Homeostasis (a Greek term meaning same state), is the maintenance of constant conditions in the internal environment of the body despite large swings in the external environment. Functions such as blood pressure, body temperature, respiration rate, and blood glucose levels are maintained within a range of normal values around a set point despite constantly changing external conditions. For instanc…
The number of human beings on Earth has increased enormously during the past several millennia, but especially during the last two centuries: from 1850 to 1950 the human population doubled, from 1.265 billion to 2.516 billion, and has more than doubled from 1950 to the present. Moreover, it is likely that the human population—presently at over 6.215 billion—will continue to increase.…
Honeycreepers are 14 living species of birds in the family Drepanididae, which occur only on the Hawaiian and Laysan Islands and nearby islands in the equatorial Pacific Ocean. Unfortunately, a further eight species of honeycreepers have recently become extinct as a result of ecological changes that humans have caused to the habitats of these birds. In addition, at least half of the surviving spec…
As their name suggests, honeyeaters are often found near flowering plants feeding on nectar. All of the Honeyeaters have slender, pointed bills with a long, brushlike tongue that is used to sip the nectar. However, there are many variations of the bill shape, depending on the specific diet of each species. Species with longer bills are usually feeding from tubular flowers, while those with shorter…
The hoopoe (Upupa epops) is the only species in its family, the Upupidae. This species breeds in northwestern Africa, on Madagascar, throughout the Middle East, and in southern Europe and southern Asia. Its usual habitats are open forests, savannas, grasslands, and some types of cultivated lands and parks. Some populations of hoopoes are sedentary, while others are migratory. Hoopoes have a body l…
A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. More simply, each horizon contains chemicals, such as rust-like iron oxides, or soil particles that differ from adjacent layers. Soil scientists generally name these horizons (from top to bottom) "O," "A," "B," "C,�…
Grasses are monocotyledonous plants in the family Poaceae (also known as Gramineae). There are as many as 10,000 species of grasses distributed among more than 600 genera. The richest genera of grasses are the panic-grasses (Panicum spp.) with 400 species, the bluegrasses (Poa spp.) and love-grasses (Eragrostis spp.) with 300 species each, and the needle-grasses (Stipa spp.) with 200 species. Spec…
While the origins of geometry are likely to remain a matter of pure speculation, the elaborate written cultures of ancient Egypt and Babylon provide a wealth of information about the uses of geometry. Area and volume measurements abound in work connected with taxation, the provision of cities, and large-scale building works. Sometimes the Babylonians' evidence (which survives because they w…
The concept of gesture suffers to some extent from insufficiently defined and imprecisely drawn outlines of what we understand by this term. The Oxford English Dictionary defines gesture as any "significant movement of limb or body"; Merriam Webster's Collegiate Dictionary defines it as "any movement of the body, or part of the body, that expresses or emphasizes an idea…
Jewish family in Nazi ghetto, Lublin, Poland, c. 1941. Although the term ghetto has evolved to serve as a name for any poor urban neighborhood, it was originally used in the sixteenth century to denote an area in which Jews were forced to live. AP/WIDE WORLD PHOTOS The name of a district in sixteenth-century Venice where Jews were required to live, ghetto came to be the name for any segrega…
Reflection on gifts and their paradoxes goes back to the Bible, where humans are reminded that everything they have is a gift from God, for which they must be grateful and which should inspire them to give to others. Yet the Scriptures also condemn gifts, as those to judges, which corrupt or harm. Greek literary texts describe a wide range of gift practices, from the honorable gifts among warriors…
Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into …
Globalization represents a process of rapid intensification of broad economic, political, and cultural interconnectedness among the different actors in the global system. The process is qualitatively different from the global expansion of product markets that is inherent to the capitalist socioeconomic system and emerges during certain epochs of capitalism under conditions of political and economi…
Grasshoppers are plant-eating insects characterized by long hind legs designed for locomotion by jumping. Like all insects, the body of grasshoppers is divided into three main parts: head, thorax, and abdomen. On the head are two antennae for feeling and detecting scent, and two compound eyes comprised of many optical units called facets, each of which is like a miniature eye. The chewing mouthpar…
Hornbills are medium- to large-sized, large-billed, long-tailed birds of tropical forests, savannas, and grasslands, comprising the family Bucerotidae. The 45 species of hornbills are distributed widely through the tropical regions of Africa and Asia. Most hornbills live in forests, and nest in holes in trees, while the species of open habitats nest in cavities in hollow trees or in holes in cliff…
Globalization is a far-ranging topic. It has as many perspectives as commentators. Western views of globalization often focus on economics and politics, while Eastern views often focus on philosophy and culture. Two Canadian scholars, Marshall McLuhan and Harold Innis, seem to bridge both East and West in their studies. …
Grasslands are environments in which herbaceous species, especially grasses, make up the dominant vegetation. Natural grasslands, commonly called prairie, pampas, shrub steppe, palouse, and many other regional names, occur in regions where rainfall is sufficient for grasses and forbs but too sparse or too seasonal to support tree growth. Such conditions occur at both temperate and tropical latitud…
The horse chestnut and buckeyes (Aesculus spp.) are various species of angiosperm trees in the family Hippocastaneae. There are about 20 species of trees and shrubs in this family, occurring widely in temperate, angiosperm forests of Europe, Asia, and North America. The horse chestnut and buckeyes have seasonally deciduous, oppositely arranged, palmately compound leaves, which means that the five …
Gravitational lenses are accidental natural arrangements of gravity, light, and distant astronomical objects that create altered images of the those objects. Commonly, a lens is a piece of glass shaped so as to bend light passing through it. In the process, it alters the image of the light source as observed through the lens. A gravitational lens bends light using gravity rather than glass. Gravit…
Horsehair or gordian worms are unusual invertebrates in the phylum Nematomorpha. These very long, thin creatures have a superficial resemblance to animated horse hairs, hence their common name. Often, horsehair worms occur in seemingly inextricable tangles of two or more individuals, especially during the breeding season, which is generally in the springtime. The second common name of these animal…
The plurality of available sources, from the Nag Hammadi texts to the writings of those fathers of the church, the so-called heresiologists who fought against Gnosticism as a heresy, makes difficult a reconstruction both of its origins and its history. The use of the category "Gnosticism" has been criticized because it provides an overview that hides the complexity of ancient histori…
Horses are members of the family Equidae, which includes the wild asses of Africa and Asia and the zebras of African plains and mountains. The origins of horse-like mammals have been traced back some 55 million years to a small dog-sized, plant-eating animal known as Hyracotherium. More recently, during the Pliocene and Miocene periods (which ended some 1.5-2 million years ago) horses and their re…
Philosophical accounts of "the good" are, broadly speaking, accounts of what it is to be an object of value—especially of moral value. A systematic study of these accounts is aided by such distinctions as the following. …
The activities characterized as Greek science cover a wide range of practices and theories that do not correspond to modern science in a simple or meaningful way. The boundaries between disciplines were fluid in the ancient period and the definition of subjects and methodologies were discussed vigorously. Hence, it is often futile to try to draw firm boundaries between subjects such as philosophy,…
Most contemporary understandings of happiness are hedonic: happiness is a state of feeling most precisely defined by the subject of the feeling. Happiness in this sense is subjective and can be of brief duration. Ancient discussions of happiness, however, revolve around the Greek term eudaimonia, and while this word is commonly translated as "happiness," it has a different meaning an…
Gravity is a force of attraction that exists between every pair of objects in the Universe. This force is proportional to the mass of each object in each pair, and inversely proportional to the square of the distance between the two; thus, …
Harmony is derived from the classical Greek harmonia (meaning a joint between the planks of a ship or a joining of those planks). From the beginning, the term was also used in its current metaphorical sense, that of a combination of parts or related things to form a consistent whole or an agreement. …
Often referred to as a living fossil, the horseshoe crab has changed very little in over 400 million years. Related to spiders, this animal is easily identified by the large greenish brown, helmet-like dorsal plate, called either the cephalothorax or prosoma. A separate plate covers its abdomen. A long tail spine, referred to as the caudal spine or telson, extends from its abdomen. Measured from t…
As a concept, hate has several interrelated dimensions. It attempts to provide historical, psychological, and sociocultural depth to the forms of hostility and animosity that the term hate ostensibly defines, and to make the idea clear in terms of its linguistic usages. As such, it faces obstacles that often appear insuperable. Nonetheless, "hate crimes"—criminal acts and beha…
Horsetails are a group of relatively primitive, vascular plants in the genus Equisetum, family Equisetaceae, subdivision Sphenophytina. The sphenophytes have an ancient evolutionary lineage occurring as far back as the Devonian period. These plants were most abundant and diverse in species about 300 million years ago, during the late Devonian and early Carboniferous periods. Fossils from that time…
Health and disease seem at first glance to be obvious and opposing concepts. We are either healthy or suffering from some disease. In practice, however, health and disease are neither clearly defined nor mutually exclusive. Asthmatics and diabetics have won Olympic gold medals, and amputees can live to a ripe old age. "Healthy" people in their eighties cannot do things they could eas…
Aspects of heaven and hell cross religious traditions. Paradise can be a city, a palace, a court, a garden, a vision of God, a mystical diagram, or an ineffable concept. Physical, indeed sexual, terms and images express the soul's union with God. In Hell, fires, dragons, serpents, stench, cacophony, torturers, and their paraphernalia abound. Christian, Islamic, Zoroastrian, and Japanese sou…
The word horticulture comes from Latin and refers to the cultivation of gardens. There are three main branches of the science of growing plants: forestry, agronomy, and horticulture. Forestry is concerned with the cultivation of stands of trees for their commercial and ecological uses. Agronomy involves the large-scale cultivation of crops, such as wheat, cotton, fruits, and vegetables. Horticultu…
The ideas on heaven and hell are closely associated with human imagination about the afterlife in Asian civilizations. The images of heaven and hell, however, vary greatly in different regions such as in the southern, central, and eastern parts of the Asian continent, and they have changed significantly throughout history. There is no such thing as a unique Asian mind or a collective Asian concept…
Hot spots are a common term for plumes of magma welling up through the crust (Earth's outermost layer of rock) far from the edges of plates. To understand what hot spots are and why they are important, some understanding of the theory of plate tectonics is necessary. This widely accepted theory proposed by Alfred Wegener in 1912 states that the crust is composed of huge plates of rock that …
Hedonism is a modern word derived from the Greek hedone, or "pleasure." As a philosophical position, moral hedonism justifies pleasure as a good, or even the good. Its history can be traced back to Hellenistic philosophy. Now, for Plato, Aristotle, and the Stoics, hedone is both the first and the worst candidate. Only the Epicureans, after the Cyrenaics, accept it fully as the condit…
The Great Barrier Reef lies off the northeastern coast of Australia and is both a scientific wonder and an increasingly popular tourist attraction. It has been described as "the most complex and perhaps the most productive biological system in the world." The Great Barrier Reef is the largest structure ever made by living organisms including human beings, consisting of the skeletons …
A hovercraft is a vehicle that can be used to journey over water and land. Unlike a boat, which floats on the water, a hovercraft is suspended above the water on a cushion of air. This also allows a hovercraft to move over land and float over small depressions such as a ditch or over waves. A powerful and specially designed fan creates the air cushion that is part of the hovercraft. For this reaso…
Given the complexity of his thought, it is not surprising that Hegel's philosophy has been interpreted in a number of different and often opposed ways. As such, while many philosophical movements might be described as "Hegelian," there is no univocal sense of this term, nor any unanimity about what the proper interpretation of Hegel's idealism involves. Perhaps because …
The greatest common factor (or greatest common divisor) of a set of natural numbers is the largest natural number that divides each member of the set evenly (with no remainder). For example, 6 is the greatest common factor of the set because 1246 = 2, 1846 = 3, and 3046 = 5. …
Hegemony, from the Greek hegemón (guide, ruler, leader) and hegemonia (rule, leadership), denotes the preeminent influence a state, social class, group, or individual exercises over others. Today it is especially associated with the Italian Marxist and revolutionary Antonio Gramsci (1891–1937), for whom it is the core and organizing concept of his social and political ideas. More recen…
Heresy is derived from a Greek word literally meaning "a choice." St. Irenaeus (c. 120 to 140–c. 200 to 203) defined heresy as deviation from the standard of sound doctrine. This definition provided a model for subsequent conceptions of heresy. Referring to the Greek word, St. Jerome (c. 347–419 or 420) wrote that each one chooses the rule that one judges to be the best…
Traditionally understood as the art of interpretation (ars hermeneutica) that provided rules for the interpretation of sacred texts, hermeneutics today serves to characterize a broad current in contemporary continental philosophy that deals with the issues of interpretation and stresses the historical and linguistic nature of our world-experience. Since this characterization is also valid for cont…
Hierarchy (Greek hierarchia; from hieros, sacred archein, rule) is a kind of order, supposing existence of higher (or more sacred) and lower (less sacred) levels of reality. Order, in its turn, is a linkage of fundamentally different elements by means of general laws, so that the whole is greater than a simple arithmetical sum of its parts. Since order presupposes difference, it often takes the fo…
Grebes are aquatic birds that make up the family Podicipedidae. This is the only family in the order Podicipitiformes, a rather unique group of birds that is not Western grebes (Aechmophorus occidentalis) displaying across the water. Photograph by Phil Dotson/The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. closely related to other living orders, an…
Hinduism, the religion of nearly one billion people mostly of South Asian provenance or descent, is notoriously difficult to define or even to describe with accuracy and comprehensiveness. Like all complex and ancient religious traditions, it is problematic to speak about Hinduism as if it were one monolithic religion rather than merely a label for many different traditions. The conglomeration of…
Floating in orbit approximately 380 miles (612 km) above the earth, the 12.5-ton Hubble Space Telescope has peered farther into the Universe than any telescope before it. The Hubble, which was launched in 1990, has produced images with unprecedented resolution at visible, near-ultraviolet, and near-infrared wavelengths since its originally faulty optics were corrected in 1993. Although ground-base…
Historical and dialectical materialism are doctrines in the philosophy of history and in metaphysics, respectively. They were developed within the Marxist tradition and refer to ideas found in the works of Karl Marx (1818–1883). However, neither term Friedrich Engels (1820–1895). Engels, a friend and collaborator of Karl Marx, believed that the concept of private property was to …
Historicism (German Historismus, French historisme, Italian storicismo) is a term of Romantic origins associated first with the German Historical Schools and then more generally with historical method as applied to all the arts and sciences and to human life. "Historicism" appeared first in a fragment of Novalis, who contrasted it with other methods (chemical, mathematical, artistic,…
Economic history emerged in the late nineteenth century as an academic field devoted to the study of past economic phenomena and processes. Since then it has undergone significant changes in terms of its thematic and theoretical concerns, analytical methodologies and language, and the spatial and temporal scales in which it is framed. Distinctive national and regional approaches and traditions can…
The key to the conceptualization of history is the effort to essentialize the past, first in a Herodotean or Thucydidean narrative, and then in more rational, conjectural, philosophical, and religious ways. The "past" became (in Arnold Toynbee's phrase) an "intelligible field of study," and anthropomorphism as well as religious beliefs and ethnic prejudices came …
To understand the greenhouse effect, Earth's energy budget must be known. An energy budget is an account of all of the energy coming into and leaving a system and of any energy that is stored in (or produced by) the system itself. Almost all of the energy coming to Earth from space has been radiated by the closest star, the Sun. The Sun emits electromagnetic energy at a rate and spectral qu…
An artificial chromosome is a deoxyribonucleic acid (DNA) containing structure that is assembled from many different components of naturally occurring chromosomes. …
Codes of honor are all-pervasive in human societies, but the modern study of honor as an academic formulation originated in the Mediterranean region, and especially in the work of anthropologists working in Spain and Greece. Julio Caro Baroja, J. G Peristiany, and Julian Pitt-Rivers wrote some of the seminal and most influential works on the concept of honor. They placed the honor complex in the M…
The groundhog or woodchuck (Marmota monax) is a husky, waddling rodent in the squirrel family Sciuridae, order Rodentia. The groundhog is a type of marmot (genus Marmota), and is also closely related to the ground squirrels and gophers. The natural habitat of the groundhog is forest edges and grasslands, ranging from the eastern United States and Canada through much of the Midwest, to parts of the…
Human chorionic gonadotropin (HCG) is a glyco-protein hormone produced by the extraembryonic tissue of the early human embryo. After fertilization, the human zygote undergoes cleavage followed by the formation of a blastocyst. The blastocyst is a hollow sphere constructed of an inner cell mass, which becomes the embryo proper, and a trophoblast, which is embryonic tissue that will contribute to th…
Ancient and modern Arabs, as well as other ethnic groups of Muslim and Mediterranean peoples, adopted ideas of honor that reinforce the ties of an individual to his or her tribal clan or extended family. One type of honor, sharaf, applies to men and is attained through maintenance of a family's reputation, hospitality, generosity, chivalry, bravery, piety, and, sometimes, nobility or politi…
Groundwater occupies the void space in a geological strata. It is one element in the continuous process of moisture circulation on Earth, termed the hydrologic cycle. Almost all groundwater originates as surface water. Some portion of rain hitting the earth runs off into streams and lakes, and another portion soaks into the soil, where it is available for use by plants and subject to evaporation b…
Human capital refers to the knowledge, skills, and capabilities of individuals that generate economic output. Human capital averages about two-thirds of the total value of the capital of most economies, which includes land, machinery, and other physical assets as well as the skills and talents of people. The value of human capital is often apparent after physical destruction, as during World War I…
A group is a simple mathematical system, so basic that groups appear wherever one looks in mathematics. Despite the primitive nature of a group, mathematicians have developed a rich theory about them. Specifically, a group is a mathematical system consisting of a set G and a binary operation * which has the following properties: Note that commutativity is not required. That is, it need not be true…
The idea of human rights posits that human beings, regardless of extrinsic differences in circumstance (nationality, class, religion) or physical condition (race, gender, age), possess a basic and absolute dignity that must be respected by governments and other people. Sometimes these rights claims have been grounded in systems of positive law, sometimes in conceptions of human nature or divine cr…
Grouse (and ptarmigan) are medium-sized birds in the family Tetraonidae, order Galliformes. Grouse and ptarmigan are often hunted for food and sport, and are sometimes broadly referred to as upland gamebirds because they are not hunted in wetlands, as are ducks and geese. Grouse are ground-dwelling birds with a short, turned-down bill. They have long, heavy feet with a short elevated fourth toe be…
An oocyte is an unfertilized egg. Oocytes and spermatozoa are called gametes, and represent different cells that fuse their genes to form a new cell, the fertilized egg. The fertilized cell is called a zygote, and it rapidly divides into several totipotent cells (cells capable of developing into any cell type) called blastomers. Totipotent cells can be considered the opposite of differentiated cel…
The Universal Declaration of Human Rights, adopted by the General Assembly of the United Nations (U.N.) on 10 December 1948, provided the most detailed outline of the human rights and fundamental freedoms of individuals in the modern era. Furthermore, it was a milestone in that these rights and freedoms applied to every person around the world. The language of the document was, however, gender neu…
Human ecology is the study of the reciprocal inter-actions of humans with their environment. Key aspects of human ecology are demographics, resource use, environmental influences on health and society, and environmental impacts of human activities. All of these subjects are intimately linked, because increasing populations of humans require more resources, the exploitation and use of which cause i…
A common misconception of African humanism is that it is a set of values brought into, instead of emerging from, communities on the African continent. This prejudice is due primarily to the influence of modern European humanism, which is premised upon a secular naturalism as the only model of humanism. The modern European humanist tradition, which treats Christianity as the model of all religion,…
The dominant Chinese conception of humanism is the Confucian theory of ren. The term ren has been translated in various ways, including as "benevolence," "goodness," "virtue," "humanity," "humanness," and "being authoritative." These different translations indicate the complexity of this Confucian theory. Ren as th…
Growth and decay refers to a class of problems in mathematics that can be modeled or explained using increasing or decreasing sequences (also called series). A sequence is a series of numbers, or terms, in which each successive term is related to the one before it by precisely the same formula. There are many practical applications of sequences. One example is predicting the growth of human popula…
The introduction of the term humanism is commonly attributed to the German pedagogical theorist F. J. Niethammer's 1808 book, which promoted reading of the ancient classics among secondary students as a counterweight to scientific and technological training. The word soon enjoyed wide currency in many European languages, in part because the much earlier Italian term umanista was already use…
In the mid-twentieth century, Paul Oskar Kristeller (1905–1999) established the understanding of Renaissance humanism accepted by all scholars in the field. Humanists or umanisti were practitioners of the studia humanitatis or liberal arts: grammar, poetry, rhetoric, history, and moral philosophy. Their origins are traceable to the notaries who worked for courts and cities in medieval Italy…
The history of how the human species, Homo sapiens sapiens, evolved is reconstructed by evidence gathered by paleontologists, anthropologists, archeologists, anatomists, biochemists, behavioral scientists, and many other professions. The evidence comes from the record left by fossils and by extrapolation from modern primates and human hunter-gatherer tribes. Fossils are evidence of past life. In p…
The philosophy and ideology of secular humanism has its roots in Enlightenment thought and is based in large part on the Western tradition of liberalism and notions about the status and role of science in the modern world. At base it is a nontheistic belief system that upholds the prime importance of rationality, human autonomy, and democracy. The term secular humanism has come to be widely used i…
While reason tells us that it is obvious to anyone, irrespective of his or her background, that humans are bipedal, featherless creatures, other characteristics we attribute to humans are not always so obvious. Also, while we all appear to assume that humans are creatures who have minds, in contrast to other creatures, it is not quite obvious what is meant by "mind" or what happens a…
Several hormones play important roles in human growth. The major human growth hormone (hGH), or somatotropin, is a protein made up of 191 amino acids secreted by the anterior pituitary and coordinates normal growth and development. Human growth is characterized by two spurts, one at birth and the other at puberty. HGH plays an important role at both of these times. Normal individuals have measurab…
Confucius's study of ancient Chinese classics led him to believe that during the reigns of the "sage kings" Yao and Shun, China had been well governed and harmony had prevailed throughout their realms. This was accomplished not through harsh punishments or excessive regulations, but by the moral force of their personalities and their attention to social rituals. They are extol…
Studies of European views of man and of the dignity of man have been central to the history of ideas, and books continue to be published discussing Western or European views of man. Meanwhile, women, lower-class men, and people of color have delved into the scholarship to determine if a thinker's text intended man (homme in French) to be generic as in the Hebrew adam, Greek anthropos, or La…
The otherwise distinctive presentation of the male body with shortened extremities, diminutive hands, enlarged feet, elongated torso, protruding belly, and protracted phallus on the otherwise untitled Baluban sculpture Male Figure (probably nineteenth century), might at first glance appear as a "primitive" distortion of the human image. However, prolonged observation proffers the ind…
Humor is such an integral part of the human psyche that philosophers and other intellectuals have long been fascinated with its origins in and its effects on the human brain. Several early theorists have provided subject matter for continuing observation and debate. The Greek word chumoi means "juices," and the ancient Greeks used the word, from which we get the English humor (as wel…
The United States Human Genome Project (HGP) is an initiative formally launched in 1990 by the National Institutes of Health (NIH) and the U.S. Department of Energy (DOE) to better understand all aspects related to human genetic material, or deoxyribonucleic acid (DNA). DNA represents a genetic alphabet and the specific sequences that are part of DNA called genes code for various proteins by virtu…
Hygiene is defined in current English dictionaries as "the science of health." This definition, though formally correct, hides a long history of change in the word's use, from its holistic classical meaning of "individual regimens to preserve health" to its nineteenth-century connotations of "social medicine" (including lethal eugenics programs), to…
Humidity is a measure of the quantity of water vapor in the air. There are different methods for determining this quantity and those methods are reflected in a variety of humidity indexes and readings. The humidity reading in general use by most meteorologists is relative humidity. The relative humidity of air describes the saturation of air with water vapor. Given in terms of percent humidity (e.…
Iconography is the description, classification, and interpretation of the subject matter of a work of art. Derived from the Greek words eikon, meaning image or icon, and graphia, meaning description, writing, or sketch, the word iconography is one of the least understood, most abused, and most flexible terms in the English language. Its primary purpose is to understand and explicate the meaning be…
The term idealism in its broadest sense denotes the philosophical position that ideas (mental or spiritual entities) are primary and lie at the very foundation of reality, knowledge, and morality, while non-ideal entities (such as physical or material things) are secondary and perhaps even illusory. Strands of idealistic thought can be found in ancient and medieval philosophy, but modern idealism …
Guenons are small to medium-sized monkeys widespread throughout sub-Saharan Africa. These primates are classified in the infraorder of Old World simian primates (Cataffhina) and the family Cercopithecidae. Their genus, Cercopithecus, is large, very diverse, and successful. The Cercopithecidae family consists of two subfamilies: the omnivorous Cercopithecinae (including guenons, talapoin, and baboo…
The "history of ideas," phrase and concept, goes back almost three centuries to the work of J. J. Brucker (1696–1770) and Giambattista Vico (1668–1744) in the early eighteenth century, followed in the nineteenth century by Victor Cousin (1792–1867) and his eclectic and "spiritualist" philosophy. The story begins with Brucker's Historia doctri…
General problems about identity had been discussed long before the early modern period, but the problem of personal identity in the form in which it is so widely discussed today has its origin in John Locke's (1632–1704) chapter "Of Identity and Diversity," which he added to the second edition of his Essay Concerning Human Understanding (1694). Indeed, most early twenty…
Hummingbirds are small, often tiny birds of the Americas, named after the noise made by their extremely rapid wingbeats. There are 320 species of hummingbirds, which make up the family Trochilidae. Hummingbirds are spectacularly beautiful birds, because of the vivid iridescence of their feathers. They are such accomplished fliers that they can aggressively drive away much larger, predatory birds. …
Although identity has deeps roots in social psychology, sociology, bridges between them (e.g., symbolic interactionism), and related disciplines, the explicit distinction between personal and social identity, within social psychology at least, can be traced to J. C. Turner's seminal article "Towards a Cognitive Redefinition of the Group" (1982). This formed the basis for self-…
Humus is an amorphous, dark brown, organic material that is formed by the incomplete decomposition of biomass. Strictly speaking, humus is composed of organic residues that are sufficiently fragmented and decomposed by microbial and other decomposition processes that the original source of the biotic materials is no longer recognizable. Humus is mostly composed of a very complex mixture of large o…
Within Western thought the subject—that is, the self as a thinking, feeling, psycho-physiological entity—has been traditionally defined as a centered consciousness, characterized and unified by one self-defining identity. Within this tradition, a centered subjectivity was long thought to exist and function independently of the social contexts surrounding it, without significant influ…
Guillain-Barre syndrome (GBS) is a cause of progressive muscle weakness and paralysis which evolves over days or weeks, and resolves over the next several weeks or months. About 85% of patients recover completely, with no residual problems. Diagnosis of GBS is made by virtue of the cluster of symptoms (ascending muscle weakness and then paralysis) and by examining the fluid which bathes the brain …
Although Filipinos lived in the United States in the sixteenth century, the first large Asian group in the modern era arrived in the United States in the nineteenth century. Since then two inclinations have simultaneously characterized how Asian-Americans' ethnic identity has been viewed. One tendency has been to look at their identity either in terms of a collective panethnic identity as &…
Guinea fowl are seven species of medium-sized terrestrial birds in the family Phasianidae, order Galliformes, which also includes other fowl-like birds, such as the grouse, ptarmigan, turkey, quail, peafowl, and pheasants. The natural range of guinea fowl is sub-Saharan Africa, the Arabian Peninsula, and Madagascar. However, these birds have been introduced to some other places, and are commonly k…
As a collective that has lived and created its history for the most part not merely in diaspora but among a vast array of "host" peoples, Jewry across the generations has been powerfully marked by the need to negotiate, on the one hand, elements of identity understood to be shared with all Jews at all times and places and, on the other hand, cultural motifs and practices shared with …
Guinea pigs, or cavies, are about 20 species of rodents in the family Caviidae. Guinea pigs are native to South America, occurring from Colombia and Venezuela in the north, to Brazil and northern Argentina. These animals occur in rocky habitats, savannas, forest edges, and swamps, and can be rather common within their preferred habitat. Guinea pigs have a stout body, with a relatively large head, …
Placing the words images, icons, and idols together as the subject of an entry that seeks to articulate the perspective of world culture suggests their intimate relationship. Such a grouping also demands that an emphasis be placed on the particular role biblical religions have had in the development and use of the last two terms, the ideas of "icon" and "idol" that play…
The idea of imagination is sometimes thought of as a product of the Enlightenment. However, although it only came to full flower in the seventeenth, eighteenth, and nineteenth centuries, its roots are much more ancient. …
Indeed, in the early twenty-first century, Nicholas Smith has argued that Plato's views in the Republic seem to be intended to provoke thought rather than to set forth consistent views. As such, we should not be surprised that in Plato's work another "view" of immortality can be found. Plato's Symposium offers this alternative view. Here Plato's Socrates r…
Gulls are 43 species of seabirds, in the subfamily Larinae of the family Laridae, which also includes the terns. Gulls occur in a wide range of coastal habitats, ranging from inland lakes, rivers, and wetlands, to marine shores and estuaries. Their distribution is virtually world-wide, but most species occur in the Northern Hemisphere. Species of gulls range in body length from 8-32 in (20-81 cm).…
Impressionism was an artistic movement that originated in France in the 1860s and 1870s. In 1874, painters including Claude Monet, Camille Pissarro, Auguste Renoir, Alfred Sisley, Berthe Morisot, Edgar Degas, Armand Guillaumin, and Paul Cézanne participated in the first of eight independent Impressionist exhibitions held until 1886. They were eventually joined by Gustave Caillebotte and the A…
One of the most popular species of freshwater topical fish is the guppy. The first specimens were brought to the British Museum in London for description in 1859 by R. J. L. Guppy, a biologist from Trinidad (West Indies) after whom the fish is named. The species originally possessed the scientific name Lebistes reticulatus, but in 1963, the Latin name was changed to Poecilia reticulata and remains…
Huntington disease is a rare, incurable genetic disease that results in the progressive degeneration of both physical and mental abilities. Huntington disease was formly known as Huntington chorea since the most obvious symptoms involve uncontrollable body movements known as chorea. As the disease progresses, its symptoms worsen and patients eventually die of respiratory failure or complications r…
The term indigenismo encompasses a diverse array of intellectual production concerning the indigenous peoples of Latin America. The twentieth century in particular witnessed an explosion of literary, critical, and visual work on the figure of the Indian. It should be made clear from the outset that the term, although most closely associated with Mexico and Guatemala and especially with the Andean …
The Gutenberg discontinuity occurs within Earth's interior at a depth of about 1,800 mi (2,900 km) below the surface, where there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, …
A hybrid is an offspring between two different species, or the offspring between two parents of the same species that differ in one or more heritable characteristics. An example of the first kind of hybrid is a mule, a cross between a female horse (Equus caballus) and a male donkey (E. asinus). An example of the second kind is the offspring from a cross between true-breeding redand white-flowered …
Individualism endorses the principle that the ends or purposes of the human individual possess dignity and worth that take precedence over communal, metaphysical, cosmological, or religious priorities. While individualism may appeal to certain metaphysical or epistemological schools of thought such as nominalism or empiricism, it will be treated here as primarily a moral and/or political doctrine.…
Gutta percha is a rubberlike gum obtained from the milky sap of trees of the Sapotaceae family, found in Indonesia and Malaysia. Once of great economic value, gutta percha is now being replaced by plastics in many items, although it is still used in some electrical insulation and dental work. The English natural historian John Tradescant (c. 1570-1638), introduced gutta percha to Europe in the 162…
Hydra are solitary animals of the phylum Coelenterata that measure from just a few millimeters in size to more than 3.5 ft (1 m) in length. They are all thin animals that rarely measure more than 0.4 in (1 cm) in diameter. Most are cylindrical in shape, with a broadened basal disk that serves to attach the animal to some firm substrate. Most species are sessile but some can, if conditions require,…
Gymnosperms are one of the two major groups of plants that produce seeds; the other is the angiosperms. Gymnosperm literally means "naked seed," which refers to the development of seeds exposed on a flat structure, that is, not within an ovary as in the angiosperms. Gymnosperms became common about 290 million years ago and although many of the earlier types are now extinct, four kind…
Gynecology, from the Greek meaning "the study of women," is a medical specialty dealing with the health of a woman's genital tract. The genital tract is made up of the reproductive organs including the vagina, cervix, uterus, ovaries, fallopian tubes, and their supporting structures. Marked changes occur in a woman's reproductive organs upon her reaching menarche (the a…
A gyroscope is heavy disk placed on a spindle that is mounted within a system of circles such that it can turn freely. When the disk, called a flywheel, is made to spin, the gyroscope becomes extremely resistant to any change in its orientation in space. If it is mounted in gimbals, a set of pivot and frame mountings that allow it freedom of rotation about all three axes, a fast-spinning gyroscope…
The term habitat refers to the type of environment in which an organism or species occurs. For plants, habitat is mostly defined by its physical attributes (e.g., rainfall, temperature, topographic position, soil texture and moisture) and its chemical properties (e.g., soil acidity, concentrations of nutrients and toxins, oxidation reduction status). For terrestrial animals, the habitat is defined…
A primitive group of fish, hagfish (order Hyperotreti, family Myxinidae) resemble eels in their external appearance. These fish lack a backbone, jaws, true fins, and scales. Their body is tubelike and often covered in a slimy substance that is secreted from abundant glands in the skin. The body is often a pale fleshy pink, but is occasionally brown-gray above and pink below. They may reach up to 2…
The half-life of a process is an indication of how fast that process proceeds—a measure of the rate or rapidity of the process. Specifically, the half-life is the length of time that it takes for a substance involved in that process to diminish to one-half of its initial amount. The faster the process, the less time it will take to use up one-half of the substance, so the shorter the half-l…
A hydrocarbon is any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms. Carbon atoms have the unique ability to form strong bonds to each other, atom after atom. Every hydrocarbon molecule is built upon a skeleton of carbon atoms bonded to each other either in the form of closed rings or in a continuous row like links in a chain. A chain of carbon atoms may be …
Hydrocephalus, which means literally, "water on the brain," is a condition in which excessive fluid collects inside the skull. The fluid is a naturally produced liquid that normally is found in the brain. Accumulation of excessive amounts of the fluid may build pressure to levels that cause brain damage and subsequent disability. The brain rests within the natural bony vault of the c…
Hydrochlorofluorocarbons (HCFCs) are compounds made up of hydrogen, chlorine, fluorine, and carbon atoms. HCFCs and their cousins, hydrofluorocarbons (HFCs), were created in the 1980s as substitutes for chlorofluorocarbons (CFCs) for use in refrigeration and a wide variety of manufacturing processes. Because all three of these classes of compounds either destroy the stratospheric ozone layer essen…
The hydrofoil is very similar to the hovercraft, because it moves in the boundary between air and water. It avoids drag by lifting itself out of the water, using wing-shaped structures called hydrofoils that extend into the water from the craft. These hydrofoils function like the wings on a plane, creating lift and flying the hull above the surface of the water. The first person to work on this id…
A current-carrying body placed in a magnetic field with the current direction unaligned with the field experiences a force leading to a transient sidewise drift of the charge carriers of the current. This drift continues until the force is balanced by an electric field produced by the charge accumulating at points on the body's surface in the direction of the drift. At points on the body&#x…
Hydrogen is the chemical element of atomic number 1. Its symbol is H, it has an atomic weight of 1.008, its specific gravity at 32°F (0°C) is 0.0000899, and it melts at -434.7°F (-259.3°C). The boiling point of hydrogen is -423.2°F (-252.9°C), just above absolute zero. Boiling liquid hydrogen is the coldest substance known, with the exception of liquid hel…
Hallucinogens are substances that alter the user's thought processes or mood to the extent that he perceives objects or experiences sensations that in fact have no reality. Many natural and some manmade substances have the ability to bring about hallucinations. In fact, because of the ready market for such chemicals, they are manufactured in illegal chemical laboratories for sale as halluci…
Halogenated hydrocarbons are derivatives of hydrocarbons (that is, organic compounds that only contain carbon and hydrogen atoms) which include some halogen atoms within their chemical structure. The most commonly encountered halogens in halogenated hydrocarbons are fluorine and chlorine, but sometimes bromine or iodine occur, or combinations of any of these. Some halogenated hydrocarbons occur na…
Hydrogen chloride is a chemical compound composed of the elements hydrogen and chlorine. It readily dissolves in water to produce a solution called hydrochloric acid. Both substances have many important industrial applications, including those in metallurgy, and the manufacture of pharmaceuticals, dyes, and synthetic rubber. Hydrochloric acid is found in most laboratories since its strong acidic n…
Hydrogen peroxide is most widely found in homes in brown bottles containing 3% solutions (3% hydrogen peroxide and 97% water). The decomposition of hydrogen peroxide happens much faster in the presence of light so that an opaque bottle helps slow this process down. The decomposition of hydrogen peroxide can be summarized by the chemical equation: which states that two molecules of hydrogen pe…
The halogens are a group of chemical elements that includes fluorine, chlorine, bromine, iodine, and astatine. Halogen comes from Greek terms meaning "produce sea salt." None of the halogens occur naturally in the form of elements, but, except for astatine, they are very widespread and abundant in chemical compounds where they are combined with other elements. Sodium chloride, common…
Hydrogenation is a chemical reaction in which hydrogen atoms add to carbon-carbon multiple bonds. In order for the reaction to proceed at a practical rate, a catalyst is almost always needed. Hydrogenation reactions are used in many industrial processes as well as in the research laboratory, and occur also in living systems. We will look at a few examples in each category in this article. …
A halosaur is a thin, elongated fish resembling an eel. The largest of halosaurs grows to about 20 in (51 cm) long. Unlike the eel, the halosaur has a backbone composed of many vertebrae. It has somewhat large scales, numbering fewer than 30 horizontal rows on each of its sides. This fish lives close to or on the bottom of the sea and is thus referred to as a benthic fish. It feeds on the ocean fl…
Hamsters are small rodents with dense fur, a short tail, and large cheek pouches. They belong to the mammalian family Muridae, which also includes rats, mice, gerbils, voles, and lemmings. During foraging trips, hamsters use their cheek pouches to carry seeds and grains back to underground food stores that are sometimes quite large. Hamsters mostly eat plant matter, especially seeds, nuts, soft fr…
The hydrologic, or water, cycle is the continuous, interlinked circulation of water among its various compartments in the environment. Hydrologic budgets are analyses of the quantities of water stored, and the rates of transfer into and out of those various compartments. The most important places in which water occurs are the ocean, glaciers, underground aquifers, surface waters, and the atmospher…
Hydrology is the science of water. It is concerned with the occurrence and circulation of water on and within Earth, the physical and chemical properties of bodies of water, the relationship between water and other parts of the environment, and societal or economic aspects of water resources. Hydrology is an interdisciplinary field of study, and hydrologists have academic backgrounds that include …
Whenever water reacts with another chemical compound, the process is called hydrolysis. Hydrolysis differs somewhat from hydration, although the two can occur together. Hydration is the bonding of whole water molecules to an ion (a charged atom or molecule), usually a metal ion. Hydrolysis, on the other hand, involves an actual chemical reaction of the water molecule itself with another reactant. …
Hand tools can be as easily found as made, and the earliest tools used by people included sticks and rocks picked up and used as projectiles, or to pound or dig. The earliest fashioned hand tools date back to the Stone Age. Currently new technologies make hand tools that are battery-powered, so they are still portable, yet easier to use than their precursors. Tools are an extension of human limbs …
There are several early examples of hydroponics, or soil-free agriculture, including the hanging gardens of Babylon and the floating gardens of China and Aztec Mexico. Early Egyptian paintings also depict the growing of plants in water. In 1600, the Belgian Jan Baptista van Helmont (1579-1644) demonstrated that a willow shoot kept in the same soil for five years with routine watering gained 160 lb…
Hantavirus infections are infections of the lungs caused by hantaviruses. There are five known types of hantaviruses, which differ only slightly from one another. These types are: Hantaan, Seoul, Puumala, Prospect Hill, and Sin Nombre. The Sin Nombre virus was the cause of the 1993 outbreak in the Southwestern United States, which led to a greater understanding of the virus and its transmission to…
Hydrosphere refers to that portion of Earth that is composed of water. The hydrosphere represents one component of Earth's system, operating in conjunction with the solid crust (lithosphere) and the air that envelopes the planet (atmosphere). The derivation of the term hydrosphere, from the Greek words for water and ball, is truly descriptive of our world, as it reflects the abundance and i…
Hard water is water that contains large amounts of calcium, magnesium, or iron ions. Hard water is undesirable since it often has an unpleasant taste, interferes with the ability of soaps to dissolve (although some synthetic detergents dissolve well in hard water), and can cause scaling (the building up of insoluble precipitates) in pipes and hot water systems. Water hardness is most commonly the …
Hydrothermal vents are places where hot fluids (up to 752°F [400°C]) related to volcanic activity are released from the ocean floor. Because of the high pressure exerted by the water at depth on the sea floor, hydrothermal fluids can exceed 212°F (100°C) without boiling. The most visible indications of on-going volcanic activity are the plumes of hot fluids issuing from…
What makes a note from a musical instrument sound rich? The volume of the sound is determined by the amplitude of the oscillations in a sound wave, the distance individual molecules oscillate. A larger amplitude produces a louder sound and transmits more energy. The pitch of a note is the frequency or number of oscillations per second. A higher frequency produces a higher pitched note. The richnes…
Hydrozoa (phylum Coelenterata, class Hydrozoa) are coelenterates that are closely related to the hydra, sea anemones, corals, and jellyfish. Although a large majority of these species are common and widespread, they are often overlooked, as they are all small animals. The vast majority are marine species, but several freshwater hydrozoans have also been identified, for example, Cordylophora lacust…
Hyenas (or hyaenas) are African and Asian dog-like carnivores in the family Hyaenidae, order Carnivora, which also includes the dogs, cats, seals, and bears. Hyenas are very powerfully built animals with a stout head, a short snout, short ears, and powerful jaws with strong teeth, useful for crushing bones to get at the nutrients contained inside. The neck of the hyena is rather elongate, and the …
Hartebeests are even-toed hoofed antelopes in the family Bovidae, which are found throughout Africa south of the Sahara. Included among the grazing antelopes are the reedbuck, waterbuck, rhebok, addax, oryx, bluebuck, gemsbok, and roan and sable antelopes. More closely related to hartebeests are gnus, impala, topi, wildebeest, and bontebok. These are medium to large antelopes that forage for food …
A hyperbola is a curve formed by the intersection of a right circular cone and a plane (see Figure 1). When the plane cuts both nappes of the cone, the intersection is a hyperbola. Because the plane is cutting two nappes, the curve it forms has two U-shaped branches opening in opposite directions. …
Hawks (family Accipitridae) are one of the major groups of predatory birds that are active during the day. They are members of the order Falconiformes, which also includes the falcons, vultures, and osprey, and like the other Falconiformes, they have the characteristic sharp, strong claws and hooked beak suited for catching and tearing up prey. Found on all continents but Antarctica, hawks are a d…
Hypertension is high blood pressure. Blood pressure is the force of blood pushing against the walls of arteries as it flows through them. Arteries are the blood vessels that carry oxygenated blood from the heart to the body's tissues. As blood flows through arteries it pushes against the inside of the artery walls. The more pressure the blood exerts on the artery walls, the higher the blood…
Hypothermia is the intentional or accidental reduction of core body temperature to below 95°F (35°C) which, in severe instances, is fatal. Because humans are endothermic—warm-blooded creatures producing our own body heat—our core body temperature remains relatively constant at 98.6°F (37°C), even in fluctuating environmental temperatures. However, in extre…
Hazardous wastes are by-products of human activities that could cause substantial harm to human health or the environment if improperly managed. The United States Environmental Protection Agency (EPA) classifies liquid, solid, and gaseous discarded materials and emissions as hazardous if they are poisonous (toxic), flammable, corrosive, or chemically reactive at levels above specified safety thres…
Hyraxes are rabbit-sized, hoofed African mammals that surprisingly share a common ancestry with elephants and manatees, or seacows. Hyraxes were originally thought to be rodents, and were later grouped with rhinoceroses. They are now placed in an order of their own, the Hyracoidea, since they share many common features of primitive ungulates. The fossil record indicates that hyraxes were the most …
Hazels or filberts are shrub-sized woody plants in the birch family (Betulaceae) found in temperate forests of North America and Eurasia. Hazels have simple, coarse-toothed, hairy leaves that are deciduous in the autumn. Hazel species native to North America include the American hazel (Corylus americana) of the east and beaked hazel (C. cornuta) of a wider distribution. The giant filbert (C. maxim…
Ibises are grouped together with large wading birds such as storks, herons, flamingos, and spoonbills, in the order Ciconiiformes. Ibises, like most birds in this order, have long legs and a long bill for feeding on fish and aquatic animals in shallow water. They also have broad wings, a short tail, and four long toes on each foot. The 26 species of ibis share the family Threskiornithidae with the…
Hearing is the ability to collect, process and interpret sound. Sound vibrations travel through air, water, or solids in the form of pressure waves. When a sound wave hits a flexible object such as the eardrum it causes it to vibrate, which begins the process of hearing. The process of hearing involves the conversion of acoustical energy (sound waves) to mechanical, hydraulic, chemical, and finall…
Ice is the solid state of water. The great abundance of water on the surface of Earth includes a great quantity of ice in the Polar Regions and high elevations. The relative proportion of each of the three states of water on Earth is a delicately balanced equilibrium controlled by the amount of incoming solar energy and the amount of reflection, known as Aledo, from clouds, water, ice caps, etc. T…
The series of ice ages that occurred between 2.4 million and 10,000 years ago had a dramatic effect on the climate and the life forms in the tropics. During each glacial period the tropics became both cooler and drier, turning some areas of tropical rain forest into dry seasonal forest or savanna. For reasons associated with local topography, geography, and climate, some areas of forest escaped th…
A heart is a means to circulate blood through the body of an animal. Among the lower species such as insects, arachnids, and others, the heart may simply be an expanded area in a blood vessel and may occur a number of times. The earthworm, for example, has 10 such "hearts." These areas contract rhythmically to force the blood through the aorta, or blood vessel. Not until the evolutio…
The ice ages were periods in Earth's history during which significant portions of Earth's surface were covered by glaciers and extensive fields of ice. Scientists sometimes use more specific terms for an ice "age" depending on the length of time it lasts. It appears that over the long expanse of Earth history, seven major periods of severe cooling have occurred. These p…
An iceberg is a large mass of free-floating ice that has broken away from a glacier. Beautiful and dangerous, icebergs wander over the ocean surface until they melt. Most icebergs come from the glaciers of Greenland or from the massive ice sheets of Antarctica. A few icebergs originate from smaller Alaskan glaciers. Snow produces the glaciers and ice sheets so, ultimately, icebergs originate from …
The Iceman is an intact, 5,300-year-old mummy discovered September 19, 1991, in a melting glacier within the Italian Alps near Austria. The oldest human discovered in Europe, he is one of the most complete, naturally mummified humans ever found. …
Any mathematical object that, when applied by an operation, such as addition or multiplication, to another mathematical object, such as a number, leaves the other object unchanged is called an identity element. The two most familiar examples are 0, which when added to a number gives the number, and 1, which is an identity element for multiplication. More formally, an identity element is defined wi…
When a set possesses an identity element for a given operation, the mathematical system of the set and operation is said to possess the identity property for that operation. For example, the set of all functions of a variable over the real numbers has the identity element, or identity function, I(x) = x. In other words, if f(x) is any function over the real numbers, then f(I(x)) = I(f(x)) = f(x). …
Igneous rocks are formed by the cooling and hardening of molten material called magma. The word igneous comes from the Latin word igneus, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rock forms within Earth's crust; the molten material rises, filling voids in the crust, and eventually hardens. Intrusive rocks are also called plutonic rocks, …
Heart diseases (cardiovascular disease) is any abnormal organic condition of the heart or the heart and circulation. A number of conditions can lead to the development of heart disease, including angina, atherosclerosis, cardiac arrhythmia, cardiomyopathy, chronic venous insufficiency, diabetes, heart attack, high cholesterol, high homocysteine, high triglycerides, hypertension, insulin resistance…
The heart-lung machine is a device used to provide blood circulation and oxygenation while the heart is stopped. It is a means of keeping a patient alive while his heart is stopped or even removed from his body. Usually called the heart-lung machine, the device also is referred to as cardiopulmonary bypass, indicating its function as a means to substitute for the normal functions of the heart (car…
Heat exchange reflects and drives changes in energy state between two objects—or more generally systems—in thermal contact due to a difference in temperature. Heat flows from a system at higher temperature to one at lower temperature until both systems are at the same temperature. Systems at the same temperature are said to be in thermal equilibrium. The term "heat" is …
Iguanas are large, ancient, herbivorous lizards with a stocky trunk, long, slender tail, scaly skin, and a single row of spines from the nape of the neck to the tip of the tail. On either side of the head is an eye with a round pupil and with moveable lids. The well-defined snout has two nostrils, the mouth houses a short, thick tongue, and dangling beneath the chin is a "dewlap," or…
In addition to providing solutions for algebraic equations, the imaginary numbers, when combined with the real numbers, form the complex numbers. Each complex number is the sum of a real number and an imaginary number, such as (6 + 9i). The complex numbers are very useful in mathematical analysis, the study of electricity and magnetism, the physics of quantum mechanics, and in the practical field …
Heat capacity (often abbreviated Cp) is defined as the amount of heat required to raise the temperature of a given mass of a substance by one degree Celsius. Heat capacity may also be defined as the energy required to raise the temperature of one mole of a substance by one degree Celsius (the molar heat capacity) or to raise one gram of a substance by one degree Celsius (the specific heat capacity…
The heat index is a measure of how warm an average person feels as a consequence of moisture in the air compared to the actual temperature measured by a thermometer at the same time and location. Generally speaking, the higher the relative humidity, the warmer the temperature will seem to be to a person. The reason for this relationship is that the human body normally loses heat through the proces…
The immune system protects the body from diseasecausing microorganisms. It consists of two levels of protection, the non-specific defenses and the specific defenses. The non-specific defenses, such as the skin and mucous membranes, prevent microorganisms from entering the body. The specific defenses are activated when microorganisms evade the non-specific defenses and invade the body. The human bo…
Heat transfer is the net change in energy as a result of temperature differences. This energy is transferred in the direction of decreasing temperature until thermal equilibrium (equality of temperatures) is achieved. The basic mechanisms involved in this process include radiation (the transfer of energy in the form of electromagnetic waves) and conduction (the transfer of kinetic energy). Heat tr…
The heath family, or Ericaceae, contains about 100-125 genera of vascular plants comprising 3,000-3,500 species. These plants are widespread in North and South America, Eurasia, and Africa, but are rare in Australasia. Species of heaths are most diverse and ecologically prominent in temperate and subtropical regions. The most species-rich genus in the heath family are the rhododendrons (Rhododendr…
Hedgehogs are small, often spine-covered members of the insectivore family Erinaceidae. The spiny hedgehogs are 13 species in subfamily Erinaceidae. Most famed is the European hedgehog, Erinaceus europaeus, which is also a resident of New Zealand, where it was introduced. Not all members of the hedgehog family have tough spines. The moonrats, or gymnures, of Southeast Asia have coarse hair instead…
The Heisenberg uncertainty principle first formulated by German physicist Werner Heisenberg (1901–1976), has broad implications for quantum theory. The principle asserts that it is physically impossible to measure both the exact position and the exact momentum of a particle (like an electron) at the same time. The more precisely one quantity is measured, the less precisely the other is know…
Immunology is the study of how the body responds to foreign substances and fights off infection and other disease. Immunologists study the molecules, cells, and organs of the human body that participate in this response. …
The impact crater is typically the most common type of landform seen on the surface of most of the rocky and icy planets and satellites in our solar system. Impact craters form when a minor planetary body (meteoritic fragment, asteroid, or comet) strikes the surface of a larger body or major planet. A physical scar is excavated on the surface and much energy is dispersed in the process. Most impac…
The heliocentric theory argues that the Sun is the central body of the solar system and perhaps of the universe. Everything else (planets and their satellites, asteroids, comets, etc.) revolves around it. The prevailing theory of the universe at that time was a geocentric (Earth-centered) one, in which all celestial bodies were believed to revolve around Earth. This was seen as a more plausible th…
Imprinting is a term used to describe two very distinct processes. Genomic imprinting is an epigenetic chromosomal modification that describes the preferential expression of a specific parental form of a gene (allele). Imprinting is also a term used in the behavioral science to describe a learning process during which a younger animal identifies with, and adopts behaviors exhibited by, other anima…
Hematology is the study of blood and its basic biological components, including red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (erythrocytes). Hematologists study and help treat a variety of hematological malfunctions and diseases, one of the primary being the various anemias. Anemias, like sickle cell anemia, result in a loss of erythrocytes, which reduces the…
In vitro fertilization (IVF) is a procedure in which eggs (ova) from a woman's ovary are removed, fertilized with sperm in a laboratory procedure, and then the resulting fertilized egg (embryo) is returned to the woman's uterus. Human fertilization in vivo (in the living body) occurs in oviducts (fallopian tubes) of the female reproductive tract. IVF is a procedure of assisted reprod…
Geometry, the study of points, lines, and other figures in space, is a very old branch of mathematics. Its ideas were undoubtedly used, intuitively if not formally, from earliest times. Walking along a straight line toward a particular destination is the shortest way to get there; lining an arrow up with the target is the way to hit it; sitting in a circle around a fire is the most equitable way t…
Hemophilia is an inheritable disorder of the mechanism of blood clotting. Depending on the degree of the disorder present in an individual, excess bleeding may occur only after specific, predictable events (such as surgery, dental procedures, or trauma), or may occur spontaneously, with no initiating event. Various tests are available to measure, under very carefully controlled conditions, the len…
The definition of in vitro and in vivo research depends on the experimental model used. In vitro research is generally referred to as the manipulation of organs, tissues, cells, and biomolecules in a controlled, artificial environment. The characterization and analysis of biomolecules and biological systems in the context of intact organisms is known as in vivo research. The basic unit of living o…
Geomicrobiology refers to the activities of microorganisms (usually bacteria) that live beneath the surface of the Earth. The field of study is also referred to as biogeochemistry and subsurface microbiology. Habitats of the organisms include the ocean and deep within the rock that makes up Earth's crust. The study of the identities and activities of such organisms is important from a basic…
Geophysics is the study of Earth's physical character, including the solid planet, the atmosphere, and bodies of water. Geophysical investigations, therefore, often draw upon information and techniques developed in scientific disciplines such as physics, geology, and astronomy. Major areas of modern geophysical research include seismology, volcanology and geothermal studies, tectonics, geom…
Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth. In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface. These geotropisms can be …
Gerbils are rat-like rodents in the mammalian family Muridae, which also includes rats, mice, voles and lemmings. Some authorities place the gerbils in a separate family Gerbilidae, together with the pigmy gerbils. Wild gerbils are rat-sized, long-tailed rodents with rather long hind feet. Nearly all live in self-dug burrows and forage at night feeding mostly on seeds. Gerbils are probably derived…
Hemorrhagic diseases are caused by infection with certain viruses or bacteria. Viruses cause virtually all the hemorrhagic diseases of microbiological origin that arise with any frequency. The various viral diseases are also known as viral hemorrhagic fevers. Bacterial hemorrhagic disease does occur, but rarely. One example of a bacterial hemorrhagic disease is scrub typhus. Copious bleeding is th…
Incandescent light is given off when an object is heated until it glows. To emit white light, an object must be heated to at least 1,341°F (727°C). White-hot iron in a forge is incandescent, as is red lava flowing down a volcano, as are the red burners on an electric stove. The most common example of incandescence is the white-hot filament in the light bulb of an incandescent lamp. T…
Germ cells are one of two fundamental cell types in the human body. Germ cells are responsible for the production of sex cells or gametes (in humans, ovum and spermatozoa). Germ cells also constitute a cell line through which genes are passed from generation to generation. The vast majority of cells in the body are somatic cells. Indeed, the term somatic cell encompasses all of the differentiated …
Hemp, or Cannabis sativa, is a tall, annual plant that thrives in temperate and subtropical climates. It is native to central and western Asia, and is one of the oldest cultivated plants. The word "hemp" is derived from the old English word "hanf," and refers to both the plant and the long fibers that are processed from its stems. The most common use of hemp has been as…
The germ theory is a fundamental tenet of medicine that states that microorganisms, which are too small to be seen without the aid of a microscope, can invade the body and cause certain diseases. Until the acceptance of the germ theory, many people believed that disease was punishment for a person's evil behavior. When entire populations fell ill, the disease was often blamed on swamp vapor…
Henna, species Lawsonia intermis of the family Lythracea, is a perennial shrub that grows wild in northern Africa and southern Asia. The name henna, which comes from the Arabic word al kenna, refers to both the plant and the dye that comes from the leaves. The henna plant has narrow, grayish green leaves and small, sweet smelling clustered flowers that are white, yellow, or rose in color. One of t…
Germination is the process by which a seed begins its development into a mature plant. Germination begins with an increase of metabolic activity within the seed. The first visible sign of germination in angiosperms (flowering plants) is generally an enlargement of the seed, due to intake of water from the environment. The seed's covering may wrinkle and crack at this time. Soon afterward, t…
Gerontology is a branch of sociology that studies aging among populations internationally, and monitors efforts to deal with problems arising in old age. It differs from geriatrics the same way that psychology is separate from psychiatry. A psychologist's inquiries apply to general questions about how the human brain and mind work. A psychiatrist is more concerned with involving patients in…
Members of the Gesneria family, the Gesneriaceae, are herbs, shrubs, sometimes trees or woody vines. The Gesneriaceae is a large family composed of approximately 120 genera and 1,800 species. With the exception of two genera (Haberlea and Ramonda), which are native to temperate Europe, they are found only in the tropical and subtropical regions of the world. Although none are native to the United …
A geyser is an intermittent or semi-regularly periodic spout of geothermally heated groundwater and steam. Any subsurface encounter between water and heat produces a hydrothermal process. The heat is usually supplied by upwellings of magma from the mantle, the water by precipitation that percolates downward through surface rocks. Some oceanic water enters the mantle at subduction zones and becomes…
Incinerators are industrial facilities used for the controlled burning of waste materials. The largest incinerators are used to burn municipal solid wastes, often in concert with a technology that utilizes the heat produced during combustion to generate electricity. Smaller, more specialized incinerators are used to burn medical wastes, general chemical wastes such as organic solvents, and toxic w…
Gibbons are species of tropical forest apes in the family Pongidae. This family contains all of the anthropoid apes, which are the closest living relatives of humans (Homo sapiens), in terms of their anatomy, physiology, and behavior. Like other anthropoid primates, gibbons lack a tail, they have a more-or-less upright posture, and they have a well-developed brain. However, gibbons are generally r…
Indicator species are plants and animals that, by their presence, abundance, lack of abundance, or chemical composition, demonstrate some distinctive aspect of the character or quality of an environment. For example, in places where metal-rich minerals occur at the soil surface, indicator species of plants can be examined to understand the patterns of naturally occurring pollution, and they can ev…
The Gila monster (Heloderma suspectum) is a large, strikingly-colored venomous lizard. The gila monster and the Mexican beaded lizard (Heloderma horridum) are the only members of the beaded lizard family, Helodermatidae. The Gila monster occurs in rocky, semi-arid habitats from the Colorado River basin in the southwestern United States to the western regions of Mexico and Guatemala. The Gila monst…
The ginger family, Zingiberaceae, includes about 50 genera and 1,300 species of plants, a few of which have culinary or medicinal uses. The common ginger (Zingiber officinale) is one of the oldest and most commonly used spices. Ginger for these uses is obtained from the tuberous rhizome, or underground stem of the plant. The common ginger is native to Southeast Asia, where it has been cultivated f…
The ginkgo, or maidenhair tree (Ginkgo biloba) is an unusual species of gymnosperm, having broad leaves, and seasonally deciduous foliage that turns yellow and is dropped in autumn. The ginkgo is a dioecious plant, which means that male and female functions are performed by separate trees. The ginkgo is famous as a socalled "living fossil," because it is the only surviving member of …
Hepatitis is inflammation of the liver, a potentially life-threatening disease most frequently caused by viral infections but which may also result from liver damage caused by toxic substances such as alcohol and certain drugs. Hepatitis viruses identified to date occur in five types: hepatitis A (HAV), hepatitis B (HBV), hepatitis C (HCV), hepatitis D (HDV), and hepatitis E (HEV). All types are p…
An individual, in the sense of evolutionary biology, is a genetically unique organism. An individual has a complement of genetic material, encoded in its DNA (deoxyribonucleic acid), that is different from other members of its species. At the level of populations and species, this variation among individuals constitutes genetic biodiversity. …
Ginseng refers to several species of plants in the genus Panax, family Araliaceae. Ginseng is a perennial, herbaceous plant, with compound leaves that grow from a starchy root. The natural habitat of ginseng is the understory of mature angiosperm forest in the temperate zones of east Asia and eastern North America. The root of ginseng is highly valued as having many therapeutic properties by pract…
An herb is an aromatic plant that is used by people most commonly in cooking, but sometimes for medicinal purposes, as an insect repellant, as a source of dye, and sometimes for their attractive aesthetics. Herbs are not necessarily plants that are taxonomically related to each other—what these plants share is a usefulness to humans, not an evolutionary lineage. In general, herbs are non-wo…
Modern medicine has provided many breakthrough treatments for serious diseases. Some conditions, however, have eluded the healing grasp of contemporary western medicine, which emphasizes rigorous scientific investigation of therapies. In addition, rising costs of some treatments have placed modern healthcare beyond the reach of many. The drugs that routinely fill pharmacy shelves of post-industria…
Giraffes are a species of large, long-legged, long-necked ungulates in the family Giraffidae, order Artiodactyla. Giraffes are the tallest living animals on Earth. Okapis are a close relative, but these animals do not have such long legs or neck. The giraffe is a widespread animal of grasslands and savannas of sub-Saharan Africa. The okapi is a much rarer animal and occurs in tropical forest. Both…
The chemical, physical, and biological characteristics of the atmosphere inside of dwellings and in commercial and institutional buildings are influenced in numerous ways. Sometimes, effects on indoor air quality can be sufficient to cause people to experience significant discomfort, and even to become physically ill. People vary greatly in their sensitivity to air pollution, both inside and outsi…
GIS is the common abbreviation for geographic information systems, a powerful and widely used computer database and software program that allows scientists to link geographically referenced information related to any number of variables to a map of a geographical area. GIS allows its users to analyze and display data using digitized maps. In addition, GIS can generate maps and tables useful to a w…
A herbicide is a chemical used to kill or otherwise manage certain species of plants considered to be pests. Plant pests, or weeds, compete with desired crop plants for light, water, nutrients, and space. This ecological interaction may decrease the productivity and yield of crop plants, thereby resulting in economic damage. Plants may also be judged to be weeds if they interfere with some desired…
An herbivore is an animal that eats plants as its primary source of sustenance. Examples of herbivores include large mammals such as cattle, deer, sheep, and kangaroos, as well as smaller creatures such as leaf-eating insects and crustaceans that graze upon aquatic algae. However, many animals are not exclusively herbivorous. In addition to feeding mostly upon live plants, omnivorous animals such …
Glaciers are flowing masses of ice, created by years of snowfall and cold local temperatures. Approximately one tenth of the Earth is covered by glaciers. Glaciers are most numerous near the poles, covering most of Antarctica and Greenland and parts of Iceland, Canada, Russia, and Alaska; they also exist in mountainous regions on every continent except Australia. From the air, a glacier looks dece…
Glands are aggregates of specialized cells that secrete or excrete chemical substances which are used elsewhere in the body. Glands carry out regulatory, digestive, reproductive, and other functions in the body. A gland may be an independent structure or may be incorporated into another, larger, structure that has still other functions. In addition, a gland can be endocrine, secreting its hormones…
A hermaphrodite is any organism with both male and female reproductive organs which produce both male gametes (sperm) and female gametes (ova). In some animals, the simultaneous hermaphrodites, both male and female organs are functional at the same time. In other animals, the sequential hermaphrodites, one sex develops at one time, which later develops into the other sex. Examples of both strategi…
Early peoples were likely to have discovered natural glass, which is created when lightning strikes sand, and were certain to have used obsidian-a dark volcanic glass-for weapons, ornaments, and money. The first manufactured glass probably took the form either of glass beads or ceramic glaze and appeared around 4000-5000 B.C. Surviving examples of Egyptian and Mesopotamian glass objects date to ar…
Industrial minerals is a term used to describe naturally occurring non-metallic minerals that are used extensively in a variety of industrial operations. Some of the minerals commonly included in this category include asbestos, barite, boron compounds, clays, corundum, feldspar, fluorspar, phosphates, potassium salts, sodium chloride, and sulfur. Some of the mineral mixtures often considered as in…
The long-term distribution of heat and precipitation on Earth's surface is called global climate. Heat from the sun keeps the Earth's average temperature at about 60°F(16°C), within a range that allows for biological life and maintains the planet's life-sustaining reservoirs of liquid water. Astronomical variations and atmospheric shielding cause incoming solar r…
Long before the space age, people used the heavens for navigation. Besides relying on the Sun, Moon, and Stars, the early travelers invented the magnetic compass, the sextant, and the seagoing chronometer. Eventually, radio navigation in which a position could be determined by receiving radio signals broadcast from multiple transmitters, came into existence. Improved high frequency signals gave gr…
A hernia occurs when an organ or tissue bulges out of its proper location. Hernias can occur in numerous locations throughout the body, including the brain, abdomen, groin, spine, and chest. The most common types will be discussed below. Hernias can be either congenital or acquired. Congenital means that the individual was born with an abnormal opening, allowing the hernia to occur. Such a hernia …
Industrial Revolution is the name given by the German socialist author Friedrich Engels in 1844 to changes that took place in Great Britain during the period from roughly 1730 to 1850. In general, those changes involved the transformation of Great Britain from a largely agrarian society to one dominated by industry. In a broader context, the term has also been applied to the transformation of the …
Global warming refers to a long-term increase in the Earth's surface temperature that results in large-scale changes in global climate, namely redistribution of climatic zones defined by temperature, precipitation, and associated adapted ecosystems. Global climate changes, and episodes of global warming, have occurred throughout geologic history as a result of natural variations in incoming…
One important property of glycerol or glycerin is that is not poisonous to humans. Therefore it is used in foods, syrups, ointments, medicines, and cosmetics. Glycerol also has special chemical properties that allow it to be used where oil would fail. Glycerol is a thick syrup that is used as the "body" to many syrups, for example, cough medicines and lotions used to treat ear infect…
Herons, egrets, and bitterns are large, slender wading birds in the family Ardeidae, order Ciconiiformes (which also includes anhingas, storks, spoonbills, and ibises). Most of the species in the heron family have long legs, necks, and bills. These characteristics are all adaptive to hunting their prey of fish, amphibians, snakes, small mammals, and other animals living in the shallow waters of we…
Herpetology is the scientific study of amphibians and reptiles. The term "herpetology" is derived from the Greek and refers to the study of creeping things. Birds and mammals, for the most part, have legs that lift their bodies above the surface of the ground. Amphibians (class Amphibia) and reptiles (class Reptilia), with the exception of crocodiles and lizards, generally have legs …
One of the most important fisheries in the world is provided by the true herrings, which belong to the bony fish family Clupeidae. This family contains a wide variety of fishes with distinctive habits. Although most of the species are marine, a few are anadromous—that is, they spend their lives in the sea and enter rivers to spawn. Other species remain permanently in freshwater. Herrings ar…
In mathematics, an inequality is a statement about the relative order of members of a set. For instance, if S Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. is the set of positive integers, and the symbol < is taken to mean less than, then the statement 5 < 6 (read "5 is less than 6") is a true statement about the relative order of …
A glycol is an aliphatic organic compound in which two hydroxyl (OH) groups are present. The most important glycols are those in which the hydroxyl groups are attached to adjacent carbon atoms, and the term glycol is often interpreted as applying only to such compounds. The latter are also called vicinal diols, or 1,2-diols. Compounds in which two hydroxyl groups are attached to the same carbon at…
Glycolysis, a series of enzymatic steps in which the six-carbon glucose molecule is degraded to yield two three-carbon pyruvate molecules, is a central catabolic pathway in plants, animals and many microorganisms. In a sequence of 10 enzymatic steps, energy released from glucose is conserved by glycolysis in the form of adenosine triphosphate (ATP). So central is glycolysis to life that its sequen…
Inertial guidance is a navigation technology that monitors changes in location by measuring cumulative acceleration. In inertial guidance, the motion of the object in three-dimensional space is measured continuously. This enables a special computer to provide related real-time information about velocity (speed) and location. An inertial-navigation system (INS) does not use information from an exte…
Goats belong to the order Artiodactyla (genus Capra), which is made up of a number of hoofed mammals having an even number of toes. Goats have existed on Earth for at least 35 million years and, during the course of evolution, have undergone an incredibly wide radiation, both in distribution and ecology. Although the taxonomy of this group is still unclear, eight species are generally recognized a…
The term infection refers to the state where a host organism has been invaded by another organism, typically a microorganism such as a virus, bacterium, protozoa, algae, or fungus. The invader is able to elude the responses of the host that are designed to kill it. Strategies include rapid multiplication, which can overwhelm the host defenses, or escaping from the host's immune system by mu…
A Hertzsprung-Russell diagram, or H-R diagram, is a graph of stellar temperatures (plotted on the horizontal axis) and luminosities, or brightnesses (plotted on the vertical axis). H-R diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an H-R diagram for a group of stars, an astronomer can make estimates of many important stellar pro…
The goatsuckers, nightjars, and nighthawks number 70 species of birds in the family Caprimulgidae. These birds have a relatively large head, with a wide beak, and a large mouth with a seemingly enormous gape. The mouth is fringed by long, stiff bristles, and is an adaptation for catching insects in flight. The unusually large mouth of goatsuckers was once believed to be useful for suckling milk at…
A heterotroph is a creature that must ingest biomass to obtain its energy and nutrition. In direct contrast, autotrophs are capable of assimilating diffuse, inorganic energy and materials and using these to synthesize biochemicals. Green plants, for example, use sunlight and simple inorganic molecules to photosynthesize organic matter. All heterotrophs have an absolute dependence on the biological…
Infertility is a couple's inability to conceive a child after attempting to do so for at least one full year. Primary infertility refers to a situation in which pregnancy has never been achieved. Secondary infertility refers to a situation in which one or both members of the couple have previously conceived a child, but are unable to conceive again after a full year of trying. Currently, in…
The term infinity conveys the mathematical concept of large without bound, and is given the symbol ∞. As children, we learn to count, and are pleased when first we count to 10, then 100, and then 1,000. By the time we reach 1,000, we may realize that counting to 2,000, or certainly 100,000, is not worth the effort. This is partly because we have better things to do, and partly because we re…
Inflammation is a localized, defensive response of the body to injury, usually characterized by pain, redness, heat, swelling, and, depending on the extent of trauma, loss of function. The process of inflammation, called the inflammatory response, is a series of events, or stages, that the body performs to attain homeostasis (the body's effort to maintain stability). The body's infla…
In mathematics, an inflection point is a point on a curve at which the curve changes from being concave upward to being concave downward, or vice versa. A concave upward curve can be thought of as one that would hold water, while a concave downward curve is one that would not. An important qualification is that the curve must have a unique tangent line at the point of inflection. This means that t…
Gobies, belonging to the suborder Gobidioidei, are small fish that usually live off the coast in tropical and warm temperate regions. They spend the majority of their time resting on the bottom near protective cracks in coral reefs or burrows in the sand. Most species of this fish have fused pelvic fins which form a suction cup on their undersides. A goby uses this suction cup to cling to rocks so…
Hibernation is a state of inactivity, or torpor, in which an animal's heart rate, body temperature, and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer. Hibernation is an important adaptation to harsh climates, because when food is scarce, an animal…
Influenza ("the flu") is a disease caused by the influenza virus. The disease is easily spread from person to person, typically by inhaling virus that has been expelled into the air by coughing or sneezing. The virus can also be spread by touch. For example, if someone touches a doorknob that has influenza viruses clinging to it and then touches their mouth, the virus can pass into t…
Goldenseal (Hydrastis canadensis L.) is a woodland plant belonging to the family Ranunculaceae. The plant is also known as eyebalm, eyeroot, hydrastis, orangeroot, tumeric root, and yellowroot. Mainly found in the wild, goldenseal grows to a height of about 1 ft (30 cm). It has an erect, hairy stem, and produces small, greenish-white flowers that bloom in early spring, and later turn into clusters…
Gophers are small rodents. Although the name is often used popularly to refer to a variety of animals, including snakes, in the United States gophers are the pocket gophers that live in the grasslands of western Canada, eastward to the Great Lakes, and down into northern South America. Pocket gophers (family Geomyidae) have fur-lined cheek pouches that let them carry food in large quantities. Thes…
Gorillas inhabit forests of Central Africa, and are the largest and most powerful of all primates. Adult males stand 6 ft (1.8 m) upright (although this is an unnatural position for a gorilla) and weigh up to 450 lb (200 kg), while females are much smaller. Gorillas live up to about 44 years. Mature males (older than 13 years), or silverbacks, are marked by a band of silver-gray hair on their back…
Throughout most of history astronomers were confined to using optical light, the light we can detect with our eyes. The advent of electronic detectors has, in the past few decades, opened up new vistas to astronomers, allowing them to utilize the entire electromagnetic spectrum. Infrared astronomers use traditional optical telescopes equipped with special detectors that can detect infrared light. …
Heredity plays a part in almost all diseases. Recent advances in gene research have allowed a steadily increasing number of specific genes and genetic factors to be linked to a wide variety of medical complaints. There are currently approximately 6,000 known genetic diseases. Those that result from simple mutations of single genes are often referred to as hereditary diseases, and they exhibit dist…
Gourds and their relatives are various species of plants in the family Cucurbitaceae. There are about 750 species in this family divided among 90 genera. Some members of the gourd family include the cucumber, squash, melon, and pumpkin. Most species of gourds are tropical or subtropical, but a few occur in temperate climates. A few species in the gourd family produce large, edible fruits, and some…
The common or river hippopotamus (Hippopotamus amphibius) is a huge, even-toed hoofed herbivore that lives in bodies of freshwater in central and southern Africa. A second species, the pygmy hippopotamus (Choeropsis liberiensis), lives in water bodies in Western African rainforests. Both species are included in the family Hippopotamidae. The name hippopotamus means "river horse" but …
Histamines are chemicals released by cells of the immune system during the inflammatory response, which is one of the body's defenses against infection. For instance, the inflammatory response helps neutralize bacteria that enter the body when the skin is accidentally cut with a knife. In addition, the sneezing, runny nose, and itchy eyes of allergies are actually "small-scale"…
Strictly speaking, insectivores are any predators that catch and eat insects. Often, however, insectivorous predators also eat other small invertebrates, such as spiders, millipedes, centipedes, and earthworms. Some insectivores specialize in catching and feeding upon flying insects, sometimes called aeroplankton. Some prominent examples of this insectivorous feeding strategy include dragonflies, …
Historical geology is the study of changes in Earth and its life forms over time. It includes sub-disciplines such as paleontology, paleoclimatology, and paleoseismology. In addition to providing a scientific basis for understanding the evolution of Earth over time, historical geology provides important information about ancient climate changes, volcanic eruptions, and earthquakes that can be used…
Insects are invertebrates in the class Insecta, which contains 28 living orders. This class of the phylum Arthropoda is distinguished by a number of anatomical features, including an adult body that is typically divided into three parts (head, thorax, and abdomen), three pairs of segmented legs attached to the thorax, one pair of antennae, and ventilation of respiratory gases through pores called …
The hoatzin (Opisthocomus hoazin) is one of the world's most peculiar bird species. It is the sole member of its family, Opisthocomidae. It is peculiar enough to have defied taxonomists' best efforts for years. This bird lives only in the rainforests of northern South America. Its feathers are dark brown on the back and lighter below, and chestnut-colored on its sides. The skin aroun…
A graft is a horticultural term for a bud or shoot of one variety or species of plant that is positioned on the stem of another, compatible plant, in such a way that integrated growth results. The recipient plant is called the stock or rootstock, and the grafted part is referred to as the scion. A simple method for stem grafting involves both stems being cut with a sharp blade at the same acute an…
One of the major theoretical hurdles to a reachable synthesis of current theories of particles and force interactions into a grand unification theory (also known as Grand Unified Field Theory, Grand Unified Theory, or GUT) is the need to reconcile the evolving principles of quantum theory with the principles of general relativity advanced by German-American physicist Albert Einstein (1879-1955) ne…
The term insomnia applies to a variety of conditions involving lack of sleep, difficulty falling asleep, and disrupted or insufficiently restful sleeping patterns. Not only the quantity but the quality of sleep is at issue for those who research insomnia and its effects on behavior and productivity during waking hours. Primary insomnias include chronic and temporary sleeplessness. Secondary insomn…
Hodgkin's disease is a type of cancer involving tissues of the lymphatic system. The lymphatic system is a network of organs, tissues, and ducts in the human body. The lymphatic system maintains the fluid balance in the body by coordinating the draining of fluid from cells and tissues back into the bloodstream. Also, the lymphatic system aids in fighting infections caused by microorganisms,…
Grapes are various species of woody vines in the genus Vitis, family Vitaceae. This family contains about 700 species most of which occur in tropical and subtropical climates, although some occur in temperate habitats. The genus Vitis has about 50 species. Grapes are ecologically important as food for wildlife. They are also cultivated by humans in large quantities, mostly for the production of ta…
An instinct is a stereotyped, species-typical behavior that appears fully functional the first time it is performed, without the need for learning. Such behaviors are usually triggered by a particular stimulus or cue, and are not readily modified by subsequent experience. For instance, a kangaroo rat instantly performs an automatic escape jump maneuver when it hears the sound of a striking rattles…
Members of the holly family (Aquifoliaceae) are shrubs and trees with small, white or pale green, unisexual flowers. The family consists of four genera with 419 species, of which 400 species are members of the holly genus, Ilex. The family Aquifoliaceae is a member of the class Magnoliopsida (dicotyledons), division Magnoliophyta (the angiosperms, or flowering plants). …
Insulin is a hormone secreted by the pancreas gland, one of the glands in the endocrine system. Insulin, working in harmony with other hormones, regulates the level of blood sugar (glucose). An insufficient level of insulin secretion leads to high blood sugar, a disease called diabetes mellitus. Endocrine glands are ductless glands; that is, they pour their products (hormones) directly into the bl…
In mathematics, a graph is a geometric representation, a picture, of a relation or function. A relation is a subset of the set of all ordered pairs (x,y) for which each x is a member of some set X and each y is a member of another set Y. A specific relationship between each x and y determines which ordered pairs are in the subset. A function is a similar set of ordered pairs, with the added restri…
Holography is defined as a method of producing a three-dimensional (3-D) impression of an object. The recording and the image it brings to life are each referred to as holograms. This impression is taken by splitting a beam of coherent (that is, uniform over distance as well as over time) radiation along two paths. One is known and stays undisturbed, to act as a reference. Another strikes the obje…
