Birds known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, a strong yet lightweight skeleton. Birds range in size from the 5 cm bee hummingbird to the 2.75 m ostrich. They rank as the world's most numerically-successful class of tetrapods, with ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are less developed depending on the species. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites and diverse endemic island species of birds; the digestive and respiratory systems of birds are uniquely adapted for flight. Some bird species of aquatic environments seabirds and some waterbirds, have further evolved for swimming; the fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs.
The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of powered flight, many retained primitive characteristics like toothy jaws in place of beaks, long bony tails. DNA-based evidence finds that birds diversified around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages, but birds those in the southern continents, survived this event and migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics; some birds corvids and parrots, are among the most intelligent animals. Many species annually migrate great distances. Birds are social, communicating with visual signals and bird songs, participating in such social behaviours as cooperative breeding and hunting and mobbing of predators.
The vast majority of bird species are monogamous for one breeding season at a time, sometimes for years, but for life. Other species have breeding systems that are polygynous or polyandrous. Birds produce offspring by laying eggs, they are laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching; some birds, such as hens, lay eggs when not fertilised, though unfertilised eggs do not produce offspring. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs and feathers. Songbirds and other species are popular as pets. Guano is harvested for use as a fertiliser. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them.
Recreational birdwatching is an important part of the ecotourism industry. The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds; this was done by excluding most groups known only from fossils, assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Gauthier identified four different definitions for the same biological name "Aves", a problem. Gauthier proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below, he assigned other names to the other groups. Aves can mean all archosaurs closer to birds than to crocodiles Aves can mean those advanced archosaurs with feathers Aves can mean those feathered dinosaurs that fly Aves can mean the last common ancestor of all the living birds and all of its descendants (a "c
Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints, is a structural component of the rib cage, the ear, the nose, the bronchial tubes, the intervertebral discs, many other body components. It is not as hard and rigid as bone; the matrix of cartilage is made up of glycosaminoglycans, collagen fibers and, elastin. Because of its rigidity, cartilage serves the purpose of holding tubes open in the body. Examples include the rings such as the cricoid cartilage and carina. Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance, rich in proteoglycan and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in relative amounts of collagen and proteoglycan. Cartilage does not contain blood nerves. Nutrition is supplied to the chondrocytes by diffusion.
The compression of the articular cartilage or flexion of the elastic cartilage generates fluid flow, which assists diffusion of nutrients to the chondrocytes. Compared to other connective tissues, cartilage has a slow turnover of its extracellular matrix and does not repair. In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix. Following the initial chondrification that occurs during embryogenesis, cartilage growth consists of the maturing of immature cartilage to a more mature state; the division of cells within cartilage occurs slowly, thus growth in cartilage is not based on an increase in size or mass of the cartilage itself. The articular cartilage function is dependent on the molecular composition of the extracellular matrix; the ECM consists of proteoglycan and collagens.
The main proteoglycan in cartilage is aggrecan, which, as its name suggests, forms large aggregates with hyaluronan. These aggregates hold water in the tissue; the collagen collagen type II, constrains the proteoglycans. The ECM responds to compressive forces that are experienced by the cartilage. Cartilage growth thus refers to the matrix deposition, but can refer to both the growth and remodeling of the extracellular matrix. Due to the great stress on the patellofemoral joint during resisted knee extension, the articular cartilage of the patella is among the thickest in the human body; the mechanical properties of articular cartilage in load-bearing joints such as the knee and hip have been studied extensively at macro and nano-scales. These mechanical properties include the response of cartilage in frictional, compressive and tensile loading. Cartilage displays viscoelastic properties. Lubricin, a glycoprotein abundant in cartilage and synovial fluid, plays a major role in bio-lubrication and wear protection of cartilage.
Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore, cartilage damage is difficult to heal; because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is replaced by fibrocartilage scar tissue. Over the last years and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement. Bioengineering techniques are being developed to generate new cartilage, using a cellular "scaffolding" material and cultured cells to grow artificial cartilage. Several diseases can affect cartilage. Chondrodystrophies are a group of diseases, characterized by the disturbance of growth and subsequent ossification of cartilage; some common diseases that affect the cartilage are listed below. Osteoarthritis: Osteoarthritis is a disease of the whole joint, however one of the most affected tissues is the articular cartilage.
The cartilage covering bones is thinned completely wearing away, resulting in a "bone against bone" within the joint, leading to reduced motion, pain. Osteoarthritis affects the joints exposed to high stress and is therefore considered the result of "wear and tear" rather than a true disease, it is treated by arthroplasty, the replacement of the joint by a synthetic joint made of a stainless steel alloy and ultra high molecular weight polyethylene. Chondroitin sulfate or glucosamine sulfate supplements, have been claimed to reduce the symptoms of osteoarthritis but there is little good evidence to support this claim. Traumatic rupture or detachment: The cartilage in the knee is damaged but can be repaired through knee cartilage replacement therapy; when athletes talk of damaged "cartilage" in their knee, they are referring to a damaged meniscus and not the articular cartilage. Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
Costochondritis: Inflammation of cartilage in the ribs, causing chest pain. Spinal disc herniation: Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content; the hernia compresses the adjacent nerves and causes back pain. Relapsing polychondritis: a destruction aut
Aesthetics is a branch of philosophy that deals with the nature of art and taste and with the creation or appreciation of beauty. In its more technical epistemological perspective, it is defined as the study of subjective and sensori-emotional values, or sometimes called judgments of sentiment and taste. Aesthetics studies how artists imagine and perform works of art, it studies how they feel about art—why they like some works and not others, how art can affect their moods and attitude toward life. The phrase was coined in English in the 18th century. More broadly, scholars in the field define aesthetics as "critical reflection on art and nature". In modern English, the term aesthetic can refer to a set of principles underlying the works of a particular art movement or theory: one speaks, for example, of the Cubist aesthetic; the word aesthetic is derived from the Greek αἰσθητικός, which in turn was derived from αἰσθάνομαι (aisthanomai, meaning "I perceive, sense" and related to αἴσθησις. Aesthetics in this central sense has been said to start with the series of articles on “The Pleasures of the Imagination” which the journalist Joseph Addison wrote in the early issues of the magazine The Spectator in 1712.
The term "aesthetics" was appropriated and coined with new meaning by the German philosopher Alexander Baumgarten in his dissertation Meditationes philosophicae de nonnullis ad poema pertinentibus in 1735. Aesthetics, a not tidy intellectual discipline, is a heterogeneous collection of problems that concern the arts but relate to nature. Even though his definition in the fragment Aesthetica is more referred to as the first definition of modern aesthetics. Aesthetics is for the artist; some separate aesthetics and philosophy of art, claiming that the former is the study of beauty while the latter is the study of works of art. However, most Aesthetics encompasses both questions around beauty as well as questions about art, it examines topics such as aesthetic objects, aesthetic experience, aesthetic judgments. For some, aesthetics is considered a synonym for the philosophy of art since Hegel, while others insist that there is a significant distinction between these related fields. In practice, aesthetic judgement refers to the sensory contemplation or appreciation of an object, while artistic judgement refers to the recognition, appreciation or criticism of art or an art work.
Philosophical aesthetics has not only to speak about art and to produce judgments about art works, but has to give a definition of what art is. Art is an autonomous entity for philosophy, because art deals with the senses and art is as such free of any moral or political purpose. Hence, there are two different conceptions of art in aesthetics: art as knowledge or art as action, but aesthetics is neither epistemology nor ethics. Aestheticians compare historical developments with theoretical approaches to the arts of many periods, they study the varieties of art in relation to their physical and culture environments. Aestheticians use psychology to understand how people see, imagine, think and act in relation to the materials and problems of art. Aesthetic psychology studies the creative process and the aesthetic experience. Aesthetics examines our affective domain response to an object or phenomenon Judgments of aesthetic value rely on our ability to discriminate at a sensory level. However, aesthetic judgments go beyond sensory discrimination.
For David Hume, delicacy of taste is not "the ability to detect all the ingredients in a composition", but our sensitivity "to pains as well as pleasures, which escape the rest of mankind." Thus, the sensory discrimination is linked to capacity for pleasure. For Immanuel Kant, "enjoyment" is the result when pleasure arises from sensation, but judging something to be "beautiful" has a third requirement: sensation must give rise to pleasure by engaging our capacities of reflective contemplation. Judgments of beauty are sensory and intellectual all at once. Kant observed of a man "If he says that canary wine is agreeable he is quite content if someone else corrects his terms and reminds him to say instead: It is agreeable to me," because "Everyone has his own taste"; the case of "beauty" is different from mere "agreeableness" because, "If he proclaims something to be beautiful he requires the same liking from others. Roger Scruton has argued similarly. Viewer interpretations of beauty may on occasion be observed to possess two concepts of value: aesthetics and taste.
Aesthetics is the philosophical notion of beauty. Taste is a result of an education process and awareness of elite cultural values learned through exposure to mass culture. Bourdieu examined how the elite in society define the aesthetic values like taste and how varying levels of exposure to these values can result in variations by class, cultural background, education. According to Kant, beauty is universal. In the opinion of Władysław Tatarkiewicz, there are
Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, snakes, lizards and their extinct relatives. The study of these traditional reptile orders combined with that of modern amphibians, is called herpetology; because some reptiles are more related to birds than they are to other reptiles, the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade. For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living Diapsids; the earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods that became adapted to life on dry land. Some early examples include Casineria. In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the Cretaceous–Paleogene extinction event wiped out the pterosaurs, plesiosaurs and sauropods, as well as many species of theropods, including troodontids, dromaeosaurids and abelisaurids, along with many Crocodyliformes, squamates.
Modern non-avian reptiles inhabit all the continents except Antarctica, although some birds are found on the periphery of Antarctica. Several living subgroups are recognized: Testudines, 350 species. Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades – the fetus develops within the mother, contained in a placenta rather than an eggshell; as amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm to the saltwater crocodile, Crocodylus porosus, which can reach 6 m in length and weigh over 1,000 kg.
In the 13th century the category of reptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, assorted amphibians, worms", as recorded by Vincent of Beauvais in his Mirror of Nature. In the 18th century, the reptiles were, from the outset of classification, grouped with the amphibians. Linnaeus, working from species-poor Sweden, where the common adder and grass snake are found hunting in water, included all reptiles and amphibians in class "III – Amphibia" in his Systema Naturæ; the terms "reptile" and "amphibian" were interchangeable, "reptile" being preferred by the French. Josephus Nicolaus Laurenti was the first to formally use the term "Reptilia" for an expanded selection of reptiles and amphibians similar to that of Linnaeus. Today, the two groups are still treated under the same heading as herptiles, it was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, Pierre André Latreille erected the class Batracia for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians and mammals.
The British anatomist Thomas Henry Huxley made Latreille's definition popular and, together with Richard Owen, expanded Reptilia to include the various fossil "antediluvian monsters", including dinosaurs and the mammal-like Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into mammals and ichthyoids, he subsequently proposed the names of Ichthyopsida for the latter two groups. In 1866, Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, that reptiles and mammals were united by the amniotic egg; the terms "Sauropsida" and "Theropsida" were used again in 1916 by E. S. Goodrich to distinguish between lizards and their relatives on the one hand and mammals and their extinct relatives on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, other features, such as the structure of the forebrain.
According to Goodrich, both lineages evolved from an earlier stem group, Protosauria in which he included some animals today considered reptile-like amphibians, as well as early reptiles. In 1956, D. M. S. Watson observed that the first two groups diverged early in reptilian history, so he divided Goodrich's Protosauria between them, he reinterpreted Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida included Procolophonia, Millerosauria, Squamata, Rhynchocephalia
The skull roof, or the roofing bones of the skull, are a set of bones covering the brain and nostrils in bony fishes and all land-living vertebrates. The bones are part of the dermatocranium. In comparative anatomy the term is used on the full dermatocranium. In general anatomy, the roofing bones may refer to the bones that form above and alongside the brain and neurocranium, in human anatomy, the skull roof refers to the skullcap. Early armoured fish did not have a skull in the common understanding of the word, but had an endocranium, open above, topped by dermal bones forming armour; the dermal bones evolved into a fixed unit overlaying the endocranium like a heavy "lid", protecting the animal's head and brain from above. Cartilaginous fish whose skeleton is formed from cartilage lack a continuous dermal armour and thus have no proper skull roof. A more or less full shield of fused dermal bones was common in early bony fishes of the Devonian, well developed in shallow water species. In early sarcopterygians the skull roof was composed of numerous bony plates around the nostrils and behind each eye.
The skull proper was joined by the bones of the operculum. The skull itself was composed rather loosely, with a joint between the bones covering the brain and the snout; this joint disappeared in the evolving labyrinthodonts, at the same time the number of bones were reduced and the operculum disappeared. In frogs and salamanders the skull roof has large openings. Only in caecilians can a full covering skull roof be found, an adaption for burrowing; the skull roof in lungfish is composed of a number of bony plates that are not compared to those found in early amphibians. In most ray-finned fishes the skull is reduced to a series of loose elements, a skull roof as such is not found; the pattern of plates of the labyrinthodonts formed that basis for that seen in all land-living vertebrates. The roof itself formed a continuous cover over the whole of the head, leaving only openings for nostrils, eyes and a parietal eye between the parietal bones; this type of skull was inherited by the first reptiles as they evolved from labyrinthodont stock in the Carboniferous.
This type of skull roof without any above openings behind the eyes is called anapsid. Today anapsid skulls are only found in turtles, though this may be a case of secondary loss of the post orbital openings. In two groups of early reptiles the skull roof evolved post orbital openings to allow for greater movement of the jaw muscles; the two groups evolved the openings independently: The Synapsids having one opening on each side low on the side of the skull, between the zygomatic bone and the elements above. The Diapsids having two openings on each side, the two openings separated by an arch formed from processes of the postorbital and squamosal bones; the synapsids are the mammals. In mammals, the side opening is closed by the sphenoid bone, so that the skull roof appear whole, despite the temporal opening. All other reptiles and the birds are diapsids
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.