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Eye: Location, Protection, Structure, Mechanism of vision, Adaptation and Accommodation

Eyes are our gateways to the world around us. We are unable to picture any object, person, place or scenario, unless we see it with our own eyes. Out of the five senses, the sense of sight is the one that best connects us with our surroundings. Even when the other senses stop working, vision is the one that can keep us aware of what is going on around us. In fact, you will be amazed to know that 80 percent of all the impressions in our brain are perceived by our eyes and the processing of visual perception holds one-quarter of one’s brain.

Our eyes are so beautifully and technically designed that even the best quality camera cannot compete with them. Want to know more about the most efficient image capturing devices to have ever existed on this planet? Come, let’s learn about the structure of the eyes and how they help us see things. 

Table of contents

  • Location of the eye
  • Protection of the eyes
  • Structure of human eye
  • Mechanism of vision
  • Adaptation
  • Accomodation
  • Practice problems
  • FAQs

Location of the eye

The eyes are photoreceptor organs which can perceive images of objects which reflect light rays of wavelength 390-760 nm. The eyes are like hollow balls which are located in a bony socket in the skull called ‘orbit’. Each eyeball is approximately 2.5 cm in diameter and weighs about 7 grams. 

Fig: Eye sockets or orbits

Fig: Eye sockets or orbits

Fig: Side view of the hollow eyeball

Fig: Side view of the hollow eyeball

Six strap shaped muscles hold the eye in its socket and help in rotation of the eyeballs in different directions. These are the four rectus (superior, medial, lateral and inferior) and two oblique (superior and inferior) muscles.

Fig: Eye muscles

Fig: Eye muscles

Protection of the eyes

The eyeballs have a covering of fat around them which acts as a cushion and protects against mechanical shocks and injuries and allows frictionless movement of the eyeballs.The sphenoid and ethmoid bones of the skull protect the eyeballs.

Fig: Sphenoid and ethmoid bones

Fig: Sphenoid and ethmoid bones

Other than this, the eye brows, the eyelids and the eye glands also help in protecting the eye.

Eye brows

These are the supra-orbital arch-like structures that bear obliquely and outwardly growing hairs. They protect the eyeballs from dust, sweat and rain.

Eyelids

Eyelids are a pair or movable skin folds present over each eye. There is an upper and a lower eyelid covering each eyeball which frequently and involuntarily come together at intervals in an action known as blinking. Blinking not only helps in protecting the eye from the entry of foreign substances but also helps to spread the lacrimal secretion over the eyeball and moisten the cornea.

The edges of the eyelids bear stiff hairs known as eyelashes which help to prevent the entry of dust and rainwater into the eyes.

A small reddish patch is found in the inner corner of the eye, known as plica semilunaris. It is believed to represent the vestigial third eyelid or nictitating membrane in humans.

Fig: Eyelids and eyelashes

Fig: Eyelids and eyelashes

Eye glands

Glands of Moll

These are modified sweat glands present in between the bases of the eye lashes. The secretion from these glands help in lubrication.

Meibomian glands

These are modified sebaceous glands which open into the free margins of the eyelids and lubricate them. These glands also help in covering the cornea with an oil for frictionless blinking, holding tears over the cornea (front bulged part of the eyeball), hold dust particles, etc.

Lacrimal or tear glands

This gland is present in the upper part of the eye orbit. It secretes a watery fluid called tear which has lysozyme, water, sugar, amino acids, proteins, minerals, salt and urea. It is poured over the front part of the eyeball by 6-10 lacrimal ducts.

The lysozyme in tears is antibacterial in nature and protects the eyes. Apart from this, the tears also moisten and cleanse the eyeball. Tear is drained out by two lacrimal canaliculi each of which arise from the inner angle of the eye through an opening known as lacrimal punctum. 

The two lacrimal canaliculi join to form a lacrimal sac from which a nasolacrimal duct arises and passes down the tears to the nose.

Fig: Lacrimal system

Fig: Lacrimal system

Structure of the human eye

Wall of the eyeball

The wall of the eyeball is composed of three layers – outermost fibrous coat, middle vascular coat and innermost retina.

Outermost fibrous coat

The outermost layer of the eyeball is made up of two parts - sclera and cornea. Sclera or the scleroid layer is the white part of the eye made up of fibrous connective tissue rich in collagen fibres. This layer is supplied with blood vessels and provides the surface for the attachment of muscles. 

Fig: The outermost fibrous coat of eyeball

Fig: The outermost fibrous coat of eyeball

The anterior or front part of the scleroid, known as the cornea, is bulged and transparent and forms 1/6th of the outermost layer. Cornea is non vascular and is nourished by the aqueous humour, lacrimal secretion, lymph vessels and lymph capillaries. Cornea transplant surgery or keratoplasty is always successful because of its non-vascular nature due to which there is no supply of blood and the immune cells in the cornea. Thus, the foereign cornea is not rejected by our immune system.

The front part of the cornea and the exposed sclera is covered by a transparent membranous covering known as the conjunctiva. The conjunctiva has two parts - ocular (present on the front of cornea) and palpebral (present on the inner surface of the eyelids). Conjunctivitis is the inflammation of the conjunctiva due to microbial infection.

Middle vascular coat

The middle layer of the eyeball is pigmented and vascular and is divided into three parts - choroid, ciliary body and iris.

Fig: The middle vascular coat of the eyeball

Fig: The middle vascular coat of the eyeball

The choroid lines the sclera and contains maximum melanin pigment. The dark colour of the choroid helps to prevent blurring of the image due to internal reflection of light rays and helps to produce a sharp image. The choroid has a rich supply of blood vessels.

The iris is an opaque muscular pigmented and perforated diaphragm that lies in the anterior part of the middle layer. The central perforation of the iris is known as the pupil. 

The colour of the eye is due to the colour of the pigment in the iris. The iris possesses radial (dilator) and circular (sphincter) muscles which are innervated by nerves of the sympathetic and parasympathetic nervous systems. The size of the pupil is controlled by the contraction and relaxation of the radial and circular muscles of the iris in order to control the amount of light entering the eyes. In low light the pupil dilates due to contraction of the radial muscles and in bright light the pupil constricts due to contraction of the circular muscles. It is one of the mechanisms of how our eyes adapt to changes in light conditions.

Fig: Adjustment of pupil size in different light conditions

Fig: Adjustment of pupil size in different light conditions

At the junction of iris and choroid lies a thick vascular, less pigmented, ring shaped structure known as the ciliary body. The ciliary body possesses folds on the inner side known as ciliary processes. The ciliary body is composed of two types of muscles - circular and meridional. Elastic band shaped suspensory ligaments are attached to the lens on one end and the ciliary body on the other end. Together they are known as the accommodation apparatus as they help in changing the shape of the lens to focus at different distances.

Innermost retina

Retina is the innermost delicate, non-vascular, nervous coat of the eyeball. The retina has two parts - an outer pigmented part and inner nervous layer.

Fig: The retina

Fig: The retina

The pigmented part is made up of modified squamous epithelium and lies in contact with the choroid layer. The inner transparent and sensitive part of retina is made up of three layers - 

  • Outer photoreceptor layer consisting of photoreceptor cells of two types - rods and cones. It also has modified unipolar nerve cells.
  • Middle layer of bipolar nerve cells.
  • Inner layer of ganglion cells.

Fig: The layers of retina

Fig: The layers of retina

Rod cells 

The rod cells are responsible for vision in darkness or dim light, i.e, scotopic vision. There are around 120 million rod cells in our eyes. A rod cell has an outer pigmented part drawn out into a rod and the inner end contains a nucleus.

Fig: Rod cells of retina

Fig: Rod cells of retina

Rod cells possess a protein named opsin and the pigment retinene (derived from Vitamin A). In dim light, the enzyme retinene isomerase helps in the formation of a photosensitive pigment named rhodopsin which helps in absorbing light of low intensity and helps to create images in dim light. Rhodopsin is bleached in strong light and splits into opsin and retinene.

Deficiency of Vitamin A results in reduced production of rhodopsin which leads to night blindness or no vision in dim light or darkness.

Cone cells

Each cone cell has an outer pigmented part and branched inner part. Around 7 million cone cells are present in our eyes. Cone cells produce a photosensitive pigment called iodopsin. Cones are responsible for vision in bright light and they are also responsible for colour vision. 

There are three different types of cone cells for responding to three different colours of light rays - red, green and blue. The different pigments in these cells which are responsible for absorbing the light rays of different wavelengths are under the control of different genes. Defects in any one of these genes can lead to colourblindness of the following types -

  • Red colour blindness or protanopia
  • Green colour blindness or deuteranopia
  • Blue colour blindness or tritanopia

Fig: Cone cells of retina

Fig: Cone cells of retina

The red and green colour blindness is an X linked disorder as the genes for the red and green cones are present on the X chromosome. The blue colour blindness is caused due to a defect in a gene present on the 7th chromosome.

Blind spot and Yellow spot

The blind spot is the point on the retina where the optic nerve (nerve connecting the eye to the brain) leaves and the blood vessels enter. This regions has no photoreceptor cells and thus any image formed here is not perceived by the brain. Thus, it is the area of no vision or the blind spot.

Fig: Blind spot

Fig: Blind spot

Lateral to the blind spot is an area rich in xanthophyll pigments which is known as the yellow area or macula lutea and is the spot of image formation.

Fig: Macula lutea

Fig: Macula lutea

The central thinned out area of the macula lutea is slightly depressed and possesses the highest number of cone cells. This is known as the yellow spot or fovea centralis and is the point of highest vision with highest acuity and resolution. With the increase in distance from the yellow spot to iris, the number of cone cells decreases and number of rod cells increases.

Fig: Fovea centralis

Fig: Fovea centralis

Lens 

The lens is the transparent, elastic, biconvex structure which is suspended in the cavity of the eyeball, behind the pupil, with the help of the suspensory ligaments. It is covered by a thin transparent membrane known as the lens capsule. It is made up of layers of non - nucleated elongated cells and intracellular proteins. The protein comprising the lens is known as crystallin.

Fig: Lens

Fig: Lens

Aqueous Chamber

It is the chamber that lies between the cornea and the lens. This is further divided into an anterior aqueous chamber between the cornea and iris and posterior aqueous chamber between the pupil and the lens. 

The aqueous chamber is filled with a watery fluids known as the aqueous humor which is continuously secreted by the ciliary processes and excess aqueous humor is drained out through the canal of Schlemm present at the junction between the sclera and cornea.

The aqueous humor helps to maintain the shape of the eyeball and also provides nutrition to the lens and the cornea.

Fig: Aqueous chamber

Fig: Aqueous chamber

Vitreous Chamber

This chamber lies between the lens and the retina and is filled with a non-replaceable jelly-like vitreous humor which is made up of mucopolysaccharides. The vitreous humor is mainly responsible for maintaining the shape of the eyeball and maintaining even pressure within the eyeball.

Fig: Vitreous chamber

Fig: Vitreous chamber

Mechanism of vision 

The process of visual perception is a highly complicated process. It comprises three main sections –

  • Optics of the eye
  • Detection of photon and the first image processing in the retina
  • Signal transmission and hence the processing of the visual cortex of the brain

To begin with, the light reflected by any object that we see is focussed on to the retina so that a sharp image is formed on the fovea centralis. Focussing is carried out by the cornea as well as the lens. The aqueous and vitreous humor help in keeping the light rays in the proper path.the conjunctiva, cornea, aqueous humor, lens and vitreous humor refract light rays entering the eye such that the light rays converge and form an image on the retina. Maximum refraction is caused by the bulged cornea which ensures image formation exactly on the fovea centralis. The lens helps to brig about changes in focal length and fine focussing.

Fig: Image formation on retina

Fig: Image formation on retina

The light rays in visible wavelength focussed on the retina through the cornea and lens generate potentials (impulses) in rods and cones. The light stimulus perceived by rods and cones results in the dissociation of the photosensitive pigments. The photosensitive compounds (photopigments) in the human eye are composed of a protein (opsin) and retinene/retinal (an aldehyde of vitamin A). Light induces dissociation of the retinene from the protein part which results in change in the structure of opsin. Changes in opsin causes activation of the regulatory protein transducin which stimulates phosphodiesterase enzyme to convert cyclic guanosine monophosphate or cGMP to 5’-GMP. cGMP keeps Na+ channels open while 5’-GMP opens them. As a result, the membrane potential of the photoreceptor cells changes and potential differences are generated in the photoreceptor cells. 

Fig: Dissociation of photopigment

Fig: Dissociation of photopigment

This potential difference acts as a signal and helps in the generation of action potentials in the ganglion cells through the bipolar cells. These action potentials or impulses are passed from the retina to the visual cortex of the brain through the optic nerve. The visual cortex analyses the neural impulses and recognises the image formed on the retina based on earlier memory and experience.

Fig: Visual cortex

Fig: Visual cortex

Adaptation

Have you ever noticed that when you are in a room with dim lights and suddenly someone switches on the bright lights, you get blinded for some time and it takes time for your vision to adjust. The same thing happens when you have been sitting under bright lights and suddenly the power switches off. It takes time for our eyes to adjust to the darkness.

The ability of the eyes to adjust to changing conditions of bright light and darkness is known as adaptation.

Dark adaptation

Dark adaptation occurs when we enter into a dark or very dimly lit place from a brightly lit area. The size of the pupil dilates to obtain maximum light and there is reformation of rhodopsin which was bleached in the presence of bright light earlier. Thus vision in darkness is restored.

Light adaptation

Light adaptation occurs when we enter into a brightly lit place from a dark or dimly lit place. The pupil constricts to limit the amount of light entering the eyes and the rhodopsin is bleached and iodopsin helps in vision in bright light. 

Accommodation 

The ability of the eye to focus at objects at different distances is known as the power of accommodation. The least distance of distinct vision for a normal eye is 25 cm and the maximum distance is infinity.

Focussing light rays coming from different distances, on the retina for image formation, is done by changing the convexity of the lens with the help of the ciliary muscles and the suspensory ligaments which are together known as the accommodation apparatus of the eye.

Focussing on distant objects (6 metres or more)

Our eyes, in their resting condition, are adjusted to focus at distant objects. For this, the ciliary muscles are relaxed and the suspensory ligaments are stretched , making the lens less convex which helps to increase the focal length of the lens and allows it to form clear images of distant objects.

Fig: Focussing on distant objects

Fig: Focussing on distant objects

Focussing on nearby objects (within 6 metres)

For focussing on nearby objects, the ciliary muscles contract and the suspensory ligaments relax to make the lens more convex. This reduces the focal length of the lens and allows it to form sharp images of nearby objects. This condition of the eye is said to be the tension stage.

Fig: Focussing on nearby objects

Fig: Focussing on nearby objects

Practice problems

Q1. The main function of the cornea present in the human eye is

(a) structural support to the eye
(b) bends light before it reaches the lens
(c) changes the shape of the lens enabling image to be focused on the retina
(d) contains a concentrated amount of cone cells on the correct orientation

Solution: The front part of the outermost fibrous coat of the eyeball is bulged and transparent and is known as the cornea. The main function of the cornea is to cause maximum bending or refraction of light before it reaches the lens. This ensure focussing of the light rays on the retina for image formation. The lens further refracts the light rays and does fine focussing to form an image precisely on the fovea centralis of the retina. Thus, the correct option is (b).

Q2. The fovea in the mammalian eye is the centre of the visual field wherein

(a) the optic nerve exits the eye
(b) only rods are found
(c) more rods than cones are found
(d) no rods but a high density of cones occur

Solution: Lateral to the blind spot, there is a yellowish pigmented spot called macula lutea with a central depression called the yellow spot or fovea centralis. The retina thins out at the fovea. The fovea has the highest concentration of cone cells and no rod cells. Thus it is the site of best vision with highest acuity and resolution.

Thus, the correct option is (d).

Q3. The innermost layer and the most delicate layer of the eyeball where the photoreceptors are located are

(a) Choroid

(b) Sclera

(c) Cornea

(d) Retina

Solution: Retina is the innermost delicate, non-vascular, nervous coat of the eyeball. The retina has two parts - an outer pigmented part and inner nervous layer. The inner transparent and sensitive part of the retina consists of an outer photoreceptor layer consisting of photoreceptor cells of two types - rods and cones, middle layer of bipolar nerve cells and inner layer of ganglion cells. Thus, the correct option is (d).

Q4. In order to focus at an object which is 4 metres away from our eyes -

(a) both the ciliary muscles and suspensory ligaments relax

(b) the ciliary muscles contract and the suspensory ligaments relax

(c) both the ciliary muscles and suspensory ligaments contract

(d) the ciliary muscles relax and suspensory ligaments contract

Solution: For focussing on objects which are within 6 metres, the ciliary muscles contract and the suspensory ligaments relax to make the lens more convex. This reduces the focal length of the lens and allows it to form sharp images of nearby objects. 

Thus, the correct option is (b).

FAQs

Question 1. Why is red and green colour blindness more common in males?
Answer: Red and green colour blindness occurs due to defects in the genes which code for the pigments responsible for seeing red light and green light. These genes occur on the X chromosome. Human males have a single X chromosome and presence of a defective gene on the X chromosome renders them colour blind. 

However, human females have two X chromosomes and the defective genes being recessive, can express only when both the X chromosomes carry the defective genes, i.e, in homozygous condition. This occurs rarely and in the presence of a single defective X chromosome, the normal X chromosome dominates and the female becomes a carrier only.

Question 2. Why should we not watch television from a close distance?
Answer: Watching television from a close distance, for long hours, causes headache and strains our ciliary muscles. For focussing on objects closer than 6 metres from the eye, our ciliary muscles contract to make the eye lens more convex. Continued contraction can cause fatigue in the ciliary muscles leading to headache, blurry vision, eye strain, etc.

Question 3. Why does our nose run everytime we cry?
Answer: Crying results in release of tears from the lacrimal glands. Tears are drained through the nasolacrimal duct which opens into the nose, causing our nose to water when we cry.

Question 4. What is heterochromia?
Answer: Heterochromia is a condition in which a person has different coloured eyes due to different colour of iris in the two eyes. It can be caused due to genetic mutation, as a symptom of other syndromes or due to injuries to the eye. Heterochromia does not harm the individual in any way.

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Related Topics

Types of Vision

Defects in Vision 

 

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