Page 266 - Anatomy and Physiology of Farm Animals, 8th Edition

Sense Organs / 251

Visual Field and Light Path where objects are simultaneously viewed by
both eyes, is the binocular field; the visual
VetBooks.ir The part of the environment from which cortex evaluates the slightly different view

from each eye and uses the information to
light will enter the eyes and stimulate the
retinas is the visual field (Fig. 12‐15). In provide depth perception. Prey animals, on
predators and arboreal animals, such as the other hand, have lateral eyes with a much
birds and primates (for whom accurate smaller binocular field. Such eye placement
depth perception is essential), the eyes are increases the peripheral vision so that the
placed so that the visual fields overlap to combined visual field is nearly completely
varying degrees. This region of overlap, panoramic. Such vision is monocular (seen
only with one eye) and therefore lacks very
accurate depth cues, but the clear advantage
of this wide field of view for a prey animal
needs no explanation.
Light traveling from the visual field to
Binocular field
(both eyes) the retina passes through a series of trans­
parent media that refract and focus it on
the light‐sensitive retina of the posterior
part of the globe. These dioptric media
include the cornea, the aqueous, the lens,
and the vitreous body. As indicated before,
the cornea is actually the most refractive
medium of the eye, but the lens is the only
part of the light path with the ability to
change its refractive index. This property
makes it the organ of accommodation for
focus on near objects. Light entering the
Monocular field Monocular field vitreous chamber of the eye is bent by the
(left eye) (right eye)
more anterior parts of the eye in such a
way that the image that is focused on the
Blind area
retina is inverted and reversed.
The site where the ganglion cell axons
leave the eye (the optic disk) has no pho­
toreceptors and is therefore considered
the blind spot of the retina. In primates, a
region dorsolateral to the optic disk is
relatively free from large blood vessels
and especially densely packed with photo­
receptors (particularly cones). This is the
region of greatest visual acuity, the macula.
Domestic animals lack a macula.

Visual Pathways of the Brain
Figure 12-15. Visual fields of the ox as viewed
from above. The region seen by both eyes (binoc­
ular field) is the region of best depth perception, The electrical information generated by
but the nose creates a wedge‐shaped blind spot exposure of photoreceptors to light under­
directly in front of the animal. Peripheral vision goes initial neural processing within the
(the monocular fields) creates a nearly complete retina. This information ultimately leaves
circle of vision around the animal, except for a the eye via the optic nerve, the fibers of
small region directly behind. which are the axons of the ganglion cells of