The human eye: anatomy, structure and function

The human eye perceives countless visual impressions every day. The various structures of the eye have their own task – the sequence of functions means that we can perceive images and thus see. To understand this amazing feat, it is important to look at the anatomy of the eye and the complex structure of the human eye.

The anatomy and function of the various structures are closely linked, enabling us to see clearly, distinguish colors and estimate distances. In particular, the different layers of the eye, such as the cornea, retina and choroid, play a central role in this process. In this article, the components of the eye are explained in detail to illustrate how they work together to enable us to see. Read on to find out exactly how the eye works and the mechanisms involved.

Creation of a visual impression

Rays of light fall into the eye and pass through the cornea, eye chambers, pupil, lens and vitreous body before finally landing on the retina. The focal point is on the retina and an image is created. The visual impression is transmitted to the brain via the optic nerve and processed there. The result is visual perception.

An important aspect of the eye’s function is accommodation, which enables us to see objects in focus at different distances. This process is controlled by changing the shape of the lens of the eye. When we look at a close object, the ciliary muscle contracts, making the lens thicker and refracting more light to focus the image clearly. Conversely, when we look at distant objects, the muscle relaxes, making the lens flatter. This dynamic adaptation is crucial to our ability to clearly perceive and interpret both near and distant objects.

Ocular membrane as the shell of the vitreous body

The vitreous body of the eye is surrounded by several layers of eye skin: The outer ocular membrane, the middle ocular membrane and the inner ocular membrane. The respective layers in turn consist of different components:

• Outer skin of the eye: Sclera, cornea
• Middle ocular membrane: Choroid, ciliary body, iris
• Inner skin of the eye: Retina, pigment epithelium

Outer eye skin: Sclera and cornea

Sclera

The sclera provides the visible white skin of the eye, which surrounds the iris. Apart from the cornea, the sclera surrounds the entire eye. The main function of the sclera is to provide a stable shell. The sclera is also bordered by eye muscles for the movement of the eyeball and the optic nerve at the back of the eye.

Cornea (cornea)

The cornea is part of the dioptric apparatus, i.e. part of the light-refracting structures of the eye. In total, this apparatus has a refractive power of +60 diopters. At +43 diopters, the cornea represents the highest proportion of the refractive power. If the refractive power of the cornea changes, this results in defective vision. Other functions of the cornea are protection against environmental influences and the transmission of light rays. The latter is possible due to the transparent nature of the cornea. A total of five layers make up the cornea:

• Epithelium: Epithelial cells produce tear film on the eye and thus supply it with moisture from the outside.
• Bowman’s layer: A layer of connective tissue that protects the stroma from external influences.
• Stroma: At 90%, this is the main part of the cornea and ensures the stability of the cornea
• Descemet’s membrane: The most resistant and elastic layer of the cornea.
• Endothelial cell layer: Regulates the fluid balance and the exchange between aqueous humor and stroma.

Medium eye skin

Choroid (choroid)

The choroid has many blood vessels. These ensure that the adjacent retina is supplied with nutrients. The choroid also contains many pigments. The pigments have the function of shielding light rays outside the pupil. Rays of light that enter the eye through the pupil are absorbed by the pigments in the choroid – this prevents light reflections within the eyeball.

Ciliary body and zonular fibers

The ciliary body produces the aqueous humor of the eye chambers. The ring-shaped ciliary muscle surrounds the lens and is responsible for focusing objects. The ciliary muscle is connected to the lens by zonular fibers – this allows the curvature and refractive power of the lens to be adjusted (accommodation).

iris (iris)

Two eye muscles run through the iris, which regulate the size of the pupil and thus adapt the pupil to the light conditions. The iris also contains color pigments, which give the human eye its visible color. In addition to providing color, the color pigments of the iris have the function of intercepting light rays outside the pupil.

Inner eye skin

Pigment phitel

The pigment epithelium contains a high proportion of the coloring substance melanin. As with the choroid, this absorbs light rays which pass through the pupil into the interior of the eye. The pigment epithelium is also responsible for the vitamin A metabolism of the outer retina and regulates the supply of nutrients and the transport of waste from photoreceptors.

Retina

The retina is an essential component when it comes to visual perception and is also known as the “screen”. The rays of light are projected directly onto the retina, creating a sharp image. The retina is formed by several layers of nerve cells. These include photoreceptors (cones and rods). Cones are used for day and color vision. Rods are used for night and black and white vision. They ensure that stimuli are received and processed into nerve impulses.

Yellow spot (macula) and blind spot

The yellow spot (lat. macula lutea) is located on the retina next to the optic nerve. Most of the retina’s photoreceptors are located in this spot – which is why the macula is the sharpest point of vision. The macula is named after the yellow pigment lutein.

The counterpart to the macula is the blind spot. There are no photoreceptors at the exit point of the optic nerve on the retina. As a result, people are blind in this spot, but this is not perceived as impairing. The visual stimuli are automatically combined in the brain to create a complete image.

Optic nerve (Nervus opticus)

The retina merges with the optic nerve at the back of the eye. This consists of numerous nerve fibers. Surrounded by a bony optic nerve canal, the nerve impulses are transmitted through the optic nerve to the visual center in the brain.

Pupil (Pupilla)

The dark round opening in the middle of the iris is called the pupil. The rays of light enter this opening. The pupil is enlarged and reduced by the muscles in the iris and can therefore control the amount of light entering the retina. In addition to the lighting conditions, the pupil size can be altered by emotional states such as stress, fear or happiness.

Lens (Lens crystallina)

The crystalline lens is a converging lens that focuses the rays of light. The lens is flexible and can be curved with the help of the ciliary muscle. This allows the refractive power of the lens to be adjusted (accommodation). This process ensures that objects at close range can be focused on, especially for people with farsightedness at a young age. Due to the natural ageing process of the human body, the lens loses its elasticity over the course of a person’s life – resulting in presbyopia.

Vitreous body (corpus vitreum)

The vitreous body makes up a large part of the eye. It fills the inside of the eye and consists of a gel-like fluid (98% water, 2% hyaluronic acid and collagen fibers). There is a uniform internal pressure within the vitreous humor. This ensures the stability of the round shape.

Eye chamber (Camerae bubli)

There are two eye chambers, i.e. the anterior and posterior eye chambers. The anterior chamber is located between the cornea and the iris. The posterior chamber is located between the iris and the lens. Both chambers are connected by an opening between the iris and the lens. This allows the aqueous humor to be exchanged.

Schlemm’s canal is located in the chamber angle. This transports old aqueous humor into the blood. This means that the aqueous humor is completely replaced every 100 minutes. If the outflow of aqueous humor is impaired, the intraocular pressure rises.