Biomedical Engineering Reference
In-Depth Information
termed the cones, which enable us to distinguish between different wavelengths of light
but they are not as sensitive to intensity. There are three types of cones to help determine
different colors, but they require a higher intensity light to function properly. There is a
higher density of rods along the periphery of the eye, whereas there is a higher density of
cones along the center of the eye.
The interior portion of the eye is hollow and is composed of two chambers: the poste-
rior cavity and the anterior cavity, which are divided by the ciliary bodies and the lens.
The posterior cavity (also known as the vitreous chamber) contains the vitreous body. This
chamber makes up approximately 90% of the hollow portion of the eye. The vitreous body
primarily helps to maintain the shape of the eye. The vitreous body is a gel (which can be
considered as a very viscous fluid) because cells within this cavity produce and secrete a
large quantity of collagen fibers and proteoglycans. The collagens and the proteoglycans
are highly charged and can organize and absorb large quantities of water. This water is
associated closely with the charged molecules and is not as free to move as compared
with water in the bulk phase. The vitreous body is only produced during fetal develop-
ment and is not replaced during the remaining portion of life.
The anterior cavity is divided into an anterior chamber and a posterior chamber and is
filled with the biofluid aqueous humor. The anterior chamber is bounded by the cornea
and the iris, while the posterior chamber is bounded by the iris and the ciliary bodies/
lens. The aqueous humor also helps to maintain the shape of the eye, but it has other func-
tions as well. The aqueous humor is continually formed and circulates through the ante-
rior cavity as well as along the surface of the retina. Some aqueous humor enters the
vitreous chamber, and its movement through this chamber can be described by diffusion
laws better than fluid mechanics laws, because of the gel that fills the cavity. The forma-
tion of aqueous humor will be discussed extensively in
Section 10.2
, and the flow of aque-
ous humor will be discussed in
Section 10.4
.
The remaining structure of the eye that we will describe here is the lens. The lens is
held in place by ligaments attached to the ciliary bodies. The function of the lens is to
focus incoming light onto the photoreceptors located within the retina. To accomplish this
task, the shape of the lens can be altered. Cells within the lens are organized into concen-
tric circles, layered throughout the lens. These cells are all covered by a fibrous capsule
mainly composed of elastic fibers that tend to make the lens less spherical. Elastic fibers
along the periphery of the lens are associated with the ciliary body ligaments that mechan-
ically pull on the lens to make it more spherical. Cells within the interior of the lens are
termed lens fibers. These cells do not have a nucleus or other organelles, but instead are
filled with transparent proteins called crystallins. Crystallins give the lens the ability to
focus light onto the retina. In fact the aqueous humor and the vitreous body also function
as a liquid lens that helps to further focus the light onto the retina. Light is primarily
focused by changing the shape of the lens and small changes to the shape of the eye.
When viewing objects that are at a distance, the lens is flat and elongated. However, when
the object that is being viewed is close, the lens becomes more spherical. This change in
shape is accomplished by a contraction of the ciliary muscles, which pulls the choroid
layer of the eye toward the lens, causing the associated ligaments to relax. Due to this
reduced tension on the lens, the elastic lens becomes thicker and rounder to bend light
more rapidly.
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