The human eye is a remarkable and highly specialized sense organ, a wonderful optical instrument. Small and delicate, it takes an exceedingly skilled doctor such as a retinal specialist to diagnose diseases and perform operations on the eye. Let’s look into your eye and see what your eye doctor has to know and work with to keep your eyes healthy.
The eye consists of several structures that nature designed to focus images on the retina at the back of your eye. The retina is like a screen of nerve cells connected to the optic nerve’s electrical pulses and sent to the brain, where the picture is interpreted.
Your eyes work in coordination under the brain’s control, aligning themselves on an object to form a clear image on each retina. If necessary, the eyes sharpen images by altering focus in an automatic process known as accommodation.
When a pattern of light shines on the retina, it stimulates a complex flow of impulses along the brain’s optic nerves. Two optic nerves pass into the skull, meet partially, cross-over, and run back on the brain’s underside and then to the visual cortex, the area on the brain’s back surface concerned with vision.
The eyeballs are held in pads of fat within the orbits with four walls, namely the upper wall or roof, lower wall or floor, lateral or temporal wall, and medial or nasal wall. The bony eye sockets protect from external injury. Coordinated by a nerve network in the brainstem, six delicate muscles move each eyeball.
The eyeball has an opaque, fibrous, protective outer layer called the sclera (white of the eye). The sclera’s anterior part, the circular transparent and slightly protruding part of the outer coat is the cornea.
The cornea is the primary lens of the eye and has most of the focusing power. Behind the cornea is a shallow chamber full of a watery fluid called the aqueous humour. Behind this is the colored part of the eye, your Iris with its pupil, which appears black. Tiny muscles change the pupil’s size to control the light entering.
Directly behind the Iris and in contact with the crystalline lens is a circular muscle ring called the ciliary body. Suspended by delicate fibers, the ciliary body adjusts the crystalline lens shape, providing the eye with additional focusing power to which the cornea provides.
Behind the lens is the eyeball’s main cavity called the vitreous humor filled with a clear gel. At the back of the eye is the retina, which needs to have a constant oxygen and glucose supply. A thin network of blood vessels called the choroid plexus lies immediately under your retina to meet this requirement. The choroid is continuous at the front with the ciliary body and Iris, forming the uveal tract.
From the outside, the eyeball is sealed off by the conjunctiva, a flexible membrane firmly secured around the cornea’s margin but lies freely on the sclera over the front third of the globe. It’s attached to the skin at the corners of the eye and forms the lids’ inner lining with a deep cul-de-sac above and below.
This arrangement provides a permanent seal while allowing free mobility of the eyeball. In the conjunctiva, there are many small and tiny tear-secreting and mucous-producing glands. Along with oil secreting from the meibomian glands in the eyelids, they provide the critical, three-layer tear film that continuously covers the cornea and conjunctiva to protect from damage due to drying out of the cells.
Each eyelid contains about 30 meibomian glands with openings along the lid margin just behind the lashes’ roots. The glands secrete oil to prevent adhesion of the lid margin during sleep and form the tear film’s outer layer. This layer retards evaporation and helps maintain the continuity of the tear film. The blink reflex is protective and helps to spread the tear film evenly over the cornea.
A deeper look into the eyes
We have looked into the eye to a basic level of understanding. Now let us go deeper into this and see what we can see!
Cornea – The front part of the tough outer side of the eyeball.
Transparent and shaped like a thin-walled cap, the cornea is about 12 mm in diameter and less than 1 mm thick with a convex front surface like a camera lens. At its circumference, the cornea joins the sclera, which is easily seen. The cornea itself being transparent is less obvious, whereas the black pupil and the colored iris are visible beneath it.
The cornea has two main functions: to focus light rays on the retina and protect the front of the eye. The surface of the cornea is highly sensitive. A small scratch or foreign body is very painful, giving you the warning of any possible damage without neglecting. The cornea must be kept moist always by tears to remain healthy. This function is performed by the lacrimal gland and the mucous and mucous and fluid secreting cells in the eyelids and conjunctiva.
The cells that form the inner layer of the cornea in adults cannot reproduce themselves. If there is severe damage, permanent clouding of the cornea occurs because one of the cell functions is to pump excess water out of the cornea to keep it transparent.
The cornea is composed of five layers:
- Bowman’s membrane
- Substantia propria or stroma
- Descemet’s membrane
The cornea is richly supplied with nerve fibers and is very sensitive with no blood vessels.
Aqueous Humour – The anterior chamber just behind the cornea
Just behind the Cornea is a space filled with a clear fluid called the aqueous humour. The ciliary body secretes this fluid into the posterior chamber located between the Iris and the anterior portion of the crystalline lens. The aqueous humour circulates from the posterior chamber through the pupillary opening into the anterior chamber. It then drains out through the duct under the side of the cornea at the limbus. This duct is called Schlemm’s canal.
Iris – The colored part of the eye behind the cornea
The Iris is connected at its outer edge to the ciliary body and has a central hole called the pupil, through which light enters the eye and falls on the retina. The Iris is a loose framework of transparent collagen and muscle fibers, which constantly contracts and dilates to alter the pupil’s size and control the amount of light that passes through the pupil and reaches the retina.
Pupil – The circular opening in the center of the Iris
In bright conditions, the pupil constricts (narrows) to reduce the amount of light received by the eye; in dim light, the pupil dilates (widens) to allow more light to reach the retina. Muscles in the iris control contraction and dilation.
The crystalline lens is the internal optical component of the eye, responsible for adjusting the focus. It is one of the two lenses in each eye; the other is the cornea which provides most of the converging power needed to form an image on the retina.
Situated behind the Iris, the crystalline lens is held in place by bundles of strands, or Zonules, from the ciliary body. It’s about 4 mm in thickness in the center; the lens grows throughout one’s life but has more in the first year after the birth. The nucleus of the lens continues to grow in layers as life progresses. In the later part of life, the nucleus grows further and becomes harder, losing the elasticity and finally interfering with the lens’s free functioning in senile life.
Changing the curvature of the lens alters the focus. Near or distant objects can be seen sharply (accommodation), which means the eye’s refractive power can voluntarily be increased from 18 diopters to about 30 diopters. Opacification of the crystalline lens by injury or by any cause is called Cataract. The lens can even be dislocated from its original position.
A transparent jelly-like body fills the large rear compartment of the eye between the crystalline lens and the retina. The vitreous humour consists almost entirely of water. Under certain conditions, it can exert sufficient pull on the retina to cause retinal tears and retinal detachment.
Sometimes, black specks float before the eye and are seen by the average person. These are various kind of opacities that throws a shadow on the retina then appearing as dark spots in the field of vision. These are considered normal unless these opacities when in large number increase, interfere with the vision.
Extraocular muscles: Six muscles work together on the eyeball to give a range of smooth and precise movements. One end of the muscles is attached to the bony orbit, while the other end is attached to the sclera, the outer coat of the eye.
The eye can move through an arc of about 100 degrees horizontally and about 80 degrees vertically. Both eyes though separate, yet work simultaneously and with coordination. This team of muscles that controls the movement of each eye is:
- Superior rectus
- Inferior rectus
- Medial rectus
- Lateral rectus
- Superior oblique
- Inferior oblique
The four rectus eye muscles run directly from the eyeball, while the oblique muscle is attached to and pulls on the eyeball at an angle.
Retina – The light-sensitive layer that lines the back of the eye
The retina has rods and cones, specialized nerve cells that convert light energy into electrical nerve impulses and transmit them to the brain. The rods are exceptionally sensitive, responding to very dim light. Cones are less-sensitive but are responsible for color perceptions, producing impulses that vary in strength with the light’s color striking them.
Near the center of the retina is the fovea. Here, retinal blood vessels are absent, and the light-sensitive cells are packed, so that vision in this area has the highest resolution. Despite its small size, the retina is subject to various disorders, many of which seriously affect the vision or, in some cases, produce blindness.
The optic nerve comprises about one million nerve fibers that transmit impulses from the retina to the brain. Two optic nerves converge to a junction behind the eyes, where fibers from the inner halves of the retina cross-over. Nerve fibers from the right halves of both the retina pass to the right side of the occipital lobes at the back of the brain, while those from the left halves go to the left-side.
Because of the arrangement of the nerve fibers, disease or injury at any point causes a unique pattern of vision loss. Charting the pattern of visual loss allows your retinal specialist to locate damage to the nerve accurately. Optic nerve disorders include optic neuritis and papilledema, caused by pressure on the nerves from a disease within the orbit or a brain tumor.
Schedule an eye exam today!
Now that you know more about the many structures of your eye, don’t you think that it would be good to have your eye doctor examine them? With your new knowledge, you will be better able to communicate with your ophthalmologist or retinal specialist. “Hey doc, can you check out my ciliary body and vitreous humour? I hear they are important parts of my eye?” Your doctor will be impressed!