Eyes

Eye

Eyes are organs that detect light. Different kinds of light-sensitive organs are found in a variety of animals. The simplest eyes do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms but can hardly be called vision. More complex eyes can distinguish shapes and colors. The visual fields of some such complex eyes largely overlap, to allow better depth perception binocular vision, as in humans; and others are placed so as to minimize the overlap, such as in rabbits and chameleons.In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods for brightness and cones for color react to the light. They interact with each other and send messages to the brain that indicate brightness, color, and contour.The first proto-eyes evolved among animals  million years ago. Almost all animals have eyes, or descend from animals that did.In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells for color and the rod cells for low-light contrasts in the retina detect and convert light into neural signals. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye, and reducing aberrations when there is enough light.The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and 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 lens and photosensitive cells. Some eyes have up to , such sensors, which are arranged hexagonally, and which can give a full -degree field of vision. Compound eyes are very 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 multiple-image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing very wide-angle, high-resolution images.

Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the worlds most complex color 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, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.In contrast to compound eyes, simple eyes are those that have a single lens. 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 stemmata which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like snails, who cannot actually 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 distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight.

Evolution of eyes

Biologists use the theory of evolution to explain the origin and development of eyes, as well as of organs in general.The common origin monophyly of all animal eyes is established by shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye evolved some  million years ago. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, as the first predator to gain true imaging would have touched off an arms race, or rather, a phylogenetic radiation from the species with that first proto-eye, among the descendents of which, there may well have been an arms race. Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.Vision in various animals shows adaptation to environmental requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eyes in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, despite their distant common ancestry.

The earliest eyes, called eyespots, were simple patches of photoreceptor cells, or light-sensitive proteins in unicellular organisms, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the lightsource. This gradually changed as the eyespot depressed into a shallow cup shape, granting the ability to slightly discriminate directional brightness by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of slightly distinguishing dim shapes for example in the nautilus.The thin overgrowth of transparent cells over the eyes aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eyes refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.

Anatomy of the mammalian eye

Dimensions vary only - mm among individuals. The vertical diameter is  mm; the transverse being larger. At birth is it generally - mm, enlarging to .- mm by three years of age, between then and age  the eye attains its mature size. It weighs . grams and its volume . millilitres.The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic.The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball consisting of the cornea and sclera. The sclera gives the eye most of its white color. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape.The vascular tunic, also known as the tunica vasculosa oculi, is the middle vascularized layer which includes the iris, ciliary body, and choroid. The choroid contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration. The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye. The iris is seen rather than the cornea when looking straight in ones eye due to the latters transparency, the pupil central aperture of iris is black because there is no light reflected out of the interior eye. If an ophthalmoscope is used, one can see the fundus, as well as vessels especially those crossing the optic disk - the point where the optic nerve fibers depart from the eyeball - among others

The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory which includes the retina. The retina contains the photosensitive rod and cone cells and associated neurons. To maximise vision and light absorption, the retina is a relatively smooth but curved layer. It has two points at which it is different; the fovea and optic disc. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for color vision in humans, and enables high acuity, such as is necessary in reading. The optic disc, sometimes referred to as the anatomical blind spot, is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus blind. In addition to the rods and cones, a small proportion about % in humans of the ganglion cells in the retina are photosensitive through the pigment melanopsin. They are generally most excitable by blue light, about  nm. Their information is sent to the SCN suprachiasmatic nuclei, not to the visual center, through the retinohypothalamic tract which is formed as melanopsin-sensitive axons exit the optic nerve. It is these light signals which regulate circadian rhythms in mammals and several other animals. Many, but not all, totally blind individuals have their circadian rhythms adjusted daily in this way.

Anterior and posterior segments

The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment.Human Eye Anterior Segment - Magnified view seen on examination with a slit lamp under diffuse illumination showing conjunctiva overlying the white sclera, transparent cornea, pharmacologically dilated pupil and cataract chematic diagram of the human eye.Latin segmentum anterius bulbi oculi
The human eye is not a plain sphere but is like two spheres combined, a smaller, sharper curved one and a larger lesser curved sphere. The former, the anterior segment is the front sixth  of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens. Within the anterior segment are two fluid-filled spaces:

 the anterior chamber between the posterior surface of the cornea i.e. the corneal endothelium and the iris. the posterior chamber between the iris and the front face of the vitreous.Aqueous humor fills these spaces within the anterior segment and provides nutrients to the surrounding structures.Some ophthalmologists specialize in the treatment and management of anterior segment disorders and diseases.The posterior segment is the back five-sixths  of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor, retina, choroid, and optic nerve.The radii of the anterior and posterior sections are  mm and  mm, respectively. The point of junction is called the limbus.On the other side of the lens is the second humour, the aqueous humour, which is bounded on all sides: by the lens, ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer the tapetum lucidum which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.Some ophthalmologists specialise in the treatment and management of posterior segment disorders and diseases.

Extraocular anatomy

Lying over the sclera and the interior of the eyelids is a transparent membrane called the conjunctiva. It helps lubricate the eye by producing mucus and tears. It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.In many animals, including humans, eyelids wipe the eye and prevent dehydration. They spread tears on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water. Most creatures will automatically react to a threat to its eyes such as an object moving straight at the eye, or a bright light by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex.In many animals, including humans, eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple .In many species, the eyes are inset in the portion of the skull known as the orbits or eyesockets. This placement of the eyes helps to protect them from injury.In humans, the eyebrows redirect flowing substances such as rainwater or sweat away from the eye.

Function of the mammalian eye

The retina contains two forms of photosensitive cells important to vision—rods and cones—in addition to the photosensitive ganglion cells involved in circadian adjustment but not vision. Though structurally and metabolically similar, the functions of rods and cones are quite different. Rod cells are highly sensitive to light, allowing them to respond in dim light and dark conditions; however, they cannot detect color differences. These are the cells that allow humans and other animals to see by moonlight, or with very little available light as in a dark room. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different wavelengths of light, which allows an organism to see color. The shift from cone vision to rod vision is why the darker conditions become, the less color objects seem to have.The differences between rods and cones are useful; apart from enabling sight in both dim and light conditions, they have further advantages. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. The fovea gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires staring at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other celestial objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the corner of their eyes averted vision where rods also exist, and where the light is sufficient to stimulate cells, allowing an individual to observe faint objects.

Rods and cones are both photosensitive, but respond differently to different frequencies of light. They contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each color-range. The process through which these proteins go is quite similar — upon being subjected to electromagnetic radiation of a particular wavelength and intensity, the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. The breakdown results in the activation of Transducin and this activates cyclic GMP Phosphodiesterase, which lowers the number of open Cyclic nucleotide-gated ion channels on the cell membrane, which leads to hyperpolarization; this hyperpolarization of the cell leads to decreased release of transmitter molecules at the synapse.

Differences between the rhodopsin and the iodopsins is the reason why cones and rods enable organisms to see in dark and light conditions — each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, synaptic convergence means that several rod cells are connected to a single bipolar cell, which then connects to a single ganglion cell by which information is relayed to the visual cortex. This convergence is in direct contrast to the situation with cones, where each cone cell is connected to a single bipolar cell. This divergence results in the high visual acuity, or the high ability to distinguish detail, of cone cells compared to rods. If a ray of light were to reach just one rod cell, the cells response may not be enough to hyperpolarize the connected bipolar cell. But because several converge onto a bipolar cell, enough transmitter molecules reach the synapses of the bipolar cell to hyperpolarize it.Furthermore, color is distinguishable due to the different iodopsins of cone cells; there are three different kinds, in normal human vision, which is why we need three different primary colors to make a color space.A small percentage of the ganglion cells in the retina contain melanopsin and, thus, are themselves photosensitive. The light information from these cells is not involved in vision and it reaches the brain not via the optic nerve but via the retinohypothalamic tract, the RHT. By way of this light information, the body clocks inherent approximate -hour cycling is adjusted daily to natures light/dark cycle.

Accommodation

The purpose of the optics of the mammalian eye is to bring a clear image of the visual world onto the retina. Because of limited depth of field of the mammalian eye, an object at one distance from the eye might project a clear image, while an object either closer to or further from the eye will not. To make images clear for objects at different distances from the eye, its optical power needs to be changed. This is accomplished mainly by changing the curvature of the lens. For distant objects, the lens needs to be made flatter, for near objects the lens needs to be made thicker and more rounded.

Water in the eye can alter the optical properties of the eye and blur vision. It can also wash away the tear fluid—along with it the protective lipid layer—and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. Osmotic effects are made apparent when swimming in freshwater pools, becase the osmotic gradient draws water from the pool into the corneal tissue the pool water is hypotonic, causing edema, and subsequently leaving the swimmer with cloudy or misty vision for a short period thereafter. The edema can be reversed by irrigating the eye with hypertonic saline which osmotically draws the excess water out of the eye.

Acuity

Visual acuity is often measured in cycles per degree CPD, which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white–black stripe cycles. For example, if each pattern is . cm wide and is placed at  m distance from the eye, it will subtend an angle of  degree, so the number of white–black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye.For a human eye with excellent acuity, the maximum theoretical resolution would be  CPD . minute of arc per line pair, or a . mm line pair, at  m. However, the eye can only resolve a contrast of %. Taking this into account, the eye can resolve a maximum resolution of  CPD, or . minute of arc per line pair . mm line pair, at  m. A rat can resolve only about  to  CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eyes central fovea region.

A maximum resolution of the human eye in good light of . minute of arc per line pair will correspond to . lines per minute of arc. Assuming two pixels per line pair one pixel per line and a square field of  degrees, this would be equivalent to approximately ××. =  pixels in each of the X and Y dimensions, or about  megapixels.[citation needed]However, the human eye itself has only a small spot of sharp vision in the middle of the retina, the fovea centralis, the rest of the field of view being progressively lower resolution as it gets further from the fovea. The angle of the sharp vision being just a few degrees in the middle of the view, the sharp area thus barely achieves even a single megapixel resolution. The experience of wide sharp human vision is in fact based on turning the eyes towards the current point of interest in the field of view, the brain thus perceiving an observation of a wide sharp field of view.The narrow beam of sharp vision is easy to test by putting a fingertip on a newspaper and trying to read the text while staring at the fingertip — it is very difficult to read text thats just a few centimeters away from the fingertip.