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The Eye

The eye

The human eye is a highly complex organ system which is difficult to describe anatomically and physiologically. This article gives you a brief overview about the essentials of the eye.

Overview

The eye ball, or bulbus oculi, lies in the orbit, a bony cavity in the skull. The retrobulbar fat (corpus adiposum orbitae) is a fatty tissue that lines the orbit from the inside and contains the vessels and nerves. 

Overview of the eye - sagittal section

Overview of the eye - sagittal section

The eye ball consists of the eye layers, the lens and the chambers, which are all discussed below.

Sclera & cornea

The two major outer layers of the eyeball are the sclera and the cornea. The sclera represents the white of the eye. It is covered by the conjunctiva, a transparent mucous membrane which is important for the distribution of the tear film during the eye blink. Inflammation of the conjunctiv (conjunctivitis) caused by bacterial or viral infection may be painful and uncomfortably restrict the patient’s vision.

The cornea protects the eye to some extent. It is part of the optical system and mainly responsible for the refraction of light. Deformation of the cornea may lead to visual disorders.

Layers of the ocular bulb/eyeball - transverse section

Layers of the ocular bulb/eyeball - transverse section

The vitreous body is also considered part of the refractive apparatus. It is a jelly-like material filling the space behind the lens and has a similar refractive index as the cornea.

The retina and the choroidea are inner layers of the eye. Light waves striking the retina within certain wave lengths are transformed into electrical signals and transmitted to the brain via the optic nerve (cranial nerve II) and the so called “visual pathway”. These signals are then processed in the brain and converted to a picture. The choroid contains blood vessels that supply the retina.

Lens & iris

The lens plays a major role in the accommodation of the eye, the focusing of close and distant vision. Through contraction and relaxation of the ciliary muscles the lens changes its shape adjusting the refraction of light. Cataract, a common eye disorder affecting the lens (discussed further below), develops slowly over years and necessarily needs to be treated.

Close up of anterior part of the eye - sagittal section

Close up of anterior part of the eye - sagittal section

The term iris is Greek for “rainbow”. This “pinhole” reaches around the pupil and changes its diameter through contraction. By this means the iris regulates the eye’s adaptation to light and dark.The anterior and posterior eye chambers are separated from each by the iris but have a connection through the pupil. They are filled with aqueous humor which flows from the posterior to the anterior eye chamber. An obstruction to the flow causes an increased intraocular pressure which may lead to glaucoma. Due to the aqueous humor’s ability of refracting the light it is considered as part of the optical system as well.

Extraocular muscles

The eye movement is controlled by different eye muscles, known specifically as the extraocular muscles, which all originate from the orbit.

Overview of extraocular muscles - lateral view (left), anterior view (right)

Overview of extraocular muscles - lateral view (left), anterior view (right)

Cataracts

Overview

The formation of 'clouding' or an opaque area on, or within the normally clear lens is known as a cataract. Depending on its location and coverage of the lens, it may impede normal vision. 

Cataracts are most often seen in people over the age of 55-60 yeas, however occasionally can occur in young children and even infants. Most often, they develop in both eyes, but it is common for one eye to be more affected than the other.

In the healthy eye, the lens focuses light transmitted onto the retina, which sends impulses to the brain throught the optic nerve. In the case of cataracts, the lens becomes clouded, scattering the light transmistted, thereby reducing the ability of the lens to focus light correctly. 

Cataracts occur mainly due to changes in the proteins and fibers which constitute the lens itself, and generally form over an extended period of time. 

Symptoms

Symptoms of a cataract include the following:

  • hazy, or blurred vision
  • difficulty with night vision, increased sensitivity to glare of lights
  • reduced intensity of colour vision (with progressive yellowish tint to light coming into the eye)
  • Light sources may appear to have a 'halo' around them
  • 'double' vision

Types of cataracts

Capsule of the lens - transverse section

Capsule of the lens - transverse section

There are three primary classifications, or types of cataract:

  • nuclear cataract: this is when the cataract is loacted in the centre of the lens, which is known as its nucleus.
  • cortical cataract: affects the cortex of the lens, which is found beneath the outermost capsule of the lens. 
  • posterior capsular cataract: found on the posterior outer layer of the lens, can develop more rapidly than other types. 

Most cataracts can be treated surgically. This most often involves the removal of the 'clouded' lens, and replacement with a plastic/artificial one, which restores clear vision. 

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Show references

References:

  • Benninghoff/Drenckhahn: Anatomie, Band 2, 16. Auflage (2004), S. 655-701
  • Kahle: Taschenatlas der Anatomie, Band 3, 9. Auflage (2005)
  • Lippert: Anatomie Text und Atlas, 8. Auflage, Urban & Fischer Verlag/Elsevier (2006), S. 346-357
  • Schiebler: Anatomie, 9. Auflage, Springer Verlag (2005), S. 651-658
  • Photo 1: Flickr / helgabj
  • Photo 2: Flickr / Patrick J. Lynch

Author & Layout:

  • Christopher A. Becker
  • Achudhan Karunaharamoorthy

Illustrations:

  • Overview of the eye - sagittal section - Paul Kim
  • Overview of the eyeball - transverse section - Paul Kim
  • Anterior portion of the eye - sagittal section - Paul Kim
  • Overview of the extraocular muscles - Yousun Koh
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

Related Atlas Images

Eyeball

Eyelids and tunica conjunctiva

Muscles of the orbit

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