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Category: Medical

Topic: Nervous System A&P

Level: EMT

Next Unit: The Physiology of Vision

13 minute read

Components of the Eye

The eyeball itself is composed of

  • the orbit,
  • the extrinsic muscles, and
  • layers of the eyeball, including
    • the sclera,
    • the cornea,
    • the choroid layer,
    • the ciliary body,
    • suspensory ligaments,
    • the iris,
    • the pupil,
    • the lens,
    • the retina,
    • the rods,
    • the macula area,
    • the macula lutea,
    • the fovea,
    • the ganglion neurons and
    • the optic disc.


Layers of the Eyeball

The SCLERA is the white outer layer of the eyeball.

The CORNEA is the transparent layer forming the front of the eye.

The CHOROID LAYER  is the vascular layer of the eye that contains connective tissue and lies between the retina and the sclera.

The CILIARY BODY is the part of the eye that connects the iris (colored part of the eye) to the choroid and consists of the ciliary muscle, a series of ciliary processes, and the ciliary ring. A series of suspensory ligaments connect the ciliary processes to the lens of the iris.

The PUPIL is the aperture of the eye and allows light to strike the retina. The amount of light entering the pupil is controlled by the size of the pupil, which is regulated by the IRIS.

The IRIS, located in front of the lens and ciliary body and behind the cornea, is the colored part of the eye consisting of the pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by the opening for light (the pupil). 

The LENS of the eye is a transparent biconvex structure that helps (along with the cornea) to refract light to be focused on the retina, allowing for changes in focal distance. It depends on its ability to change shape so as to change the focus of vision as needed. 

Cataracts are caused by the progressive opaque change to the lens of the eye, resulting in blurred vision.

The RETINA of the eye is a layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.

  • The ROD CELLS of the retina are able to function in less intense light than the other type of photoreceptor cells in the retina, they are for vision at low light levels (scotopic vision).
  • CONE CELLS, which are found in diminishing numbers toward the outer edges of the retina, but are primarily located in a sunken area near the center of the retina. Cones with different pigments absorb light according to wavelengths and are responsible for color perception.

. Cones are active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity.

The MACULA area of the eye includes the macula lutea, is found at the center of the retina, and is responsible for the eye viewing different colors by the normal function of the area within the macula known as the fovea. The fovea contains three different types of cones with different ranges of spectral sensitivity allowing for different colors.

The GANGLION NEURONS of the eye are retinal neurons the receive impulses from bipolar neurons. The axons of ganglion neurons combine at the optic disc to form the optic nerve.

The OPTIC DISC serves as the exit point for the ganglion cell axons leaving the eye, and because there are no rods or cones overlying the disc, it creates a small blind spot in each eye. The coalescence of all of the ganglion cell axons leaving the eye to continue as the OPTIC NERVE.


Shape of the Eye

The way the cornea and lens focus light depends on the eye being of a certain shape so that the distance from the lens to the retina is a certain length. When these shapes are off, it causes refractive errors like myopia (near-sightedness), hyperopia (far-sightedness), and astigmatism (irregular curve of the cornea such that light cannot focus on a single point causing vision distortion).

The eyeball itself maintains its ball-like shape by pressure from the gel-like vitreous humor (which also keeps the retina pressed to the back of the eye).

The aqueous humor is in the anterior chamber of the eye (between the cornea and the lens). 


Hyphema: a collection of blood inside the anterior portion of the eye (the space between the cornea and the iris). Traumatic Hyphema is a common complication of blunt or penetrating injury to the eye and can result in permanent vision loss. The blood can cover most or all of the iris and the pupil, blocking vision partially or completely. Treatment includes the covering of both eyes, pain control, and rapid transport. In children should always be suspected as abuse. 

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