lecture notes

1. Humors of the Eye
• The spaces forward from the lens are divided by
the iris into two parts.
• The space behind the iris and forward from the
lens is called the posterior chamber, and the
space behind the cornea but in front of the iris is
called the anterior chamber.
• Projecting from the ciliary body and into the
posterior chamber are structures known as
ciliary processes (see Fig. 5- 19).

3. • They present a considerable surface area
because of their folded arrangement and are
well vascularized.
• They actively secrete a liquid into the posterior
chamber, the aqueous humor (Fig. 5-20).
• The aqueous humor has free communication
with the anterior chamber and thus occupies
all spaces anterior to the lens.

5. • The transparent material behind the lens that
occupies most of the volume of the eyeball (the
vitreous chamber) is called the vitreous body.
• It does not have the flow characteristics of a
liquid, rather, it is more similar to a gelatinous
mass—hence, the name vitreous body is more
appropriate than vitreous humor.
• Aqueous humor can diffuse through the mass of
the vitreous body, but only slowly.

6. • The principal flow of aqueous humor after its
formation is through the pupil into the anterior
chamber and to where it is reabsorbed at the
iridocorneal angle, which is the angle formed
where the cornea meets the iris (Fig. 5-21).
• Inasmuch as there is a constant formation of
aqueous humor, there must also be a constant
removal.
• It enters the corneoscleral meshwork at the
iridocorneal angle and is directed to aqueous
collecting veins and the scleral venous for its
return to blood.

8. • Aqueous humor functions to:
1) provide nutrition to the avascular lens and
cornea,
2) remove waste products of metabolism from these
structures, and
3) occupy space and maintain a constant distance for
the refractive parts.

9. • The pressure maintained by the aqueous humor
within the eyeball can be measured; it is about
20 mm Hg in the dog.
• This pressure maintains the normal shape and
firmness of the eyeball.
• If the reabsorption of aqueous humor is
impeded, the pressure increases.
• This situation is recognized clinically as
glaucoma and can lead to blindness if left
untreated.

10. Retina
• The innermost tunic of the eye is the nerve tunic,
or neural retina (Fig. 5-22).
• The photoreceptors, the rods and cones, are
located near the outer aspect, immediately
inward from the pigmented epithelium.
• Impulse transmission is directed inwardly toward
the vitreous.
• Considerable convergence of impulses from the
photoreceptors occurs on two interposed cell
layers, of which the innermost cell layer is that of
ganglion cells.

11. • The unmyelinated axons of the ganglion cell layer
arc toward the optic disc, also known as optic
nerve head.
• Here they become myelinated and turn to form
the optic nerve.
• The intraocular myelinated portion of the nerve
forms the optic disc.
• There are no photoreceptors overlying the disc,
hence the optic disc represents a blind spot.
• It is located ventrolateral to the posterior pole of
the eyeball.

12. • The posterior pole is the posterior location of
the optical axis, which is a line drawn from the
center point of the cornea to the center point
of the posterior sphere.
• The retinas of domestic mammals contain
mostly rods and the retinas of domestic birds
contain mostly cones.
• The rods are the photoreceptors associated
with black-and-white vision, and the cones are
those associated with color vision.

13. • The rods are extremely sensitive to light and
are used for night vision, whereas cones
function best in day vision.
• The part of the retina and all associated
structures that are visible with the
ophthalmoscope are referred to clinically as
the ocular fundus.
• The fundi of several domestic animals are
shown in Figure 5-23.

15. Chemistry of Vision
• Light that enters the eye stimulates biochemical
reactions in the rods and cones.
• Chemicals in the rods and cones decompose on
exposure to light.
• The chemical in the rods is called rhodopsin, and
the light-sensitive chemicals in the cones are
only slightly different from rhodopsin.
• The reaction scheme shown in Figure 5-24 is
characteristic of the visual cycle.

16. • Rhodopsin (also known as visual purple) is a
light-sensitive pigment in the outer part of the
rod that is located in the pigmented epithelium.
• It is composed of 11-cis-retinal (also known as
retinene) and scotopsin.
• Scotopsin is a rod protein, and the similar opsin
in cones is photopsin

18. • Exposure of rhodopsin to light energy
immediately begins its decomposition, in which a
number of unstable, short-lived (nanoseconds
for prelumirhodopsin and seconds for
metarhodopsin II) intermediates are formed.
• The final one, metarhodopsin II, triggers highly
amplified visual excitation and splits into
scotopsin and all-trans-retinal.

19. • All-trans-retinal is chemically the same as 11-cis-
retinal but has a different physical structure; it is a
straight rather than a curved molecule.
• Its conversion to 11-cis-retinal requires the presence
of the retinal enzyme isomerase.
• All-trans-retinal is converted to 11-cis-retinal, which
then recombines with scotopsin to reform
rhodopsin.

20. • Rod stimulation is believed to occur at the
instant that the rhodopsin molecule becomes
excited by light.
• The stimulation resulting from an
instantaneous flash of light can persist for
about 0.05 to 0.5 s, depending on the intensity
of the light.
• Rapidly successive flashes with alternating
intensity become fused to give the appearance
of being continuous.

21. • This effect is observed when watching motion
pictures or television.
• There is a relationship between vision and
vitamin A.
• A lack of vitamin A results in inadequate
formation of rhodopsin.
• Night vision requires optimum amounts of
rhodopsin, and its shortage, because of
vitamin A deficiency, is referred to as night
blindness.

22. Adaptation to Varying Light
• Dark adaptation refers to an adaptation to
relatively dark environments.
• Because of less light, the concentration of
rhodopsin increases, allowing for maximum
reaction to the available light.
• When first entering a dark room, one might be
almost unable to see anything, but after dark
adaptation, objects can be perceived more
readily.

23. • Light adaptation refers to an adaptation to lighter
environments.
• The higher concentration of rhodopsin decomposes
because of the abundance of light.
• The images perceived seem to be overexposed.
• Normal vision returns when the rhodopsin
concentration is balanced with the available light

24. • Concurrent with the adaptation processes are
the visual refl exes, which increase or decrease
the diameter of the pupil (see previous section).
• Consequently, not only does rhodopsin
concentration increase in the dark, but the pupil
diameter also increases to allow for maximum
light entry.
• Conversely, rhodopsin concentration decreases
in light, and pupil size decreases to minimize
light entry.

25. • The tapetum is a light-refl ecting layer of cells
of the inner choroid, located just outside the
retinal-pigmented epithelium (Fig. 5-25).
• Melanin is absent from the pigmented
epithelium of the retina where the tapetum is
present.
• The tapetum is not present throughout the
choroid and varies in size among those domestic
species in which it is present (e.g., cats, dogs,
horses, ruminants).

26. • The tapetum allows light that has just
stimulated the receptor cells to be refl ected
back onto them so that they receive another
stimulation.
• In this way greater vision is obtained, even with
minimal light.
• The refl ected light continues on a forward path
through the pupil and out of the eye again.
• This refl ected light is termed eyeshine, which is
when eyes glow at night in the presence of
light.

27. Field of Vision
• The field of vision for an animal is the spatial
area from which the complete image is formed.
• The more lateral the placement of the eyes,
the larger the field of vision.
• In fact, some animals might even see
everything around them with the exception of
objects directly behind their body, which can
still be seen with only a slight movement of the
head

28. • If the field of vision for each eye overlaps that of
the other, a binocular area of vision is formed;
conversely, a monocular area of vision is formed
if there is no overlap.
• Binocular vision provides greater depth
perception, and this is more pronounced in
animals that prey on other animals for food.
• Greater accuracy of position is necessary before
the leap. Such animals characteristically have
more forward-placed eyes (Fig. 5-26).

30. • In contrast, herbivores (plant eaters) have more
laterally placed eyes and have a wider field of
• vision; this gives greater protection while grazing
as far as predator observation is concerned (Fig.
5-27).
• In all domestic animals, regardless of how far
their eyes are situated laterally, there is some
central area of overlap providing a zone of
binocular vision.

32. • The horse has little or no accommodation.
• When observing more distant objects, the horse
increasingly raises its head and lifts up the nose.
When observing much closer objects, the horse
may arch the neck and rotate the head on one
side.
• In the past, these behaviors were explained as an
attempt to make up for the lack o
accommodation by using a rampshaped retina.

33. • This was presumed to provide a longer focal
distance for viewing downward than for
viewing along the axis of the eye.
• It was assumed that this retinal feature
accounted for the alterations in head position
whereby the animal was finding a distance
from lens to retina appropriate for focusing
the image.

34. • It is now known that the ramp-shaped retina does
not exist.
• Measurements have been made indicating that
the horse has a retina that is equidistant from the
lens except in the far dorsal and far ventral retina.
• In these peripheral regions, the retina is nearer to
the lens, not farther as proposed for the ramp-
shaped retina.

35. • Ganglion cell densities in the horse have been
mapped throughout the retina and correlated
with maximum visual acuity.
• Densities of cells are low in the periphery and
high in the ventrally placed visual streak,
which is a strong narrow region visible in the
ventral retina immediately above the optic
nerve head.

36. • In retinal regions other than the visual streak, acuity
in the horse is very low.
• Acuity is similar at any point along the narrow
streak, and the horse can see a narrow, very
circumscribed frontal and circular view.
• Because peripheral acuity is quite low, it would be
of little benefit for a horse to use any part of the
retina other than the visual streak for direct
observation

37. • When the horse lifts its head and points its
nose forward to use its binocular field to scan
the horizon, its monocular field is lessened and
lateral vision becomes more limited (see Fig. 5-
27).
• When the animal lowers its head, such that the
nose approaches the vertical, the binocular
vision is directed toward the ground for grazing
and the lateral monocular fields are again in
position to scan the lateral horizon.

38. • The horse can attend to either the frontal field with
the head raised or the lateral field with the head
lowered.
• The horse has a frontally placed blind field such that
when the nose is drawn in and the face approaches
the vertical, the animal is unable to see directly in
front.
• This situation occurs when the horse is being ridden
“on the bit” with the neck arched and the nose just
in front of the vertical.
• If a show jumping horse is to see and judge the
distance of a fence that it approaches, it must have
the ability to raise its head and direct its binocular
field forward.