- Nervous System
- Physiology
The eyes are located in the orbit formed by the seven skull bones (frontal, lacrimal, ethmoid, zygomatic, maxilla, sphenoid and palatine.)
They are two in number. (binocular vision)
The bony ridge superior to the orbit called the supraorbital ridge, protect the eyes from blows and serves as a location of the eyebrows.
The photo or light receptors are in the eye’s retina.
Before we go further into the anatomy of the eye it's better that we first define what is the role or the eye
What is vision?
Vision or sight is the perception of objects in the environment by means of the light they reflect or emit
There are two types of vision
- Central vision- This enables the visualization of objects directly in front of you
- Peripheral vision – It enables visualization of lateral objects while a person is looking forward
it employs:
- Accessory structures of the eye.
- The eyeball.
Accessory structures of the eye.
Under the anatomy of the eye, we shall start by looking at the accessory structures of the eye.
These structures include:
- Eyebrows, eyelids, and eyelashes.
- lacrimal apparatus which produces a tear.
- extrinsic muscles that move the eyes.
Eyebrows
Eyebrows have short, thick hair positioned transversely above the eye along the supraorbital ridge.
They shade the eyes from the sun and prevent perspiration or debris from falling into the eye.
Eyelashes.
They can trap debris and prevent it from entering the eye.
The sebaceous gland associated with each eyelash produces only oily secretions that lubricate the eye.
Inflammation of one of the glands is called a sty.
Eyelids.
They are a continuation of the skin and contains the skeletal muscles that enable the eyelids to close and covers the front of the eyeball.
Blinking of the eyelids keeps the eye lubricated and free of debris.
The eyelids are operated by orbicularis oculi muscle which closes the eyelids and by levator palpebrae that raises the eyelids.
The inner surface of an eyelid is lined with conjunctiva(mucous membrane).
The conjunctiva folds back to cover the anterior eye, except for the cornea which is covered by a delicate epithelium.
Conjunctiva.
The conjunctiva covers the inner surface of the eyelids and the anterior surface of the eye.
A membrane that produces mucus that lubricates the eye and prevents dryness.
Protects the eye. When the conjunctiva becomes inflamed it leads to conjunctivitis.
lacrimal apparatus.
The lacrimal apparatus consists of the lacrimal gland and lacrimal sac with its ducts.
They are located at the upper outer corner of the eyeball within the orbit.
These apparatus produce tear that flows over the eye when the eyelids are blinked.
The tears are then collected by two small ducts, pass into the lacrimal sac before draining into the nose by way of the lacrimal duct.
Tear production.
The image below demonstrates the layers of tears and functions
Extrinsic muscles.
The eye is anchored by the extrinsic muscles, whose contraction moves the eyes.
Each of these muscles originates from the bony orbit and insert by tendons to the outer layer of the eyeball.
There are three pairs of antagonistic muscles:
The eye muscles.
These pairs of extrinsic muscles include:
- First pair:
Superior rectus-rolls the eye upwards.
Inferior rectus-roll the eye downwards.
- Second pair.
Lateral rectus-turn eye outwards away from the midline.
Medial rectus-turn eye inwards towards the midline.
- Third pair:
Superior oblique-rotates the eye counterclockwise.
Inferior oblique-rotate eye clockwise.
Cranial nerves innervation of the muscles:
The muscles are controlled by the three cranial nerves namely oculomotor, abducens, and trochlear nerves.
- Oculomotor nerve innervates the superior, inferior, medial rectus and inferior oblique.
- Abducens innervates the lateral rectus muscle.
- Trochlear innervates the superior oblique.
The motor unit of these muscles is the smallest in the body.
A single motor axon serves only about 10 muscle fibers, allowing the eyeball movement to be very precise.
The eyeball
The eyeball is an elongated sphere about 2.5 cm in diameter. It has three layers or coats:
- The outer sclera. (fibrous tunic)
- The middle choroid. (vascular tunic)
- The inner retina. (sensory tunic)structures of the eyeball
The Sclera, This is a thick fibrous outer coat which gives resistance and durability to the eye.
The Choroid is a delicate vascular coat that supplies nourishment to the inner layer of the retina.
The Retina This is a photosensitive layer of the eye that translates the light impulses into an electronic impulse that travels down the optic pathways to the brain.
The sclera.
The sclera is the thickest layer and is made up of the fibrous connective tissues that is visible as the white of the eye.
The most anterior portion is the cornea, which differs from the rest of the sclera in that it is transparent.
The cornea has no capillaries, covers the iris and the pupil inside the eye and is the first part of the eye that refracts light rays.
Functions of the sclera.
Support eyeball.
It provides attachment to the muscles.
Functions of the cornea
The cornea allows the passage of the light into the eye and also focuses light.
The choroid
The choroid is the middle thin dark blue layer.
It is vascular and absorbs the stray light rays that photoreceptors have not absorbed.
The anterior portion of the choroid is modified into a more specialized structure the ciliary body and the iris.
The ciliary body is a circular muscle that surrounds the edge of the lens and is connected to the lens by the suspensory ligaments.
The lens is made up of the transparent elastic protein and like the cornea has no capillaries.
The shape of the lens is changed by the ciliary muscles which enable the eye to focus light from objects at varying distance from the eye
Just in front of the lens is the circular iris, the colored part of the eye; its pigment is a form of melanin.
What we call “eye color” is the color of the iris and is a genetic characteristic, just as skin color.
Two sets of smooth muscle fibers in the iris change the diameter of the pupil, the central opening. The contraction of the radial fibers dilates the pupil; this is a sympathetic response.
Contraction of the circular fibers constricts the pupil; this is a parasympathetic response (oculomotor nerve)
Pupillary constriction is a reflex that protects the retina from intense light or that permits more acute near vision, as when reading.
Toward the front, the choroid becomes the donut-shaped iris.
The iris regulates the size of the pupil, a hole in the center of the iris through which light enters the eyeball.
The color of the iris (and therefore the color of your eyes) correlates with its pigmentation.
Heavily pigmented eyes are brown, while lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body.
The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.
The lens, attached to the ciliary body by ligaments, divides the eye into two compartments; the one in front of the lens is the anterior compartment, and the one behind the lens is the posterior compartment.
The anterior compartment is filled with a clear, watery fluid called the aqueous humor.
A small amount of aqueous humor is continually produced each day.
Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked, and aqueous humor builds up.
If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located.
The nerve fibers begin to die due to lack of nutrients, and the person becomes partially blind.
The function of iris.
This is the colored part of the eye
It controls the light entering the eye.
Pupil (black hole)
This is the black hole in iris where light enters the eye.