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Structure of the human eye, Practice Problems and FAQs

We live in a beautiful world and are able to enjoy the things around us. Starting from the small insects to the beauty of large trees we are able to enjoy. You know that we see the beauty of our surroundings through our eyes. The eyes are the main sense organs that give a sense of vision and it connects us best with the surroundings. Can you imagine a situation where you are not able to see anything around you? Yes, we cannot imagine that. As a biology student, have you ever wondered what is present inside this fascinating structure?

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                                                     GIF: Eye

Do you know which part of an eye acts as a camera? Yes, it is an iris that regulates the amount of light entering the eye. It is regulated by constricting or dilating the pupil. Similarly, do you know which part of an eye acts as a film of a camera? It is the retina where an image is formed like a film. Like this eye possesses many parts that have their own specific functions. All these parts along with their functions we are going to discuss in depth in this article.

Table of contents

  • Human eye
  • Structure of the human eye
  • Practice Problems
  • FAQs

Human eye

Human eyes are specialised organs that are capable of receiving visual information from the surroundings and then transfer it to the brain. As photoreceptor organs, the eyes are able to see images of things that reflect light with a wavelength of 390 - 760 nm. The eyes, which resemble hollow balls, are housed in the skull's ‘orbit,’ a bony socket. Each eyeball has a diameter of roughly 2.5 cm and weighs around 7 grams.

Orbit

The orbit is described as the bony cavity that includes various structures, such as eyeball, muscles, nerves, and blood vessels. It also contains those structures that produce and drain tears. Each orbit is a pear-shaped structure that is composed of several bones. They are cushioned in fatty connective tissues.


                           Fig: Orbit

Eye muscles

There are six strap-shaped muscles that are responsible for holding the eye in its socket. These muscles help in eyeball rotation in different directions. These muscles are categorised into four rectus muscles and two oblique muscles. These are as follows:

  • Superior rectus
  • Medial rectus
  • Lateral rectus
  • Inferior rectus
  • Superior oblique
  • Inferior oblique

Superior rectus

The superior rectus originates behind the eye on the common ring tendon, inserting at the anterior (front) region of the eye. Its main function is to elevate the eye, while its minor secondary functions are adduction (inward rotation) and intorsion (rotating the top of the eye towards the nose).

Medial rectus

The medial rectus originates behind the eye on the common ring tendon, inserted at the anterior (front) region of the eye. Its primary function is adduction and side to side movement of the eye.

Lateral rectus

The greater wing of the sphenoid bone and the common ring tendon is located behind the eye, where the lateral rectus muscle originates. This muscle inserts at the anterior (front) part of the eye. It helps in the abduction and side to side movement of the eye along with the medial rectus.

Inferior rectus

The inferior rectus originates behind the eye on the common ring tendon and is inserted at the anterior (front) region of the eye. Its primary function is to depress the eye, and its minor secondary functions are extortion and adduction.

Superior oblique

The superior oblique muscle is a unique muscle. It inserts on the front, back, and lateral (ear-side) surfaces of the eye. The superior oblique muscle has its physical origin behind the eye on the lesser wing of the sphenoid bone, however, it operates as a pulley and loops back through a connective tissue sling known as the trochlea. Despite being located above the eye, its distinctive use of the trochlea grants it a primary function of intorting the eye as well as secondary functions of depression and abduction.

Inferior oblique

It connects to the lateral, inferior, and posterior portions of the eye. The medial (middle) maxillary bone is where it originates. Extorsion is its primary function, and abduction and elevation are its secondary purposes.


                                                  Fig: Muscles of an eye

Orbital bones

The eyeball's protection and proper positioning within the skull are maintained mostly by the orbit and orbital bones. It is composed of several bones of the skull that form a four-sided pyramid. The orbit's roof is made up of the orbital plate of the frontal bone, which is followed by the lesser wing of the sphenoid. The orbit's floor is composed of fragments of the maxilla, zygomatic, and palatine bones. The optic foramen is the opening through which the optic nerves return to the brain. Through this opening, the large ophthalmic artery also enters the orbit. The optic foramen is present at the nasal side of the apex. On the other hand, there is a larger hole called the superior orbital fissure through which large nerves and veins pass.


                                                 Fig: Orbit

Protective structures of the eye

The protective structures of the eye include eyebrows, eyelids, eyelashes, eye glands and adipose tissue.

Eyebrows

These resemble supra-orbital arches and are covered in hairs that are growing outward and obliquely. They shield the eyes from sweat, rain, and dust. The dense sensory innervations in the thick eyebrow hairs give them an acute awareness of tactile stimuli and aid in the detection of objects close to the eye like small insects.


                        Fig: Eye brow

Eyelids (Palpebrae)

Over each eye are a pair of moveable skin folds called eyelids present. Each eyeball has an upper and lower eyelid that covers it; these eyelids frequently and involuntarily come together at regular intervals to showcase the blinking motion. In addition to preventing foreign objects from entering the eye, blinking helps circulate lacrimal secretion throughout the eyeball and wet the cornea. The eyelid serves a variety of purposes, all of which combine to protect the eye and preserve a healthy ocular surface.

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                           GIF: Movement of eyelids

Functions of the eyelids

The functions of the eyelids are as follows:

  • Voluntary eyelid opening and closure.
  • Tonic eyelid raising for prolonged eye-opening.
  • Spontaneous and reflex blinking.
  • Protect the eyes from dust particles and small insects.
  • Helps to express oneself by closing the eyelids firmly.

Eyelashes

Eyelashes are stiff hairs on the edges of the eyelids, which help to keep dust and wetness out of the eyes. The plica semilunaris is a little reddish patch which is located in the inner corner of the eye. It is thought to be a representation of the human third eyelid or nictitating membrane.


                                     Fig: Eyelids and eyelashes

Eye glands

There are four glands present in an eye:

  • Meibomian glands
  • Glands of Zeis
  • Glands of Moll
  • Glands of Wolfring

Meibomian glands

These types of glands are present in the tarsal plate of an eye. These modified sebaceous glands lubricate the free margins of the eyelids by opening into them. These glands aid in retaining tears over the cornea (the front bulged region of the eyeball), covering the cornea with oil to prevent frictional blinking, holding dust particles, etc.

Glands of Zeis

These are unilobar sebaceous glands that open into the follicles of the eyelashes. Through the sebaceous lobule's excretory channels, these glands release an oily fluid that enters the centre of the hair follicle.

Glands of Moll

These are modified sweat glands that are located in the space between the bases of the eyelashes. These glands produce a fluid that aids in lubrication.

Glands of Wolfring

These are accessory lacrimal or tear glands present in the upper part of the eye orbit. It secretes tears.

Tears

It contains lysozymes, water, sugars, amino acids, proteins, minerals, salts, and urea, The normal pH range of tears is 6.5 to 7.6. But as per the studies of Carney and Hill, tears are more acidic in the morning (pH 7.25) than later in the day.


                        Fig: Tears

Structure of the tear glands

Six to ten lacrimal ducts pour it over the front of the eyeball. Two lacrimal canaliculi, one on each side of the inner angle of the eye, emerge through the lacrimal punctum to drain the tears. A lacrimal sac is created when the two lacrimal canaliculi unite. From this sac, a nasolacrimal duct emerges, which drains tears into the nostrils.


                                  Fig: Structure of the tear glands

Functions of tears

The following are the major functions of the tears:

  • Tears include lysozyme, an antimicrobial enzyme that safeguards the eyes.
  • Additionally, tears hydrate and clean the eyeball.
  • It protects the eye from infections.
  • It also protects the eye from irritating subjects like dust particles.
  • It aids the immune system.
  • It also occurs as the body's natural response to pain.

Adipose tissue

A layer of adipose tissue surrounds the eyeball in the orbit. It serves as a soft shock proof cushion.

Structure of the human eye

The structure and function of a human eye are very complex. Each eye continuously modifies the quantity of light it allows in, focuses on both close and far objects, and creates continuous images that are immediately transmitted to the brain. The human eye consists of the following parts:

  • Fibrous coat
  • Sclera
  • Conjunctiva
  • Cornea
  • Iris
  • Ciliary body
  • Pupil
  • Lens
  • Retina
  • Optic nerve
  • Aqueous chamber
  • Vitreous chamber


                                                   Fig: Structure of an eye

Fibrous coat

It is the outermost thick and tough coat made up of dense connective tissue. It maintains the form of an eyeball and provides a firm surface for the insertion of eye muscles. It possesses sclera and cornea.

Sclera

The white portion of the eye, known as the sclera or scleroid layer, is made up of fibrous connective tissue that is abundant in collagen fibres. Blood vessels are present in this layer, which also provide a surface for the attachment of muscles.


                                   Fig: Sclera of an eye

Cornea

Cornea is the anterior or front part of the scleroid, which is bulged and transparent. It forms the 1/6th part of the outermost layer. The aqueous humour, lacrimal secretion, lymph arteries, and lymph capillaries all supply the non-vascular cornea. Due to the cornea's non-vascular nature, which prevents blood and immune cells from supplying it, cornea transplant surgery, also known as keratoplasty, is always successful. Our immune system does not, therefore, reject the foreign cornea. This part absorbs oxygen from the air.


                                      Fig: Cornea of an eye

Conjunctiva

The conjunctiva is a translucent membranous layer that covers the front portion of the cornea as well as the exposed sclera.


                      Fig: Conjunctiva

Regions of conjunctiva

The conjunctiva possesses three regions as follows:

  • Bulbar or ocular conjunctiva - It covers the outer surface of the eye.
  • Palpebral conjunctiva - It lines the eyelids.
  • Fornix conjunctiva - It is a loose soft tissue lying at the junction between the bulbar conjunctiva and the palpebral conjunctiva.


                               Fig: Regions of conjunctiva

Conjunctivitis

The inflammation of the conjunctiva brought on by microbial infection is known as conjunctivitis or pink eye.


                   Fig: Conjunctivitis

Iris

The iris is described as a circular and colored area of the eye. It gives various shades to the eye like brown, green, blue etc. It separates the eye cavity into anterior and posterior chambers. It is present around the pupil that controls the amount of light that enters the eye. It possesses two types of muscles like circular muscles and radial muscles. The iris is responsible for regulating the amount of light that enters the eye. It dilates the pupil when the environment is dark and allows more light to enter the eye. On the contrary, it constricts the pupil when the environment is bright and allows less light to enter the eye. Thus, as the amount of light in the local environment varies, the pupil expands and contracts like the aperture of a camera lens. The pupillary sphincter and dilator muscles work together to regulate the pupil's size.


                                            Fig: Iris

Pupil

It is the little opening at the centre of the iris. It allows light to pass through and concentrate on the retina.


                                          Fig: Dilation and constriction of pupil

Ciliary body

It is a thickened ring shaped muscular structure present at the junction of choroid and iris. It contains circular and meridional muscles. The ciliary muscles change the shape of the lens when eyes focus on a near object. It also produces the fluid in the eye called aqueous humour.


                                             Fig: Ciliary body

Lens

The lens is located behind the iris. It is a biconvex and elastic body made up of crystalline proteins. It is enclosed in a thin transparent elastic membrane called lens capsule. The lens focuses light onto the retina by altering its shape. The lens changes thickness to concentrate on local objects and gets narrower to focus on distant objects due to the action of tiny muscles (known as the ciliary muscles).


                                                   Fig: Lens

Retina

The retina is described as the innermost layer which is non-vascular in nature and forms a nervous coat of the eyeball. It is composed of two parts as follows:

  • An outer pigmented part
  • The inner nervous layer


                                                 Fig: Retina

The outer pigmented part

It is composed of modified squamous epithelium and is present in contact with the choroid layer. The cells of this layer contain dark brown pigment granules and protoplasmic processes extending into the next layer. This layer reinforces the light absorbing property of choroid and reduces the scattering of light in the eye.

The inner nervous part

On the other hand, the inner nervous part is a transparent and sensitive part of the retina. The macula, a small region of the retina with millions of densely packed photoreceptors (the type called cones). It is the most sensitive component of the eye and is composed of many layers as follows:

  • Photoreceptor layer: Rods and cones are the two types of photoreceptor cells that make up the outer photoreceptor layer. Additionally, it has altered unipolar nerve cells.
  • Middle layer of bipolar nerve cells.
  • Inner layer of ganglion cells.


                                       Fig: Macula

Photoreceptor cells

The function of the photoreceptor cells in the retina is to convert the images into electrical signals. These electrical signals are then carried to the brain through the optic nerve. There are two main types of photoreceptors as follows:

  • Cones
  • Rods

Cones

Cones are mostly grouped in the macula and are responsible for sharp, detailed central vision as well as colour vision. A cone cell is composed of an outer pigmented part and a branched inner part. In human eyes, around 7 million cone cells are present. They produce a photosensitive pigment known as iodopsin. Cone cells come in three different varieties, each of which can respond to the red, green, and blue light spectrum. The pigments of these cone cells are responsible for absorbing the light rays of different wavelengths and these pigments are under the control of different genes. If any of the genes got defected, it will cause colour blindness of the following types:

  • Red colour blindness or protanopia - A condition in which the red cone cells are absent.
  • Green colour blindness or deuteranopia - This condition makes green look more red.
  • Blue colour blindness or tritanopia - In this condition the person cannot distinguish between blue and yellow colours.


                                    Fig: Cone

Rods

Rods are responsible for peripheral (side) vision and night vision (scotopic vision). Rods are more numerous than cones and far more sensitive to light, but unlike cones, they do not detect colour or support fine central vision in detail. The peripheral regions of the retina are where rods are primarily grouped. Our eyes contain about 120 million rod cells. A rod cell has a pigmented outside portion that is drawn out into a rod shape, and its nucleus is located at the inner end. Rod cells have an opsin protein and a pigment retinene which is derived from Vitamin A. Rhodopsin, a photosensitive pigment that aids in absorbing low-intensity light and aids in the creation of pictures in low light. It is formed in low light due to the enzyme retinene isomerase. Strong light causes rhodopsin to bleach and separate into opsin and retinene.


                           Fig: Rod

Difference between rods and cones

The following are the major differences between rods and cones

Rods

Cones

In rods the outer pigmented part is rod shaped

In cones the outer pigmented part is cone shaped

They are sensitive to dim light

They are sensitive to bright light

These photoreceptors contains rhodopsin pigment formed from vitamin A

These photoreceptors contains pigments like iodopsin, porphyropsin and cyanopsin

They do not help in seeing colours

They help in seeing colours.

Fig: Rod

Fig: Cone

Blind spot

The blind spot is the region of the retina where blood vessels enter and the optic nerve (the nerve that connects the eye to the brain) leaves. Since there are no photoreceptor cells in this area, the brain cannot perceive any images that are created there. As a result, it is the blind zone or area with no vision.


                                 Fig: Blind spot

Yellow spot

A region known as the yellow area, or macula lutea, is the location of image creation and is located lateral to the blind spot. The central region of the macula lutea is thin and slightly depressed. This region has the highest number of cone cells. This region is known as the yellow spot or fovea centralis and is characterised as the point of highest vision with the highest accuracy and resolution. Cone cell density declines and rod cell density rises as the distance between the yellow spot to the iris increases.


                              Fig: Yellow spot

Optic nerve

They are commonly found in the posterior portion of the eyes. All of the retina's nerve impulses that are needed for perception are transported by the optic nerves to the brain.


                                  Fig: Optic nerve

Chambers of the eyeball

The lens and its suspensory ligaments divide the cavity of the eyeball into two chambers as follows:

  • Aqueous chamber
  • Vitreous chamber

Aqueous chamber

The aqueous chamber is present between the cornea and the lens. This is further separated into a posterior aqueous chamber located between the pupil and the lens and an anterior aqueous chamber situated between the cornea and iris. The ciliary processes continuously secrete a liquid known as aqueous humour, which fills the aqueous chamber. Excess aqueous humour drains out through the canal of Schlemm, which is located at the junction of the sclera and cornea. The aqueous humour not only nourishes the cornea and lens but also aids in maintaining the shape of the eyeball.


                              Fig: Aqueous chamber

Vitreous chamber

The vitreous chamber is present between the lens and the retina. This chamber is filled with a non-replaceable jelly-like substance called a vitreous humour and it is composed of mucopolysaccharides. The vitreous humour is primarily responsible for keeping the eyeball in shape and ensuring consistent pressure inside the eyeball.


                                   Fig: Vitreous chamber

Practice Problems

  1. Which structure in the eye gives shape to the eyeball?
  1. Sclera
  2. Aqueous humour
  3. Vitreous humour
  4. Choroid

Solution: The vitreous humour is primarily responsible for keeping the eyeball in shape and ensuring consistent pressure inside the eyeball. It is filled with vitreous humour. It is present between the lens and the retina. Hence, the correct option is c.

2. From the given options, which are the external structures of an eye?

  1. Vitreous Humour
  2. Cornea
  3. Iris
  4. Both (b) and (c)

Solution: The external structures of an eye are those that are visible externally. These structures include:

  • Sclera
  • Conjunctiva
  • Cornea
  • Iris
  • Pupil

Cornea is the anterior or front part of the sclera, which is bulged and transparent. The iris is described as a circular and colored area of the eye. It is present around the pupil that controls the amount of light that enters the eye. Hence, the correct option is d.

3. In humans, the eyeball is moved by _______________.

  1. four rectus and two oblique muscles
  2. four oblique and two rectus muscles
  3. the ciliary muscles
  4. the ciliary body and optic peduncle

Solution: There are six strap-shaped muscles that are responsible for holding the eye in its socket. These muscles help in eyeball rotation in different directions. These muscles are categorised into four rectus muscles and two oblique muscles as follows:

  • Superior rectus
  • Medial rectus
  • Lateral rectus
  • Inferior rectus
  • Superior oblique
  • Inferior oblique

Hence, the correct option is a.

4. The image of an object forms on which structure in the human eye?

  1. Cornea
  2. Iris
  3. Pupil
  4. Retina

Solution: The retina is an inner lining in the back of the eye that is sensitive to light. It serves as the screen on which an object's image is created. Light is detected by sensitive cells present inside of it, which then translate it into electrical impulses. The outer layer of the eye is known as the cornea and is responsible for light refraction. The iris, a coloured, muscular portion of the eye that surrounds the pupil and regulates the quantity of light passing through it. It gives eyes their colour. Light enters the eye through an aperture called the pupil, which is a small, black dot in the middle of the eye. Hence, the correct option is d.

5. Write down the differences between aqueous humour and vitreous humour?

Answer: The following are the major differences between the aqueous humour and vitreous humour.

Aqueous humour

Vitreous humour

It occur in the anterior aqueous chamber

It occurs in the posterior vitreous chamber

It is a watery fluid

It is a viscous fluid

It is drained of into the canal of Schlemm and then into the blood

It is not drained off or replaced

It contains most of the diffusible substances along with water

It contains 90% water and many substances like proteins, collagen fibres, hyaluronic acids, vitrein etc

Obstruction in its flow may damage retina and may cause glaucoma

It does not flow

It maintains the shape of the cornea

It maintains the shape of the eyeball

It supports the lens and supplies nutrition to lens and cornea

It supports lens and retina

Fig: Aqueous humour

Fig: Vitreous humour

FAQs

  1. Why do owls have better vision than humans?

Answer: The owls have better vision than humans because they can rotate their eyes at 270 degrees without rotating their bodies. Behind the retina in the owl’s eye, there is another layer called the tapetum lucidum present which is able to catch any light that may pass through the retina and bounces it back to the sensitive rods. Moreover, they have a large number of rod cells as compared to humans which makes them more sensitive to dim light.

  1. Which eye colour is most and least common in human populations?

Answer: The most common eye colour is brown in the human population. Blue and hazel are less common. The least common eye colour is green.


                         Fig: Eye colour in human beings

  1. What is the radius of a normal human eyeball?

Answer: The diameter of a normal human eyeball is 2.3 cm and therefore, the radius is half of it, i.e. 1.15 cm.

  1. What is the range of vision for a normal human eye?

Answer: For a normal human eye, the range of vision lies between the near point and far point, i.e. from 25 cm to infinity.

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