Catfish include some 2,500 species of mostly freshwater fish characterized by two to four pairs of whiskers or barbels around their mouth. Many species have spines on the dorsal fins and near the gills. In some species these spines may contain poison. Catfish belong to the bony fish order Siluriformes, and are mainly freshwater forms with representatives throughout the world. Most species of catfi…
Catheters are long, flexible tubes that are inserted into the body for various purposes, either to remove an unwanted substance or to instill nourishment or medication. A relatively large catheter can be passed through the nose, down the throat and into the stomach to remove the contents of the stomach; for example, if someone has consumed a poisonous substance, a catheter can be used to remove a …
The cathode is one of the two electrodes that are present in any system in which electricity is entering and leaving a region; the other electrode is called the anode. The electric current enters through one of the electrodes and leaves through the other. Two general kinds of systems employ electrodes: vacuum tubes (also called gas discharge tubes) and electrochemical cells. In a vacuum tube, the …
A cation is any atom or group of atoms that has a net positive charge. While matter is electrically neutral overall, ionic compounds are matter that is composed of positively-charged and negatively-charged particles called ions. An ion is any atom or group of atoms with an overall electrical charge. According to the laws of physics, opposite charges attract, so the oppositelycharged ions attract e…
Cats are mammals in the family Felidae of the order Carnivora, which includes all of the carnivores. The highly predatory instincts of species in the cat family are easily seen in domestic cats, for even well-fed individuals will aggressively hunt small mammals and birds. The cat family includes both large species (jaguar, leopard, lion, and tiger) and small ones (bobcat, lynx, ocelot, and serval)…
Cattails or reedmaces are about 10 species of monocotyledonous plants in the genus Typha, comprising the family Typhaceae. Cattails are tall, herbaceous, aquatic plants, growing from stout rhizomes located in shallow sediments of wetlands. The leaves of cattails are long and strap-like, sheathing at the base of the plant, while the spike-like inflorescence is borne by a long cylindrical shoot. The…
The cattle family, Bovidae, is a widespread group of mammals which also includes the goats, sheep, gazelles, antelopes, and goat-antelopes. Of the 107 species currently recognized within this family, just 12 are wild cattle. Even the large muskox (Ovibos moschatus), which looks quite cow-like, is more closely related to the goats than to cattle. Cattle are generally characterized by their large si…
Philosophers have theorized about causation since well before Aristotle, who distinguished several types of causation: efficient, material, final, and formal. For example, a wood carving is made by an artist (the efficient cause) by chiseling a piece of wood (the material cause) for the purpose of creating a beautiful object (the final cause), arriving at something that has the properties of a woo…
Cauterization is the application of heat, mechanically or chemically, to prevent or stop bleeding. It is widely used in surgery to hold bleeding to a minimum and speed the surgical process. Birthmark removal by cauterization with an argon laser. Photograph by Alexander Tsiaras. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. Searing areas of bleeding wi…
A cave is a naturally occurring hollow area inside the earth. Most caves are formed by some type of erosional process. The most notable exception is hollow lava tubes such as those in Hawaii. The formation of caves depends upon geologic, topographic, and hydrologic factors. These factors determine where and how caves develop, as well as their structure and shape. The study of caves is called spele…
Many species of fish have evolved to living under strange conditions, but few are more intriguing than those that have adapted to living in complete darkness. Some of these fish have developed a tendency to live at great depths in the ocean where no light penetrates, while others have found refuge in equally dim locations such as caves, wells, and subterranean streams. This specialization to livin…
Celestial coordinates locate objects on the sky, which is considered to be an infinitely large (celestial) sphere. The four conventional celestial coordinate systems are defined. These are based on the earth's rotation, which produces an apparent westward rotation of the celestial sphere around the NCP and SCP. An object's arc distance along its hour circle from the celestial equator…
Modern celestial mechanics began with Isaac New ton's generalization of Kepler's laws published in his Principia in 1687. Newton used his three laws of motion and his law of universal gravitation to do this. The three generalized Kepler's law are: where G is the Newtonian gravitations factor, and from his second law of motion, Newton derived the following general form of …
The celestial sphere is an imaginary projection of the Sun, Moon, planets, stars, and all astronomical bodies upon an imaginary sphere surrounding Earth. The celestial sphere is a useful mapping and tracking remnant of the geocentric theory of the ancient Greek astronomers. Although originally developed as part of the ancient Greek concept of an Earth-centered universe (i.e., a geocentric model of…
The cell is the smallest living component of organisms and is the basic unit of life. In multicellular living things, a collection of cells that work together to perform similar functions is called a tissue; various tissues that perform coordinated functions form organs; and organs that work together to perform general processes form body systems. The human digestive system, for example, is compos…
Like all living things, the various types of cells in plants, animals, and the many different cell types in humans must eventually die. Cell death occurs in one of two ways. Cells can be killed by the effects of physical, biological, or chemical injury. Additionally, cells are induced to kill themselves. Cell suicide is also referred to as apoptosis (from the Greek words apo, meaning from, and pto…
Cell division is the process where a single living cell splits to become two or more distinct new cells. All cells divide at some point in their lives. Cell division occurs in single-celled organisms like bacteria, in which it is the major form of reproduction (binary fission), or in multicellular organisms like plants, animals, and fungi. Many cells continually divide, such as the cells that line…
Electrochemical cells are devices based on the principle that when a chemical oxidation-reduction reaction takes place, electrons are being transferred from one chemical species to another. In one type of electrochemical cell called a voltaic or galvanic cell, these electrons are deliberately taken outside the cell and made to flow through an electric circuit to operate some kind of electrical dev…
The cell is bound by an outer membrane that, in accord with the fluid mosaic model, is comprised of a phospholipid lipid bilayer with proteins—molecules that also act as receptor sites—interspersed within the phospholipid bilayer. Varieties of channels exist within the membrane. There are a number of internal cellular membranes that partially partition the intercellular matrix, and t…
Medical science depends on the staining of cells in tissues to make accurate diagnoses of a wide range of diseases from cholera to sexually transmitted diseases, to parasitic diseases and skin infections. Staining techniques performed routinely in microbiological laboratories include gram's stain, acid-fast stains, acridine orange, calcofluor white, toluidine blue, methylene blue, silver st…
Respiration occurs in three stages. The first stage is glycolysis, which is a series of enzyme-controlled reactions that degrades glucose (a 6-carbon molecule) to pyruvate (a 3-carbon molecule) which is further oxidized to acetylcoenzyme A (acetyl CoA). Amino acids and fatty acids may also be oxidized to acetyl CoA as well as glucose. During electron transport, much of the energy represented by th…
Cellular telephone technology is also called cellular radio. The cellular radio network became fully operational in North America in 1978. This technology relies on the distribution of what are called cell sites over a wide geographical area. Each cell site consists of a radio transceiver and a controller that sends and receives signals from the mobile phones in the area to a telephone switch. The…
Censorship comprises many methods of preventing the publication or dissemination of speech, printed matter, art, theater, music, electronic media, or other forms of expression. The most common subjects that are censored are religion, politics, and sex. The usual justification is that such expression is subversive, blasphemous, heretical, obscene, pornographic, or otherwise offensive or harmful. Ce…
Centipedes (phylum Arthropoda, class Chilopoda) occur throughout the world in both temperate and tropical regions where they live in soil and humus and beneath fallen logs, bark, and stones. Because they lack a hard outer skeleton, centipedes are confined to moist environments in order to maintain water balance. Many species are therefore active only at night, remaining sheltered during the day. M…
A centrifuge is a device for separating two or more substances from each other by using centrifugal force. Centrifugal force is the tendency of an object traveling around a central point to continue in a linear motion and fly away from that central point. Centrifugation can be used to separate substances from each other becausematerials with different masses experience different centrifugal forces…
Ceramic materials are usually understood to be compounds of metallic and nonmetallic elements, though some are actually ionic salts, and others are insulators. These materials can be very complicated, as are for example clays, spinels, and common window glass. Many ceramic compounds have very high melting points. Ceramics have a wide range of applications. They have been used as refractories, abra…
The Cerenkov effect is the emission of light from a transparent substance like water or glass when a charged particle, such as an electron, travels through the material with a speed faster than the speed of light in that material. The Cerenkov effect was discovered by Russian experimentalist P. A. Cerenkov in 1934 and explained by Russian theorists I. Y. Tamm and I. M. Frank. All three scientists …
Modern biologists believe that life first appeared in the sea; from these marine beginnings, land-dwelling organisms such as mammals gradually evolved. Cetaceans have returned to the marine environment after an ancestral period on land. As evidence of their terrestrial pedigree, consider that a whale fetus possesses four limb buds, a pelvis, tail, and forelimbs with five fingers like any land mamm…
Chachalacas, curassows, and guans are 42 species of birds that make up the family Cracidae. These birds are in the order Galliformes, which also includes the grouse, pheasants, quail, guinea fowl, and turkey. Curassows, chachalacas, and guans (or cracids) are believed to represent a relatively ancient and primitive lineage within this order. Fossil members of this family are known from deposits in…
Chaos theory is used to model the overall behavior of complex systems. Despite its name, chaos theory is used to identify order in complex and otherwise seemingly unpredictable systems. Chaos theory is used to understand explosions, complex chemical reactions (e.g., the Belousov-Zhabotinsky oscillating reaction that yields a red solution that turns blue at varying intervals of time), and many biol…
A chelate is a type of coordination compound in which a single metallic ion is attached by coordinate covalent bonds to a molecule or an ion called a ligand. The term chelate comes from the Greek word chela, meaning "crab's claw." The term clearly describes the appearance of many kinds of chelates, in which the ligand surrounds the central atom in a way that can be compared to…
Chemical evolution describes chemical changes on the primitive Earth that gave rise to the first forms of life. The first living things on Earth were prokaryotes with a type of cell similar to present-day bacteria. Prokaryote fossils have been found in 3.4-million-year-old rock in the southern part of Africa, and in even older rocks in Australia, including some that appear to be photosynthetic. Al…
Chemical oxygen demand is related to biochemical oxygen demand (BOD), another standard test for assaying the oxygen-demanding strength of waste waters. However, biochemicaloxygen demand only measures the amount of oxygen consumed by microbial oxidation and is most relevant to waters rich in organic matter. It is important to understand that COD and BOD do not necessarily measure the same types of …
Chemical reactions describe the changes between reactants (the initial substances that enter into the reaction) and products (the final substances that are present at the end of the reaction). Describing interactions among chemical species, chemical reactions involve a rearrangement of the atoms in reactants to form products with new structures in such a way as to conserve atoms. Chemical equation…
Chemical warfare involves the use of natural or synthetic substances to incapacitate or kill an enemy or to deny them the use of resources such as agricultural products or screening foliage. The effects of the chemicals may last only a short time, or they may result in permanent damage and death. Most of the chemicals used are known to be toxic to humans or plant life. Other normally benign (mild)…
Chemistry is the science that studies why materials have their characteristic properties, how these particular qualities relate to their simplest structure, and how these properties can be modified or changed. The term chemistry is derived from the word alchemist, which finds its roots in the Arabic name for Egypt al-Kimia. The Egyptians are credited with being the first to study chemistry. They d…
Where and when did chemistry originate? Some chemists would identify ancient Egypt as the birthplace of chemistry because of that culture's glassworks, cosmetics, and mummification techniques. Advocates of this theory might also refer to a possible etymology of the word chemistry from the Egyptian word for black. Other historians place the origins of chemistry amid ancient Greek theories of…
Chemoreception is the biological recognition of chemical stimuli, by which living organisms collect information about the chemistry of their internal and external environments. Chemoreception has three sequential stages: detection, amplification, and signaling. In detection, a molecule typically binds to a chemoreceptor protein on the surface of a cell, changing the shape of the chemoreceptor. All…
Chestnuts are species of trees in the genus Castanea, family Fagaceae. They are species of temperate hardwood (angiosperm-dominated) forests found in the Northern Hemisphere and are indigenous to eastern North America and Eurasia. Species in the genus Castanea can grow to be 100 ft (30 m) tall. They have simple leaves with a broadly toothed margin and sweet-smelling, yellowish, insect-pollinated, …
A chi-square analysis is best illustrated using an example in which data from a population is categorized with respect to two qualitative variables. Table 1 shows a sample of patients categorized with respect to two qualitative variables, namely, congenital heart defect (CHD; present or absent) and karyotype (trisomy 21, also called Down syndrome, or trisomy 13, also called Patau syndrome). The cl…
Between 1966 and 1977, members of the Mexican-American community engaged in a period of widespread political activism akin to other civil rights and antiwar movements of the 1960s. The resulting challenges and concurrent mentalities became the Chicano movement, or as it is now known in recognition of the equally important participation of Chicanas, the Chicana/o movement. During this period large-…
Chickenpox, a disease characterized by skin lesions and low-grade fever, is common in the United States and other countries located in areas with temperate climates. The incidence of chickenpox is extremely high-almost everyone living in the United States contracts chicken-pox, usually during childhood, but sometimes in adulthood. In the United States, about 3.9 million people a year contract chic…
Diseases that are more common among children than among adults are referred to as childhood diseases. That is not to say that adults cannot or will not contract these illnesses; but usually children contract these diseases and form the immunity against them that will protect them as adults. In fact, some of these diseases may be quite uncomplicated in children, but may be life-threatening when con…