How the Human Eye Works

The human eye is a “living camera” (Figure 1). The main parts of the eye are:

The Cornea – “lens” of the camera (bends light to focus images)

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Figure 1
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Figure 6

The Iris/Pupil – the adjustable “aperture” which regulates light entering the camera

The Lens – the second “lens” of the camera (when this lens becomes cloudy, we call it a “cataract”)

The Vitreous cavity – the empty space of the camera box; in the eye this space is filled with the clear Vitreous gel.

The Retina – the film of the camera where the image “picture” is formed.

The Optic Nerve – the “cable” which sends the “picture” information to the brain.

Vision loss can occur from problems with the lenses of the eye (corneal cloudiness, cataracts), the vitreous (blood in the vitreous), the retina (retinal detachment, retinal edema (swelling), loss of blood flow), the optic nerve (edema, loss of blood flow), or the visual center of the brain (stroke).

The Retina is a very complex tissue that lines the inside wall of the eye like wall paper (Figure 2). The most sensitive part of the Retina is a small area in the back called the Macula (Figure 3). While the Macula represents only 2% of the Retina, it is the part that provides “central vision”, the vision for reading and recognizing faces among other things. The other 98% of the Retina provides peripheral or “side” vision.

Some other things to know about the Retina:

The Retina is translucent, like wax paper. It lies on top of a layer of cells called the Retinal Pigment Epithelium (Figure 4). It is the underlying RPE layer that gives the inside of the eye its orange or brownish color.

The Retina has several cell layers which perform different functions (Figure 5). The Photoreceptor Layer lies just on top of the RPE. Photoreceptors are the cells that sense and respond to images (such as words on a page, faces, etc.). The Nerve Fiber Layer is on the inside of the Retina. These fibers come together to form the Optic Nerve, which sends visual signals to the Brain.

Because the Retina is translucent like wax paper, light rays can be used to generate cross-sectional pictures of the Retina. This is called Optical Coherence Tomography, which gives us important information regarding the anatomy of the Retina (Figure 6).

The Photoreceptors receive nutrients and oxygen through the RPE. If the Photoreceptor layer detaches from the RPE layer, the Photoreceptors stop working and these cells may become injured or die. That is what happens with a Retinal Detachment and is the reason Retinal Detachment may lead to blindness (Figure 6).

Anything else which alters the anatomy of the Retina or damages its cells may caused vision loss such as a Macular Pucker (wrinkling of the macula) or Macular Edema (swelling) (Figure 6, lower left).

 
 
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