Eye

July 30, 2020 Rima Anatomy and Physiology 0

  • Human eye is one of the most remarkable sensory system. Eyes are like windows to the outside world, which helps us to capture, adjust and transform light into a chemical code that can be deciphered only by brain.
  • Among the five senses (vision or sight, smell, touch, hear and taste), eyesight is the most valued mode of perception (fear of lost), according to a survey.
  • The eyes are in the orbit formed by seven skull bones (frontal, lacrimal, zygomatic, ethmoid, maxilla, palatine and sphenoid). They are two in number (binocular vision). The human eye has a 200-degree viewing angle and can 10 million colors and shades.

What is vision?

  • Vision or sight is perception of objects in environment by the means of light they reflect or emit. There are two types of vision:
    • Central vision– this enables visualization of object which are directly in front of us.
    • Peripheral vision– this enables visualization of lateral objects when we are looking forward.

Different parts of the eye

Figure- cross-section of human eyeball viewed from above (Source- Dave Carlson)

  • Human eye can be divided into 3 layers; outer layers, middle layer and inner layer.
  • Outer layer consists of the cornea and sclera, middle layer is composed of the iris, the ciliary body and choroid and the inner region consist of retina, a complex layered structure of neurons. Each part has their own function and importance. Let’s lean about these all parts in detail.

Cornea and sclera

  • Sclera is outermost layer of the eyeball and is white and opaque. It is made up of fibrous connective tissue. Extraocular muscle which helps in movement of eyeball are attached to eyeball at sclera.
  • At the front of the eyeball, the sclera becomes cornea. It is the clear, dome-like, most anterior part of the eye in front of iris and pupil. Light rays should pass through the cornea to reach the lens.
  • Cornea can be divided into 5 layers. These 5 layers from front to back are corneal epithelium, Bowman’s layer, corneal stroma, descemet’s membrane and the corneal endothelium.
  • The junction between the white sclera and clear cornea is called limbus.

Function of sclera

  • It surrounds the eye and give shape to eye.
  • Its tough, fibrous nature protect eye from external damage.

Function of cornea

  • Cornea filters most damaging UV rays from sun.
  • It provides around 65-75 % of focusing power of eyes. Remaining focusing power is provided by lens, present behind the pupil.

Conjunctiva

  • Conjunctiva is a thin transparent layer of tissue covering the front of eye. It contains visible blood vessels that we can see against the white background of the sclera.

Function– It keeps bacteria and foreign material from getting behind the eye.

Choroid

Choroid is the middle layer of eyeball located between sclera and retina. It is vascular and absorb the stray lights that are not absorbed by photoreceptors.

Function- It provides nutrients and oxygen to outer surface of retina.

Anterior chamber

  • Anterior chamber is the space between the cornea’s innermost surface and iris. It is also known as anterior cavity. It is filled with aqueous humor.

Posterior chamber

  • It is located opposite to anterior chamber and consist of narrow space behind peripheral part of iris and in front of suspensory ligament of the lens and ciliary processes.
  • It is filled with fluid- vitreous humor which exerts fluid pressure and keeps the retina layers pressed together.

Iris and pupil

  • The choroid continues at the front of the eyeball to from iris. It is a flat, thin, ring shaped structure sticking into the anterior chamber. The central opening of iris is called pupil.
  • Iris is the colored part of the eye. The color is due to pigment which is a form of melanin. Eye color is the color of the iris and is genetic characteristic. Heavily pigmented eyes are brown whereas lightly pigmented eyes are blue or green.
  • The iris contains circular muscle around the pupil. Contraction of circular muscle makes the pupil smaller (this occurs in bright light) and its relaxation makes the pupil larger (in darkness).
  • The black color of pupil is because light that passes through pupil is absorbed by retina and is not reflected back. It may become cloudy or pale due to formation of cataract.

Function– The function of iris is to control the size of pupil and control the amount of light entering the eye. The function of pupil is to allow the light to enter the eye, so it can be focused on retina to begin the process of sight.

Ciliary body and ciliary muscles

  • Ciliary body lies between anterior border of the choroid and the iris. Ciliary muscles are located inside the ciliary body. The ciliary muscles contract or relax to change the shape of lens.
  • It also contains ciliary epithelium which produces aqueous humor. Aqueous humor is clear, transparent fluid that circulate in anterior chamber. It provides oxygen and nutrients to inner eye and maintain fluid pressure.

Function– The main 3 functions of ciliary body is aqueous humor production, accommodation and maintenance of lens zonules.

Lens zonulas

  • It is a suspensory ligament that hold the lens suspended in place. It connects lens to the ciliary body and allow lens to change shape.

Lens

  • Lens is a clear, biconvex, transparent and flexible disc made of proteins called crystalline. It is located just behind iris and people.
  • It can change its shape and change the focal length of eye to help focus objects at different distances.

Function– It helps to refract light to be focused on the retina.

Retina

  • Retina is the innermost and light sensitive part of eye. It is located in posterior part of eye, surrounding the vitreous cavity.
  • It contains photoreceptors called rods and cones. Cones help us to detect color. They are sensitive to different wavelengths of light and are able to distinguish colors. Rods are very sensitive to light but don’t see colors. They enable use to see in poor light (at nighttime).
  • Retina acts like film of camera to form image. Photoreceptor cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.
  • Fovea is the specific part of retina where cone cells are densely packed. It is responsible for providing sharp central vision.

Optic nerve

  • It is network of nerve cells which receive impulses from the nerve fiber layer on the retina.

Function– It transfer nerve impulses from retina to brain.

Accessory organs of eye

  • The accessory structures of eye include:
    • Eyebrows, eyelashes and eyelids.
    • Lacrimal apparatus which produces tears.
    • Extrinsic muscle that move the eyes.

References

  1. Willoughby CE, Ponzin D, Ferrari S, Lobo A, Landau K, Omidi Y. Anatomy and physiology of the human eye: effects of mucopolysaccharidoses disease on structure and function – a review. Clinical and Experimental Ophthalmology. 2010; 38 (s1): 1-62.
  2. Jaichandran VV. Ophthalmic regional anaesthesia: A review and update. Indian J Anaesth. 2013 Jan-Feb; 57(1): 7–13.
  3. Remington, Lee Ann. Clinical Anatomy and Physiology of The Visual System, 3rd Edition. Butterworth-Heinemann, 2012.
  4. Cassel GH, Billig MD, and Randall, HG. The Eye Book: A Complete Guide to Eye Disorders And Health. The Johns Hopkins University Press, 1998.
  5. Sliney D., Wolbarsht M. (1980) Review of Anatomy and Physiology of the Eye and Skin. In: Safety with Lasers and Other Optical Sources.
  6. https://www.ncbi.nlm.nih.gov/books/NBK470322/