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23de58ffabf5bfa6b49266c6058bd10d | https://www.britannica.com/science/cuttlebone | Cuttlebone | Cuttlebone
…internal calcified shell called the cuttlebone. The approximately 100 species of cuttlefish range between 2.5 and 90 cm (1 to 35 inches) and have somewhat flattened bodies bordered by a pair of narrow fins. All species have eight arms and two longer tentacles that are used in capturing prey and…
Cuttlefishes, because of the large cuttlebone, are less active animals and spend most of their life lying on or hovering slightly above the bottom. Both jet and fins are used, the latter more frequently.
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5637d5a79ec7c38696c8f9726506a863 | https://www.britannica.com/science/CV-carbonaceous-chondrite | CV carbonaceous chondrite | CV carbonaceous chondrite
…that it belongs to the CV group of the first table and petrologic type 3 of the second table.
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017ec6e5e7aa84f17f0be5df7aea7073 | https://www.britannica.com/science/cyanogen-chloride | Cyanogen chloride | Cyanogen chloride
Cyanogen chloride, made by the reaction of chlorine with hydrocyanic acid or its salts, is a liquid that has been suggested for use as a military poison gas. Cyanogen bromide is formed by the reaction of bromine with salts of hydrocyanic acid; it is a…
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04d42df11350f122516349a419321c5c | https://www.britannica.com/science/cyathium | Cyathium | Cyathium
…are in cup-shaped clusters called cyathia, each of which seems to be a single female flower but actually consists of a single pistil surrounded by several male flowers, each of which has a single stamen. These clusters of reduced flowers are enclosed by an involucre (whorl) of bracts (modified leaves)…
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0b11c55f61448cfa0796aa0c3dfb9dd6 | https://www.britannica.com/science/cybernetics | Cybernetics | Cybernetics
Cybernetics, control theory as it is applied to complex systems. Cybernetics is associated with models in which a monitor compares what is happening to a system at various sampling times with some standard of what should be happening, and a controller adjusts the system’s behaviour accordingly.
The term cybernetics comes from the ancient Greek word kybernetikos (“good at steering”), referring to the art of the helmsman. In the first half of the 19th century, the French physicist André-Marie Ampère, in his classification of the sciences, suggested that the still nonexistent science of the control of governments be called cybernetics. The term was soon forgotten, however, and it was not used again until the American mathematician Norbert Wiener published his book Cybernetics in 1948. In that book Wiener made reference to an 1868 article by the British physicist James Clerk Maxwell on governors and pointed out that the term governor is derived, via Latin, from the same Greek word that gives rise to cybernetics. The date of Wiener’s publication is generally accepted as marking the birth of cybernetics as an independent science.
Wiener defined cybernetics as “the science of control and communications in the animal and machine.” This definition relates cybernetics closely with the theory of automatic control and also with physiology, particularly the physiology of the nervous system. For instance, a “controller” might be the human brain, which might receive signals from a “monitor” (the eyes) regarding the distance between a reaching hand and an object to be picked up. The information sent by the monitor to the controller is called feedback, and on the basis of this feedback the controller might issue instructions to bring the observed behaviour (the reach of the hand) closer to the desired behaviour (the picking up of the object). Indeed, some of the earliest work done in cybernetics was the study of control rules by which human action takes place, with the goal of constructing artificial limbs that could be tied in with the brain.
In subsequent years the computer and the areas of mathematics related to it (e.g., mathematical logic) had a great influence on the development of cybernetics—for the simple reason that computers can be used not only for automatic calculation but also for all conversions of information, including the various types of information processing used in control systems. This enhanced ability of computers has made possible two different views of cybernetics. The narrower view, common in Western countries, defines cybernetics as the science of the control of complex systems of various types—technical, biological, or social. In many Western countries particular emphasis is given to aspects of cybernetics used in the generation of control systems in technology and in living organisms. A broader view of cybernetics arose in Russia and the other Soviet republics and prevailed there for many years. In this broader definition, cybernetics includes not only the science of control but all forms of information processing as well. In this way computer science, considered a separate discipline in the West, is included as one of the component parts of cybernetics.
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4e1bc905ef5f94722ce1eba326e7baff | https://www.britannica.com/science/cyclamate | Cyclamate | Cyclamate
Cyclamate, odourless white crystalline powder that is used as a nonnutritive sweetener. The name usually denotes either calcium cyclamate or sodium cyclamate, both of which are salts of cyclohexylsulfamic acid (C6H11NHSO3H). These compounds are stable to heat and are readily soluble in water. Cyclamates have a very sweet taste, with about 30 times the sweetening power of sucrose. They are used as sweeteners in baked goods, confections, desserts, soft drinks, preserves, and salad dressings. They are often combined with saccharin to produce a synergistic sweetening effect.
Cyclamates were discovered by Michael Sveda in 1937. They have no caloric value and are poorly absorbed from the gastrointestinal tract. However, a variable amount is hydrolyzed by bacteria in the intestinal tract to form cyclohexylamine, which is a potential carcinogen. Two scientific studies prior to 1970 linked cyclamates to the production of cancerous tumours in the bladders of rats. This led to an immediate ban on use of the compounds in many countries. Subsequent research, however, failed to establish convincing evidence of carcinogenic properties of cyclamates. This has led to reapproval of their use in many countries, though they remain banned in South Korea and the United States.
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6ab0307a6e9eefa50ad0c3831aefa45d | https://www.britannica.com/science/cyclic-disease-transmission | Cyclic disease transmission | Cyclic disease transmission
This is an example of cyclic disease transmission. The relationship between the parasitic disease organism and its two hosts, vertebrate and insect, is a result of evolutionary adaptation. However, it is not known whether the trypanosome was originally a fly parasite that spread to humans and other vertebrates or whether…
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e0837bdcc6d855e3e93e4a22833ea95b | https://www.britannica.com/science/cyclical-shift-substitution-cipher | Cyclical-shift substitution cipher | Cyclical-shift substitution cipher
…has discovered to his embarrassment, cyclical-shift substitution ciphers are not secure, nor is any other monoalphabetic substitution cipher in which a given plaintext symbol is always encrypted into the same ciphertext symbol. Because of the redundancy of the English language, only about 25 symbols of ciphertext are required to permit…
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682a4874a52179799210833d8e411d45 | https://www.britannica.com/science/cyclin-dependent-kinase | Cyclin-dependent kinase | Cyclin-dependent kinase
…family of key enzymes, the cyclin-dependent kinases (CDKs), which participate in many cell functions. By 2001 about a half dozen other CDKs were identified in humans.
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6dd6708ddbd838377d7a583d81e739bd | https://www.britannica.com/science/cyclophosphamide | Cyclophosphamide | Cyclophosphamide
…are nitrogen mustards (chlorambucil and cyclophosphamide), cisplatin, nitrosoureas (carmustine, lomustine, and semustine), alkylsulfonates (busulfan), ethyleneimines (thiotepa), and triazines (dacarbazine).
…or chemotherapeutic drugs, such as cyclophosphamide or methotrexate, may be used. GPA can be brought into remission through prompt treatment, in some cases without long-term maintenance with drug therapies.
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d8f9cfebc42533d4c82b1d3361122ced | https://www.britannica.com/science/cylindrical-projection | Cylindrical projection | Cylindrical projection
Cylindrical projection, in cartography, any of numerous map projections of the terrestrial sphere on the surface of a cylinder that is then unrolled as a plane. Originally, this and other map projections were achieved by a systematic method of drawing the Earth’s meridians and latitudes on the flat surface. But this method produces distortion, so a map projection today may be created using any of a number of mathematical methods. The familiar Mercator projection (q.v.) is a cylindrical projection.
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faff9d7f5c10739de994757e3af02911 | https://www.britannica.com/science/Cyrillid-meteor-shower | Cyrillid meteor shower | Cyrillid meteor shower
The Cyrillid shower of 1913 had no radiant (the meteoroids seemed to enter the atmosphere from a circular orbit around Earth) and was named for St. Cyril of Alexandria, on whose feast day (formerly celebrated on February 9) the shower was observed. The great Leonid meteor…
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369284095e7bd8c0059885aa5739e592 | https://www.britannica.com/science/cyst | Cyst | Cyst
Cyst, in biology, enclosed sac within body tissues, having a distinct membrane and generally containing a liquid material. In the life cycle of certain parasitic worms, a cyst develops around the larval form within the muscle tissue of the host animal.
Although the majority of cysts are benign, several varieties may be malignant or precancerous. Benign cysts often require removal because they interfere with surrounding organs. Cysts form from a proliferation of epithelium, the tissue making up the skin and the linings of the blood vessels and body cavities, and may become detached from surrounding structures so that they move freely. The material inside can consist of natural body secretions, abnormal products from the breakdown of natural secretions and structural proteins, or, in case of infection, bacteria, larval parasites, and microbial products.
Several organs, including the kidney, liver, and breast, are particularly susceptible to cyst formation and may become filled with numerous cysts of various sizes. In some cases, these cystic diseases are themselves dangerous, or they may obscure more serious, underlying diseases.
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07b30495d41a253385019ad98ea00a45 | https://www.britannica.com/science/cysticercus | Cysticercus | Cysticercus
…(encysts) and is called a cysticercus, or bladder worm. If the cysticercus is eaten alive in raw meat, it attaches itself to the host’s intestine and develops directly into a mature adult.
…is the pork tapeworm larva, Cysticercus, which causes nodules in the muscle tissue and brain. The organism grows, lays its eggs, and then dies. The nodes become calcified and may be seen on X rays.
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377364f9bfae89a79f559b65aab7a50f | https://www.britannica.com/science/cystinuria | Cystinuria | Cystinuria
Cystinuria, hereditary error of metabolism characterized by the excessive excretion into the urine of four amino acids: cystine, lysine, arginine, and ornithine. The main clinical problem of cystinuria is the possibility of cystine stone formation in the kidney; unlike lysine, arginine, and ornithine, which are freely soluble, cystine is only slightly soluble in urine, and when urine volume decreases, particularly at night, this amino acid may form stones. These may be reduced or eliminated by forcing fluids, alkalization, or, in severe cases, dietary restriction of foods containing methionine and cystine. Cystinuria is believed to be caused by a genetic defect in the transport system of the kidney tubule, which normally reabsorbs the four amino acids into the body circulation. In some forms of cystinuria, intestinal transport may be similarly affected. It is estimated that approximately one in 600 persons excretes abnormally large quantities of cystine, which can be detected by a simple test. Cystinuria is transmitted by an autosomal recessive gene: unaffected carriers of the trait who mate may expect, on the basis of chance, to have one affected offspring out of four.
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177c346d2b224155efdb50070eb82374 | https://www.britannica.com/science/cystolith | Cystolith | Cystolith
… arranged in opposite pairs, with cystoliths (enlarged cells containing crystals of calcium carbonate) in streaks or protuberances in the vegetative parts. The bisexual flowers are frequently bilaterally symmetrical and are usually enclosed by leaflike bracts, often coloured and large. Sepals and petals number five or four each and are often…
…(Moraceae) is the presence of cystoliths, deposits of calcium carbonate inside enlarged epidermal (surface) cells. They are visible as dots or variously shaped marks, especially in pressed, dried leaves. They may serve as some kind of protection from leaf-eating insects or other animals. Calcium carbonate deposits occur in another form…
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330f3ce32e53129137a83a4e7b7c0d62 | https://www.britannica.com/science/cytokinesis | Cytokinesis | Cytokinesis
Cytokinesis, in biology, the process by which one cell physically divides into two cells. Cytokinesis represents the major reproductive procedure of unicellular organisms, and it occurs in the process of embryonic development and tissue growth and repair of higher plants and animals. It generally follows nuclear doubling, whether in mitosis or in meiosis. Prokaryotes reproduce principally by binary fission, in which the mother cell enlarges until it divides into two identical daughter cells, with cytokinesis representing the physical division into the two daughter cells.
In animal cells cytokinesis is achieved through the constriction of the cell by a ring of contractile microfilaments consisting of actin and myosin, the proteins involved in muscle contraction and other forms of cell movement. In plant cells the cytoplasm is divided by the formation of a new cell wall, called the cell plate, between the two daughter cells. Once the daughter cells are separated, cellulose synthesis begins, and the cell plate becomes a primary cell wall.
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9146310909be5c851498ded5ea9b1ac3 | https://www.britannica.com/science/D-lines | D-lines | D-lines
D-lines, in spectroscopy, a pair of lines, characteristic of sodium, in the yellow region of the spectrum. Their separation is too small to be detected with a spectroscope of low resolving power. The line is the fourth prominent absorption line in the Sun’s spectrum, starting from the red end, and accordingly is designated by the letter D. It has been resolved into two components, D1 and D2, corresponding to wavelengths 5895.924 and 5889.950 Å (angstrom = 10-10 metre), respectively. An emission line appearing in the chromosphere, D3, of wavelength 5875.62 Å, has been discovered. This line is caused by helium.
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9fa11bc1f1a750db8772c70967e00e05 | https://www.britannica.com/science/dactylozooid | Dactylozooid | Dactylozooid
Some colonies possess dactylozooids, tentacleless polyps heavily armed with nematocysts that seem primarily concerned with defense. Gonozooids develop reproductive structures called gonophores. Members of the order Siphonophora, free-floating colonial hydrozoans, display an even greater variety of polymorphs. These include gas-filled floats called pneumatophores,
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c3adf913fc8bb4cb56f931af3f9b6d66 | https://www.britannica.com/science/daily-reference-value | Daily reference value | Daily reference value
Daily reference value (DRV), set of numerical quantities developed by the U.S. Food and Drug Administration for the dietary intake of energy-containing macronutrients, including carbohydrates, cholesterol, fat, fibre, saturated fatty acids, potassium, protein, and sodium. In the United States the DRVs for adults and children over age four, based on a diet of 2,000 calories per day, are 65 grams of fat (or about 30 percent of all calories), just under one-third of which (20 grams) should be saturated fat; 300 grams of carbohydrates (or 60 percent of calories); 50 grams of protein (10 percent of calories); 300 mg of cholesterol; 25 grams of fibre; 2,400 mg of sodium; and 3,500 mg of potassium. Canada has similar DRVs, with slight differences concerning saturated fats and trans fats. In some countries, recommended dietary intake is equivalent to DRV.
DRVs have played an important role in food labeling. In the United States the DRV is one of two types of reference values, the second being the reference daily intake (RDI) for vitamins and minerals. RDI and DRV are combined under daily value (DV) on food labels. The fat content in a single serving of a food or beverage product, and how that amount figures into the fat DRV, often is of particular interest to consumers.
DRVs allow people to assess whether their diet exceeds the amount of macronutrients that they should be eating. DRVs also serve an especially important role as a reference point for people with dietary problems.
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6b54c4e4422887fa522d544a66ac20f7 | https://www.britannica.com/science/Dalradian-Series | Dalradian Series | Dalradian Series
Dalradian Series, sequence of highly folded and metamorphosed sedimentary and volcanic rocks of late Precambrian to Early Cambrian age, about 540 million years old, that occurs in the southeastern portions of the Scottish Highlands of Great Britain, where it occupies a belt 720 kilometres (450 miles) long.
Containing no fossils over most of its outcrop area, the Dalradian has yielded rare specimens of the trilobite genus Pagetides, a Lower Cambrian form known from North America, collected from strata near the top of the series. Thus, it is known that the upper portion of the Dalradian is Lower Cambrian, but the position of the Precambrian–Cambrian boundary is uncertain.
Metamorphism (chemical and physical alteration from elevated temperatures and mechanical stresses in the Earth’s crust) related to the Caledonian orogenic (mountain-building) episode, has not obscured the original nature of Dalradian sedimentary types. Primary features are still evident in quartzites, calcareous limes, conglomerates, and graywackes, whereas fine-grained sedimentary rocks are represented by slates, phyllites, and schists. Current bedding can be seen in the graywackes and quartzites. Combined thicknesses of the Dalradian are thought to approach 9,100 metres (30,000 feet) and probably represent geosynclinal (downward flexures in the Earth’s crust) accumulations.
Dalradian sequences are also known from Ireland, especially in northern Donegal.
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da605dbc409575e2f11c32cf5e9a359d | https://www.britannica.com/science/Dalton-minimum | Dalton minimum | Dalton minimum
Dalton minimum, also called Modern minimum, period of reduced sunspot activity that occurred between roughly 1790 and 1830. It was named for the English meteorologist and chemist John Dalton.
Sunspot activity waxes and wanes with over about an 11-year cycle. During the Dalton minimum, the solar cycle continued; however, the number of sunspots at the peak of the solar cycles was about one-third of that observed during normal solar cycles. The three solar cycles that occurred during the Dalton minimum also had unusually long periods of sunspot inactivity.
As with the Maunder minimum (1645–1715), a period of almost no sunspots that coincided with the coldest part of the Little Ice Age, the Dalton minimum coincided with a period of cooler temperatures around the world. (Some of the decrease in temperatures can be attributed to the massive eruption of Mount Tambora on the island of Sumbawa in present-day Indonesia in April 1815. Parts of the Northern Hemisphere experienced sporadic periods of heavy snow and killing frost through June, July, and August 1816, which came to be known there as the “year without a summer.”) However, the physical mechanism that explains how changes in activity on the Sun affect Earth’s climate is unknown, and such episodes, however suggestive, do not prove that lower sunspot numbers produce cooling.
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a2be4e97773ccefc76d0e2695f4660d4 | https://www.britannica.com/science/Daltons-law | Dalton's law | Dalton's law
Dalton’s law, the statement that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual component gases. The partial pressure is the pressure that each gas would exert if it alone occupied the volume of the mixture at the same temperature.
This empirical relation was stated by the English chemist John Dalton in 1801. It follows from the kinetic theory of gases under the assumption of a perfect (ideal) gas and assumes no chemical interaction between the component gases. It is approximately valid for real gases at sufficiently low pressures and high temperatures.
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2dc726c7c5de68b9766e35336fa214a0 | https://www.britannica.com/science/damped-vibration | Damped vibration | Damped vibration
The simple harmonic oscillations discussed above continue forever, at constant amplitude, oscillating as shown in Figure 3 between A and −A. Common experience indicates that real oscillators behave somewhat differently, however. Harmonic oscillations tend to die away as time goes on.…
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