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• The eye

Author: Maria Yiallouros, erstellt am: 2016/11/21, Editor: Maria Yiallouros, English Translation: PD Dr. med. Gesche Tallen, Last modification: 2016/11/21

The eye is a sensory organ. It collects light from the visible world around us and converts it into nerve impulses. The optic nerve transmits these signals to the brain, which forms an image so thereby providing sight.

Human eyes primarily consist of two globe-shaped structures, the eyeballs, which are surrounded by the the bony sockets of the skull, the orbits. The orbits are covered with fatty and fibrous tissue to protect the eye. Additional structures protecting the eye include the eyelids, the outer coating layer of the eye (fibrous tunic), the conjunctiva, and the lacrimal glands. Six special muscles that insert at different sites outside the eyeball work together to control eye movement.

Each eyeball houses the following parts of the eye:

• the three coating layers: the outer, middle and inner coat
• the inner part of the eyeball: it contains the lens and the vitreous body and is divided into the anterior and the posterior chamber.

The following chapters will explain anatomy and function of the three coats as well as of the inner part of the eyeball.

Layers of the eye

The eyeball is surrounded by a three-layered wall, the three coats of the eye. They consist of different tissue and serve different functions.

Outer coat (fibrous tunic)

The eye’s outer layer is made of dense connective tissue, which protects the eyeball and maintains its shape. It is also known as the fibrous tunic.

The fibrous tunic is composed of the sclera and the cornea. The sclera covers nearly the entire surface of the eyeball. With its external surface being white-coloured, it is commonly known as the “white of the eye”. The sclera provides attachments for the muscles that control the eye’s movement (see above).

The transparent cornea occupies the front center part of the external tunic. It serves as the eye’s “window”, which lets the light in and bends its rays, thereby providing most of the eye’s focusing power.

The anterior, visible part of the sclera as well as the inner surface of the eyelids are covered by the conjunctiva, a mucous membrane that helps lubricating the eye together with the tears made by the lacrimal glands, thus protecting the eye from drying out.

Middle coat (vascular tunic)

The middle layer of tissue surrounding the eye, also known as the vascular tunic or „uvea“, is formed – from behind forward – by the choroid, the ciliary body, and the iris.

The choroid takes up the posterior five-sixths of the bulb and is mainly comprised of blood vessels. Its major functions are oxygen supply and nutrition for the eye. A dark pigment, melanin, occurs throughout the choroid in order to help limiting uncontrolled reflection within the eye, which would potentially result in the perception of confusing images.

The anterior part of the choroid passes into the ciliary body, one function of which is anchoring the lens in place. The ciliary body contains a muscle (ciliary muscle), which can change the shape of the lens for adjustment to far or near sight, respectively, thereby controlling the so-called refractive power of the lens (accomodation). Additional functions of the ciliary body are the production, secretion, and outflow of aquaeous humour (the latter via the so-called „Schlemm’s canal“), a watery fluid that fills both the anterior and the posterior chambers of the eye (see below).

The iris, which is connected to the anterior part of the ciliary body, covers the top of the lens. Similar to the aperture of a camera, it controls how much light is let into the eye. The iris forms a circular, thin structure within the eyeball that regulates the size and the diameter of the pupil. It also contains pigments, the amount of which determines a person’s eye colour. For example, in children with blue eyes, the iris contains less pigment than in brown-eyed kids.

Inner coat

The third and inner coat of the eye is the retina, which is responsible for the perception of images – vision.

The retina is a light-sensitive layer of nervous tissue composed of multiple sensory cells, so-called light- or photoreceptor cells, as well as associated nerve cells and other types of cells, all working together to make a person see.

For vision, there are two types of photoreceptor cells: rods and cones. Rods provide the perception of black-and-white vision, mostly in dim light, whereas cones help to see colors in daylight.

The light and colour impulses received by these photoreceptors are transmitted to the associated nerve cells of the retina, which, on their part, send these signals – via the optical nerve – to the visual centre (visual cortex) of the brain.

The point where the optic nerve fibers depart from the eyeball (optical disc) does not contain any photosensitive cells; it is, thus, insensitive to light and termed the “blind spot”.

Directly opposite the lens, the retina contains a small yellowish area, the “macula lutea”. Its central part (fovea centralis) is densely packed with cone cells for colour perception. At this point, the sense of vision is the most accurate and detailed.

The inner part of the eyeball

The inner part of the eyeball consists of the lens, the vitreous body and the two eye chambers.

The lens

The lens is a transparent olive-shaped structure in the eye that has no blood vessels. Lens and cornea (see above) work together to focus the light rays passing through the eyeball to the back of the eye, that is, to the retina, by bending or refracting them, thereby creating clear images of the environment perceived from different distances.

By adjusting its shape and size, the lens can change the focus. This process is called accomodation. Accomodation is possible thanks to the lens’ elastic capsule as well as to the lens fibers, which connect with the ciliary muscle (see middle layer of the eye).

The vitreous body (vitreous humour, vitreous)

The vitreous is a clear gelatinous mass held by collagen fibers. It is situated between lens and retina and comprises about two thirds of the entire eyeball. By pushing the retina towards the choroid, the vitreous promotes keeping the retina in place.

Anterior and posterior eye chamber

The anterior chamber of the eye is located between the iris and the cornea (see above). The posterior chamber is the space between parts of the iris and the lens. Both chambers are filled with aquaeous fluid to nourish cornea and lens.

How the eye works

The human eye is a complex optical system that basically works like a camera: the iris serves as the aperture that controls the amount of light rays reaching cornea and lens (photographic objective), and the retina works as the film.

Bending of light rays by cornea and lens serves to create sharp images on the retina. These images ultimately trigger nerve impulses, which are transmitted to the brain where the images are perceived and interpreted.