Human Eye: Anatomy, Structure, and Function - Class XII

Smooth muscle cells of the frog iris sphincter contain rhodopsin and contract in response to light. It is the colored part of the eye, which regulates the size of the pupil controlling the entry of light into the eye.

The colored part of the eye is called as the iris. It controls light levels inside the eye. The round opening in the center of the iris is called as the pupil. It is photosensitive in nature and is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size depending on the presence or absence of light.
Therefore, the correct answer is option A.

Rhodopsin, also known as visual purple, is a light-sensitive receptor protein. It is a biological pigment in photoreceptor cells of the retina. 

The ciliary muscle is located in the eye and surrounds the lens. It is smooth circular muscle, that relaxes or tightens the zonules to enable the lens to change shape for focusing. When it contracts, it pulls forward, moving to a frontal portion to relax the fibers, that hold the lens in place; this process also allows it to take on a more spherical shape to provide short range focus.

• The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs.
• The optics of the eye create a focused two-dimensional image of the visual world on the retina, which translates that image into electrical neural impulses to the brain to create the visual perception.
• The retina serving a function analogous to that of the film or image sensor in a camera. Hence, the retina  is the image of an object formed in the human eye.

The part of the eye responsible for its shape is sclera. It is the opaque, fibrous, tough and the protective outer layer of the eye.

• The anterior chamber of the eyeball is the part of the human eye with the chamber at right.
  
• The anterior chamber is the aqueous humor-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium.

The eye spot (stigma) is a light-sensitive organelle found in certain unicellular algae, such as Chlamydomonas, used for phototaxis.

The cornea is the clear, dome-shaped outer surface of the eye covering the front of the iris, pupil, and anterior chamber, functioning as a clean window that helps focus light.

Umbraculum is a modification of iris which serves to reduce the excessive glare in daytime. It is found in animals like camels. It is also known as corpus nigra.

Many suspensory ligaments are attached radially around the lens that serve to pull the edges of lens edges toward the outer circle of the eyeball. They maintain the relatively flat shape of the lens under normal conditions of the eye.

The horse has the largest eye of any land mammal. Its visual abilities are directly related to the animal's behavior and the fact that the horse is a flight animal.
Thus, the correct answer is option B.

Supporting cells of the retina is Muller cells also known as Muller glia. They are a type of retinal glial cells, first recognized and described by Heinrich Muller. They are found in the vertebrate retina, which serves as support cells for the neurons of the retina.

The retina makes up the inner layer and occupies only the posterior two-third of the eye. The retina consists of several layers of cells, including the rods and cones, the sensory cells, that respond to light. The tips of the rods and cones are embedded in a pigmented layer of cells on the back of the retina. The pigment helps prevent light from scattering in the back of the eye. 

Ostriches have the biggest eyes of all land mammals. The ostrich needs large eyes for its terrestrial lifestyle sharing the savanna as it does with an alarming army of fearsome predators. It is a flightless bird with strong hindlimbs to run.

The meibomian glands (or tarsal glands) are a special kind of sebaceous gland at the rim of the eyelids inside the tarsal plate, responsible for the supply of meibum, an oily substance, that prevents evaporation of the eye's tear film. Meibum prevents tear spillage onto the cheek, trapping tears between the oiled edge and the eyeball, and makes the closed lids airtight. There are approximately 50 glands on the upper eyelids and 25 glands on the lower eyelids. Dysfunctional meibomian glands often cause dry eyes.

Retina is the light-sensitive tissue lining the back of our eye. It is the black pigment in the eye, which reduces the internal reflection. Thereby, it controls the impact of the bright light entering our eye. Function of retina is to receive light that the lens has focused, convert the light into neural signals, and send these signals on to the brain for visual recognition. Hence option A is correct.

The almond-shaped glands in the eyes present above each eyeball are called lacrimal glands. Thes glands are called tear gland because they secrete the fluid called tears. These glands have their openings between the eyelashes. Water, salts and bactericidal protein such as lysozyme are components of tear. The secretion of this glands is involved in the clearance of dust, grit and microorganisms in the cornea. They are also involved in the lubrication of eyes and eyelashes. Thus the correct answer is option C.

Glands of Zeis are modified sebaceous gland located on the margin of eyelids and produces oil into the follicles of eye lashes. 

Rods and cones are photoreceptors in the human retina. Rods are responsible for vision at low light levels (scotopic vision). They do not mediate colour vision and have a low spatial acuity. Cones are active at higher light levels (photopic vision), are capable of colour vision and are responsible for high spatial acuity. So, the correct answer is 'Rods are responsible for scotopic vision and Cones are responsible for photopic vision'.

Rods and cones are used to process light, in a human eye, their number is estimated to be 5-7 million cones and 110-130 million rods. Cones have iodopsin and are found mainly in the central area of the retina (fovea), while rods containing rhodopsin are found in the peripheral retina. Cones sense colour and need more light than rods to work well. Cones are most helpful in the normal or bright light. There are 3 types of cones - red, green, and blue - to help you see different ranges of colour. Together, these cones sense combinations of light waves, that enable our eyes to see millions of colours. The light-sensitive photopigments are made of the protein opsin and the chromophore retinene. Bleaching of iodopsin in cones is similar to rhodopsin in rods, only rods contain a higher concentration of visual pigment than cones, so more light is needed to cause an action potential to be fired in cones ( a threshold for cones is higher than rods). In other words, rods are mainly used for dim light vision, cones for bright light vision.

Rods and cones are the photoreceptors present in retina of the eye. They have their own bipolar neurons and photo-sensitive pigments.

The fovea centralis is the central portion of the human retina, that has the most acute vision and the best color perception. The cones have 3 types of specific photopigments - red, green and blue, or RGB. Most of the color sensitive cones are packed in the fovea centralis.
The colored part of the eye is called as the iris. It controls light levels inside the eye similar to the aperture on a camera. The round opening in the center of the iris is called as the pupil. The iris is embedded with tiny muscles, that dilate (widen) and constrict (narrow) the pupil size.
The lens, by changing shape, functions to change the focal distance of the eye, so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina, which is then transmitted to the brain by the optical nerve.

Vultures are highly reliant on their sensory systems for the rapid detection and localization of carrion before other scavengers can exploit the resource. Vulture has acute vision. Retina has a comb like pectin in birds.

The human eye is sensitive to violet (400 nm) to red (700 nm ) light only that can generate photometric effect i.e. visual sensation on an eye. So, the correct answer is option B.

Ciliary body and the attached suspensory ligament change the shape (flattened or round) of the lens through relaxation and contraction of ciliary muscles and thereby the focal length to focus on the distant and near object. Contraction of ciliary muscles releases the suspensory ligaments and allow the lens to become round (convex) while relaxation of these muscles makes the lens taut. Since ciliary muscles are involved in accommodation required for close vision, loss of their contraction would not cause loss of vision but would affect the close vision only. The amount of light entering retina is controlled by the iris. 

The choroid is a layer of eyeball. The eyeball has three coats namely sclera, choroid, and retina. The choroid is middle dark pigmented thin vascular layer of the eyeball and is characterized by the presence of many blood vessels. Retina forms the innermost layer of the eye. 

Compound eyes are the characteristic feature of insects. The compound eye of insects consists of many ommatidia, each of which is a separate visual unit that portrays each object into several broken pieces. This type of vision is called as mosaic vision. Among the given options, housefly is an insect and has ommatidia as a unit of an eye. Pheretima (annelid) and pila and sepia (molluscs) do not have compound eyes. 

The choroid is the part of the vascular coat in eyes. It also contains blood vessels that provide nourishment to the eye. It is pigmented towards a rear of the eye and absorbs light. It is a dark brown in colour. It darkens the cavity of the eyeballs to prevent internal reflection of light that might blur the image.

• The word ''Opthalmology'' comes from the Greek roots ''Ophthalmol'' means ''Eye'' and ''Logos'' means ''Study''. Hence Ophthalmology literally means the ''The science of eyes''. 
• An Ophthalmologist is a specialist in medical and surgical eye problems.
• Hence Ophthalmology is the study of Caring, diagnosis and cure of eye diseases.
• So, the correct answer is 'Ophthalmology'.

The ciliary body is the structure in the eye that releases a transparent liquid (called the aqueous humour) within the eye. The ciliary body also contains the ciliary muscle, which changes the shape of the lens when your eyes focus on something. This process is called accommodation.

The photosensitive compound present in vision is called rhodopsin, also known as visual purple which consists of a large protein called opsin and retinal. Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Retinal allows certain microorganisms to convert light into metabolic energy.
So, the correct answer is option A.

When eye is donated or transplanted only cornea part of the eye is transplanted to the needy person.

Neural adaptation or sensory adaptation is a change over time in the responsiveness of the sensory system to a constant stimulus. It is usually experienced as a change in the stimulus that is, the responsiveness to constant stimulus is decreased or stop

1. The first thing light encounters when it enters the eye is the cornea, a protective clear covering over the pupil and iris. The cornea bends the light and begins to form an image.
2. Light passes from the cornea to the pupil, the dark circle in the centre of the iris, which is the coloured portion of the eye. The pupil regulates the amount of light that will enter the inner eye based on environmental conditions: It dilates, growing bigger to receive more light under dim lighting conditions, and shrinks in response to bright light. This response is quicker in young individuals and tends to slow with increasing age.
3. From the pupil, light waves travel to the lens of the eye. The lens is a clear, flexible structure that focuses an upside-down image onto the retina. It is flexible so that it can focus on images that are close or far away. Eye injuries, normal variations in the eye and age can distort the lens, making it difficult to focus on nearby or faraway objects -- you see the objects, but details are hazy. Late in life, the lens can also become clouded and form cataracts that make images seem hazy and dim.
4. The lens focuses light and images on the retina, a layer of light-sensitive cells at the back of the eye. It is made up of two kinds of photoreceptor cells: cones and rods. The cones transmit colour and sharp images. The concentration of cones is low on the sides of the retina and increases as the cones approach the centre of the retina or the macula. The rods are more sensitive to light and are more numerous than cones; They let you see when lighting is dim, although what you see lacks colour and clear details.
5. Once the retina senses the image, it sends impulses to the optic nerve at the back of the eye. The optic nerve then transmits them to special areas in the brain, which automatically flips the upside-down image so that it becomes upright again. Disease or injury can damage the optic nerve, resulting in varying degrees of blindness.

Cornea (vii) bends light, Iris (iii) controls light, Lens (vi) focuses light, Optic nerves (v) transmit to CNS, Pupil (i) provides opening, Ciliary muscles (iv) alter shape, Fovea (ii) transducens light. Matching these leads to A.

Rhodopsin is found in specialized light receptor cells called rods. These are light-sensitive tissue present in the retina. They provide vision in low light. Other light receptor cells in the retina, called cones, are responsible for vision in bright light.

Retina is the innermost layers of the eyeball. The cornea is the transparent anterior window of the outer layer of the eyeball located within otherwise opaque sclera. Iris is the colored part of the eye that imparts characteristic color of eyes to different people. 

In the eye of the bird, a comb-like structure is present. It is called as pectin. It is soft and vascular. It is a rectangular plate. It projects into the vitreous humor from the blind spot. So, the correct answer is option B.

Iris is the part of the eye, which acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye. The iris works like a shutter in a camera. It has the ability to enlarge and shrink, depending on how much light is entering the eye.
The two eyelids act to protect the front of the eye and excessive light by their closure. The inner layer of the cornea is made up of transparent tissue, which allows light to pass. The pupil is the dark opening in the centre of the coloured iris, that controls how much light enters the eye. The ciliary body is a structure located behind the iris which produces aqueous fluid that fills the front part of the eye and thus maintains the eye pressure. It also allows focusing of the lens. 

In the rods of fresh water fishes, rhodopsin is replaced by the purple photolabile pigment porphyropsin. This participates in a retinal cycle identical in form with that of rhodopsin, but in which new carotenoids replace retinene and vitamin A.

The back of the eye is lined by a layer called as the retina, which acts like the film inside a camera. The retina is a thin layer of nerve tissue, that contains photoreceptors. Photoreceptors convert light rays into electrical impulses, which are then sent through the optic nerve to the brain, where an image is perceived. As with a camera, if the film is bad in the eye (i.e., the retina is damaged or diseased), no matter how well the rest of the eye is functioning, a good picture is not possible.

The ora Serrata is a toothlike junction between the peripheral retina and the pars plana and is composed of forward extensions of the retina and backward extensions of the ciliary body. So, the correct answer is option A.

A myopic eyed person focuses the light rays from a distant object in front of the retina, it can not focus distant objects sharply on the retina. Emmetropic eyes are characterized by sharp focusing of parallel light rays from distant objects on the retina when the ciliary muscle is completely relaxed to make the person see all the distant objects clearly. Hypermetropic eyes can not sufficiently bend the parallel light rays from the distant object to focus it on the retina in time.

The compound eyes of arthropods are composed of units called as ommatidia. An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells.

The retina contains the cells, that respond to light. These specialized cells are called as photoreceptors. There are 2 types of photoreceptors in the retina: rods and cones. 

In order to reduce the amount of light entering into the eye, pupils constrict, and this constriction helps to decrease the strong light, which can be harmful to cells in the retina. The retina is the innermost layer of the eye which contains photoreceptor cells, that are useful for the formation of the image and perceiving an object. Another advantage of constriction of the pupil is to improve the clarity of image by blocking some of the rays of light, which can optically cause distortion of the image. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.

Rhodopsin is a light-sensitive receptor protein. It is a biological pigment in photoreceptor cells of the retina. It is the primary pigment found in rod photoreceptors. They are extremely sensitive to light, enabling vision in low-light conditions. 

Refraction is the phenomenon which makes image formation possible by the eye as well as by cameras and other systems of lenses. Most of that refraction in the eye takes place at the first surface since the transition from the air into the cornea is the largest change in index of refraction which the light experiences. About 80% of the refraction occurs in the cornea and about 20% in the inner crystalline lens.

The retina is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision, while cones support the daytime vision and the perception of colour. The third type of photoreceptor, the intrinsically photosensitive ganglion cell, is important for reflexive responses to bright daylight. Neural signals from the rods and cones undergo processing by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells, whose axons form the optic nerve. 

The abducens nerve innervates the lateral rectus eye muscle that turns the eye outward away from the midline. 

All photoreceptors share a common visual pigment: a mixture of a vitamin A-based chromophore (retinal) and a seven-transmembrane-helix apoprotein (opsin). Together, they form rhodopsin. The pigments belong to a special class of receptors called as G protein-coupled receptors, characterised by signaling through heterotrimeric G proteins. 

The plica semilunaris in humans is a small fold of tissue on the inside corner of the eye. It is the vestigial remnant of the nictitating membrane, an organ that is fully functional in some other species of mammals.

At extremely low intensities of stimuli, when only rods are stimulated, the retina shows a variable sensitivity to light according to its wavelength, being most sensitive at about 5000 angstroms, the absorption maximum of the rod visual pigment, rhodopsin. There is a characteristic shift in the maximum sensitivity from 5000 angstroms for scotopic (night) vision to 5550 angstroms for photopic (day) vision, called Purkinje shift.

Six extraocular muscles facilitate eye movement. These muscles arise from the common tendinous ring in the orbit, the eye cavity and attach to the eyeball. The six muscles are the lateral, medial, inferior and superior rectus muscles and the inferior and superior oblique muscles. The muscles, when contracting, cause movement of the eyeball, by pulling the eyeball towards the muscle.

Neuroectoderm gives rise to the following compartments of the eye: retina, epithelial lining of ciliary body and iris, optic nerves. Surface ectoderm produces the following parts: lens, corneal epithelium, skin of eyelid.

The harderian gland is a gland found within the eye's orbit, which occurs in tetrapods (reptiles, amphibians, birds and mammals) that possess a nictitating membrane. The gland can be compound tubular or compound tubuloalveolar, and the fluid it secretes (mucous, serous or lipid) varies between different groups of animals. In some animals it acts as an accessory to the lacrimal gland, secreting fluid that eases movement of the nictitating membrane. 

Vascular tunic is the middle coat of the eyeball consisting of highly vascular and heavily pigmented connective tissue consisting of a posterior choroid layer and an anterior ciliary body and the iris. Choroid is the posterior portion of the vascular middle tunic of the eyeball, lined on the inside by a layer of darkly pigmented cells; it provides the main blood supply to the back of the eye and gives rise to the ciliary body and the iris anteriorly.

Nictitating membrane is also called as the third eye of rabbit and is found in medial canthus (inner corner) of an eye. This is swept closed across the eye in the direction from beak to ear to protect the eye as well as to clean and lubricate it with the moisture of lachrymal gland. 

The pecten or pecten oculi is a comb-like structure of blood vessels, belonging to the choroid in the eye of a bird. It is a non-sensory, pigmented structure that projects into the vitreous body from the point, where the optic nerve enters the eyeball. The pecten nourishes the retina and controls the pH of the vitreous body.

Iris is the part of the eye, which is responsible for one's eye color. It acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye.

Meibomian gland are present in the inner surface of the eyelids. They secrete an oily substance which spreads over the cornea and prevents the cornea from drying. These glands prevent the friction between two eyelids. These glands are present in animals like rabbits. They help in frictionless movement and blinking of the eyelids. 

The retina is the thin light-sensitive membrane lining the inner eyeball towards the back of the eye. When the light goes into the eye it must pass through the lens (a biconvex lens), which inverts the image we are seeing. When we look at an image, light bounces it into our eye. As it enters our eye and passes through the lens the image gets inverted- reversed and flipped the other way- so that the image on our retina looks like an upside down object going from right to left instead of left to right. Once this image is set on the retina, cone cells distinguish the colour and detail, while rod cells distinguish movement and shades of grey. 

The lacrimal gland lies within the orbit on the outer portion of the upper eye. The gland continually secretes tears, which moisten, lubricate, and protect the eye. Meibomian gland is a type of gland in the eyelids, that makes a lubricant called as sebum, which is discharged through tiny openings in the edges of the lids. It is responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's tear film and frictionless blinking.

The large round lens of the frog gives the animal a large field of view. The frog is naturally nearsighted (myopic) to -6 diopters giving it a focus of approximately 6 inches. Frogs and toads can change their focus by moving the lens out towards the cornea. 

Frogs use their regular set of eyelids to keep their eyes moist and clean. They have a bottom lid that stays still and an upper lid that blinks. Frogs also have a third eyelid that serves a different purpose. This one, called as the nictitating membrane, is a semitransparent lid that covers the eye completely, helping the frog see underwater and hide from predators. 

The tapetum lucidum is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina it reflects visible light back through the retina, increasing the light available to the photoreceptors. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores that hunt their prey at night, while others are deep sea animals. Choroidal guanine tapetum is seen in Elasmobranchii (skates, rays, and sharks) and chimaeras. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.

The conjunctiva is the thin film composed of non-keratinized stratified columnar epithelium and goblet cells that line the inside of the eyelids and covers the white of the eye called as the sclera. The conjunctiva lubricates the produces mucus and tears in small amounts to lubricate the eye and also prevents the entry of microbes into the eye.
Therefore, the correct answer is option C.

The person wearing spectacles with concave lenses for correcting vision suffers from myopia (near sightedness). In such a person, the image of distant object will be formed in front of retina, if he is not using the glasses.

The structures of the eye that deflect light is called refractive medium. Light is actually refracted through four media in the eye. The first is the cornea (the clear layer on the outside of the eye), then the aqueous humour (liquid), then the lens (which can change shape slightly to allow objects near and far to focus), and then there is the vitreous humour (liquid filling most of the inside of the eye) all of which light needs to pass through before it reaches the retina. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. Since, pupil only contacts and dilates, therefore, it is the one not a refractive medium of the eye. So, the correct answer is option D.