Page 265 - Anatomy and Physiology of Farm Animals, 8th Edition

250 / Anatomy and Physiology of Farm Animals

In clinical practice the retina is routinely This arrangement – often called a “ramp
viewed through the pupil with an ophthal­
VetBooks.ir moscope; the portion of the retina that retina” – is supposed to account for a
horse’s tendency to toss his head while
can be viewed in this manner is called the
fundus (Fig. 12‐14). looking at something odd or alarming,
the idea being that head movements
The retina has many histological layers. would bring the object into focus on the
One of these is the photoreceptor layer, in appropriate part of the retina and that
which are found the specialized neural accommodation by the lens is not the
receptor cells of the visual system, the rods primary focusing tool of the horse. It is
and cones. Each photoreceptor features an attractive idea for the same reason
an external segment that consists of orderly much folk wisdom surrounding horses
stacks of flattened, disk‐shaped plasma is attractive: it sounds like it explains a
membrane. These are replete with photo­ peculiarly equine behavior. But the
pigment, the molecules that are sensitive ramp retina theory has been unambigu-
to photons of light. Photoreceptors have ously debunked. Horses focus their eyes
one of two basic shapes: either a tall cylinder for close‐up vision like all animals do to
or a shorter, tapering stack. The cylindrical greater or lesser effect: they change the
photoreceptors are rods, equipped with a shape of the lens.
photopigment that renders them sensitive
to very low levels of light. The tapering
photoreceptors are cones, each expressing Lens
one of three known photopigments with
sensitivity to a specific wavelength of light. The lens is a transparent proteinaceous
Cones are, therefore, color receptors. As a biconvex disk suspended between the
rule, animals with nocturnal habits have posterior chamber and the vitreous chamber.
retinas populated primarily with rods, It is surrounded by an elastic capsule
whereas those with diurnal habits (e.g., that serves as an attachment site for the
primates and birds) have more cones, par­ suspensory ligaments at the lens’s equator.
ticularly in the region that receives light Cells of the lens are called lens fibers,
from the center of the visual field. and they are generated continuously
Most domestic animals have some color throughout life. As these are added to the
vision. Their retinal cones are primarily outside of the lens, older lens fibers are
sensitive to blue and yellow wavelengths pushed toward the center, resulting in a
of light, with few or no red‐sensitive lamellar arrangement of fibers, often
cones. Vision in these species is probably likened to the layers of an onion.
rich in blue, green, and yellow tones. As the animal ages, more lens fibers
The flapping red cape wielded by the are added to the outside of the lens,
matador therefore attracts the bull by its compressing the fibers at the center.
movement and not by its color. These old cells harden and after middle
The photoreceptors synapse with other age begin to lose their transparency. This
neurons of the retina, and visual informa­ change is lenticular sclerosis. Although
tion undergoes early neural processing the sclerotic lens looks milky, lenticular
within the retina. Ultimately, the axons of sclerosis does not usually interfere with
neurons called ganglion cells project to vision. More severe changes in the lens
the optic disk, where they exit the globe fibers can produce significant opacity
and become the optic nerve. that interferes with transmission of light
A story that has been retold for many and therefore vision. These opacities are
years is that the equine retina slopes in cataracts. Cataracts can be congenital or
the posterior part of the eye in such a can develop as a consequence of injury
way that some parts are closer and others to the lens or metabolic disease (e.g.,
more distant from the front of the eye. diabetes mellitus).