In optics, an aperture is a hole or an opening through which light travels. More the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An optical system has many openings or structures that limit the ray bundles; these structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, the aperture stop is the stop that determines the ray cone angle and brightness at the image point. In some contexts in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope, the aperture stop is the edges of the objective lens or mirror. One speaks of a telescope as having, for example, a 100-centimeter aperture. Note that the aperture stop is not the smallest stop in the system.
Magnification and demagnification by lenses and other elements can cause a large stop to be the aperture stop for the system. In astrophotography, the aperture may be given as a linear measure or as the dimensionless ratio between that measure and the focal length. In other photography, it is given as a ratio. Sometimes stops and diaphragms are called apertures when they are not the aperture stop of the system; the word aperture is used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy, for example, a photometric aperture around a star corresponds to a circular window around the image of a star within which the light intensity is assumed; the aperture stop is an important element in most optical designs. Its most obvious feature is; this can be either unavoidable, as in a telescope where one wants to collect as much light as possible. In both cases, the size of the aperture stop is constrained by things other than the amount of light admitted. Smaller stops produce a longer depth of field, allowing objects at a wide range of distances to all be in focus at the same time.
The stop limits the effect of optical aberrations. If the stop is too large, the image will be distorted. More sophisticated optical system designs can mitigate the effect of aberrations, allowing a larger stop and therefore greater light collecting ability; the stop determines. Larger stops can cause the intensity reaching the film or detector to fall off toward the edges of the picture when, for off-axis points, a different stop becomes the aperture stop by virtue of cutting off more light than did the stop, the aperture stop on the optic axis. A larger aperture stop requires larger diameter optics, which are more expensive. In addition to an aperture stop, a photographic lens may have one or more field stops, which limit the system's field of view; when the field of view is limited by a field stop in the lens vignetting results. The biological pupil of the eye is its aperture in optics nomenclature. Refraction in the cornea causes the effective aperture to differ from the physical pupil diameter.
The entrance pupil is about 4 mm in diameter, although it can range from 2 mm in a brightly lit place to 8 mm in the dark. In astronomy, the diameter of the aperture stop is a critical parameter in the design of a telescope. One would want the aperture to be as large as possible, to collect the maximum amount of light from the distant objects being imaged; the size of the aperture is limited, however, in practice by considerations of cost and weight, as well as prevention of aberrations. Apertures are used in laser energy control, close aperture z-scan technique, diffractions/patterns, beam cleaning. Laser applications include Q-switching, high intensity x-ray control. In light microscopy, the word aperture may be used with reference to either the condenser, field iris or objective lens. See Optical microscope; the aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film's or image sensor's degree of exposure to light.
A fast shutter will require a larger aperture to ensure sufficient light exposure, a slow shutter will require a smaller aperture to avoid excessive exposure. A device called a diaphragm serves as the aperture stop, controls the aperture; the diaphragm functions much like the iris of the eye – it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture, the greater the distance from the plane of focus the subject matter may be while
An organ system is a group of organs that work together as a biological system to perform one or more functions. Each organ system does a particular job in the body, is made up of certain tissues; these specific systems are studied in anatomy. They are present in many types of animals. Respiratory system: the organs used for breathing, the pharynx, bronchi and diaphragm. Digestive system: digestion and processing food with salivary glands, stomach, gallbladder, intestines and anus. Cardiovascular system: and channeling blood to and from the body and lungs with heart and blood vessels. Urinary system: kidneys, ureters and urethra involved in fluid balance, electrolyte balance and excretion of urine. Integumentary system: skin, hair and nails. Skeletally system: structural support and protection with bones, cartilage and tendons. Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal gland, thyroid and adrenal glands.
Lymphatic system: the transfer of lymph between tissues and the blood stream. Includes the functions of immune responses and the development of antibodies. Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living; the job of the circulatory system is to move blood, oxygen, carbon dioxide, hormones, around the body. It consists of the heart, blood vessels and veins. Immune system: protects the organism from foreign bodies. Nervous system: collecting and processing information with brain, spinal cord, peripheral nervous system and sense organs. Sensory systems: visual system, auditory system, olfactory system, gustatory system, somatosensory system, vestibular system. Muscular system: allows for manipulation of the environment, provides locomotion, maintains posture, produces heat. Includes skeletal muscles, smooth muscles and cardiac muscle. Reproductive system: the sex organs, such as ovaries, fallopian tubes, vagina, mammary glands, testes, vas deferens, seminal vesicles and prostate
Long ciliary nerves
The long ciliary nerves, two or three in number, are given off from the nasociliary nerve as it crosses the optic nerve. The nasociliary nerve that the long ciliary nerves branch from is itself a branch of the ophthalmic branch of the trigeminal nerve, they accompany the short ciliary nerves from the ciliary ganglion, pierce the posterior part of the sclera, running forward between it and the choroid, are distributed to the iris and cornea. The long ciliary nerves provide sensory innervation including the cornea. In addition, they contain sympathetic fibers from the superior cervical ganglion to the dilator pupillae muscle; the sympathetic fibers to the dilator pupillae muscle travel in the nasociliary nerve but there are sympathetic fibers in the short ciliary nerves that pass through the ciliary ganglion without forming synapses. Short ciliary nerves This article incorporates text in the public domain from page 888 of the 20th edition of Gray's Anatomy
The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that miss exiting the narrow pupil. In humans the pupil is round, but other species, such as some cats, have vertical slit pupils, goats have horizontally oriented pupils, some catfish have annular types. In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop; the image of the pupil as seen from outside the eye is the entrance pupil, which does not correspond to the location and size of the physical pupil because it is magnified by the cornea. On the inner edge lies a prominent structure, the collarette, marking the junction of the embryonic pupillary membrane covering the embryonic pupil; the pupil is a hole located in the centre of the iris of the eye that allows light to strike the retina.
It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that miss exiting the narrow pupil. The iris is a contractile structure, consisting of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, the iris regulates the amount of light by controlling the size of the pupil; this is known as the pupillary light reflex. The iris contains two groups of smooth muscles; when the sphincter pupillae contract, the iris constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract; these muscles are sometimes referred to as intrinsic eye muscles. The sensory pathway is linked with its counterpart in the other eye by a partial crossover of each eye's fibers; this causes the effect in one eye to carry over to the other. The pupil gets narrower in light; when narrow, the diameter is 2 to 4 millimeters.
In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8 mm. In any human age group there is however considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4 mm to 9 mm with different individuals. After 25 years of age the average pupil size decreases, though not at a steady rate. At this stage the pupils do not remain still, therefore may lead to oscillation, which may intensify and become known as hippus; the constriction of the pupil and near vision are tied. In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; when bright light is shone on the eye, light sensitive cells in the retina, including rod and cone photoreceptors and melanopsin ganglion cells, will send signals to the oculomotor nerve the parasympathetic part coming from the Edinger-Westphal nucleus, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.
This is the pupillary light reflex, an important test of brainstem function. Furthermore, the pupil will dilate. If the drug pilocarpine is administered, the pupils will constrict and accommodation is increased due to the parasympathetic action on the circular muscle fibers, atropine will cause paralysis of accommodation and dilation of the pupil. Certain drugs cause constriction such as opioids. Other drugs, such as atropine, LSD, MDMA, psilocybin mushrooms and amphetamines may cause pupil dilation; the sphincter muscle has a parasympathetic innervation, the dilator has a sympathetic innervation. In pupillary constriction induced by pilocarpine, not only is the sphincter nerve supply activated but that of the dilator is inhibited; the reverse is true, so control of pupil size is controlled by differences in contraction intensity of each muscle. Another term for the constriction of the pupil is miosis. Substances that cause miosis are described as miotic. Dilation of the pupil is mydriasis. Dilation can be caused by mydriatic substances such as an eye drop solution containing tropicamide.
A condition called bene dilitatism occurs when the optic nerves are damaged. This condition is typified by chronically widened pupils due to the decreased ability of the optic nerves to respond to light. In normal lighting, people afflicted with this condition have dilated pupils, bright lighting can cause pain. At the other end of the spectrum, people with this condition have trouble seeing in darkness, it is necessary for these people to be careful when driving at night due to their inability to see objects in their full perspective. This condition is not otherwise dangerous; the size of the pupil can be a symptom of an underlying disease. Dilation of the pupil is known as mydriasis and contraction as miosis. Not all variations in size are indicative of disease however. In addition to dilation and contraction caused by light and darkness, it has been shown that solving simple multiplication problems affects the size of the pupil; the simple act of recollection can dilate the size of the pupil, however when the brain is required to process at a rate above its maximum capacity, the pupils contract.
There is evidence that pupil size is related to the extent of positive or negative emotional arousal experienced by a person. Not all animals
Color, or colour, is the characteristic of human visual perception described through color categories, with names such as red, yellow, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. Color categories and physical specifications of color are associated with objects through the wavelength of the light, reflected from them; this reflection is governed by the object's physical properties such as light absorption, emission spectra, etc. By defining a color space, colors can be identified numerically by coordinates, which in 1931 were named in global agreement with internationally agreed color names like mentioned above by the International Commission on Illumination; the RGB color space for instance is a color space corresponding to human trichromacy and to the three cone cell types that respond to three bands of light: long wavelengths, peaking near 564–580 nm. There may be more than three color dimensions in other color spaces, such as in the CMYK color model, wherein one of the dimensions relates to a color's colorfulness).
The photo-receptivity of the "eyes" of other species varies from that of humans and so results in correspondingly different color perceptions that cannot be compared to one another. Honeybees and bumblebees for instance have trichromatic color vision sensitive to ultraviolet but is insensitive to red. Papilio butterflies may have pentachromatic vision; the most complex color vision system in the animal kingdom has been found in stomatopods with up to 12 spectral receptor types thought to work as multiple dichromatic units. The science of color is sometimes called chromatics, colorimetry, or color science, it includes the study of the perception of color by the human eye and brain, the origin of color in materials, color theory in art, the physics of electromagnetic radiation in the visible range. Electromagnetic radiation is characterized by its intensity; when the wavelength is within the visible spectrum, it is known as "visible light". Most light sources emit light at many different wavelengths.
Although the spectrum of light arriving at the eye from a given direction determines the color sensation in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color sensation, although such classes would vary among different species, to a lesser extent among individuals within the same species. In each such class the members are called metamers of the color in question; the familiar colors of the rainbow in the spectrum—named using the Latin word for appearance or apparition by Isaac Newton in 1671—include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate wavelengths for various pure spectral colors; the wavelengths listed are as measured in vacuum. The color table should not be interpreted as a definitive list—the pure spectral colors form a continuous spectrum, how it is divided into distinct colors linguistically is a matter of culture and historical contingency.
A common list identifies six main bands: red, yellow, green and violet. Newton's conception included a seventh color, between blue and violet, it is possible that what Newton referred to as blue is nearer to what today is known as cyan, that indigo was the dark blue of the indigo dye, being imported at the time. The intensity of a spectral color, relative to the context in which it is viewed, may alter its perception considerably; the color of an object depends on both the physics of the object in its environment and the characteristics of the perceiving eye and brain. Physically, objects can be said to have the color of the light leaving their surfaces, which depends on the spectrum of the incident illumination and the reflectance properties of the surface, as well as on the angles of illumination and viewing; some objects not only reflect light, but transmit light or emit light themselves, which contributes to the color. A viewer's perception of the object's color depends not only on the spectrum of the light leaving its surface, but on a host of contextual cues, so that color differences between objects can be discerned independent of the lighting spectrum, viewing angle, etc.
This effect is known as color constancy. Some generalizations of the physics can be drawn, neglecting perceptual effects for now: Light arriving at an opaque surface is either reflected "specularly", scattered, or absorbed – or some combination of these. Opaque objects that do not reflect specularly have their color determined by which wavelengths of light they scatter strongly. If objects scatter all wavelengths with r
Eyes are organs of the visual system. They provide organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs and arthropods; the simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, sufficient for the entrainment of circadian rhythms.
From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex. Complex eyes can distinguish colours; the visual fields of many organisms predators, involve large areas of binocular vision to improve depth perception. In other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision; the first proto-eyes evolved among animals 600 million years ago about the time of the Cambrian explosion. The last common ancestor of animals possessed the biochemical toolkit necessary for vision, more advanced eyes have evolved in 96% of animal species in six of the ~35 main phyla. In most vertebrates and some molluscs, the eye works by allowing light to enter and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye; the cone cells and the rod cells in the retina detect and convert light into neural signals for vision.
The visual signals are transmitted to the brain via the optic nerve. Such eyes are roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and an iris; the eyes of most cephalopods, fish and snakes have fixed lens shapes, focusing vision is achieved by telescoping the lens—similar to how a camera focuses. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye; each sensor has its own photosensitive cell. Some eyes have up to 28,000 such sensors, which are arranged hexagonally, which can give a full 360° field of vision. Compound eyes are sensitive to motion; some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing different, high-resolution images.
Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the world's most complex colour vision system. Trilobites, which are now extinct, had unique compound eyes, they used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods; the number of lenses in such an eye varied, however: some trilobites had only one, some had thousands of lenses in one eye. In contrast to compound eyes, simple eyes are those. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision; some insect larvae, like caterpillars, have a different type of simple eye which provides only a rough image, but can possess resolving powers of 4 degrees of arc, be polarization sensitive and capable of increasing its absolute sensitivity at night by a factor of 1,000 or more. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot "see" in the normal sense.
They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can no more; this enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents—in this way the bearers can spot hot springs and avoid being boiled alive. There are ten different eye layouts—indeed every technological method of capturing an optical image used by human beings, with the exceptions of zoom and Fresnel lenses, occur in nature. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for any behaviour or environment; the only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°.
Superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to
Embryology is the branch of biology that studies the prenatal development of gametes and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology. Embryology has a long history. Aristotle proposed the accepted theory of epigenesis, that organisms develop from seed or egg in a sequence of steps; the alternative theory, that organisms develop from pre-existing miniature versions of themselves, held sway until the 18th century. Modern embryology developed from the work of von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end. In bilateral animals, the blastula develops in one of two ways that divide the whole animal kingdom into two halves. If in the blastula the first pore becomes the mouth of the animal, it is a protostome.
The protostomes include most invertebrate animals, such as insects and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula; the gastrula with its blastopore soon develops three distinct layers of cells from which all the bodily organs and tissues develop: The innermost layer, or endoderm, give rise to the digestive organs, the gills, lungs or swim bladder if present, kidneys or nephrites. The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, blood system; the outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, skin or carapace and hair, bristles, or scales. Embryos in many species appear similar to one another in early developmental stages; the reason for this similarity is. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.
Drosophila melanogaster, a fruit fly, is a model organism in biology on which much research into embryology has been done. Before fertilization, the female gamete produces an abundance of mRNA - transcribed from the genes that encode bicoid protein and nanos protein; these mRNA molecules are stored to be used in what will become the developing embryo. The male and female Drosophila gametes exhibit anisogamy; the female gamete is larger than the male gamete because it harbors more cytoplasm and, within the cytoplasm, the female gamete contains an abundance of the mRNA mentioned. At fertilization, the male and female gametes fuse and the nucleus of the male gamete fuses with the nucleus of the female gamete. Note that before the gametes' nuclei fuse, they are known as pronuclei. A series of nuclear divisions will occur without cytokinesis in the zygote to form a multi-nucleated cell known as a syncytium. All the nuclei in the syncytium are identical, just as all the nuclei in every somatic cell of any multicellular organism are identical in terms of the DNA sequence of the genome.
Before the nuclei can differentiate in transcriptional activity, the embryo must be divided into segments. In each segment, a unique set of regulatory proteins will cause specific genes in the nuclei to be transcribed; the resulting combination of proteins will transform clusters of cells into early embryo tissues that will each develop into multiple fetal and adult tissues in development. Outlined below is the process that leads to tissue differentiation. Maternal-effect genes - subject to Maternal inheritance Egg-polarity genes establish the Anteroposterior axis. Zygotic-effect genes - subject to Mendelian inheritance Segmentation genes establish 14 segments of the embryo using the anteroposterior axis as a guide. Gap genes establish 3 broad segments of the embryo. Pair-rule genes define 7 segments of the embryo within the confines of the second broad segment, defined by the gap genes. Segment-polarity genes define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves.
Homeotic genes use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type. Humans are deuterostomes. In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception, the developing human is called a fetus; as as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo – a preformed, miniature infant, or homunculus – that becomes larger during development. Until the birth of modern embryology through observation of the mammalian ovum by von Baer in 1827, there was no clear scientific understanding of embryology. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available; the competing explanation of embryonic development was epigenesis proposed 2,000 years earlier by