Anatomy
Of The EyeIn 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. The eye ball is very mobile and this is done through the action
of the 6 eye muscles. This gross
anatomy of the eye site tells more about this and 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 to know when it comes to 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. |
