Human Eye Structure and Function

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