Structure and Functions of the Eye.pptx

1. Structure and Functions of the
Eye

2. • The eye is composed of the eyeball (globe), the
optic nerve, and the accessory structures, which
comprise the eyelids, conjunctivae, lacrimal
apparatus, and the muscles of the eyeball.

3. Tunics of the Eyeball
• The eyeball has three distinct layers or coats,
known as tunics.
• The outermost external coat (fi brous tunic) is
the supporting layer of the eyeball and is
composed of the anterior cornea and posterior
sclera.
• The sclera is the tough, white part of the fi
brous tunic.
• The middle coat is the vascular tunic and is
composed of the choroid, ciliary body, and iris.

4. • The innermost tunic is the light-sensitive retina
(nerve tunic).
• It consists of several layers; three of its layers are
cells.
• The light-sensitive cell layer consists of the rods
(black-and-white vision) and the cones (color
vision).
• These receptors convert light to a nerve impulse.
The retina is black because of the presence of
melanin.
• This black pigmentation not only assists in the
absorption of light, but also prevents
uncontrolled reflection to other parts of the eye.

6. Cornea
• The cornea is the transparent, forward continuation
of the sclera.
• It is transparent to allow for the entrance of light.
• In a crosssectional view of the cornea, fi ve layers can
be identifi ed from the outside to the inside.
• 1) the anterior epithelium (stratifi ed squamous
nonkeratinizing epithelium),
• 2) subepithelial basement membrane (Bowman
membrane),
• 3 ) substantia propria, or stroma,
• 4) posterior limiting lamina (Descemet membrane),
and 5) posterior endothelium (Descemet
endothelium).

7. • There is greater light transmission if the surface
area ratio of cornea to sclera is increased.
• Nocturnal animals (nighttime) have relatively
larger corneas than diurnal animals (daytime).
• About 17% of the eyeball in the dog is cornea (a
diurnal animal), whereas about 30% of the eyeball
in the cat is cornea (a nocturnal animal).

8. • About 90% of the corneal thickness is a result of
collagen fi bers (called stroma).
• The collagen fibers have an orderly, laminated
arrangement, and this is related to the transparent
nature of the cornea.
• The cornea is avascular (without blood supply) so
that blood vessels do not interfere with the inward
transmission of light.
• The cornea is supplied abundantly with
nonmyelinated nerve fi bers, bare nerve endings
that enter from the limbus and penetrate the
outer epithelial layer.

9. • The cornea is one of the most sensitive tissues of the
body.
• Transparency of the cornea depends further on the
degree of its hydration; the normal transparent cornea
contains less water than it is able to imbibe.
• Increased uptake of water with a consequent reduction
in transparency can occur as a result of damage to
either the anterior epithelium or the posterior
endothelium or to a reduction of oxygen.
• If this occurs, a rearrangement of the collagen fi bers
results, causing the cornea to become cloudy or white.
• Other causes of corneal cloudiness or whiteness are
thinning from increased intraocular tension, disruption
by trauma, or replacement by scar tissue.

10. The Lens and Accommodation
• The lens is positioned between the cornea and
retina.
• It is attached by suspensory ligaments (zonular
fi bers) to the ciliary body, which is a thickened
forward ridge of the choroid that circumscribes
the eyeball (Fig. 5-19).
• The ciliary body contains three sets of smooth
muscle fi bers (ciliary muscles), with each set
oriented in a different direction

11. • Normal ciliary muscle tone results in normal
vision with a lens that is convex and that has a
focal distance (distance from the lens to the
retina) where the image is focused on the retina.
• Convex lenses converge light rays.
• A more convex lens would shorten the focal
distance, and a less convex lens would lengthen
the focal distance.
• Accordingly, when nearer objects are viewed,
the focal distance is in back of the retina and the
lens must become more convex to shorten the
focal distance and bring it to focus on the retina.

12. • This is accomplished by increased ciliary muscle
contraction.
• Because the ciliary muscles encircle the eyeball,
contraction decreases the tension on the lens
ligaments making them more slack (see Fig. 5-19).
• Because of its elastic capsule, the reduced tension
on the ligaments allows the lens to assume a more
convex confi guration and the focal distance is
shortened.
• When more distant objects are viewed, the focal
distance is in front of the retina and the lens must
become less convex (more divergent) in order to
increase the focal distance to the retina

13. • Greater relaxation of the ciliary muscles occurs
and more tension is applied to the elastic lens
capsule thereby making the lens less convex,
and the focal distance is increased because of
the greater divergence.
• The adjustments needed for near and far
objects are known as accommodation.
• Accommodation among domestic animals
seems limited.
• This is thought to be true because of the
sparseness of ciliary muscles, except in the cat.

14. • In the cat, the convexity of the lens increases
during accommodation for near objects to the
extent that it may compress the iris anteriorly to
the cornea.
• Visual acuity is the extent to which details and
forms of objects can be perceived accurately.
• A foveal region is an area of the retina where
there is high visual acuity and is characterized by
a pit (fovea centralis).
• In primates and birds it contains only cone
(color) cells.
• Domestic mammals lack foveas but do have
areas with high visual acuity called visual streaks.

15. Iris
• The amount of light allowed to enter the eye is
controlled by the iris, which is the colored part
of the eye (see Fig. 5-19).
• The allowed opening, of varying size, is called
the pupil.
• The pupil is horizontal in the domestic
herbivores and pig, vertical and elliptic in the
cat, and circular in the dog.

16. • The iris contains two sets of smooth muscles:
• 1) circularly arranged fi bers, innervated by the
parasympathetic division of the autonomic
nervous system; and
• 2) radially arranged fi bers, innervated by the
sympathetic division.
• Contraction of the circularly arranged fi bers
decreases the size of the pupil and allows less
light to enter the eye, whereas contraction of
the radially arranged fibers increases the size of
the pupil and allows more light to enter the eye.