Visual System Anatomy

For the purposes of this discussion, only a basic overview of the anatomy and physiology of the visual system will be presented.    

Figure 1. Cross Section of the Eye

 

Anterior Segment (front of the eye):  Starting with the most anterior (front) structure, the first thing that involves the visual process is the lids. From an anatomical point of view, the lids provide protection to the eyeball with a bony plate called the tarsus. Imbedded in the tarsus are glands that produce tears. 

 

The next structure is the cornea, a transparent membrane that covers the eye. The cornea is kept moist and clear by blinking and pulling the tears over its surface. The cornea is responsible for bending light as it starts toward the back of the eye. The cornea is clear and cannot be seen without special eye instruments; it is connected to the sclera, the white of the eye. The clear gel found behind the cornea is referred to as the aqueous. The aqueous is constantly circulating from the eye to the blood stream through little holes.

 

Next is the iris, the colored part of the eye. The pupil is a hole in the middle of the iris. The pupil gets larger and smaller depending on the amount of light hitting the eye.  The last structure included in the front part of the eye is the lens, the clear firm material responsible for bringing an object being viewed into focus. It is controlled by the ciliary muscle. When the ciliary muscle contracts, the lens gets thicker and we are able to focus on near objects. As people age, their lenses get harder and they are unable to focus on near objects like newspaper print.

 

Posterior Segment (back part of the eye):  As light enters the eye it encounters a gel- like substance known as the aqueous or the “meat of the eye.”  The next area is the delicate clear retinal tissue. This is where the basic image takes form and starts the transmission to the brain. The center and most important two millimeters of the retina is the macula. The macular area is responsible for our central vision or visual acuity, color vision and depth perception. All of the visual information is gathered up and sent back to the brain via the optic nerve. There are more than a million nerve fibers in each eye that gather up and form the optic nerve. About 80 percent of these fibers end up in the farthest back part of our brain known as the occipital cortex. The other 20 percent go to areas responsible for balance. Most of our vision occurs at the occipital lobe. It is here where vision is enhanced and it is also where vision can be lost in the traumatized brain. 

Figure 2. The eye and its extra-ocular muscles

 

Eye Muscles:  Each eye is controlled by six individual muscles. Each muscle is responsible for keeping the eye fixated in a direction and also for coordinating the individual eye with the other eye. There are four basic muscles for up-and-down and in-and-out motion. These are the rectus muscles (superior, inferior, lateral and medial). The other two muscles are the oblique muscles (superior and inferior). These muscles are responsible for rotational movements that, when combined with the rectus muscles, allow us to look in any direction.