Of The Eye
In order to understand the various eye conditions, it is important
to develop a general understanding of the "nuts and bolts" of the eye
and the adjacent structures.
Physicians call this the "anatomy". I
will use various sites that are freely available to you on the Internet to demonstrate
the basic principles. This link of the eye
anatomy describes how the rays from an image enter the eye and form
an image on the back of the eye, called retina. This link of the function
of the lens describes how through accommodation the rays from an image
enter the eye and form an image on the back of the eye, called retina, depending
on whether you focus on a near or far target. As you look at the outer structures
of the eye, notice that the eyeball is protected by the eyelids that will close
in milliseconds by powerful blinking reflexes when any object threatens to hit
the cornea. The outer layer of the eyeball is white (called sclera) and this can
be seen around the iris that is colored and is clearly visible through a normal
cornea. The opening of the iris is determined by the brightness of the environment
(pupil usually wide open in darkness or when in a sympathetic mood, but the pupil
is narrow in bright light). The pupil size is also under the influence of drugs
where narcotics make the pupil small and tropicamide or phenylephrine makes it
wide. Here is another anatomy site that shows how the beam of light gets through
the eye lens and the vitreous body to the retina in the back of the eye. It also
shows the anterior and posterior eye chambers that are important to know about
when it comes to any abnormality of the circulation of the internal eye fluid.
When the pressure in this system builds up, glaucoma can develop.
ball is very mobile and this is done through the action of the 6
eye muscles. This link
tells more about the way how the macula and the rest of the retina are intricately
shaped and connected to the optic nerve.
A complex nervous pathway connects
the optic nerve impulses to the appropriate part of the brain after the nerve
fibers crossed in the optic chiasm. The optic pathways have a very structured
way of being processed in the brain as is depicted
schematically here (from the Merck Manual).
This knowledge is important when it comes to interpreting the
location of strokes in certain parts of the brain. A person who develops a stroke
in the back part of the brain (occipital area) cannot process the visual images,
because the processing unit in the brain is defective. We know from studies with
microelectrodes during brain surgery that the eyes in a case like this are processing
images normally (cortical or central blindness).
Other parts of the anatomy
of the eye will be further discussed in each of the subchapters.