Human Eye Structure and Function

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Questions and Answers

What is the function of the eyelids and eyelashes?

Prevent entry of dust, excessive light, wind, and foreign bodies from entering the eye (correct)

Distribute warmth

Lubricate the eye

Create tears

Rods in the retina provide color vision.

False

What is the function of the choroid?

The choroid contains blood vessels and dark pigments that absorb excessive light rays, preventing excessive refraction in the eye and blurring. Additionally, the blood vessels in the choroid supply food and oxygen to the cells of the retina.

The yellow spot in the retina contains mainly __________.

cones Signup and view all the answers

What is the shape of a convex surface?

Bulgy Signup and view all the answers

What happens to light rays when they pass through a concave surface?

They diverge Signup and view all the answers

What is the process called when the eye forms a finely-focused image on the retina?

Accommodation Signup and view all the answers

What happens to the lens when the eye is focused on a close object?

It becomes more convex Signup and view all the answers

When does the process of accommodation take place?

When viewing objects nearer than 6 m Signup and view all the answers

What is the state of the ciliary muscles during distance vision?

Relaxed Signup and view all the answers

What is the shape of the lens during distance vision?

Less convex Signup and view all the answers

What happens to the suspensory ligament during close vision?

It slackens Signup and view all the answers

What is the correct order of light passing through the eye?

Cornea, Aqueous humour, Pupil, Lens, Vitreous humour, Retina Signup and view all the answers

How many times is light refracted during its passage into the eye?

Three times Signup and view all the answers

What is the shape of the lens that bends light to converge on the retina?

Convex Signup and view all the answers

What is the result of light stimulation on photoreceptors?

Breaking down of photopigments Signup and view all the answers

Where do the impulses from the photoreceptors travel to?

Optic nerve Signup and view all the answers

What is the result of the overlap of visual fields from both eyes?

A single three-dimensional image Signup and view all the answers

What is the characteristic of the cornea that affects its refractive power?

It has the same curvature always and its refractive power is unchanging Signup and view all the answers

What is the characteristic of light that is refracted by convex or concave surfaces?

It is bent Signup and view all the answers

Study Notes

Eye Structure and Protection

The eye is situated in a bony socket, which protects it from mechanical injury.
The bony socket is formed by the cranium, cheek, and brow bones.
Eyelids and eyelashes prevent entry of dust, excessive light, wind, and foreign bodies into the eye.
Fat deposition at the back of the eye acts as a shock absorber.
The eye is held in position by six extrinsic muscles.

Tears and Lachrymal Fluid

Tears wash away dust, destroy germs, and prevent dessication.
Lachrymal fluid lubricates the eye, allowing it to move freely.
Glands of Meibomian open on the inner margin of the eyelids, secreting an oily fluid.

Conjunctiva and Sclera

The conjunctiva is an extension of the eyelid lining, with pain receptors that cause the eye to close when foreign matter enters.
It also lubricates the eye to prevent dessication.
The sclera is a tough, white, inelastic layer made of connective tissue, covering the back of the eye.
It protects internal parts of the eye, attaches to the six extrinsic muscles, and maintains the eye's shape.

Cornea and Choroid

The cornea is continuous with the sclera, more convex, and has no blood vessels, making it transparent and allowing light rays to pass through.
The greatest refraction (bending of light) occurs in the cornea.
The choroid is a thin, dark layer containing blood vessels and dark pigments, which absorb excessive light and prevent blurring.
The choroid supplies food and oxygen to the retina cells.

Ciliary Body and Iris

The ciliary body is an extension of the choroid, containing circular ciliary muscles that control the lens curvature during accommodation.
The iris is a circular, colored curtain with a hole in the middle (pupil), controlling the amount of light entering the eye.
The iris contains brown pigments and circular and radial muscles that work antagonistically.

Pupillary Mechanism

In bright light, circular muscles contract, and radial muscles relax, constricting the pupil size.
In dim light, circular muscles relax, and radial muscles contract, dilating the pupil size.

Retina and Photoreceptors

The retina is the innermost layer of the eye, receiving focused light.
It consists of two photoreceptors: rods and cones.
Rods are thin, elongated cells containing rhodopsin, providing night vision, peripheral vision, and detecting movement.
Cones are fatter cells containing iodopsin, providing color vision and sharp, clear vision.

Blind Spot and Yellow Spot

The blind spot is the area where the optic nerve leaves the eyeball, lacking rods and cones.
The yellow spot (macula lutea) contains mainly cones, providing the highest visual acuity.
The fovea centralis is the center of the yellow spot, containing only cones.

Lens and Cavities

The lens is a round, biconvex, flexible, and transparent structure with no blood vessels.
It changes shape to allow sharp, precise focusing of light onto the retina.
The posterior cavity is larger than the anterior cavity, filled with vitreous humor, and supports the lens and retina.
The anterior cavity is filled with aqueous humor, providing nutrients and oxygen to the lens and cornea, and carrying away metabolic waste.

Pathway of Light Rays and Image Formation

Light passes through the following structures in the eye: cornea, aqueous humour, pupil, lens, vitreous humour, and retina, exciting the photoreceptors
Photoreceptors convert light stimuli into nerve impulses, which are transmitted to the brain
Light is refracted (bent) three times: through the cornea, on entering the lens, and on leaving the lens
The cornea has a fixed curvature, providing a constant refractive power
The lens is elastic and can change its curvature, bending light to converge on the yellow spot on the retina
The resulting image is smaller, upside down, and reversed

Stimulation of Photoreceptors

Rods and cones in the retina are stimulated by light, breaking down photo pigments
This breakdown generates electrical impulses in the photoreceptors

Pathway of Nerve Impulses

Impulses from photoreceptors travel along two layers of neurons
The axons of ganglion neurons form the optic nerve, which leaves the eye at the blind spot and carries impulses to the cerebral cortex
Impulses are interpreted as vision in the occipital lobe of the cerebral cortex

Binocular Vision

Both eyes have overlapping visual fields, allowing for depth perception and judging distance and size of objects
The brain combines information from both eyes to form a single three-dimensional image

Basics of Light Refraction

Light is bent/refracted by convex or concave surfaces
Convex surfaces cause light to converge (come together) at a point, while concave surfaces cause light to diverge (bend outward)
The shape of the lens changes to accommodate close or distant vision

The Process of Accommodation

Accommodation is the process of producing a finely-focused image on the retina
It is carried out by the action of the ciliary muscles, which change the shape of the lens
The more convex the lens, the more light rays are bent
The flatter the lens, the less light rays are bent

Accommodation for Close Vision

Close vision occurs when viewing objects nearer than 6 m
The eye accommodates by making active adjustments to form a clear, sharp image on the retina
The lens bulges out more to bend light rays and focus the image on the retina

Accommodation for Distant Vision

Distant vision occurs when viewing objects farther than 6 m
The eye accommodates by making adjustments to form a clear, sharp image on the retina
The ciliary muscles are relaxed, the suspensory ligaments tighten, and the lens is as thin as it gets (less convex)
Less light is refracted, and a focused image falls on the yellow spot

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Description

Learn about the protective features of the human eye, including the bony socket, eyelids, and tears, and their functions in maintaining eye health.