active galactic nucleus (AGN), small region at the centre of a galaxy that emits a prodigious amount of energy in the form of radio, optical, X-ray, or gamma radiation or high-speed particle jets. Many classes of “active galaxies” have been identified—for example, quasars, radio galaxies, and

solar heating: In active heating, mechanical means are used to store, collect, and distribute solar energy in buildings in order to provide hot water or space heating. The sunlight falling on a building’s collector array is converted to heat, which is transferred to a carrier fluid (usually a…

permafrost: Active wedges, inactive wedges, and ice-wedge casts: Active ice wedges are those that are actively growing. The wedge may not crack every year, but during many or most years cracking does occur, and an increment of ice is added. Ice wedges require a much more rigorous climate to grow than does permafrost.…

immune system: Active immunization: Active immunization aims to ensure that a sufficient supply of antibodies or T and B cells that react against a potential infectious agent or toxin are present in the body before infection occurs or the toxin is encountered. Once it has been primed,…

immune system: Active immunization: Active immunization aims to ensure that a sufficient supply of antibodies or T and B cells that react against a potential infectious agent or toxin are present in the body before infection occurs or the toxin is encountered. Once it has been primed,…

epistemology: Aristotle: …passive intellect, the second the active intellect, of which Aristotle speaks tersely. “Intellect in this sense is separable, impassible, unmixed, since it is in its essential nature activity.…When intellect is set free from its present conditions, it appears as just what it is and nothing more: it alone is immortal…

permafrost: …in summer is called the active layer. The thickness of the active layer depends mainly on the moisture content, varying from less than a foot in thickness in wet, organic sediments to several feet in well-drained gravels.

Raymond Neil Wilson: This technique of active optics was first tried on the New Technology Telescope at La Silla, Chile, in 1989 and was later used in 1998 for the four telescopes that make up the Very Large Telescope on Cerro Paranal, Chile.

ecological restoration: Implementation: Restorationists must then engage in active restoration, which starts or accelerates the recovery process or attempts to change the site’s ecological succession.

Stephen Crane: …wrote the first draft of Active Service, a novel of the Greek war. He finally returned to Cora in England nine months after his departure and settled in a costly 14th-century manor house at Brede Place, Sussex. Here Cora, a silly woman with social and literary pretensions, contributed to Crane’s…

mass shooting, as defined by the U.S. Federal Bureau of Investigation (FBI), an event in which one or more individuals are “actively engaged in killing or attempting to kill people in a populated area. Implicit in this definition is the shooter’s use of a firearm.” The FBI has not set a minimum

enzyme: Mechanism of enzyme action: …of the enzyme, called the active site, binds to the substrate. The active site is a groove or pocket formed by the folding pattern of the protein. This three-dimensional structure, together with the chemical and electrical properties of the amino acids and cofactors within the active site, permits only a…

solar prominence: …are of two main types, active and quiescent. Active prominences erupt quickly and have lifetimes lasting from several minutes to a few hours. They are associated with sunspot groups and, like these, are correlated in numbers and activity with the solar cycle. Quiescent prominences tend to emerge smoothly and subside…

sonar: In active sonar systems an acoustic projector generates a sound wave that spreads outward and is reflected back by a target object. A receiver picks up and analyzes the reflected signal and may determine the range, bearing, and relative motion of the target. Passive systems consist…

transducer: The active transducers generate electric current or voltage directly in response to stimulation. An example is the thermocouple; here, the fact that a current will flow in a continuous circuit of two metals, if the two junctions are at different temperatures, is used to generate electricity.…

biophysics: Biological membranes: Ussing’s definition of active transport made possible an understanding, at the cellular level, of the way in which ions and water are pumped into and out of living cells in order to regulate the ionic composition and water balance in cells, organs, and organisms. The molecular mechanism by…

voice: …in languages are those of active, passive, and middle voice. These distinctions may be made by inflection, as in Latin, or by syntactic variation, as in English. The active-passive opposition can be illustrated by the following sentences:

nervous system: Axon: The thickened areas are called presynaptic dense projections, or active zones.

rocket and missile system: Active: With active guidance, the missile would track its target by means of emissions that it generated itself. Active guidance was commonly used for terminal homing. Examples were antiship, surface-to-air, and air-to-air missiles that used self-contained radar systems to track their targets. Active guidance had…

electronic game: Early home video consoles: Activision, founded in 1979 by former Atari game designers David Crane and Alan Miller and entertainment executive Jim Levy, was the first company exclusively focused on game software. By 1983, however, the flood of game titles for the leading home consoles had led to a…

Activision Blizzard is an American developer and manufacturer of electronic games. The company was formed in 2008 by the merger of Activision, an entertainment software publisher that traced its roots to the original Atari game console, and Vivendi Games, the parent company of Blizzard

judicial activism: The term activism is used in both political rhetoric and academic research. In academic usage activism usually means only the willingness of a judge to strike down the action of another branch of government or to overturn a judicial precedent, with no implied judgment as to whether…

What happens when an investor takes an active role in steering the direction of a publicly traded company? Activist investors are typically empowered by amassing a significant number of shares in a target company. They are characterized by a drive to reform public companies to their liking. The

Konni Zilliacus: …was an organizer of the Activist Opposition Party, an underground revolutionary group; he also served as editor of its widely disseminated newspaper Fria Ord (“Free Word”). Beginning with the Russo-Japanese War and the first stirrings of unrest in Russia, Zilliacus limited his smuggling almost exclusively to arms, not only for…

activities of daily living (ADLs), any task that commonly is completed by most persons, that is performed habitually or repeatedly at regular intervals, and that often serves as a prerequisite for other activities. Examples of ADLs include dressing, eating, attending to hygiene, toileting, and

Jadids organized New Method schools at the primary and secondary level, teaching pupils by modern pedagogical methods rather than by the rote learning that had been used in traditional schools. For the literate, Jadids published numerous short-lived newspapers and lithographed or printed many

liquid: Fugacity: …must be used, called the activity. The activity is given by the product of the mole fraction xi and the activity coefficient γi. The equation is:

activity, in radioactive-decay processes, the number of disintegrations per second, or the number of unstable atomic nuclei that decay per second in a given sample. Activity is determined by counting, with the aid of radiation detectors and electronic circuits, the number of particles and photons

economics: Postwar developments: …development of linear programming and activity analysis, which opened up the possibility of applying numerical solutions to industrial problems. This advance also introduced economists to the mathematics of inequalities (as opposed to exact equation). Likewise, the emergence of growth economics promoted the use of difference and differential equations.

activity coefficient, in chemistry, the ratio of the chemical activity of any substance to its molar concentration. The measured concentration of a substance may not be an accurate indicator of its chemical effectiveness, as represented by the equation for a particular reaction, in which case an

labour economics: Activity rates: The employed labour force may be characterized by particular activity rates. An activity rate is the proportion of the whole number in a given age and sex group—for example, females aged 30–34—who work for gain. Among males, activity rates in the earlier years…

business finance: Financial ratio analysis: Activity ratios, relating to the turnover of such asset categories as inventories, accounts receivable, and fixed assets, show how intensively a firm is employing its assets. A firm’s primary operating objective is to earn a good return on its invested capital, and various profit ratios…

accounting: Cost finding: …third method of cost finding, activity-based costing, is based on the fact that many costs are driven by factors other than product volume. The first task is to identify the activities that drive costs. The next step is to estimate the costs that are driven by each activity and to…

Amazon Cooperation Treaty Organization (ACTO), international organization founded to promote the preservation of the Amazon basin and regulate Amazonian development through international cooperation. The Amazon Cooperation Treaty was drafted and signed on July 3, 1978, by Bolivia, Brazil, Colombia,

Acto Adicional of 1834, amendment to the Brazilian constitution of 1824 that abolished some of that charter’s extremely centralist and authoritarian aspects. It was enacted as a concession to federalists and republicans who threatened to sunder the nation. The abdication of the unpopular Brazilian

Manoel de Oliveira: In the feature-length Acto da primavera (1963; “Rite of Spring”), which documents a small village’s performance of a Passion play, Oliveira mused on the complexities of artistic representation by introducing fictional elements and depicting on-screen his own role in the film’s creation.

protein: The muscle proteins: Actomyosin is a complex molecule formed by one molecule of myosin and one or two molecules of actin. In muscle, actin and myosin filaments are oriented parallel to each other and to the long axis of the muscle. The actin filaments are linked to each…

Ealing: …de Actune sold land in Acton to St. Paul’s Cathedral. In late 1642, after the Battle of Edgehill during the English Civil Wars, defense forces at Acton, Brentford, and Turnham Green (at the present border between Ealing and Hounslow) barred the access of Charles I’s forces to London. Acton became…

John Emerich Edward Dalberg Acton, 1st Baron Acton was an English Liberal historian and moralist, the first great modern philosopher of resistance to the state, whether its form be authoritarian, democratic, or socialist. A comment that he wrote in a letter, “Power tends to corrupt, and absolute

Hillingdon: …of the main campus of Brunel University. Founded in 1928 as Acton Technical College, it became the Brunel College of Advanced Technology in 1962 and was later granted university status. It incorporated the Shoreditch College of Education in 1980 (adding the Runnymede campus) and the West London Institute of Higher…

WhatsApp: …was founded in 2009 by Brian Acton and Jan Koum. Koum, a Ukrainian immigrant who had just begun attending classes at San Jose State University, and Acton, a Stanford-educated programmer, met while Acton was working for Internet pioneer Yahoo! in 1997. Koum was soon offered a job with the Mountain…

John Emerich Edward Dalberg Acton, 1st Baron Acton was an English Liberal historian and moralist, the first great modern philosopher of resistance to the state, whether its form be authoritarian, democratic, or socialist. A comment that he wrote in a letter, “Power tends to corrupt, and absolute

Sir John Francis Edward Acton, 6th Baronet was the commander of the naval forces of Tuscany and then of Naples. As prime minister of Naples, he allied that kingdom with England and Austria in the period of the French Revolution. Finding the French Navy unappreciative of his skills, Acton, the son

jacana: …variably black or reddish; the African jacana (Actophilornis africanus); the Australian lotus bird (Irediparra gallinacea) of New Guinea and the eastern Australian coast; and the pheasant-tailed jacana (Hydrophasianus chirurgus), of India and the Philippines, a handsome black, yellow, and white bird that acquires long tail feathers in breeding season.

actor-manager system, method of theatrical production dominant in England and the U.S. in the 19th century, consisting of a permanent company formed by a leading actor who chose his or her own plays, took a leading role in them, and handled business and financial arrangements. The advantages of

Bruno Latour: This approach became known as actor-network theory, and its influence soon spread beyond Latour’s field of science and technology studies. Latour’s work exasperated many practicing scientists with its denial of the existence of objective truths and its claims to have unmasked science as a social process and debunked its pretenses…

The Actors Studio, prestigious professional actors’ workshop in New York City whose members have been among the most influential performers in American theatre and film since World War II. It is one of the leading centres for the Stanislavsky method of dramatic training. (Read Lee Strasberg’s 1959

Ian McKellen: In 1971 he cofounded the Actors’ Company, a collective of actors who had equal say in the choosing and casting of plays and in the recruiting of directors. He left the group in 1974 to join the Royal Shakespeare Company at the invitation of Barton.

Acts of the Apostles, fifth book of the New Testament, a valuable history of the early Christian church. Acts was written in Greek, presumably by St. Luke the Evangelist. The Gospel According to Luke concludes where Acts begins, namely, with Christ’s Ascension into heaven. Acts was apparently

Harry The Minstrel: …and Valiant Champion Sir William Wallace, Knight of Elderslie, which is preserved in a manuscript dated 1488. He has been traditionally identified with the Blind Harry named among others in William Dunbar’s The Lament for the Makaris (“poets”) and with a “Blin Hary” who is listed from time to time…

John Foxe: …Puritan preacher and author of The Book of Martyrs, a graphic and polemic account of those who suffered for the cause of Protestantism. Widely read, often the most valued book beside the Bible in the households of English Puritans, it helped shape popular opinion about Roman Catholicism for at least…

Gondophernes: …first known from the apocryphal Acts of Judas Thomas the Apostle, which told that St. Thomas visited the court of Gondophernes, where he was put in charge of building a royal palace but was imprisoned for spending the construction money on charitable purposes. Meanwhile, according to the story, Gad, the…

St. Joseph of Arimathea: In the apocryphal Gospel of Nicodemus (or Acts of Pilate; 4th/5th century), Jews imprison Joseph after Jesus’ burial, but he is released by the risen Lord, thus becoming the first witness of the Resurrection. In Robert de Boron’s verse romance Joseph d’Arimathie (c. 1200), he is entrusted with…

St. Blaise: Subsequent legends, notably the apocryphal Acts of St. Blaise, claim that, before Blaise was made bishop, he was a physician possessed of wonderful healing power. Numerous miracles were attributed to him, including the cure of diseased beasts during his refuge, thus accounting for his also being the patron saint of…

Christianity: The early liturgy, the calendar, and the arts: …adventure was satisfied by apocryphal Acts of the Apostles, recounting their travels, with continence replacing love. Justin and Irenaeus did not write for high style but simply to convey information. Apologists hoping for well-educated readers, however, could not be indifferent to literary tastes. By 200 ce the most graceful living…

Acts of the Apostles, fifth book of the New Testament, a valuable history of the early Christian church. Acts was written in Greek, presumably by St. Luke the Evangelist. The Gospel According to Luke concludes where Acts begins, namely, with Christ’s Ascension into heaven. Acts was apparently

mystery play: In France a single play, The Acts of the Apostles by Arnoul and Simon Gréban, contained 494 speaking parts and 61,908 lines of rhymed verse; it took 40 days to perform. They died out in many areas with the Reformation.

Dēnkart, 9th-century encyclopaedia of the Zoroastrian religious tradition. Of the original nine volumes, part of the third and all of volumes four through nine are extant. The surviving portion of the third book is a major source of Zoroastrian theology. It indicates that later Zoroastrianism had

patristic literature: Late 2nd to early 4th century: …works accepted as scripture—and the Acts of the Scillitan Martyrs (180) of Africa.

Australian Council of Trade Unions (ACTU), the dominant association and governing body of the trade union movement in Australia, established in May 1927. Membership grew significantly when the Australian Workers’ Union joined the ACTU in 1967. Two other mergers with federations of white-collar

insurance: Limitations on amount recoverable: …either full replacement cost or actual cash value (ACV). Under the former, the owner suffers no reduction in loss recovery due to depreciation of the property from its original value. This basis applies if the owner took out coverage that is at least equal to a named percentage—for example, 80…

sin: ” Actual sin is sin in the ordinary sense of the word and consists of evil acts, whether of thought, word, or deed. Original sin (the term can be misleading) is the morally vitiated condition in which one finds oneself at birth as a member of…

Ed Ruscha: …depicting the institution in flames; Actual Size (1962), an image of a flying can of Spam (a precooked luncheon meat) beneath the Spam logo; Large Trademark with Eight Spotlights (1962), a dramatic representation of the Twentieth Century-Fox logo, and Oof (1963), a straightforward rendering of the expression used to communicate…

metaphysics: Aristotelianism: form and matter, potentiality and actuality, and cause (see Aristotle: Physics and metaphysics). Whatever happens involves some substance or substances; unless there were substances, in the sense of concrete existents, nothing whatsoever could be real. Substances, however, are not, as the name might suggest, mere parcels of matter; they are…

commodity trade: Primary commodity markets: …technically as trade in “actuals”), or it may be conducted by means of futures contracts. A futures contract is an agreement to deliver or receive a certain quantity of a commodity at an agreed price at some stated time in the future. Trade in actuals has declined considerably and…

commodity trade: Primary commodity markets: …technically as trade in “actuals”), or it may be conducted by means of futures contracts. A futures contract is an agreement to deliver or receive a certain quantity of a commodity at an agreed price at some stated time in the future. Trade in actuals has declined considerably and…

actuary, one who calculates insurance risks and premiums. Actuaries compute the probability of the occurrence of various contingencies of human life, such as birth, marriage, sickness, unemployment, accidents, retirement, and death. They also evaluate the hazards of property damage or loss and the

automation: Computer process control: …of some optimizing strategy, (3) actuation of such devices as valves, switches, and furnaces that enable the process to implement the optimal strategy, and (4) generation of reports to management indicating equipment status, production performance, and product quality. Today computer process control is applied to many industrial operations, two of…

Albert Camus: Camus’s literary career: …reprinted in abridged form in Actuelles III (1958), drew attention (15 years in advance) to many of the injustices that led to the outbreak of the Algerian War in 1954. Camus took his stand on humanitarian rather than ideological grounds and continued to see a future role for France in…

criminal law: The elements of crime: …voluntary act or omission (actus reus), accompanied by (2) a certain state of mind (mens rea). An act may be any kind of voluntary human behaviour. Movements made in an epileptic seizure are not acts, nor are movements made by a somnambulist before awakening, even if they result in…

Amalgamated Clothing and Textile Workers Union (ACTWU), former union of garment and apparel workers in the United States and Canada. It was formed in 1976 by the merger of the Amalgamated Clothing Workers of America (ACWA), a large union representing workers in the men’s clothing industry, with the

acouchy, (genus Myoprocta), either of two species of South American rodents that resemble the small tropical-forest-dwelling hoofed animals of Africa and Asia (see royal antelope; chevrotain). Weighing 1 to 1.5 kg (2.2 to 3.3 pounds), acouchys are 30 to 39 cm (12 to 15 inches) long, with a very

Roy Acuff was an American vocalist, songwriter, and fiddle player, called the “King of Country Music,” who in the mid-1930s reasserted the mournful musical traditions of Southeastern rural whites and became a national radio star on the “Grand Ole Opry” broadcasts. Turning his attention to music

Roy Acuff was an American vocalist, songwriter, and fiddle player, called the “King of Country Music,” who in the mid-1930s reasserted the mournful musical traditions of Southeastern rural whites and became a national radio star on the “Grand Ole Opry” broadcasts. Turning his attention to music

Roy Acuff: In 1942 he organized Acuff-Rose Publishing Company, the first publishing house exclusively for country music, with songwriter Fred Rose. Following an unsuccessful bid for the Tennessee governorship in 1948, Acuff continued to record extensively from the 1950s on, lending authenticity to the new boom in country music with such…

mollusk: Critical appraisal: …by the more appropriate term Aculifera. All other mollusks are included in the subphylum Conchifera (shell-bearers). The familiar division of the Gastropoda into the subclasses Prosobranchia, Opisthobranchia, and Pulmonata is no longer widely accepted. Similarities in the morphology of the nervous system suggest that the opisthobranchs and pulmonates should be…

Ciudad Acuña, city, northern Coahuila estado (state), northeastern Mexico. The city is on the Rio Grande (Río Bravo del Norte) just across the U.S.-Mexico border from Del Rio, Texas, and is a port of entry. Ciudad Acuña is also a commercial and manufacturing centre for the agricultural hinterland.

Rosario de Acuña was a Spanish playwright, essayist, and short-story writer known for her controversial liberal views. Little is known of Acuña’s early life. One of Spain’s few women playwrights, she was considered radical for her willingness to address such issues as religious fanaticism, atheism,

Ronald Acuña, Jr. is a superstar baseball outfielder for the Atlanta Braves who is the only player in Major League Baseball history to hit at least 40 homers and steal at least 70 bases in a single season. Acuña is one of the most exciting players of his generation, a rare five-tool athlete who

Ronald Acuña, Jr. is a superstar baseball outfielder for the Atlanta Braves who is the only player in Major League Baseball history to hit at least 40 homers and steal at least 70 bases in a single season. Acuña is one of the most exciting players of his generation, a rare five-tool athlete who

Rosario de Acuña was a Spanish playwright, essayist, and short-story writer known for her controversial liberal views. Little is known of Acuña’s early life. One of Spain’s few women playwrights, she was considered radical for her willingness to address such issues as religious fanaticism, atheism,

Wu Li was a Chinese painter who was a member of the orthodox school of “literati painting” (wenrenhua) in the early Qing period. Wu became a convert to Catholicism and in 1681 went to Macao Island (on the southeast coast of China), where, without family obligations after the deaths of his mother

acupressure, alternative-medicine practice in which pressure is applied to points on the body aligned along 12 main meridians (pathways), usually for a short time, to improve the flow of qi (life force). Though often referred to by its Japanese name, shiatsu, it originated in China thousands of

acupuncture, ancient Chinese medical technique for relieving pain, curing disease, and improving general health. It was devised before 2500 bce in China and by the late 20th century was used in many other areas of the world. Acupuncture consists of the insertion of one or several small metal

Honda Motor Company, Ltd.: From motorcycles to automobiles: Honda also launched Acura in 1986, establishing the first luxury division of a Japanese automaker. The company’s other major product areas include farm machinery, small engines, and power equipment. Honda is a major Japanese exporter to the United States and other parts of the world. It operates assembly…

technology of photography: Resolving power and acutance: …measured physically to give an acutance value, which is preferred to resolving power as a criterion of a film’s sharpness performance. Fine-grain films with thin emulsions yield the highest acutance.

polio, acute viral infectious disease of the nervous system that usually begins with general symptoms such as fever, headache, nausea, fatigue, and muscle pains and spasms and is sometimes followed by a more serious and permanent paralysis of muscles in one or more limbs, the throat, or the chest.

cystitis: Acute cystitis: Acute, or common, cystitis is caused by bacterial infection, frequently as part of a general urinary tract infection (UTI). The mucous membrane of the bladder becomes swollen and red and bleeds. Small ulcers can develop, the surface layer can shred, and small clear cysts (sacs…

alcoholism: Acute diseases: Alcohol intoxication produces a wide variety of disturbances of neuromuscular and mental functions and of body chemistry. In addition, the intoxicated person is more liable to accidents and injuries. Alcoholics—who chronically experience severe intoxication—are said to be 30 times more liable to fatal…

virus: Infectious patterns: …have also been linked to acute flaccid myelitis, a polio-like disease characterized by sudden muscle weakness and paralysis.

Bright disease: Acute glomerulonephritis is characterized by severe inflammation, renal (kidney) insufficiency, swelling, increased blood pressure, and severe back pain. Recovery is usually fairly complete after an episode of acute glomerulonephritis, but minor infections may do further damage to the kidneys and bring on the subacute and…

inflammation: Although acute inflammation is usually beneficial, it often causes unpleasant sensations, such as the pain of a sore throat or the itching of an insect bite. Discomfort is usually temporary and disappears when the inflammatory response has done its job. But in some instances inflammation can…

Guillain-Barré syndrome: Pathophysiology: Demyelinating forms include acute inflammatory demyelinating polyneuropathy (AIDP), which is the most commonly occurring version of Guillain-Barré syndrome. Axonal forms include acute motor axonal neuropathy (AMAN) and acute motor and sensory axonal neuropathy (AMSAN). Miller Fisher syndrome is a rare and often rapidly developing variant of the syndrome…

metabolic disease: Porphyrias: One common form is acute intermittent porphyria, which is caused by a deficiency of the enzyme porphobilinogen deaminase. Symptoms usually arise during adolescence, and hormonal changes (e.g., menstruation), alcohol ingestion, certain foods, and some drugs may exacerbate the condition. Diagnosis is made by detecting porphyrins in the

blood disease: Leukemia: In acute lymphocytic anemia (ALL), most frequently seen in children, the cells are immature forms of the lymphatic series of cells. In acute myelogenous leukemia (AML), the predominant cells are the youngest recognizable precursors (myeloblasts) of the neutrophils of the blood. In a third and the…

otitis media, inflammation of the lining of the middle ear. Otitis media is one of the most common infections in childhood, with about three-quarters of children affected by age three. There are three types of otitis media. Acute otitis media commonly develops in association with an infection of

Guillain-Barré syndrome: Pathophysiology: …motor axonal neuropathy (AMAN) and acute motor and sensory axonal neuropathy (AMSAN). Miller Fisher syndrome is a rare and often rapidly developing variant of the syndrome that has three defining characteristics: areflexia (loss of tendon reflexes), ataxia (loss of limb coordination), and ophthalmoplegia (muscle weakness in the eyes that results…

Guillain-Barré syndrome: Pathophysiology: Axonal forms include acute motor axonal neuropathy (AMAN) and acute motor and sensory axonal neuropathy (AMSAN). Miller Fisher syndrome is a rare and often rapidly developing variant of the syndrome that has three defining characteristics: areflexia (loss of tendon reflexes), ataxia (loss of limb coordination), and ophthalmoplegia (muscle…

blood disease: Leukemia: In acute myelogenous leukemia (AML), the predominant cells are the youngest recognizable precursors (myeloblasts) of the neutrophils of the blood. In a third and the least common variety, acute monocytic leukemia, the immature cells appear to be precursors of the monocytes of the blood. Myelogenous and…

Vincent gingivitis, acute and painful infection of the tooth margins and gums that is caused by the symbiotic microorganisms Bacillus fusiformis and Borrelia vincentii. The chief symptoms are painful, swollen, bleeding gums; small, painful ulcers covering the gums and tooth margins; and