posts. Some trainees take extra time to obtain MSc, MD or PhD degrees and to undertake clinical fellowships in the UK, Australia and the United States.

Pakistan

In Pakistan, after MBBS, a four-year full-time residency program leads to an exit-level FCPS examination in ophthalmology, held under the auspices of the College of Physicians and Surgeons, Pakistan. The tough examination is assessed by both highly qualified Pakistani and eminent international ophthalmic consultants. As a prerequisite to the final examinations, an intermediate module, an optics and refraction module, and a dissertation written on a research project carried out under supervision is also assessed. Moreover, a two-and-a-half-year residency program leads to an MCPS while a two-year training of DOMS is also being offered. For candidates in the military, a stringent two-year graded course, with quarterly assessments, is held under Armed Forces Post Graduate Medical Institute in Rawalpindi. The M.S. in ophthalmology is also one of the specialty programs. In addition to programs for doctors, various diplomas and degrees for allied eyecare personnel are also being offered to produce competent optometrists, orthoptists, ophthalmic nurses, ophthalmic technologists, and ophthalmic technicians in this field. These programs are being offered notably by the College of Ophthalmology and Allied Vision Sciences in Lahore and the Pakistan Institute of Community Ophthalmology in Peshawar. Subspecialty fellowships are also being offered in the fields of pediatric ophthalmology and vitreoretinal ophthalmology. King Edward Medical University, Al Shifa Trust Eye Hospital Rawalpindi, and Al- Ibrahim Eye Hospital Karachi have also started a degree program in this field.

Philippines

Ophthalmology is a considered a medical specialty that uses medicine and surgery to treat diseases of the eye. There is only one, duly recognized by the PMA and the PCS, professional organization in the country: the Philippine Academy of Ophthalmology (PAO). PAO and the state-standard Philippine Board of Ophthalmology (PBO) regulates ophthalmology residency programs and board certification. To become a general ophthalmologist in the Philippines, a candidate must have completed a Doctor of Medicine degree (MD) or its equivalent (e.g. MBBS), have completed an internship in Medicine, have passed the physician licensure exam, and completed residency training at a hospital accredited by the Philippine Board of Ophthalmology (accrediting arm of PAO). Attainment of board certification in ophthalmology from the PBO is essential in acquiring privileges in most major health institutions. Graduates of residency programs can receive further training in ophthalmology subspecialties, such as neuro-ophthalmology, retina, etc. by completing a fellowship program which varies in length depending on each program's requirements.

United Kingdom

In the United Kingdom, three colleges grant postgraduate degrees in ophthalmology. The Royal College of Ophthalmologists (RCOphth) and the Royal College of Surgeons of Edinburgh grant MRCOphth/FRCOphth and MRCSEd/FRCSEd, (although membership is no longer a prerequisite for fellowship), the Royal College of Glasgow grants FRCS. Postgraduate work as a specialist registrar and one of these degrees is required for specialization in eye diseases. Such clinical work is within the NHS, with supplementary private work for some consultants. Only 2.3 ophthalmologists exist per 100,000 population in the UK - fewer pro rata than in any other nation in the European Union.

United States

In the United States, four years of residency training after medical school are required, with the first year being an internship in surgery, internal medicine, pediatrics, or a general transition year. Optional fellowships in advanced topics may be pursued for several years after residency. Most currently practicing ophthalmologists train in medical residency programs accredited by the Accreditation Council for Graduate Medical Education or the American Osteopathic Association and are board-certified by the American Board of Ophthalmology or the American Osteopathic Board of Ophthalmology and Otolaryngology. United States physicians who train in osteopathic medical schools hold the Doctor of Osteopathic Medicine (DO) degree rather than an MD degree. The same residency and certification requirements for ophthalmology training must be fulfilled by osteopathic physicians.

Physicians must complete the requirements of continuing medical education to maintain licensure and for recertification. Professional bodies like the American Academy of Ophthalmology and American Society of Cataract and Refractive Surgery organize conferences, help physician members through continuing medical education programs for maintaining board certification, and provide political advocacy and peer support.

src: previews.123rf.com

Subspecialties

Ophthalmology includes subspecialities which deal either with certain diseases or diseases of certain parts of the eye. Some of them are:

• Anterior segment surgery
• Retinal ophthalmology, which emphasizes such things as laser treatment of the retina and actual retinal surgery
• Cataracts - not usually considered a subspecialty per se, since most general ophthalmologists perform cataract surgery
• Cornea, ocular surface, and external disease
• Glaucoma
• Medical retina, deals with treatment of retinal problems through non-surgical means.
• Neuro-ophthalmology
• Ocular oncology
• Oculoplastics and orbit surgery
• Ophthalmic pathology
• Pediatric ophthalmology/strabismus (misalignment of the eyes)
• Refractive surgery
• Uveitis
• Immunology
• Veterinary formal specialty training programs in veterinary ophthalmology now exist in some countries.
• Vitreo-retinal surgery, deals with surgical management of retinal and posterior segment diseases and disorders. Medical retina and vitreo-retinal surgery sometimes together called posterior segment subspecialisation.

Ophthalmic surgery

src: www.kumc.edu

Notable ophthalmologists

18th-19th centuries

• Theodor Leber discovered Leber's congenital amaurosis, Leber's hereditary optic neuropathy, Leber's miliary aneurysm, and Leber's stellate neuroretinitis.
• Sir William Adams (UK) was the founder of Exeter's West of England Eye Infirmary.
• Carl Ferdinand von Arlt (1812-1887), the elder (Austrian), proved that myopia is largely due to an excessive axial length, published influential textbooks on eye disease, and ran annual eye clinics in needy areas long before the concept of volunteer eye camps became popular. His name is still attached to some disease signs, e.g., von Arlt's line in trachoma. His son Ferdinand Ritter von Arlt, the younger, was also an ophthalmologist.
• Jacques Daviel (France) claimed to be the 'father' of modern cataract surgery in that he performed extracapsular extraction instead of needling the cataract or pushing it back into the vitreous. He is said to have carried out the technique on 206 patients in 1752-53, of which 182 were reported to be successful. These figures are not very credible, given the total lack of both anaesthesia and aseptic technique at that time.
• Franciscus Donders (1818-1889) (Dutch) published pioneering analyses of ocular biomechanics, intraocular pressure, glaucoma, and physiological optics. He made possible the prescribing of combinations of spherical and cylindrical lenses to treat astigmatism.
• Joseph Forlenze (1757-1833) (Italy), specialist in cataract surgery, became popular during the First French Empire, healing, among many, personalities such as the minister Jean-Étienne-Marie Portalis and the poet Ponce Denis Lebrun. He was nominated by Napoleon "chirurgien oculiste of the lycees, the civil hospices and all the charitable institutions of the departments of the Empire". He was known also for his free interventions, mainly in favour of poor people.

• John Frederick France (1817-1900), in 1847, succeeded the late Mr John Morgan as lecturer on Ophthalmic Surgery at the school. In 1855, he was elected a Fellow of the Royal College of Surgeons, and was also a Fellow of the Society of Antiquaries. France edited, with notes, the second edition of Morgan's work on Lectures on Diseases of the Eye (1848) and was a voluminous writer himself on ophthalmic subjects. To the Guys Hospital Reports he contributed 17 papers between 1848 and 1861, and numerous other papers appeared in various periodical publications. He was one of the authors who supported the theory of causal connection between diabetes and cataract, which at that time was still questioned by many physicians. He used, for fixation of the eyeball, a simple artery forceps (without spring lock) in 27 patients and had success in all of them. France also published clinical observations about eye injuries, paralysis of the pupil, and ptosis. In the same journal (October 1845), he reported his successful extraction of a traumatic calcified cataract from the anterior chamber.
• Albrecht von Graefe (1828-1870) (Germany) Along with Helmholtz and Donders, one of the 'founding fathers' of ophthalmology as a specialty. He was a brilliant clinician and charismatic teacher who had an international influence on the development of ophthalmology, and was a pioneer in mapping visual field defects and diagnosis and treatment of glaucoma. He introduced a cataract extraction technique that remained the standard for over 100 years, and many other important surgical techniques such as iridectomy. He rationalised the use of many ophthalmically important drugs, including mydriatics and miotics. He also was the founder of one of the earliest ophthalmic societies (German Ophthalmological Society, 1857) and one of the earliest ophthalmic journals (Graefe's Archives of Ophthalmology). He was probably the most important ophthalmologist of the 19th century.
• Allvar Gullstrand (Sweden) was a Nobel Prize-winner in 1911 for his research on the eye as a light-refracting apparatus. He described the 'schematic eye', a mathematical model of the human eye based on his measurements known as the 'optical constants' of the eye. His measurements are still used today.
• Hermann von Helmholtz, a great German polymath, invented the ophthalmoscope (1851) and published important work on physiological optics, including colour vision (1850s).
• Julius Hirschberg (Germany) in 1879 became the first to use an electromagnet to remove metallic foreign bodies from the eye and in 1886 developed the Hirschberg test for measuring strabismus.
• Socrate Polara (1800-1860, Italy) founded the first dedicated ophthalmology clinic in Sicily in 1829, entirely as a philanthropic endeavor; later he was appointed as the first director of the ophthalmology department at the Grand Hospital of Palermo, Sicily, in 1831 after the Sicilian government became convinced of the importance of state support for the specialization.
• Herman Snellen (Netherlands) introduced the Snellen chart to study visual acuity.
• Sir Arthur Conan Doyle (United Kingdom) was a Scottish writer, primarily of the Sherlock Holmes stories. He was trained in ophthalmology, but apparently never practiced.
• Jose Rizal (Philippines), a Philippines' national hero, was an ophthalmologist. One of his works was an operation on both his mother's eyes for removal of cataracts.

20th-21st centuries

• William Horatio Bates (1860-1931) (United States) was creator of the unorthodox Bates method, and credited for being the founder of the Natural Vision Improvement movement.
• Vladimir Petrovich Filatov (1875-1956) (Ukraine) contributed to the medical world the tube flap grafting method, corneal transplantation, and preservation of grafts from cadaver eyes and tissue therapy. He founded the Filatov Institute of Eye Diseases and Tissue Therapy, Odessa, one of the leading eye-care institutes in the world.
• Ignacio Barraquer (1884-1965) (Spain), in 1917, invented the first motorized vacuum instrument (erisophake) for intracapsular cataract extraction. He founded of the Barraquer Clinic in 1941 and the Barraquer Institute in 1947 in Barcelona, Spain.
• Tsutomu Sato (Japan) Pioneer in incisional refractive surgery, including techniques for astigmatism and the invention of radial keratotomy for myopia.
• Jules Gonin (1870-1935) (Switzerland) was the "father of retinal detachment surgery".
• Sir Harold Ridley (United Kingdom), in 1949, may have been the first to successfully implant an artificial intraocular lens after observing that plastic fragments in the eyes of wartime pilots were well tolerated. He fought for decades against strong reactionary opinions to have the concept accepted as feasible and useful.
• Charles Schepens (Belgium) was the "father of modern retinal surgery" and developer of the Schepens indirect binocular ophthalmoscope whilst at Moorfields Eye Hospital. He was the founder of the Schepens Eye Research Institute in Boston, Massachusetts. This premier research institute is associated with Harvard Medical School and Massachusetts Eye and Ear Infirmary.
• Marshall M. Parks was the "father of pediatric ophthalmology".
• José Ignacio Barraquer (1916-1998) (Spain) was the "father of modern refractive surgery". In the 1960s, he developed lamellar techniques, including keratomileusis and keratophakia, as well as the first microkeratome and corneal microlathe.
• Tadeusz Krwawicz (Poland), in 1961, developed the first cryoprobe for intracapsular cataract extraction.
• Svyatoslav Fyodorov (Russia) was the "father of ophthalmic microsurgery". He improved and popularized the radial keratotomy, invented a surgical cure for cataract, and developed the scleroplasty.
• Charles Kelman (United States) developed the ultrasound and mechanized irrigation and aspiration system for phacoemulsification, first allowing cataract extraction through a small incision.
• Wui Seng, Quah (Malaysia) developed first radial-enhanced laser treatment for glaucoma patients leading to the development of the modern glaucoma patient program.
• Ioannis Pallikaris (Greece) performed the first laser-assisted intrastromal keratomileusis (LASIK) surgery.
• Fred Hollows (New Zealand/Australia) pioneered programs in Nepal, Eritrea, and Vietnam, and among Australian aborigines, including the establishment of cheap laboratory production of intraocular lenses in Nepal and Eritrea.
• Marco Abbondanza (Italy) developed the mini asymmetric radial keratotomy for keratoconus and astigmatism, and popularized the cross-linking.
• Ian Constable (Australia) founded the Lions Eye Institute in Perth, Western Australia, the largest eye research institute in the Southern Hemisphere and home to 10 ophthalmologists.
• Rand Paul (United States) is a current member of the United States Senate from Kentucky.
• L. L. Zamenhof (Poland) created the Esperanto language.
• Bashar al-Assad (Syria) is both the President of Syria and a doctor of ophthalmology who completed his residency in a London hospital.
• Syed Modasser Ali (Bangladesh) is an ophthalmic surgeon who used to be the Director-General of Health Services for the government of Bangladesh. He wrote the first book on community ophthalmology (public eye health).
• Dr Albrecht Hennig (Germany) pioneered the "fish-hook" technique enabling his team to complete more than 48,000 cataract operations in Nepal in 2004. He was nominated as "Eye Health Hero" at the International Agency for the Prevention of Blindness 9th General Assembly in September 2012.
• Dr. Paul T. Finger (United States) pioneered the use of palladium-103 plaque radiation to treat choroidal melanoma and three-dimensional and high-frequency ultrasound to image intraocular tumors.
• Herbert Lightfoot Eason (1874-1948) was superintendent at Guy's Hospital, London, president of the General Medical Council, and vice chancellor (1935-1937) of the University of London.
• J. William Harbour (United States) is an ocular oncologist and retinal surgeon who specializes in the treatment of intraocular cancers, including ocular melanoma, lymphoma, and retinoblastoma.

src: previews.123rf.com

See also

src: clinicadrdaniel.com

References

src: previews.123rf.com

External links

• EyeWiki
• Multilingual illustrated Dictionary of Ophthalmology

Source of the article : Wikipedia