Anatomical terms of location
Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. All vertebrates have the same basic body plan – they are bilaterally symmetrical in early embryonic stages and bilaterally symmetrical in adulthood; that is, they have mirror-image left and right halves if divided down the middle. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other organisms as well. While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic, differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories. Standardized anatomical and zoological terms of location have been developed based on Latin and Greek words, to enable all biological and medical scientists to delineate and communicate information about animal bodies and their component organs though the meaning of some of the terms is context-sensitive.
The vertebrates and Craniata share a substantial heritage and common structure, so many of the same terms are used for location. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way. For humans, one type of vertebrate, anatomical terms may differ from other forms of vertebrates. For one reason, this is because humans have a different neuraxis and, unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position, thus what is on "top" of a human is the head, whereas the "top" of a dog may be its back, the "top" of a flounder could refer to either its left or its right side. For invertebrates, standard application of locational terminology becomes difficult or debatable at best when the differences in morphology are so radical that common concepts are not homologous and do not refer to common concepts. For example, many species are not bilaterally symmetrical. In these species, terminology depends on their type of symmetry.
Because animals can change orientation with respect to their environment, because appendages like limbs and tentacles can change position with respect to the main body, positional descriptive terms need to refer to the animal as in its standard anatomical position. All descriptions are with respect to the organism in its standard anatomical position when the organism in question has appendages in another position; this helps avoid confusion in terminology. In humans, this refers to the body in a standing position with arms at the side and palms facing forward. While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to be worth changing. Many anatomical terms can be combined, either to indicate a position in two axes or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position, both anterior and lateral to the body axis. In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays pass from their source to patient's anterior body wall through the body to exit through posterior body wall.
There is no definite limit to the contexts in which terms may be modified to qualify each other in such combinations. The modifier term is truncated and an "o" or an "i" is added in prefixing it to the qualified term. For example, a view of an animal from an aspect at once dorsal and lateral might be called a "dorsolateral" view. Again, in describing the morphology of an organ or habitus of an animal such as many of the Platyhelminthes, one might speak of it as "dorsiventrally" flattened as opposed to bilaterally flattened animals such as ocean sunfish. Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral"; such terms sometimes used to be hyphenated. There is however little basis for any strict rule to interfere with choice of convenience in such usage. Three basic reference planes are used to describe location; the sagittal plane is a plane parallel to the sagittal suture. All other sagittal planes are parallel to it, it is known as a "longitudinal plane".
The plane is perpendicular to the ground. The median plane or midsagittal plane is in the midline of the body, divides the body into left and right portions; this passes through the head, spinal cord, and, in many animals, the tail. The term "median plane" can refer to the midsagittal plane of other structures, such as a digit; the frontal plane or coronal plane divides the body into ventral portions. For post-embryonic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical. A longitudinal plane is any plane perpendicular to the transverse plane; the coronal plane and the sagittal plane are examples of longitudinal planes. A transverse plane known as a cross-section, divides the body into cranial and caudal portions. In human anatomy: A transverse plane is an X-Z plane, parallel to the ground, which s
Anatomy is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of living things, it is an old science. Anatomy is inherently tied to developmental biology, comparative anatomy, evolutionary biology, phylogeny, as these are the processes by which anatomy is generated over immediate and long timescales. Anatomy and physiology, which study the structure and function of organisms and their parts, make a natural pair of related disciplines, they are studied together. Human anatomy is one of the essential basic sciences; the discipline of anatomy is divided into microscopic anatomy. Macroscopic anatomy, or gross anatomy, is the examination of an animal's body parts using unaided eyesight. Gross anatomy includes the branch of superficial anatomy. Microscopic anatomy involves the use of optical instruments in the study of the tissues of various structures, known as histology, in the study of cells.
The history of anatomy is characterized by a progressive understanding of the functions of the organs and structures of the human body. Methods have improved advancing from the examination of animals by dissection of carcasses and cadavers to 20th century medical imaging techniques including X-ray and magnetic resonance imaging. Derived from the Greek ἀνατομή anatomē "dissection", anatomy is the scientific study of the structure of organisms including their systems and tissues, it includes the appearance and position of the various parts, the materials from which they are composed, their locations and their relationships with other parts. Anatomy is quite distinct from physiology and biochemistry, which deal with the functions of those parts and the chemical processes involved. For example, an anatomist is concerned with the shape, position, blood supply and innervation of an organ such as the liver; the discipline of anatomy can be subdivided into a number of branches including gross or macroscopic anatomy and microscopic anatomy.
Gross anatomy is the study of structures large enough to be seen with the naked eye, includes superficial anatomy or surface anatomy, the study by sight of the external body features. Microscopic anatomy is the study of structures on a microscopic scale, along with histology, embryology. Anatomy can be studied using both invasive and non-invasive methods with the goal of obtaining information about the structure and organization of organs and systems. Methods used include dissection, in which a body is opened and its organs studied, endoscopy, in which a video camera-equipped instrument is inserted through a small incision in the body wall and used to explore the internal organs and other structures. Angiography using X-rays or magnetic resonance angiography are methods to visualize blood vessels; the term "anatomy" is taken to refer to human anatomy. However the same structures and tissues are found throughout the rest of the animal kingdom and the term includes the anatomy of other animals.
The term zootomy is sometimes used to refer to non-human animals. The structure and tissues of plants are of a dissimilar nature and they are studied in plant anatomy; the kingdom Animalia contains multicellular organisms that are motile. Most animals have bodies differentiated into separate tissues and these animals are known as eumetazoans, they have an internal digestive chamber, with two openings. Metazoans do not include the sponges. Unlike plant cells, animal cells have neither chloroplasts. Vacuoles, when present, are much smaller than those in the plant cell; the body tissues are composed of numerous types of cell, including those found in muscles and skin. Each has a cell membrane formed of phospholipids, cytoplasm and a nucleus. All of the different cells of an animal are derived from the embryonic germ layers; those simpler invertebrates which are formed from two germ layers of ectoderm and endoderm are called diploblastic and the more developed animals whose structures and organs are formed from three germ layers are called triploblastic.
All of a triploblastic animal's tissues and organs are derived from the three germ layers of the embryo, the ectoderm and endoderm. Animal tissues can be grouped into four basic types: connective, epithelial and nervous tissue. Connective tissues are fibrous and made up of cells scattered among inorganic material called the extracellular matrix. Connective tissue holds them in place; the main types are loose connective tissue, adipose tissue, fibrous connective tissue and bone. The extracellular matrix contains proteins, the chief and most abundant of, collagen. Collagen plays a major part in maintaining tissues; the matrix can be modified to form a skeleton to protect the body. An exoskeleton is a thickened, rigid cuticle, stiffened by mineralization, as in crustaceans or by the cross-linkin
Feathers are epidermal growths that form the distinctive outer covering, or plumage, on birds, other extinct species of dinosaurs, pterosaurs. They are considered the most complex integumentary structures found in vertebrates and a premier example of a complex evolutionary novelty, they are among the characteristics. Although feathers cover most of the bird's bodies, they arise only from certain well-defined tracts on the skin, they aid in flight, thermal insulation, waterproofing. In addition, coloration helps in protection. Plumology is the name for the science, associated with the study of feathers. Feathers are among the most complex integumentary appendages found in vertebrates and are formed in tiny follicles in the epidermis, or outer skin layer, that produce keratin proteins; the β-keratins in feathers and claws — and the claws and shells of reptiles — are composed of protein strands hydrogen-bonded into β-pleated sheets, which are further twisted and crosslinked by disulfide bridges into structures tougher than the α-keratins of mammalian hair and hoof.
The exact signals that induce the growth of feathers on the skin are not known, but it has been found that the transcription factor cDermo-1 induces the growth of feathers on skin and scales on the leg. There are two basic types of feather: vaned feathers which cover the exterior of the body, down feathers which are underneath the vaned feathers; the pennaceous feathers are vaned feathers. Called contour feathers, pennaceous feathers arise from tracts and cover the entire body. A third rarer type of feather, the filoplume, is hairlike and are associated with contour feathers and are entirely hidden by them, with one or two filoplumes attached and sprouting from near the same point of the skin as each contour feather, at least on a bird's head and trunk. In some passerines, filoplumes arise exposed beyond the contour feathers on the neck; the remiges, or flight feathers of the wing, rectrices, the flight feathers of the tail are the most important feathers for flight. A typical vaned feather features a main shaft, called the rachis.
Fused to the rachis are a series of branches, or barbs. These barbules have minute hooks called barbicels for cross-attachment. Down feathers are fluffy because they lack barbicels, so the barbules float free of each other, allowing the down to trap air and provide excellent thermal insulation. At the base of the feather, the rachis expands to form the hollow tubular calamus which inserts into a follicle in the skin; the basal part of the calamus is without vanes. This part is embedded within the skin follicle and has an opening at the base and a small opening on the side. Hatchling birds of some species have a special kind of natal down feathers which are pushed out when the normal feathers emerge. Flight feathers are stiffened so as to work against the air in the downstroke but yield in other directions, it has been observed that the orientation pattern of β-keratin fibers in the feathers of flying birds differs from that in flightless birds: the fibers are better aligned along the shaft axis direction towards the tip, the lateral walls of rachis region show structure of crossed fibers.
Feathers insulate birds from water and cold temperatures. They may be plucked to line the nest and provide insulation to the eggs and young; the individual feathers in the wings and tail play important roles in controlling flight. Some species have a crest of feathers on their heads. Although feathers are light, a bird's plumage weighs two or three times more than its skeleton, since many bones are hollow and contain air sacs. Color patterns serve as camouflage against predators for birds in their habitats, serve as camouflage for predators looking for a meal; as with fish, the top and bottom colors may be different, in order to provide camouflage during flight. Striking differences in feather patterns and colors are part of the sexual dimorphism of many bird species and are important in selection of mating pairs. In some cases there are differences in the UV reflectivity of feathers across sexes though no differences in color are noted in the visible range; the wing feathers of male club-winged manakins Machaeropterus deliciosus have special structures that are used to produce sounds by stridulation.
Some birds have a supply of powder down feathers which grow continuously, with small particles breaking off from the ends of the barbules. These particles produce a powder that sifts through the feathers on the bird's body and acts as a waterproofing agent and a feather conditioner. Powder down has evolved independently in several taxa and can be found in down as well as in pennaceous feathers, they may be scattered in plumage as in the pigeons and parrots or in localized patches on the breast, belly, or flanks, as in herons and frogmouths. Herons use their bill to break the powder down feathers and to spread them, while cockatoos may use their head as a powder puff to apply the powder. Waterproofing can be lost by exposure to emulsifying agents due to human pollution. Feathers can become waterlogged, causing the bird to sink, it is very difficult to clean and rescue birds whose feathers have been fouled by oil spills. The feathers of cormorants soak up water and help to reduce buoyancy, thereby allowing the birds to swim submerged.
Bristles are stiff. Rictal bristles are found around bill, they may serve a similar purpose to e
The uropygial gland, informally known as the preen gland or the oil gland, is a bilobed sebaceous gland possessed by the majority of birds. It is located dorsally at the base of the tail and is variable in both shape and size. In some species, the opening of the gland has a small tuft of feathers to provide a wick for the preen oil, it is a holocrine gland enclosed in a connective tissue capsule made up of glandular acini that deposit their oil secretion into a common collector tube ending in a variable number of pores, most two. Each lobe has a central cavity that collects the secretion from tubules arranged radially around the cavity; the gland secretion is conveyed to the surface via ducts that, in most species, open at the top of a papilla. From uropygium: Mediaeval Latin, from Ancient Greek οὐροπύγιον, from οὐρά "tail" and πυγή "rump"; the gland is invariably present at embryonic stages, whereas it can be vestigial in adults of certain orders, families and species. Some or all species in at least nine families of birds lack a uropygial gland the ones unable to fly or the ones that produce powder down for feather maintenance.
These include kiwis, ostriches, cassowaries, bustards and doves, amazon parrots and woodpeckers. These birds find other means to stay clean and dry, such as taking a dust bath. Researchers have been unable to correlate the presence or absence of the uropygial gland with factors such as distribution, ecology, or flightlessness; the uropygial gland secretes an oil through the dorsal surface of the skin via a grease nipple-like nub or papilla. The oil contains a complex and variable mixture of substances formed of aliphatic monoester waxes, formed of fatty acids and monohydroxy wax-alcohols. However, some types of diester waxes called uropygiols and containing hydroxyfatty acids and/or alkane-diols exist in the secretions of the uropygial gland of some groups of birds. Preen gland secretion of some birds have shown to be antimicrobial, while others are not antimicrobial; some birds harbor bacteria in their preen gland, which to date, have been isolated from preen glands. Some of those bacteria add to the antimicrobial properties of preen wax.
A bird will transfer preen oil to its body during preening by rubbing its beak and head against the gland opening and rubbing the accumulated oil on the feathers of the body and wings, on the skin of the feet and legs. Tailward areas are preened utilizing the beak, although some species, such as budgerigars, use the feet to apply the oil to feathers around the vent. Emperor Frederick II, in his thirteenth-century treatise on falconry, was the first to discuss the function of the uropygial gland of birds, he believed that its product not only oiled the plumage but provided a poison, introduced by the claws of hawks and owls thus bringing quicker death to their prey. However, studies in 1678 on the question of the toxic nature of the uropygial gland secretion found no evidence to support Frederick's contention. Several researchers have reported differences in the relative gland weights attributing them to factors like seasonal changes, body weight, inter-individual variations, sex. Significant differences are found in the relative gland size between males and females in most species, however, no coherent explanation has as yet been found for these results.
Many ornithologists believe the function of the uropygial gland differs among various species of birds. The preen oil is believed to help maintain the integrity of the feather structure. In waterbirds, the oil maintains the flexibility of feathers and keeps feather barbules from breaking; the interlocking barbules, when in good condition. In some species, preen oil is believed to maintain the integrity of the horny beak and the scaly skin of the legs and feet, it has been speculated that in some species, the oil contains a precursor of vitamin D. Some researchers have postulated that the change in preen oil viscosity may be related to the formation of the more brilliant plumage required for courtship, although research did not find support for this idea; the results of other studies suggest that the gland in females may be involved in the production and secretion of lipids with female pheromone activity. The uropygial gland is developed in many waterbirds, such as ducks, pelicans and in the osprey and the oilbird.
A study examining the gland's mass relative to body weight in 126 bird species showed the absence of a clear-cut correlation between the gland's mass and the degree of birds' contact with water. Anecdotal reports indicating that the waterproofing effect of the hydrophobic uropygiols might be increased by electrostatic charge to the oiled feather through the mechanical action of preening are not supported by scientific studies; the taxonomic richness of avian louse burdens covaries positively with uropygial gland size across avian taxa, suggesting coevolution between gland size and parasite biodiversity. The hoopoe uropygial gland harbours symbiotic bacteria whose excretions reduce the activity of feather-degrading bacteria and thus help to preserve the plumage. In vitro studies suggest that the preen oils of ro
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population, it is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules. The scientific theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species. Evolution by natural selection was first demonstrated by the observation that more offspring are produced than can survive.
This is followed by three observable facts about living organisms: 1) traits vary among individuals with respect to their morphology and behaviour, 2) different traits confer different rates of survival and reproduction and 3) traits can be passed from generation to generation. Thus, in successive generations members of a population are more to be replaced by the progenies of parents with favourable characteristics that have enabled them to survive and reproduce in their respective environments. In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution with classical genetics, which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation. All life on Earth shares a last universal common ancestor that lived 3.5–3.8 billion years ago. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilised multicellular organisms.
Existing patterns of biodiversity have been shaped by repeated formations of new species, changes within species and loss of species throughout the evolutionary history of life on Earth. Morphological and biochemical traits are more similar among species that share a more recent common ancestor, can be used to reconstruct phylogenetic trees. Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from the field or laboratory and on data generated by the methods of mathematical and theoretical biology, their discoveries have influenced not just the development of biology but numerous other scientific and industrial fields, including agriculture and computer science. The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles; such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura.
In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. This was part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and gave examples of how new types of living things could come to be. In the 17th century, the new method of modern science rejected the Aristotelian approach, it sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types.
John Ray applied one of the more general terms for fixed natural types, "species," to plant and animal types, but he identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. The biological classification introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan. Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism; the first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809, which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.
These ideas were cond
Vision is the most important sense for birds, since good eyesight is essential for safe flight, this group has a number of adaptations which give visual acuity superior to that of other vertebrate groups. The avian eye resembles that of a reptile, with ciliary muscles that can change the shape of the lens and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, movement is limited within the eye's bony socket. In addition to the two eyelids found in vertebrates, it is protected by a third transparent movable membrane; the eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds. Some bird groups have specific modifications to their visual system linked to their way of life. Birds of prey have a high density of receptors and other adaptations that maximise visual acuity; the placement of their eyes gives them good binocular vision enabling accurate judgement of distances.
Nocturnal species have tubular eyes, low numbers of colour detectors, but a high density of rod cells which function well in poor light. Terns and albatrosses are amongst the seabirds which have red or yellow oil droplets in the colour receptors to improve distance vision in hazy conditions; the eye of a bird most resembles that of the reptiles. Unlike the mammalian eye, it is not spherical, the flatter shape enables more of its visual field to be in focus. A circle of bony plates, the sclerotic ring, surrounds the eye and holds it rigid, but an improvement over the reptilian eye found in mammals, is that the lens is pushed further forward, increasing the size of the image on the retina. Most birds can not move their eyes. Birds with eyes on the sides of their heads have a wide visual field, useful for detecting predators, while those with eyes on the front of their heads, such as owls, have binocular vision and can estimate distances when hunting; the American woodcock has the largest visual field of any bird, 360° in the horizontal plane, 180° in the vertical plane.
The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third concealed eyelid that sweeps horizontally across the eye like a windscreen wiper; the nictitating membrane covers the eye and acts as a contact lens in many aquatic birds when they are under water. When sleeping, the lower eyelid rises to cover the eye in most birds, with the exception of the horned owls where the upper eyelid is mobile; the eye is cleaned by tear secretions from the lachrymal gland and protected by an oily substance from the Harderian glands which coats the cornea and prevents dryness. The eye of a bird is larger compared to the size of the animal than for any other group of animals, although much of it is concealed in its skull; the ostrich has the largest eye of any land vertebrate, with an axial length of 50 mm, twice that of the human eye. Bird eye size is broadly related to body mass. A study of five orders showed that eye mass is proportional to body mass, but as expected from their habits and visual ecology and owls have large eyes for their body mass.
Behavioural studies show that many avian species focus on distant objects preferentially with their lateral and monocular field of vision, birds will orientate themselves sideways to maximise visual resolution. For a pigeon, resolution is twice as good with sideways monocular vision than forward binocular vision, whereas for humans the converse is true; the performance of the eye in low light levels depends on the distance between the lens and the retina, small birds are forced to be diurnal because their eyes are not large enough to give adequate night vision. Although many species migrate at night, they collide with brightly lit objects like lighthouses or oil platforms. Birds of prey are diurnal because, although their eyes are large, they are optimised to give maximum spatial resolution rather than light gathering, so they do not function well in poor light. Many birds have an asymmetry in the eye's structure which enables them to keep the horizon and a significant part of the ground in focus simultaneously.
The cost of this adaptation is. Birds with large eyes compared to their body mass, such as common redstarts and European robins sing earlier at dawn than birds of the same size and smaller body mass. However, if birds have the same eye size but different body masses, the larger species sings than the smaller; this may be. Overnight weight loss for small birds is 5-10% and may be over 15% on cold winter nights. In one study, robins put on more mass in their dusk feeding. Nocturnal birds have eyes optimised for visual sensitivity, with large corneas relative to the eye's length, whereas diurnal birds have longer eyes relative to the corneal diameter to give greater visual acuity. Information about the activities of extinct species can be deduced from measurements of the sclerotic ring and orbit depth. For the latter measurement to be made, the fossil must have retained its three-dimensional shape, so activity pattern cannot be determined with confidence from flattened specimens like Archaeopteryx, which has a complete sclerotic ring but no orbit depth measurement.
The main structures of the bird eye are similar to those of other vertebrates. The outer layer of the eye consists of the transparent cornea at the front, two layers of scle